Eye Vision Testing System and Eyewear Using Micromachines

Article Eye Vision Testing System and Eyewear Using Micromachines

Nabeel A. Riza *, M. Junaid Amin and Mehdi N. Riza

Received: 7 September 2015 ; Accepted: 29 October 2015 ; Published: 6 November 2015 Academic Editors: Pei-Cheng Ku and Jaeyoun (Jay) Kim

School of Engineering, University College Cork, College Road, Cork, Ireland; [email protected] (M.J.A.); [email protected] (M.N.R.) * Correspondence: [email protected]; Tel.: +353-21-490 (ext. 2210)

Abstract: Proposed is a novel eye vision testing system based on micromachines that uses micro-optic, micromechanic, and microelectronic technologies. The micromachines include a programmable micro-optic lens and aperture control devices, pico-projectors, Radio Frequency (RF), optical wireless communication and control links, and energy harvesting and storage devices with remote wireless energy transfer capabilities. The portable lightweight system can measure eye refractive powers, optimize light conditions for the eye under testing, conduct color-blindness tests, and implement eye strain relief and eye muscle exercises via time sequenced imaging. A basic eye vision test system is built in the laboratory for near-sighted (myopic) vision spherical lens refractive error correction. Refractive error corrections from zero up to ´5.0 Diopters and ´2.0 Diopters are experimentally demonstrated using the Electronic-Lens (E-Lens) and aperture control methods, respectively. The proposed portable eye vision test system is suited for children’s eye tests and developing world eye centers where technical expertise may be limited. Design of a novel low-cost human vision corrective eyewear is also presented based on the proposed aperture control concept. Given its simplistic and economical design, signiﬁcant impact can be created for humans with vision problems in the under-developed world.

Keywords: vision correction; ophthalmic examination; eyewear; eye test; low cost eyewear

1. Introduction A first focus of this paper is on efficient ophthalmic tests where eye examinations of young patients with vision limitations are carried out in order to ensure that prescriptions for the eyeglasses or contact lenses of the corrective lenses may be quickly issued to them [1–3]. Today, these examinations are carried out through the use of phoropters, bulky optical instruments which patients are asked to look through while the optometrist manually changes certain settings and combinations of lenses until the patient is able to see a perfectly clear image of an eye chart across the room. There are several issues with this kind of examination, one of these being that these tests, depending on the patient, can take a long time (possibly up to 15 min). Another issue is that patients often have trouble distinguishing between the effects of certain lenses in the phoropter, leading the optometrist having to just pick one of them based on subjective instincts. Thus, these modern day eye evaluation methods are not very suitable for young children (aged 4–10 years). Not only do the test’s subjectivity and the required patient participation make it harder for younger patients to get an accurate prescription, it could also be very difficult for a younger child to sit still for the duration of the examination. Because small children have limited patience and perseverance for these types of examinations, their parents may prefer to wait until they are older before taking them to the eye doctor. However, this means that existing vision defects will remain undiscovered and uncorrected for several years,

Micromachines 2015, 6, 1690–1709; doi:10.3390/mi6111449 www.mdpi.com/journal/micromachines Micromachines 2015, 6, 1690–1709 possibly allowing them to grow in severity. With the increased use of internet-based visual display platforms, there is a concern about myopic progression in children caused by greater lengthening of the eye that in some cases can be so rapid that it can cause retinal detachment or bleeding of the eye via abnormal blood vessel growth. Inaccessibility of widespread eye care facilities in developing countries due to the high costs linked with the necessary technical expertise requirements to operate bulky and costly modern eye test equipment leads to a further urgency to develop a portable and automated eye vision test system. For these reasons, a lightweight and electrically programmable basic eye vision evaluation system is proposed for replacing the classic mechanics-based phoropter so as to enable a more cost effective, time-efficient, easier to operate, and “child friendly” early vision testing experience [4,5]. A second focus of the present paper is to propose and demonstrate the working principles of a new kind of ultra-low cost eye vision test system and its related large scale economically-produced eyewear that can be easily designed, fabricated, and provided at low cost to people from the disadvantaged world like parts of Asia, Africa, and the Middle East. Traditional eyewear is based on relatively expensive optical lenses either in spectacles or as contact lenses. The foundations of this new eye vision test system and eyewear is aperture-controlled imaging that is able to produce adequate in-focus vision for human eyes with refractive power limitations without the use of traditional and expensive refraction power corrective lenses in eyewear. The proposed novel eyewear designs are limited to specified vision ranges, limited refractive powers, and require adequate lighting levels. Nevertheless, it is expected that the high affordability of the proposed new technology for eye vision testing and care will improve the sight and hence lives of peoples from the underdeveloped world, a key motivation of the current work. The paper begins with a history of the use of E-lenses for human vision care starting with the proposal to use E-lens-based spectacles for sight correction and later the use of E-lenses to perform human eye vision testing. Next, the proposed eye vision test system design is described based on micromachine-based devices. Three modes of the eye test system operations are described, including the basic optical principles governing these operational modes. Low cost vision correction eyewear design is described without the use of refractive lens-optics. Basic experiments demonstrating the principles of the proposed ideas are presented and the paper ends with a final conclusion.

2. History of Electronic Lenses in Human Vision Care As aforementioned, eye vision correction measurement is traditionally done using bulky opto-mechanical systems called phoropters [6,7] that commonly have large moving parts and minimal handheld portability (see Figure1). It would be highly desirable to have a vision (for prescription) test instrument that has minimal large moving parts and uses compact low power consumption optical and electronic devices. Although the design of liquid variable focus lenses has been around since the early 1940s, such as via the 1942 U.S. Patent by Flint [8], the desire to use mechanically programmable optical lens devices to replace spectacles for everyday use has been around since the 1960s. For example, Basil M. Wright (see Figure2) was awarded a 1968 patent proposing the use of mechanically controlled liquid variable focus lenses for spectacle eyewear [9,10]. In 1979, Sato of Japan [11] proposed the use of electrically programmable optical lens devices, now called Electronic lenses or E-lenses based on Liquid Crystals (LCs) to form electrically controlled varifocal spectacles worn by people who need both distance and reading glasses. In 1980, Dwight W. Berreman was issued a patent for an LC E-lens design suitable for use in eyeglasses [12]. Sato also proposed the use of LC E-Lens for auto-focus cameras, but Sato, Berreman and Wright did not propose E-lenses for eye refractive power vision tests and measurements. In 1981, LC-based lenses in spectacles were also proposed to control the light transmission passing to the eyes to control image brightness [13].

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(GE-CRD) fabricated novel electronically controlled LC lens devices [17], including for eye correctionMicromachines measurement 2015, 6, page–page applications [18,19]. Specifically, to the best of our knowledge, Riza, in 1989, first proposed the use of the LC E-Lens for eye exams (see Figure 3) to measure refractive power (GE-CRD)Micromachines fabricated2015, 6, 1690–1709 novel electronically controlled LC lens devices [17], including for eye change [18]. correction measurement applications [18,19]. Specifically, to the best of our knowledge, Riza, in 1989, first proposed the use of the LC E-Lens for eye exams (see Figure 3) to measure refractive power change [18].

Figure 1.Figure Example 1. Example of a typical of a typical commercial commercial phoropter phoropter systemsystem commonly commonly used used today today to measure to measure eye eye refractiveFigurerefractive errors1. Example errorsto determine of to a determine typical human commercial human eyewear eyewear phoropter prescriptions system [7 ].commonly[7]. used today to measure eye refractive errors to determine human eyewear prescriptions [7].

Figure 2. Basic design of the 1968 Basil M. Wright proposed for a mechanically controlled variable FigureFigure 2. Basic 2. Basic design design of the of the1968 1968 Basil Basil M. M. Wright Wright prop proposedosed forfor aa mechanicallymechanically controlled controlled variable variable focalfocal length focallength spectacles length spectacles spectacles based based based on on liquid onliquid liquid movement movement [10]. [[10].10].

Since the 1960s, the Radio Corporation of America (RCA) and later the parent company General Electric (GE), pioneered the use of LCs for designing optical displays such as in watches, avionics, and television systems, and also for designing optical switches [14–16]. Starting in the mid-1980s, the LC Display (LCD) lab at the General Electric Corporate Research and Development Center (GE-CRD) fabricated novel electronically controlled LC lens devices [17], including for eye correction measurement applications [18,19]. Speciﬁcally, to the best of our knowledge, Riza, in 1989, ﬁrst proposed the use of the LC E-Lens for eye exams (see Figure3) to measure refractive power change [18]. In addition, apart from using an electronic LC lens in a phoropter to get new eyewear refractive readings [18], two dimensional (2-D) optical Spatial Light Modulator (SLM) devices via LC and micromachined or Micro-electromechanical Systems (MEMS) devices plus bias lenses have been proposed in 1995 by Riza, not only for eye refractive power measurements (both positive and negative Diopters), but also for color blindness tests as well as to provide the capability to perform eye strain relief and eye muscle exercises [20,21]. Because 2-D SLMs are optically phase programmable devices, astigmatic refractive error measurement in the human eye was proposed in these SLM-based eye vision test systems. In addition, Riza proposed the use of switching to different optical paths in the eye vision system to measure eye refractive power [20,21]. In 1998, Riza introduced the design of a fast response digital LC E-lens for numerous 3-D beamforming applications including vision [22–24].

1692 Figure 3. Basic design of the 1989 Nabeel A. Riza proposed E-lens based human eye vision test

system. Adapted from [18]. Figure 3. Basic design of the 1989 Nabeel A. Riza proposed E-lens based human eye vision test 3 system. Adapted from [18]. 3 Micromachines 2015, 6, page–page

(GE-CRD) fabricated novel electronically controlled LC lens devices [17], including for eye correction measurement applications [18,19]. Specifically, to the best of our knowledge, Riza, in 1989, first proposed the use of the LC E-Lens for eye exams (see Figure 3) to measure refractive power change [18].

Figure 1. Example of a typical commercial phoropter system commonly used today to measure eye refractive errors to determine human eyewear prescriptions [7].

Figure 2. Basic design of the 1968 Basil M. Wright proposed for a mechanically controlled variable focal length spectaclesMicromachines 2015 based, 6, 1690–1709 on liquid movement [10].

Figure 3. Basic designFigure 3. Basic of designthe 1989 of the 1989 Nabeel Nabeel A.A. Riza Riza proposed prop E-lensosed based E-lens human eye based vision test human system. eye vision test Adapted from [18]. system. Adapted from [18]. Independent use of the LC E-Lens for eye exams to measure refractive power change was patented in 1994 (see Figure4) by Piosenka and3 Leahy [25] and licensed by E-Vision (same as Pixel-Optics). Interestingly, the same basic idea was patented by E-Vision in 2003 by Blum and Dustin [26]. The use of liquid-based lenses using mechanical pressure to change lens focal length has also been proposed for phoropters in a patent by Quaglia in 1995 [27] and later in 2010 by the group of Peyghambarian [28]. In 1996, an LC diffractive-optic E-Lens had been designed [29] and in 2006, the Peyghambarian group suggested the use of the LC diffractive-optic E-Lens for eye exams to measure eye refractive power change [30]. In 2003, a team from Lawrence Livermore National Lab demonstrated an adaptive optics phoropter where a Shack-Hartmann sensor was used to measure the aberrations of the eye and an LC SLM was used to compensate these eye aberrations [31]. More recently, works have been conducted to improve both LC and liquid device design and performance in phoropter systems. Large 20 mm diameter E-lenses have been designed with LC and electromagnetic force controlled liquid materials with 3 Diopters and ˘25 Diopters powers, respectively [32]. A double-layered LC E-lens has been designed that has polarizer-free operations [33]. In addition, aberrations in an astigmatic liquid E-lens have been compensated using a Shack-Hartmann adaptive optics system [34]. These recent E-lens device technologies and adaptive optic system advances can be incorporated in the proposed eye vision system using micromachines to enable user scenario-dependent optimal performance.

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In addition, apart from using an electronic LC lens in a phoropter to get new eyewear refractive readings [18], two dimensional (2-D) optical Spatial Light Modulator (SLM) devices via LC and micromachined or Micro-electromechanical Systems (MEMS) devices plus bias lenses have been proposed in 1995 by Riza, not only for eye refractive power measurements (both positive and negative Diopters), but also for color blindness tests as well as to provide the capability to perform eye strain relief and eye muscle exercises [20,21]. Because 2-D SLMs are optically phase programmable devices, astigmatic refractive error measurement in the human eye was proposed in these SLM-based eye vision test systems. In addition, Riza proposed the use of switching to different optical paths in the eye vision system to measure eye refractive power [20,21]. In 1998, Riza introduced the design of a fast response digital LC E-lens for numerous 3-D beamforming applications including vision [22–24]. Independent use of the LC E-Lens for eye exams to measure refractive power change was patented in 1994 (see Figure 4) by Piosenka and Leahy [25] and licensed by E-Vision (same as Pixel-Optics). Interestingly, the same basic idea was patented by E-Vision in 2003 by Blum and Dustin [26]. The use of liquid-based lenses using mechanical pressure to change lens focal length has also been proposed for phoropters in a patent by Quaglia in 1995 [27] and later in 2010 by the group of Peyghambarian [28]. In 1996, an LC diffractive-optic E-Lens had been designed [29] and in 2006, the Peyghambarian group suggested the use of the LC diffractive-optic E-Lens for eye exams to measure eye refractive power change [30]. In 2003, a team from Lawrence Livermore National Lab Micromachinesdemonstrated2015 an, 6, 1690–1709adaptive optics phoropter where a Shack-Hartmann sensor was used to measure the aberrations of the eye and an LC SLM was used to compensate these eye aberrations [31].

(a) (b)

Figure 4. Basic design of the 1994 U.S. Patent by Gerald V. Piosenka and Peter J. Leahy that proposed Figure 4. Basic design of the 1994 U.S. Patent by Gerald V. Piosenka and Peter J. Leahy that proposed E-lens spectacles based human eye vision test system [25]. E-lens spectacles based human eye vision test system [25].

More recently, works have been conducted to improve both LC and liquid device design and 3. Proposed Eye Vision System using Micromachines performance in phoropter systems. Large 20 mm diameter E-lenses have been designed with LC and electromagneticToday micromachine-based force controlled liquid microelectronic, materials with micromechanic, 3 Diopters and and ±25 micro-optic Diopters powers, device technologiesrespectively [32]. have A evolved double-layered to a very LC high E-lens degree, has be providingen designed energy that efﬁcient, has polarizer-free compact and operations lightweight [33]. sub-systems.In addition, Inaberrations particular, Radioin an Frequency astigmatic (RF) li wirelessquid E-lens control have is commonplace been compensated for many handheldusing a systemsShack-Hartmann and highly adaptive reliable liquid-basedoptics system Electronically [34]. These recent Controlled E-lens Variable device technologies Focus Lens (ECVFL) and adaptive (also calledoptic system an E-lens) advances micro-optic can be devices incorporated have entered in the proposed the commercial eye vision arena, system for purposes using micromachines such as using electro-wettingto enable user scenario-dependent technology and electromagnetic optimal performance. actuation of elastic polymer membranes [35,36]. In addition, MEMS devices including optical MEMS devices have reached commercial maturity, including3. Proposed applications Eye Vision in System ﬁber-optical using Internet Micromachines and inertial guidance applications [37]. Hence, what is proposedToday next micromachine-based is a basic design of microelectronic, a human eye vision microm testechanic, system and that micro-optic combines the device use oftechnologies advanced micro-optic,have evolved micromechanic, to a very high degree, and microelectronic providing energy technologies efficient, compact to deliver and lightweight a portable, sub-systems. lightweight systemIn particular, that can Radio measure Frequency eye refractive (RF) wireless powers, control conduct is commonplace color-blindness for tests, many and handheld also implement systems eyeand strain highly relief reliable and liquid-based eye muscle exercisesElectronically via time Contro sequencedlled Variable imaging. Focus Lens (ECVFL) (also called an E-lens)Shown micro-optic in Figure devices5a is thehave basic entered design the ofco themmercial proposed arena, eye for vision purposes test systemsuch as using using micromachines. The test system eyewear per eye includes an ECVFL (E-lens), a bias lens (BL), and novel frontend smart optics module. The dioptric4 power of the BL depends on the refractive power performance of the E-lens, as well as the refractive power testing range desired for the eye test system. The smart optics module shown in Figure5b can include an aperture control device, color control device, optical throughput/attenuation control device, polarization control device, and energy-harvesting device. The use or physical deployment of these smart devices depends on the level of eye test features required for the test system scenario. The smart optical devices can be stackable, ultra-thin, electrically controlled optical cells that are fabricated using a variety of LC materials (e.g., polymer dispersed LCs to form the attenuation control device), ultra-thin glass substrates, and thin polarization optics. Multiple functionalities for light control can be within a single ultra-thin device. These devices can also be fabricated using other technologies such as liquids, silicon MEMS, and liquid MEMS. When electronically controlled for desired best vision, e.g., 20/20 setting, the described test eyewear combines with the eye’s internal refractive optic (Eye Lens and Cornea) to project a clear image on the retina while optimizing operations of all devices in the light path from test scene on screen to the eye retina. RF/optical wireless communication between the test eyewear and the Personal Computer (PC) controls the smart optics, as well as the ECVFL focal length settings. The eyewear is also equipped with energy harvesting and storage devices to enable wireless remote energy transfer to power components such as the ECVFL.

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(a)

(b)

Figure 5.5. (a(a) )Basic Basic design design of of the the proposed proposed novel novel eye eye vision vision test system using micro-optics, micromechanics, andand microelectronics.microelectronics. The system frontend encompasses the smart optics for light conditioning inin thethe contextcontext ofof thethe speciﬁcspecific propertiesproperties ofof thethe humanhuman eyeeye beingbeing tested.tested. The test system eyewear includes devices that enable wireless communication and control links, energy harvesting, energy storage,storage, andand remoteremote energyenergy delivery.delivery. BL:BL: biasbias lens.lens. PC: personalpersonal computer.computer. ( b) NovelNovel frontendfrontend smartsmart opticsoptics designdesign whichwhich includes an aperture control device,device, color control device, optical throughput/attenuationthroughput/attenuation control control device, device, polarizatio polarizationn control device, and energy harvesting device.

The Pico Projector, which can use a micro MEMS chip (e.g., TI Digital Light Processing chip), Modern ECVFLs come in different sizes with specific power requirements and are based on a micro LC chip (e.g., Liquid Crystal on Silicon (LCoS)), or a laser scanner, displays the examination different technologies including electrowetting [35], LCs [24,30], deformable mirror membrane [38], content on a screen. One can envision mounting a micro sized projector as part of the eyewear. The and others. For example, Optotune’s EL-6-18 device [36] utilizes optical fluids and a polymer projector is connected to the PC, which communicates with the eyewear optics via wireless membrane to achieve variable focus action in less than 2 ms with a power consumption ranging communication. Depending on eye examination requirements and prior patient information, the between 0 to 350 mW depending on driver current, which is governed by desired focal length projected display can either be static images/text, video, or a hybrid data set to initiate appropriate requirements. For the proposed system using this particular Optotune ECVFL model, ECVFL patient response. Depending on patient feedback, the smart optics are adjusted via PC control operational power needs to be supplied from micro-electronic components mounted on the proposed according to pre-set instructions. The visual data sets are synchronized with both smart optics eyewear. One option to provide this power is via a micro-sized battery, possibly via a rechargeable control and patient feedback such that the test system automatically converges to the eye’s inherent photocell. The photocell can be designed to be sensitive to a particular wavelength band depending refractive error accurately, enabling allocation of correct subscription eyeglasses. This means the on the lighting environment for the vision test system. Various wireless power transfer techniques proposed system does not necessarily need technically trained opticians to oversee eyesight diagnosis. Note that the PC can be replaced by modern-day handheld devices such as smartphones 1695 6 Micromachines 2015, 6, 1690–1709 are gaining popularity and can serve as a viable option. Magnetic coupling technology [39–41] is able to wirelessly transfer up to 60 W over a 2 m range, though the size of the receiver would need to be customized for the proposed eyewear application with its low power requirements. Solar cells for energy harvesting applications as well as micro-sized rechargeable batteries [42–44] can prove to be a relatively inexpensive option in the long run. Once power is supplied, communication via a wireless link needs to be established to convey information necessary for the programmable micro-optics. This communication link can either be RF Wi-Fi or via recently proposed optical energy efficient wireless indoor links [45–47] with an appropriate interface to the programmable optics. In addition, the wireless optical link can also naturally be used for remote energy transfer such as via the spatially controlled light from the eye safe LED or laser source. In summary, for a complete functioning system, the various components on the proposed eyewear and their interconnections need to be designed and optimized to keep the overall dimensions of the eyewear to a minimum weight and size while delivering the required performance for the eye vision system. The Pico Projector, which can use a micro MEMS chip (e.g., TI Digital Light Processing chip), a micro LC chip (e.g., Liquid Crystal on Silicon (LCoS)), or a laser scanner, displays the examination content on a screen. One can envision mounting a micro sized projector as part of the eyewear. The projector is connected to the PC, which communicates with the eyewear optics via wireless communication. Depending on eye examination requirements and prior patient information, the projected display can either be static images/text, video, or a hybrid data set to initiate appropriate patient response. Depending on patient feedback, the smart optics are adjusted via PC control according to pre-set instructions. The visual data sets are synchronized with both smart optics control and patient feedback such that the test system automatically converges to the eye’s inherent refractive error accurately, enabling allocation of correct subscription eyeglasses. This means the proposed system does not necessarily need technically trained opticians to oversee eyesight diagnosis. Note that the PC can be replaced by modern-day handheld devices such as smartphones or tablets. Decreasing price of wireless technology and its wider availability means that this simple to use system is likely suitable for deployment in medical centers and shops for self-testing. The ease and speed of use to provide vision test measurements will enhance usage with children, a key motivation for the proposed Figure5 system. The Figure5 eye test system can operate in three different modes to measure the needed eye refractive power and to achieve non-blurred clear vision of the test target. The first mode is the classical refractive error measurement method similar to a mechanical phoropter where except for mechanically changing fixed power lenses in the eye vision path, an E-lens without macro-mechanical motion creates lenses of different powers at the eye spectacle positions to determine when the patient feels that he/she has seen the best non-blurred image of the test target such as a 20/20 vision standard when seeing line 8 in the chart. Figure6 shows a typical standard eye vision Snellen chart [48] that is used today in phoropter systems where this eye chart is viewed by a patient from a distance of 20 ft (or 10 ft when using a mirror in the vision path). Based on the smallest text on a line that the patient is able to read, a quantification of the patient’s near sightedness is determined for both the patient’s uncorrected eye and the E-lens refraction corrected eye. The E-lens is driven by an electrical control signal (e.g., current) that is calibrated to spherical lens power values. Thus by noting the current of the E-lens that gives the best non-blurred vision to the patient, the patient’s corrective lens prescription is determined directly in Diopters (D) with a standard 0.25 D step size. Here it is assumed that apertures of devices used are much larger than the natural aperture of the open pupil in the eye so that the pupil functions as the limiting aperture for the eye test vision system.

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Figure 6. A standard eye Snellen vision test chart used today in phoropter systems, which quantifies Figure 6. A standard eye Snellen vision test chart used today in phoropter systems, which quantiﬁes the eye near sighted refractive power levels based on the text line the patient is able to read as a the eye near sighted refractive power levels based on the text line the patient is able to read as a non-blurred clear image. non-blurred clear image.

The second mode of operation proposed for the Figure5 eye vision test system is based on controlled optical aperture control. In this mode, the E-lens and BL are removed from the system, leaving only the aperture-controlled device in the smart frontend optics. In effect, an extremely simple and cost effective eye vision system is formed that relies on the operation of a single micromechanical aperture device. Figure7 shows the basic principle of aperture-controlled myopic eye vision testing explained using a variable aperture in front of the(a eye,) and imaging of two points labeled as point 1 and point 2 in the far-ﬁeld test target. As shown in Figure7a, as the eye is myopic with a certain given myopic refractive power, light rays from points 1 and 2 focus along a slightly curved contour that is before the retina (shown as a ﬂat screen). Thus, one larger spot is formed at the retina location as these two imaged spots partly overlap in space. In effect, the two spatially separated spots in the viewed test image appear as one blurred large spot; hence the patient under eye test is unable to see a clear focused image on his/her eye retina of two spatially independent spots. In Figure7b, the aperture in front of the eye is made smaller to the extent that now the two spots appearing at the retina are smaller in size and appear separated from each other(b) on the retina, thus creating an image of points 1 andFigure 2 in the7. Principle viewed testof aperture-controlled image that is now eye seen vision and testing considered explained a non-blurred via geometrical image. optics, In effect,a the simplemyopic act eye, of a squeezing variable aperture the optical in front aperture of the eye size and at theimaging entrance of two of points the eye in createsthe test thetarget. ability (a) for a myopicAperture eye is to large see (orso points focus on)1 and objects 2 create that phys otherwiseically inseparable would appear optical toospots small on the to seeretina; with (b) clear vision.Aperture This simple is smaller act hasso points been naturally1 and 2 create known phys toically human separable eyesfor and perhaps smaller thousandsoptical spots of on years the and oftenretina people enabling squint clear their vision eyes of when the two trying points to focustest ta onrget. small Note distantthat these objects. spots are In thenot Figureof the smallest5 proposed eye visionsize that system, wouldthe be observed power of by aperture-controlled the retina if the eye lens clearer did visionnot have is myopia harnessed (see usingFigure a 8). micromachine electronically controlled variable aperture device that is calibrated to provide equivalent corrective dioptricBecause powers human for a eyes near-sighted have certain eye. limiting For a human charac eyeteristics to clearly designed view by atest biology, letter such in a blackas retinal and whitecell resolution eye chart, and the eye eye cavity should length be able between to resolve eye twolens blackand retina, dots with there a are white limits dot to in how between powerful the two an blackapproach dots. can For be example, in dioptric the numbers letter “U” for can eye be vision considered testing made using up aperture of two parallelcontrol. blackA basic vertical analysis lines to separateddetermine bythis a limit white can space. be conducted Points 1 andas follow 2 in Figures [1,2].7 A can normal be considered eye lens of representing focal length theFN focuses vertical a blackfar field line object widths onto in a the point letter image “U” on while the theretina white giving gap the (dot) highest between detectable points spatial 1 and 2resolution in the target for spacethe observed can be considered image. This as is an shown additional in Figu invisible/whitere 8a where the dot aperture in a line diameter of three dots is D ina and the targetthe distance space. between the lens and the retina is ds. It can be seen for Figure 8a that:

ds = FN (1) 1697 8 Micromachines 2015, 6, page–page

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Figure 6. A standard eye Snellen vision test chart used today in phoropter systems, which quantifies To seethe the eye letter near “U”,sighted the refractive retina should power be levels able based to spatially on the resolvetext line allthe three patient dots is able (two to black read onesas a and one whitenon-blurred one between clear image. the two black dots).

(a)

(b)

Figure 7. Principle of aperture-controlled eye vision testing explained via geometrical optics, a Figure 7. Principle of aperture-controlled eye vision testing explained via geometrical optics, a myopic myopic eye, a variable aperture in front of the eye and imaging of two points in the test target. (a) eye, a variable aperture in front of the eye and imaging of two points in the test target. (a) Aperture is Aperture is large so points 1 and 2 create physically inseparable optical spots on the retina; (b) large so points 1 and 2 create physically inseparable optical spots on the retina; (b) Aperture is smaller Aperture is smaller so points 1 and 2 create physically separable and smaller optical spots on the so points 1 and 2 create physically separable and smaller optical spots on the retina enabling clear visionretina enabling of the two clear points vision test of target. the two Note points that test these target. spots Note are that not ofthese the spots smallest are sizenot of that the would smallest be observedsize that would by the be retina observed if the eyeby the lens retina did not if th havee eye myopia lens did (see not Figure have 8myopia). (see Figure 8).

Because human eyes have certain limiting characteristics designed by biology, such as retinal Because human eyes have certain limiting characteristics designed by biology, such as retinal cell resolution and eye cavity length between eye lens and retina, there are limits to how powerful an cell resolution and eye cavity length between eye lens and retina, there are limits to how powerful an approach can be in dioptric numbers for eye vision testing using aperture control. A basic analysis to approach can be in dioptric numbers for eye vision testing using aperture control. A basic analysis to determine this limit can be conducted as follows [1,2]. A normal eye lens of focal length FN focuses a determine this limit can be conducted as follows [1,2]. A normal eye lens of focal length F focuses far field object onto a point image on the retina giving the highest detectable spatial resolutionN for a far ﬁeld object onto a point image on the retina giving the highest detectable spatial resolution for the observed image. This is shown in Figure 8a where the aperture diameter is Da and the distance the observed image. This is shown in Figure8a where the aperture diameter is Da and the distance between the lens and the retina is ds. It can be seen for Figure 8a that: between the lens and the retina is ds. It can be seen for Figure8a that: ds = FN (1) ds “ FN (1)

If the normal eye lens is replaced by an abnormal8 (i.e., myopic) eye lens of focal length FAB, where FAB < FN, the far ﬁeld object focuses as a point located at a distance ∆L from the retina as shown in Figure8b. As a result, a blur spot of diameter db forms on the retina. From Figure8b, it can be seen that: ∆L “ ds ´ FAB (2)

Since ds = FN, Equation (2) is written as:

∆L “ FN ´ FAB (3)

Using geometry, db, ∆L, FAB and Da are related by:

d D b “ a (4) ∆L FAB

Equation (4) is rearranged to give: db Da “ F (5) AB ∆L

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(a) (b)

FigureFigure 8. ( 8.a)( Opticala) Optical ray ray diagrams diagrams for for an eyeeye lenslens system system having having a normal a normal eye eye lens lens and (andb) optical (b) optical ray ray diagramsdiagrams for for an an eye eye lens lens system system having ananabnormal abnormal eye eye lens. lens.

The expression for ∆L from Equation (3) is substituted in Equation (4), giving:

db Da “ FAB (6) FN ´ FAB

Let the optical power in Diopters (1/m) of a normal lens be DN = 1/FN and let the optical power of an abnormal lens be DAB = 1/FAB. Therefore, Equation (6) can be expressed in terms of dioptric powers as: 1 db db Da “ , Da “ DN (7) DAB 1 ´ 1 DAB ´ DN DN DAB Figure 9. Optical ray diagram of a single´ lens system¯ showing rays from two objects striking the retina. Equation (7) relates the aperture diameter Da with the optical powers of both the normal and abnormal eyes. The next step is to ﬁnd an expression for d . To do this, refer to Figure9 which In Figure 9, in order for the retina to be able to resolve bthe two object points 1 and 2, db must be shows two point objects, separated by H12, being imaged by an eye lens located a distance dt from less than or equal to L12. That is: the objects, and forming their respective image blur spots on the retina. A ray from each object point

passes through the center of the eye lens beforedL strikingb  12 the retina. The angle between the two rays (11) (one from each object point as shown in Figure9 is α, and the distance between the centers of the two dFtan blur spots formed on the retina is L12. The diameterbN of each blur spot is db. From trigonometry, α is (12) written as: Combining Equations (7) and (12) gives: H α “ tan´1 12 (8) d tanˆ t ˙ Da  Also from trigonometry, L12 can be written as: (13) DDAB N

In order to resolve two object pointsL12 “ind sthetanα test target used in the proposed aperture(9) control-based eye vision test system, Equation (13) shows that the aperture diameter must be less Since ds = FN from Equation (1), Equation (9) can be written as: than some threshold value. Like any optical imaging system with a variable aperture such as in the proposed eye test system, diffraction affectsL12 “theFN spatialtanα resolving power of the system based(10) on specific system parameters. Specifically, the diameter Ddiff of the airy disk (smallest diffraction spot) formed Inon Figure the retina9, in orderdue to for the the proposed retina to be eye able vision to resolve testing the system two object is given points by: 1 and 2, db must be less than or equal to L12. That is: db ď L12 F (11) Ddiff 2.44  (14) Da db ď FNtanα (12) where λCombining is the wavelength Equations of (7) light, and (12)F is gives:the focal length of the eye lens system and Da is the diameter of the variable aperture. Using typical values of F = 55 Diopters, λ = 550 nm (center of visible band), tanα and a minimal Da of 0.2 mm deployed in Dthea ď eye test system, one gets Ddiff of 122 μm. Given(13) that the DAB ´ DN eye under test is not expected to spatially resolve test images with features as small as 122 μm in diameter, the effect of optical diffraction can be considered negligible. With regards to visual acuity that is a measure of the degree to clearly observ1699e images using the proposed eye test system, a number of factors come into play such as the scene object size and lighting conditions as well as aberrations in the optical system, which interestingly reduce for smaller optical apertures [1]. Indeed, the high resolution imaging performance of the proposed aperture control eye vision test method and corresponding eyewear is somewhat limited when compared to classic

10 Micromachines 2015, 6, page–page

(a) (b)

Figure 8. (a) Optical ray diagrams for an eye lens system having a normal eye lens and (b) optical ray Micromachines 2015, 6, 1690–1709 diagrams for an eye lens system having an abnormal eye lens.

Figure 9. OpticalFigure ray 9. Optical diagram ray diagram of a single of a single lens lens system system showingshowing rays rays from twofrom objects two striking objects the striking retina. the retina.

In Figure 9,In in order order to resolve for the two objectretina points to be in theable test to target resolve used in the the proposedtwo object aperture points control-based 1 and 2, db must be eye vision test system, Equation (13) shows that the aperture diameter must be less than some less than or equal to L12. That is: threshold value. Like any optical imaging system with a variable aperture such as in the proposed eye test system, diffraction affects the spatial resolving power of the system based on speciﬁc system dLb  12 (11) parameters. Speciﬁcally, the diameter Ddi f f of the airy disk (smallest diffraction spot) formed on the retina due to the proposed eye vision testing system is given by: dFbNtan (12) F D « 2.44λ (14) Combining Equations (7) and (12) gives:di f f D ˆ a ˙

where λ is the wavelength of light, F is the focal lengthtan  of the eye lens system and Da is the diameter of the variable aperture. Using typical valuesDa  of F = 55 Diopters, λ = 550 nm (center of visible band), (13) DDAB N and a minimal Da of 0.2 mm deployed in the eye test system, one gets Ddi f f of 122 µm. Given that the eye under test is not expected to spatially resolve test images with features as small as In order122 µtom inresolve diameter, two the effectobject of optical points diffraction in the can test be considered target used negligible. in Withthe regardsproposed to aperture control-basedvisual eye acuity vision that istest a measure system, of theEquation degree to (13) clearly shows observe that images the using aperture the proposed diameter eye test must be less than some thresholdsystem, a number value. of factorsLike any come optical into play imaging such as the system scene object with size a andvariable lighting aperture conditions such as as in the proposed eyewell test as aberrations system, in diffraction the optical system, affects which the interestingly spatial reduceresolving for smaller power optical of aperturesthe system [1]. based on Indeed, the high resolution imaging performance of the proposed aperture control eye vision test specific systemmethod parameters. and corresponding Specifically, eyewear is the somewhat diameter limited D whendiff of compared the airy to disk classic (smallest prescription-lens diffraction spot) formed on thebased retina hardware due where to the large proposed aperture customeye vision lens design testing eyewear system is engaged. is given This by: concludes the description for the second mode of operation proposed for the Figure5 eye vision test system. The third mode of the proposed eye vision test systemF is a hybrid mode that engages both Ddiff 2.44  (14) the E-lens first mode and the aperture-controlled secondD mode of the system. For example, to measure eye refractive errors up to two Diopters, mode 2 isa deployed and for higher refractive power measurements, the E-lens system first mode is activated. So for weaker refractive power corrections, where λ is thethe E-lenswavelength is not engaged. of light, This F is can the be focal a benefit length when of using the eye today’s lens E-lens system technologies and Da since is the diameter of the variablegeneration aperture. and stability Using oftypical weak E-lensing values canof F be = considered 55 Diopters, electronically λ = 550 harder nm (center than operating of visible band), and a minimalE-lenses Da forof 0.2 higher mm refractive deployed powers. in Finally,the eye the test choice system, of operations one gets mode D useddiff of for 122 the proposedμm. Given that the eye vision test system will depend on many parameters including the patient testing scenario as well eye under test is not expected to spatially resolve test images with features as small as 122 μm in the economics of the local eye testing site. diameter, the effectThe second of op operationstical diffraction mode of the can eye be test considered system also empowersnegligible. a new With type regards of myopic to eye visual acuity that is a measurecorrective of eyewear the degree whose design to clearly using multiple observ aperturese images and using principles the of proposed operation is showneye test in system, a number of Figurefactors 10 .come The diameter into playDm of thesuch circular as the aperture scene holes object in the eyewearsize and is dictatedlighting by theconditions corrective as well as refractive power needed by the myopic patient to get clear (e.g., 20/20) vision. The eye vision test aberrations conductedin the withoptical the patientsystem, will givewhich the needed interest prescriptioningly reduce for the eyewear for smaller aperture diameter.optical Asapertures a [1]. Indeed, thesingle high aperture resolution in eyewear imaging restricts performance the visual field of of view the (FOV) proposed to a narrow aperture field, multiple control apertures eye vision test method and corresponding eyewear is somewhat limited when compared to classic 1700 10 Micromachines 2015, 6, page–page

prescription-lens based hardware where large aperture custom lens design eyewear is engaged. This concludes the description for the second mode of operation proposed for the Figure 5 eye vision test system. The third mode of the proposed eye vision test system is a hybrid mode that engages both the E-lens first mode and the aperture-controlled second mode of the system. For example, to measure eye refractive errors up to two Diopters, mode 2 is deployed and for higher refractive power measurements, the E-lens system first mode is activated. So for weaker refractive power corrections, the E-lens is not engaged. This can be a benefit when using today’s E-lens technologies since generation and stability of weak E-lensing can be considered electronically harder than operating E-lenses for higher refractive powers. Finally, the choice of operations mode used for the proposed eye vision test system will depend on many parameters including the patient testing scenario as well the economics of the local eye testing site. The second operations mode of the eye test system also empowers a new type of myopic eye Micromachinescorrective2015 eyewear, 6, 1690–1709 whose design using multiple apertures and principles of operation is shown in Figure 10. The diameter Dm of the circular aperture holes in the eyewear is dictated by the corrective refractive power needed by the myopic patient to get clear (e.g., 20/20) vision. The eye vision test are addedconducted in 2-D with space the patient in the eyewearwill give the to increaseneeded pres thecription overall for eye the FOV. eyewear The aperture spacing diameter. between As the a clear aperturessingle denoted apertured xinin eyewear the horizontal restricts plane the visual and d yfieldin the of verticalview (FOV) plane, to isa determinednarrow field, by multiple the desired patientapertures viewing are maximum added in target2-D space distance in the toeyewear prevent to FOVincrease overlap, the overall which eye would FOV. causeThe spacing overlap of viewedbetween images the per clear aperture apertures (Figure denoted 10b). dx Note in the that horizontal when wearing plane and such dy proposedin the vertical low costplane, eyewear, is the userdetermined may need by the to rolldesired his/her patient eyes viewing left to maximum right and target up and distance down to to prevent ﬁll in FOV all images overlap, to which construct would cause overlap of viewed images per aperture (Figure 10b). Note that when wearing such wide FOV imaging. Another aspect when using such eyewear is the need for adequate lighting. This proposed low cost eyewear, the user may need to roll his/her eyes left to right and up and down to is because compared to traditional eyewear; the proposed aperture eyewear involves a decreased fill in all images to construct wide FOV imaging. Another aspect when using such eyewear is the apertureneed diameter, for adequate which lighting. reduces This the is because amount compar of lighted entering to traditional the eye. eyewear; The rationale the proposed behind aperture using the eyeweareyewear in optimal involves light a decreased conditions aperture is to ensure diameter, that wh theich reduced reduces lightthe amount levels of entering light entering the eye the do not handicapeye. The the rationale user in any behind way. using For the scenes eyewear in low in optimal lighting light conditions, conditions aperture is to ensure control that the can reduced deteriorate visionlight by affectinglevels entering scene brightnessthe eye do levels.not handicap Therefore, the ituser isrecommended in any way. For to usescenes the in proposed low lighting aperture eyewearconditions, in well-lit aperture regions control to ensure can deteriorate that its use vision won’t by signiﬁcantlyaffecting scene degrade brightness the levels. quality Therefore, of vision. it For instance,is recommended bright electronic to use the displays proposed or aperture high sunlight eyewear areas in well-lit would regions provide to ensure good that lighting its use won’t levels for operationssignificantly of this degrade low cost the eyewear. quality of Therefore, vision. Fordeployment instance, bright in Sunelectronic rich Asiandisplays and or Africanhigh sunlight countries areas would provide good lighting levels for operations of this low cost eyewear. Therefore, would be suitable for the proposed eyewear. deployment in Sun rich Asian and African countries would be suitable for the proposed eyewear.

(a) (b)

Figure 10. (a) Design and (b) principle of wide FOV operation of a new type of low cost myopia Figure 10. (a) Design and (b) principle of wide FOV operation of a new type of low cost myopia correction eyewear using aperture control and multiple distributed apertures. correction eyewear using aperture control and multiple distributed apertures. 4. Experimental Demonstration 4. Experimental Demonstration To demonstrate the potential eyesight testing capability of the proposed vision test system, the Toexperimental demonstrate design the potentialshown in eyesightFigure 11 testing is impl capabilityemented in of the the laboratory. proposed visionNote that test this system, the experimental design shown in Figure 11 is11 implemented in the laboratory. Note that this proof-of-concept experiment demonstrates spherical refractive error correction only. Optotune ECVFL devices are used to simulate a human eye of different myopic powers viewing a test image through another ECVFL device that simulates a patient’s eyewear. A complementary metal oxide semiconductor (CMOS) chip sensor is placed at a distance equivalent to ds from the eye lens system comprising of a pair of 20 D lenses and an ECVFL labeled as ECVFL 1. The deployed CMOS sensor model number is IDS UI-1250LE-M-GL having a 5.3 µm pixel size. In comparison, the diameter of a single cone cell in the fovea of the human eye is on average between 1 to 4 µm [2]. Therefore, the deployed CMOS pixel size is in close proximity to that of the human eye. However, it must be noted that unlike the CMOS sensor, the human eye’s optical sensing resolution depends on various factors including angle of viewing, lighting conditions, and viewing distances. The ECVFL 1 is used to control the power of the myopic eye during the experiment, thereby simulating an eye having varying degrees of refractive error. A mechanically variable aperture of diameter Da is located a distance da from the eye lens system. Another ECVFL, labeled as ECVFL 2, is placed a distance de2 from the aperture. The ECVFL 2 represents the corrective lens needed to restore blurred vision. A standard Snellen chart is used as a vision test target, ds is 14.2 mm, da is 9.5 mm, de2 is 11 mm and dt is 100 cm. The ECVFL 1 model deployed in the experiment is Optotune’s EL-10-30 having a

1701 Micromachines 2015, 6, page–page Micromachines 2015, 6, page–page proof-of-concept experiment demonstrates spherical refractive error correction only. Optotune proof-of-conceptECVFL devices are experiment used to simulate demonstrates a human spherical eye of different refractive myopic error powers correction viewing only. a testOptotune image ECVFLthrough devices another are ECVFL used todevice simulate that asimulates human eye a patient’sof different eyewear. myopic A powers complementary viewing ametal test image oxide throughsemiconductor another (CMOS) ECVFL chipdevice sensor that issimulates placed at a a patient’s distance eyewear.equivalent A to complementary ds from the eye metal lens system oxide semiconductorcomprising of a(CMOS) pair of 20chip D lensessensor andis placed an ECVFL at a distance labeled equivalentas ECVFL 1.to Theds from deployed the eye CMOS lens system sensor comprisingmodel number of a ispair IDS of UI-1250LE-M-GL 20 D lenses and anhaving ECVFL a 5.3 labeled μm pixel as ECVFL size. In 1. comparison, The deployed the CMOS diameter sensor of a modelsingle numbercone cell is in IDS the UI-1250LE-M-GL fovea of the human having eye ais 5.3 on μ averagem pixel size.between In comparison, 1 to 4 μm [2]. the Therefore,diameter of the a singledeployed cone CMOS cell in pixel the foveasize is of in theclose human proximity eye is to on th ataverage of the humanbetween eye. 1 to However, 4 μm [2]. it Therefore,must be noted the deployedthat unlike CMOS the CMOS pixel sizesensor, is in the close human proximity eye’s opti to thcalat sensingof the human resolution eye. However,depends on it variousmust be factorsnoted thatincluding unlike anglethe CMOS of viewing, sensor, thelighting human conditions, eye’s opti andcal sensing viewing resolution distances. depends The ECVFL on various 1 is usedfactors to includingcontrol the angle power of viewing,of the myopic lighting eye conditions, during the and experiment, viewing distances. thereby simu The latingECVFL an 1 eyeis used having to controlvarying the degrees power of of refractive the myopic error. eye A duringmechanically the experiment, variable aperture thereby ofsimu diameterlating anDa eyeis located having a varyingdistance degrees da from ofthe refractive eye lens system.error. A Another mechanically ECVFL, variable labeled aperture as ECVFL of 2,diameter is placed D aa is distance located dae2 Micromachinesdistancefrom the d aperture.a 2015from, 6 ,the 1690–1709 The eye ECVFL lens system. 2 represents Another the ECVFL, corrective labeled lens as needed ECVFL to 2,restore is placed blurred a distance vision. d Ae2 fromstandard the aperture.Snellen chart The isECVFL used as 2 arepresents vision test the target, corrective ds is 14.2 lens mm, needed da is 9.5 to mm,restore de2 blurredis 11 mm vision. and d tA is standard100 cm. TheSnellen ECVFL chart 1 is model used as deployed a vision testin the target, experi ds isment 14.2 ismm, Optotune’s da is 9.5 mm, EL-10-30 de2 is 11 having mm and a focal dt is focal length F , while Optotune’s EL-6-18 is used as ECVFL 2 having focal length F . SM1D12C Iris 100length cm. F Thee1, while eECVFL1 Optotune’s 1 model EL-6-18deployed is inused the asexperi ECVFLment 2 ishaving Optotune’s focal lengthEL-10-30 Fee2 . havingSM1D12C a focal Iris DiaphragmlengthDiaphragm Fe1, while fromfrom ThorlabsOptotune’sThorlabs (Newton,(Newton, EL-6-18 NJ,NJ, is USA)usedUSA) isasis deployeddeployedECVFL 2as ashaving thethe mechanicallymechanically focal length controlledcontrolled Fe2. SM1D12C aperture,aperture, Iris with all D values measured using Aerospace Vernier calipers. The height of the largest letter “E” on Diaphragmwith all Daa valuesfrom Thorlabs measured (Newton, using Aerospace NJ, USA) Vernier is deployed calipers. as the The mechanically height of the controlled largest letter aperture, “E” on awitha SnellenSnellen all D chartachart values placedplaced measured 66 mm awayaway using isis 87.5Aerospace87.5 mmmm [[48].48 Vernier]. Therefore,Therefore, calipers. forfor The thethe 1height1 mm targettarget of the distancedistance largest asas letter usedused “E” inin the theon experiment,aexperiment, Snellen chart thethe placed deployeddeployed 6 m Snellen Snellenaway is chart chart87.5 ismm is scaled scaled [48]. to Therefore,to have have its its largest forlargest the “E” 1“E” m to targetto be be 87.5/6 87.5/6 distance mmmm as == 14.6used14.6 mm mmin the inin height.experiment,height. FigureFigure the 1212 deployed showsshows aa topSnellentop viewview chart ofof thethe is laboratoryscaledlaboratory to have experimentalexperimental its largest setup.“E”setup. to FigurebeFigure 87.5/6 1313 mm showsshows = 14.6 imagesimages mm ofinof theheight.the 11 cmcm Figure diameterdiameter 12 shows modelmodel a EL-10-30 EL-10-3top view0 ECVFL ECVFLof the laboratory 11 andand thethe 6 6experimental mmmm diameterdiameter setup. modelmodel Figure EL-6-18EL-6-18 13 ECVFLshowsECVFL images 22 usedused toofto introducetheintroduce 1 cm diameter eyeeye refractive refractive model error error EL-10-3 and and0 dioptric dioptricECVFL power1 power and the correction, correction, 6 mm diameter respectively. respectively. model EL-6-18 ECVFL 2 used to introduce eye refractive error and dioptric power correction, respectively.

Figure 11. Experimental design of the proposed basic E-lens based eye vision test system assembled Figure 11. Experimental design of the proposed basic E-lens based eye vision test system assembled Figurein the laboratory11. Experimental using commercially design of the availableproposed micromachines. basic E-lens based eye vision test system assembled in the laboratory using commercially available micromachines. in the laboratory using commercially available micromachines.

Figure 12. Laboratory experimental setups (top view) used to demonstrate the principles of the Figure 12. Laboratory experimental setups (top view) used to demonstrate the principles of the Figureproposed 12. eyeLaboratory vision test experimental system using setupsmicromac (tophines view) based used on to E-lenses demonstrate and aperture the principles control. of the proposed eye vision test system using micromachines based on E-lenses and aperture control. proposed eye vision test system using micromachines based on E-lenses and aperture control. Micromachines 2015, 6, page–page 12 12

(a) (b)

Figure 13. Images of Electronically Controlled Variable Focus Lens (ECVFL) 1 (a) and ECVFL 2 (b) used Figure 13. Images of Electronically Controlled Variable Focus Lens (ECVFL) 1 (a) and ECVFL 2 in the experiment. (b) used in the experiment.

To demonstrate the first mode of operations of the proposed Figure 5 eye vision test system that implements eye refractive error correction via1702 ECVFL 2, Da is fixed to an average human pupil diameter of 5 mm with Fe1 initially set to 15 D resulting in a total eye lens system power of 55 D. This results in a clear image of the Snellen chart acquired by the CMOS sensor with the image recorded and viewed via the connected laptop, with Fe2 set to 0 D (infinite focal length). The first step in the experiment is to acquire Fe1 values corresponding to varying degrees of myopia. Note that since myopia (nearsightedness) [49] is measured by the optical power (in negative Diopters) of the corrective lens (in this case ECVFL 2) which helps focus the image on the retina, Fe2 is first varied via ECVFL 2 current control from 0.00 to −5.00 D in 0.25 D steps signifying the diﬀerent degrees of myopia. For each Fe2 setting, Fe1 is altered to restore the clear image of the Snellen chart. A clear image is defined in the experiment as an image in which the letters on the 20/20 line of the Snellen chart are resolvable by the experimenter viewing the laptop display that provides the CMOS sensor generated image. These acquired Fe1 values represent the optical power of the myopic eye for which the different degrees of myopia are given by Fe2 values. As an example, a person with −1.0 D myopia requires spectacles with a prescription of −1.0 D (i.e., a corrective lens of focal length Fe2), but it does not necessarily mean that the eye lens system has a refractive error of −1.0 D. Once all Fe1 values are noted for varying degrees of myopia, Fe2 is set to give 0.0 D and Fe1 is altered electronically to induce a refractive error in the eye, simulating a patient’s eye with varying degrees of known myopia. Depending on the induced refractive error, subsequent control of ECVFL 2, which represents part of the programmable vision test eyewear, allows correction of the induced error. The CMOS chip sensor snapshots are captured for each 0.25 D step of myopia. These images show what the myopic eye would see for different degrees of refractive error. Figure 14 shows such images for refractive errors of −0.50 D, −2.00 D, and −3.0 D error values along with ECVFL 2 corrected images. The list of induced myopic refractive errors via Fe1 and corresponding Fe2 values along with respective ECVFL current settings are summarized in Table 1. Note that ECVFL 2 is equally capable of going into the positive range of powers (above 0.0 Diopters) by increasing the current supplied to it above 65.6 mA. These positive ECVFL 2 dioptric powers would be useful in testing far-sightedness/hyperopia and presbyopia in patients. Although, positive range of powers wasn’t considered for this proof of concept experiment because hyperopia and related vision problems do not normally occur among younger patients, it is fully realizable using the proposed Figure 5 system. The demonstrated resolution of 0.25 D can also be enhanced using smaller ECVFL drive current steps that will in-turn give smaller focal length change. With optimized ECVFL technologies suited for the desired range of focal lengths, the patient’s eyes can be diagnosed with greater accuracy. In conjunction with the smart optics described in the previous section, the proposed eyewear system has potential in taking the eyesight diagnosis process to the next level of vision systems impacting human health. Next, demonstrated is the second mode of operations of the proposed Figure 5 eye vision test system comprising eye refractive error correction via aperture control. For this experiment, ECVFL 2 is removed from the vision path because easy access is obtained to vary and measure the aperture setting without ECVFL 2. Also, ECVFL 2 is not needed for this part of the experiment. The ECVFL 1 13 Micromachines 2015, 6, 1690–1709

To demonstrate the first mode of operations of the proposed Figure5 eye vision test system that implements eye refractive error correction via ECVFL 2, Da is fixed to an average human pupil diameter of 5 mm with Fe1 initially set to 15 D resulting in a total eye lens system power of 55 D. This results in a clear image of the Snellen chart acquired by the CMOS sensor with the image recorded and viewed via the connected laptop, with Fe2 set to 0 D (infinite focal length). The first step in the experiment is to acquire Fe1 values corresponding to varying degrees of myopia. Note that since myopia (nearsightedness) [49] is measured by the optical power (in negative Diopters) of the corrective lens (in this case ECVFL 2) which helps focus the image on the retina, Fe2 is first varied via ECVFL 2 current control from 0.00 to ´5.00 D in 0.25 D steps signifying the different degrees of myopia. For each Fe2 setting, Fe1 is altered to restore the clear image of the Snellen chart. A clear image is defined in the experiment as an image in which the letters on the 20/20 line of the Snellen chart are resolvable by the experimenter viewing the laptop display that provides the CMOS sensor generated image. These acquired Fe1 values represent the optical power of the myopic eye for which the different degrees of myopia are given by Fe2 values. As an example, a person with ´1.0 D myopia requires spectacles with a prescription of ´1.0 D (i.e., a corrective lens of focal length Fe2), but it does not necessarily mean that the eye lens system has a refractive error of ´1.0 D. Once all Fe1 values are noted for varying degrees of myopia, Fe2 is set to give 0.0 D and Fe1 is altered electronically to induce a refractive error in the eye, simulating a patient’s eye with varying degrees of known myopia. Depending on the induced refractive error, subsequent control of ECVFL 2, which represents part of the programmable vision test eyewear, allows correction of the induced error. The CMOS chip sensor snapshots are captured for each 0.25 D step of myopia. These images show what the myopic eye would see for different degrees of refractive error. Figure 14 shows such images for refractive errors of ´0.50 D, ´2.00 D, and ´3.0 D error values along with ECVFL 2 corrected images. The list of induced myopic refractive errors via Fe1 and corresponding Fe2 values along with respective ECVFL current settings are summarized in Table1. Note that ECVFL 2 is equally capable of going into the positive range of powers (above 0.0 Diopters) by increasing the current supplied to it above 65.6 mA. These positive ECVFL 2 dioptric powers would be useful in testing far-sightedness/hyperopia and presbyopia in patients. Although, positive range of powers wasn’t considered for this proof of concept experiment because hyperopia and related vision problems do not normally occur among younger patients, it is fully realizable using the proposed Figure5 system. The demonstrated resolution of 0.25 D can also be enhanced using smaller ECVFL drive current steps that will in-turn give smaller focal length change. With optimized ECVFL technologies suited for the desired range of focal lengths, the patient’s eyes can be diagnosed with greater accuracy. In conjunction with the smart optics described in the previous section, the proposed eyewear system has potential in taking the eyesight diagnosis process to the next level of vision systems impacting human health. Next, demonstrated is the second mode of operations of the proposed Figure5 eye vision test system comprising eye refractive error correction via aperture control. For this experiment, ECVFL 2 is removed from the vision path because easy access is obtained to vary and measure the aperture setting without ECVFL 2. Also, ECVFL 2 is not needed for this part of the experiment. The ECVFL 1 is set to current values corresponding to varying degree of myopia shown in Table1 and the aperture diameter Da is reduced until the 20/20 line on the Snellen chart on the CMOS sensor acquired image seen on the computer is just readable by the experimenter. Two experimenters called Subject A and Subject B are chosen to carry out the decision making process on whether the 20/20 line is readable on decreasing Da values. Sample CMOS chip acquired images for induced refractive error values of ´0.50 D and ´2.00 D along with respective aperture control vision corrected images are shown in Figure 15. Table2 summarizes the acquired Da data for both experimenters for a simulated eye refractive error range of 0.00 D to ´2.00 D. Refractive errors worse than ´2.00 D can also be corrected to some degree but are not considered here because the aperture diameters were too small to measure using the Vernier caliper used in the experiment.

1703 Micromachines 2015, 6, 1690–1709

The experimental Da data in Table2 for both Subject A and Subject B is compared to the theoretically derived Equation (13) Da curve for refractive error of up to ´2.00 D. This theoretical curve is given by Equation (13) in which FN = 55 D and FAB is varied from 55 to 57 D to estimate the 0.00 D to ´2.00 D range of myopia. For Equation (13), α is found using Equation (8) where dt is 1 m. To find H12, first the height of the letters on the 20/20 line of the target Snellen chart is found which is 10 times smaller than the height of the largest “E” on the 20/200 line, i.e., (height of letters on 20/20 line) = (height of letters on 20/200 line)/10 = 14.6/10 mm = 1.46 mm. In order to read these letters, one needs to resolve the highest spatial frequency variation in these letters. This is given by the second letter “E” on the 20/20 line, which has the top arm followed by a white space, which is then followed by the middle arm as the highest spatial frequency variation on this line. Hence H12 is given by theMicromachines distance 2015 covered, 6, page–page by these three dots (two black and one white), so H12 = height of letters on 20/20 line ˆ fraction of this height covered by three dots = 1.46 ˆ (3/5) mm « 0.88 mm. Using is set to current values corresponding to varying degree of myopia shown in Table 1 and the this H , α is calculated using Equation (8) to be 0.05 Rad. Using this α value, the Equation (13) D is 12 aperture diameter Da is reduced until the 20/20 line on the Snellen chart on the CMOS sensor a plotted andacquired shown image in Figure seen on 16 the as computer a coarse is just dashed readable curve by the with experimenter. plus symbols, Two experimenters along with called experimental data from SubjectSubject A Aand (solid Subject curve B are chosen with dotto carry symbols) out the decision and Subject making Bprocess (fine-dashed on whether curve the 20/20 with circular symbols). Theline is difference readable on indecreasing the experimental Da values. Sample data CMOS acquired chip acquired via Subject images A for and induced Subject refractive B is due to the error values of −0.50 D and −2.00 D along with respective aperture control vision corrected images subjective natureare shown of in the Figure readability 15. Tableof 2 summarizes the 20/20 the line acquired and is D aa commondata for both data experimenters representation for a in human vision experimentalsimulated eye studies refractive [50 error]. A range pattern of 0.00 different D to −2.00 from D. Refractive the letters errors deployedworse than − in2.00 a D standard can vision test chart mayalso be corrected used to to discern some degree the degreebut are not of clarityconsidered of here a target. because the aperture diameters were too small to measure using the Vernier caliper used in the experiment.

(a)

(b)

(c)

Figure 14. Images acquired by CMOS sensor acting as retina for varying degrees of ECVFL 1 induced Figure 14. Imagesmyopia of acquired (a) −0.50 D, by (b) CMOS−2.00 D, sensorand (c) −3.00 acting D, all as corrected retina forusing varying ECVFL 2. degrees of ECVFL 1 induced myopia of (a) ´0.50 D, (b) ´2.00 D, and (c) ´3.00 D, all corrected using ECVFL 2.

1704 14 Micromachines 2015, 6, 1690–1709

Table 1. Table shows focus and current settings for both ECVFL 1 and ECVFL 2 for eye refractive error range from 0.00 to ´5.00 D. Micromachines 2015, 6, page–page

Induced Myopic ECVFL 2 current setting Corresponding ECVFL Table 1. Table shows focusECVFL and1 current Current settings for both ECVFL 1 and ECVFL 2 for eye refractive Refractive Error via for vision correction 2 focal length F error range from 0.00 to −5.00setting D. (mA) e2 ECVFL 1 (Diopter) (mA) (Diopter) Induced Myopic Refractive ECVFL 1 Current ECVFL 2 current setting Corresponding ECVFL 2 0.00 142.01 65.6 0.00 Error via ECVFL 1 (Diopter) setting (mA) for vision correction (mA) focal length Fe2 (Diopter) ´0.25 146.26 64.3 ´0.25 0.00 142.01 65.6 0.00 ´0.50 148.13 63.1 ´0.50 −0.25 146.26 64.3 −0.25 ´0.75 150.03 61.8 ´0.75 −0.50 148.13 63.1 −0.50 ´1.00 152.61 60.5 ´1.00 −0.75 150.03 61.8 −0.75 ´1.25 158.47 59.3 ´1.25 −1.00 152.61 60.5 −1.00 ´1.50 161.04 58.0 ´1.50 ´1.75−1.25 167.62158.47 56.759.3 −´1.251.75 ´2.00−1.50 173.84161.04 55.458.0 −´1.502.00 ´2.25−1.75 182.00167.62 54.256.7 −´1.752.25 ´2.50−2.00 185.72173.84 52.955.4 −´2.002.50 ´2.75−2.25 189.01182.00 51.754.2 −´2.252.75 ´3.00−2.50 195.87185.72 50.452.9 −´2.503.00 ´3.25−2.75 198.23189.01 49.151.7 −´2.753.25 ´3.50−3.00 210.00195.87 47.950.4 −´3.003.50 ´3.75−3.25 216.82198.23 46.649.1 −´3.253.75 ´4.00−3.50 220.11210.00 45.347.9 −´3.504.00 ´4.25−3.75 224.33216.82 44.146.6 −´3.754.25 ´4.50−4.00 230.15220.11 42.845.3 −´4.004.50 ´4.75−4.25 234.20224.33 41.544.1 −´4.254.75 ´5.00−4.50 239.49230.15 40.342.8 −´4.505.00 −4.75 234.20 41.5 −4.75 −5.00 239.49 40.3 −5.00

(a)

(b)

Figure 15. CMOS sensor acquired images showing induced refractive error values of (a) −0.50 D and Figure 15. CMOS sensor acquired images showing induced refractive error values of (a) ´0.50 D and (b) −2.00 D, along with respective aperture control vision corrected images. The aperture values for (b) ´(a2.00) and D, (b along) corrected with vision respective as measured aperture by controlSubject A vision are 4.58 corrected mm and images. 0.75 mm, The respectively. aperture values for (a) and (b) corrected vision as measured by Subject A are 4.58 mm and 0.75 mm, respectively.

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Table 2. Table shows induced myopic refractive errors and corresponding Da values needed to restore corrected vision for Subject A and Subject B.

Induced Myopic Subject A: D required Subject B: D required ECVFL 1 Current a a Refractive Error via for vision correction for vision correction setting (mA) ECVFL 1 (Diopter) (Diopter) (Diopter) 0.00 243.02 5.00 5.00 ´0.25 146.26 4.80 4.70 ´0.50 148.13 4.58 3.92 ´0.75 150.03 3.92 2.86 ´1.00 152.61 3.22 2.16 Micromachines ´20151.25, 6, page–page 158.47 2.56 1.62 ´1.50 161.04 2.08 1.28 ´1.75 167.62 1.26 0.98 given aperture´2.00 diameter, one must 173.84 take into account the aperture 0.75 separation parameters 0.86 dx and dy in both the transverse (x-y) directions for a given maximum viewing distance dmax.

Figure 16.16.Data Data plot plot showing showing vision vision test systemtest system aperture aperture diameter diameter needed needed to correct to forcorrect eye refractive for eye refractive error. The theoretical Equation (13) acquired aperture diameter Da is shown as a error. The theoretical Equation (13) acquired aperture diameter Da is shown as a coarse-dashed line withcoarse-dashed plus symbols line andwith experimental plus symbols data and for experimental Subject A is showndata for as Subject a solid A line is withshown dot as symbols a solid andline Subjectwith dot B symbols as ﬁne-dashed and Subject line with B as circle fine-dashed symbols. line with circle symbols.

An experimental test of the proposed Figure 10 eyewear is also conducted demonstrating the clearer vision using aperture control and the effect of aperture separation on the FOVs seen by the sensor representing the retina. Custom masks having apertures each of diameter Dm are constructed from black sheets and placed at the location of ECVFL 2 in the Figure 11 set-up with Da set to 5 mm and ´1.75 D of myopia induced in the eye lens system using ECVFL1. Three masks are designed called mask A, mask B and mask C. Mask A has a single aperture of Dm = 1.5 mm, mask B has two apertures of the 1.5 mm diameters with a separation of 3 mm and mask C has two apertures of 1.5 mm diameters that are separated by 6 mm. Figure 17 shows the deployed custom-made eyewear Figure 17. Test eyewear comprising of eye-pieces having 1.5 mm diameter apertures separated by having eye pieces made with mask B (left) and mask C (right). 3 mm (left) and 6 mm (right) distances. Figure 18a shows the CMOS sensor chip acquired eye test chart image obtained without any mask present in the deployed test eyewear. Figure 18b shows an image viewed through mask A giving the expected clearer image demonstrating non-blurred image viewing aperture control. Figure 18c shows the CMOS chip acquired image when using mask B. This image shows laterally offset eye charts that results due to the slightly overlapping FOVs that occurs due to the two apertures in mask B with a gap of 3 mm between the two apertures. Finally, Figure 18d is captured using mask C where the apertures are separated by a larger distance of 6 mm resulting in FOVs that are separated and thus result in a clear non-blurred view of the Snellen chart through one of the mask apertures. Therefore, in the design of the proposed eyewear using multiple apertures of a given aperture diameter, one must take(a) into account the aperture separation(b) parameters dx and dy in both the transverse (x-y) directions for a given maximum viewing distance dmax.

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Figure 18. CMOS chip acquired images for eyewear vision test system having an eye lens refractive error of −1.75 D with test eyewear having (a) no mask; (b) mask A having a single aperture of diameter 1.5 mm; (c) mask B having two 1.5 mm diameter apertures separated by 3.00 mm creating overlapping FOVs; and (d) mask C having two 1.5 mm diameter apertures separated by 6.00 mm creating non-overlapping FOVs. 17 Micromachines 2015, 6, page–page

given aperture diameter, one must take into account the aperture separation parameters dx and dy in Micromachines 2015, 6, page–page both the transverse (x-y) directions for a given maximum viewing distance dmax.

given aperture diameter, one must take into account the aperture separation parameters dx and dy in both the transverse (x-y) directions for a given maximum viewing distance dmax.

Figure 16. Data plot showing vision test system aperture diameter needed to correct for eye

refractive error. The theoretical Equation (13) acquired aperture diameter Da is shown as a Micromachines 2015, 6, 1690–1709 coarse-dashedFigure 16. Data line plot with showing plus symbols vision and test experimental system aperture data diameterfor Subject needed A is shown to correct as a solidfor eye line

withrefractive dot symbols error. andThe Subjecttheoretical B as fine-dashedEquation (13) line acquired with circle aperture symbols. diameter Da is shown as a coarse-dashed line with plus symbols and experimental data for Subject A is shown as a solid line with dot symbols and Subject B as fine-dashed line with circle symbols.

Figure 17. Test eyewear comprising of eye-pieces having 1.5 mm diameter apertures separated by

Figure 17.3 mmTest (left) eyewear and 6 mm comprising (right) distances. of eye-pieces having 1.5 mm diameter apertures separated by 3 mm (left)Figure and 6 17. mm Test (right) eyewear distances. comprising of eye-pieces having 1.5 mm diameter apertures separated by 3 mm (left) and 6 mm (right) distances.

(a) (b)

Figure 18. CMOS chip acquired(c) images for eyewear vision test system(d) having an eye lens refractive errorFigure of 18.−1.75 CMOS D with chip testacquired eyewear images having for eyewear (a) no mask;vision (testb) masksystem A having having an a eyesingle lens aperture refractive of Figure 18.diametererrorCMOS of 1.5− chip1.75 mm; D acquired (withc) mask test Beyewear images having twohaving for 1.5 eyewear (mma) no diameter mask; vision (aperturesb) testmask system A separated having having a by single 3.00 an mmaperture eye creating lens of refractive error of ´overlapping1.75diameter D with1.5 FOVs; mm; test ( cand) mask eyewear (d) Bmask having havingC havingtwo 1.5 ( twoamm) no 1.5diameter mm mask; diameter apertures (b)mask apertures separated A havingseparated by 3.00 mm aby single creating6.00 mm aperture of diameter 1.5creatingoverlapping mm; non-overlapping (c) FOVs; mask and B having( dFOVs.) mask twoC having 1.5 mmtwo 1.5 diameter mm diameter apertures apertures separated separated by 6.00 3.00 mm mm creating creating non-overlapping FOVs. overlapping FOVs; and (d) mask C having two17 1.5 mm diameter apertures separated by 6.00 mm 17 creating non-overlapping FOVs.

5. Conclusions The proposed system for eye vision testing is a unique design using micromachines based on micro-optics, micromechanics, and micro-electronics. The system design includes RF/optical data communications capability and remote wireless energy harvesting and energy storage as well as smart light conditioning modules for optical attenuation, aperture, and light polarization control. The eye vision test system is particularly suited for rapid eye vision tests for children, given child eye vision characteristics including child behavior and psychology. The proposed eye test system can operate in three different modes including vision testing using variable aperture, and variable lens focus controls. Basic experiments are conducted to demonstrate the operational principles of the proposed novel eye test system. Economical design eyewear based on aperture control is proposed and experimentally demonstrated for myopic eyes. Future work relates to the design and demonstration of the proposed eye vision system for complete refractive error correction, along with its full smart optics and optical and RF wireless capabilities, including energy harvesting and remote energy transfer, capture, and storage. It is expected that such a system can lead to a lightweight, energy efﬁcient, user friendly, economical and reliable instrument for wide spread deployment around the world.

Author Contributions: M. Junaid Amin and Mehdi N. Riza designed and performed the experiments and also conducted the theory and data analysis; Nabeel A. Riza conceived the designs and wrote the paper; M. Junaid Amin and Mehdi N. Riza revised the paper.

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Conﬂicts of Interest: The authors declare no conﬂict of interest.

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