energies

Article LED Uniform Illumination Using Double Linear Fresnel for Energy Saving

Ngoc Hai Vu * ID , Thanh Tuan Pham ID and Seoyong Shin *

Department of Information and Communication Engineering, Myongji University, San 38-2 Nam-dong, Yongin 449-728, Korea; [email protected] * Correspondence: [email protected] (N.H.V.); [email protected] (S.S.); Tel.: +82-102-709-6483 (S.S.)

Received: 9 November 2017; Accepted: 6 December 2017; Published: 11 December 2017

Abstract: We present a linear Fresnel design for -emitting diode (LED) uniform illumination applications. The LED source is an array of LEDs. An array of collimating lens is applied to collimate output from the LED array. Two linear Fresnel lenses are used to redistribute the collimated beam along two dimensions in the illumination area. Collimating lens and linear surfaces are calculated by geometrical and nonimaging optics. The collimated beam output from the collimating lens array is divided into many fragments. Each fragment is refracted by a segment of Fresnel lens and distributed over the illumination area, so that the total beam can be distributed to the illumination target uniformly. The simulation results show that this design has a compact structure, high optical efficiency of 82% and good uniformity of 76.9%. Some consideration of the energy savings and optical performance are discussed by comparison with other typical light sources. The results show that our proposed LED system can reduce energy consumption five-times in comparison to using a conventional fluorescent lamp. Our research is a strong candidate for low cost, energy savings for indoor and outdoor lighting applications.

Keywords: geometric optical design; illumination design; light-emitting diodes; nonimaging optics

1. Introduction In recent years, the use of light-emitting diodes (LEDs) for lighting has become more popular because of their energy saving, long lifespan, compact volume, high color-rendering index and environmental benefits [1,2]. Although LED manufactures and leading suppliers have celebrated the advantages of LEDs in the industry publications and their catalogues [3], the semi-spherical radiation pattern of LEDs is a drawback that renders them rarely used directly for illumination purposes that require high uniformity. To solve this problem, some secondary optical elements are utilized to redistribute light from the LED source to the target position efficiently and uniformly. Recently, many researchers have been concentrating on the development of the LED diffusers, which can improve the cost-efficiency, energy savings and uniform distribution of LED lighting systems. In most cases, the secondary optical component is designed for a single LED [2]. This element design involves several basic methods. Shi et al. presented the design of an LED rectangular uniform illumination lens based on the freeform optics, the lighting-energy conservation law, the edge- principle and Snell’s law [4]. Rabl et al. proposed a 3D reflector for uniform far-field illumination. This reflector was designed using tailored edge-ray theory [5]. A combination of a Fresnel lens and a micro-lens array for LED illumination was proposed by Wang et al. [6]. The Fresnel lens for the collimation of LED irradiation and the micro-lens array for the uniform distribution of the collimated beam were designed according to geometrical optics and nonimaging optics. A group from Yonsei University, Korea, improved the illuminance uniformity, color uniformity and flux efficiency of LED illumination by using a modified Fresnel lens with an optimized groove angle [7]. However, most

Energies 2017, 10, 2091; doi:10.3390/en10122091 www.mdpi.com/journal/energies Energies 2017, 10, 2091 2 of 15 methods that use secondary optics design involve complex calculations, which result in complicated and costly LED lens designs. In addition, because of the limited optical power output from a single LED, many LED lighting modules are required to illuminate a large area, and this requirement increases the cost of the total illumination system and installation. Another approach is using an LED array as a light source. For the purpose of illumination, achieving a good uniform distribution of LED array sources is important. Zhouping Su’s group proposed a numerical optimization method for designing an LED array for achieving a good uniform illumination distribution on the target plane. A simulated annealing algorithm is used to optimize an LED array arrangement. In a report from Moreno et al. [8], different array configurations with optimum LED-to-LED spacing were used to obtain a uniform distribution. The optimum LED-to-LED spacing was achieved using the analytical method This group also designed a spherical LED array, which could distribute LED light uniformly over a large area [9]. Whang presented a method for designing an LED array with an arbitrary view angle for a uniform illumination distribution [10]. Zong Qin studied the uniform illumination condition for an LED array with a large view angle [11]. Most methods for the uniform illumination of an LED array are based on the calculation and optimization of LED positions, intensities and emitting directions. Although the advantage of these methods is that they can achieve high optical efficiency because they do not use any secondary optics for redistribution, they are only suitable for some specific purposes because of the great complexity of the analytical method. These methods are difficult for wide use in indoor and outdoor lighting, which requires flexibility in design. Therefore, these studies have not been commercialized successfully on the market to date. In this study, we propose an alternative approach to the uniform illumination of an LED array by designing simple secondary optics components. The LED bare chips are integrated into an array using chip-on-board (COB) technology. An array of is placed on the top LED array to collect and collimate the light output from an LED array. The is a plano-convex lens, which is developed with a special aspherical convex design using the freeform optics design method. The distributor was used to redistribute the LED light uniformly throughout the illumination area. The distributor was composed of two linear Fresnel lenses placed perpendicularly to each other. The linear Fresnel lens was designed by using the simultaneous multiple surface (SMS) method and the commercial MATLAB software. The simplicity and the flexibility of the design method and system configuration enable this research to be applied for various lighting purposes. The remainder of the paper is organized as follows: Section2 discusses the LED illumination system design. In Section3, an LED illumination system is modeled by using the LightTools TM software (Synopsys Inc., San Jose, CA, USA) to analyze the performance of this system. This section also discusses the applications for indoor and outdoor illumination purposes. The conclusions are presented in Section4.

2. LED Uniform Distribution System Design The design for a uniform LED illumination system is presented in this section. Our goal is to design a lighting system with a simple shape and a low fabrication cost using an array of LEDs. By constraining the design to be compatible with a simple fabrication process such as the molding method, we propose a novel uniform illumination system for an LED array source by using a freeform plano-convex lens as the light collimator and a double layer of linear Fresnel lenses as the uniform light distributor. The physical layout of the system is shown in Figure1. The upper layer is a rectangular array of LED chips integrated on a printed circuit board (PCB) using COB technology. The light emitted from the LED array is collimated by the collimation layer, which is an array of plano-convex lenses. Most conventional LED diffusers, which are reflectors or lenses, do not provide an effective solution to redistribute the collimated beam at the illumination target. In this study, we propose the design of the double layers of linear Fresnel lenses to spread the collimated beam for uniform illumination. The details of the system such as optical elements and their parameters are discussed below. Energies 2017, 10, 2091 3 of 15 Energies 2017, 10, 2091 3 of 15 Energies 2017, 10, 2091 3 of 15

Figure 1. Physical layout of the uniform illumination system for a light-emitting diode (LED) array. FigFigureure 1. Physical layout of the uniform illum illuminationination syste systemm for a light-emittinglight-emitting diode (LED)(LED) array.

2.2. LED Array 2.1.2.2. LED Array In our proposed system shown in Figure 1, the array of collimators (plano-convex lenses) is InIn ourour proposedproposed systemsystem shownshown inin FigureFigure1 1,, the the array array of of collimators collimators (plano-convex (plano-convex lenses)lenses) isis attached to collimate the LED output light from an array of LED chips. It is difficult to make an array attached toto collimatecollimate thethe LED LED output output light light from from an an array array of of LED LED chips. chips It. It is is difficult difficult to to make make an an array array of of LED using the conventional LED chips, which have an end capsule placed right on top of the LEDof LED using using the conventionalthe conventional LED LED chips, chips which, which have anhave end an capsule end capsule placed rightplaced on right top ofon the top emitting of the emitting surface. The LED COB technology has been developed recently, and it gives us a solution to surface.emitting Thesurface. LED The COB LED technology COB technology has been has developed been developed recently, recently, and it givesand it us gives a solution us a solution to solve to solve this problem. In this study, we optimized certain aspects of the design space using the thissolve problem. this problem. In this study,In this we study, optimized we optimized certain aspects certain of aspects the design of the space design using space the parameters using the parameters from the commercially available LED COB of ProPhotonix Company (Cork, Ireland). The fromparameters the commercially from the commercially available LED available COB of LED ProPhotonix COB of ProPhotonix Company (Cork,Company Ireland). (Cork, The Ireland) LED COB. The LED COB technology is a method of mounting a bare LED chip in direct contact with the substrate technologyLED COB technology is a method is of a mountingmethod of amounting bare LED a chip bare in LED direct chip contact in direct with contact the substrate with the to substrate fabricate to fabricate LED arrays. COB technology allows a high packing density because of the small size of LEDto fabricate arrays. LED COB arrays technology. COBallows technology a high allows packing a high density packing because density of the because small size of the of the small LED size chip, of the LED chip, thus resulting in higher intensity and easy coupling with the collimator. We employed thusthe LED resulting chip, thus in higher resulting intensity in higher and easyintensity coupling and easy with coupling the collimator. with the We collimator. employed We a rectangular employed a rectangular array of LEDs on board to couple with the collimator layer. arraya rectangular of LEDs array on board of LEDs to couple on board with to the couple collimator with the layer. collimator layer. In our optical design and simulations, 100 LED chips with a size of 1 mm × 1 mm are arranged InIn ourour opticaloptical designdesign and and simulations, simulations, 100 100 LED LED chips chips with with a sizea size of of 1 mm1 mm× 1× mm1 mm are are arranged arranged in in a rectangular array with a lattice of l = 10 mm, as shown in Figure 2. Therefore, the total length of ain rectangular a rectangular array array with with a lattice a lattice of l of= 10l = mm,10 mm, as shownas shown in Figurein Figure2. Therefore, 2. Therefore, the totalthe total length length of the of the system is 100 mm × 100 mm. We use LightToolsTM to model the LED array with ray tracing. This systemthe system is 100 is mm100 mm× 100 × 100 mm. mm. We useWe LightToolsuse LightToolsTM toTM model to model the LEDthe LED array array with with ray tracing. ray tracing. This LEDThis LED array is used for the design of the collimator and the distributer. arrayLED array is used is used for the for design the design of the of collimator the collimator and the and distributer. the distributer.

FigureFigure2. 2.A A rectangularrectangular arrayarray ofof LEDLED chipschips waswas integratedintegrated intointo thethe printedprinted circuitcircuit boardboard (PCB)(PCB) asas aa Figure 2. A rectangular array of LED chips was integrated into the printed circuit board (PCB) as a lightlight source source for for the the illumination illumination system. system. light source for the illumination system.

2.2.2.3. CollimatorCollimator 2.3. Collimator TheThe collimatorcollimator isis anan opticaloptical elementelement thatthat collectscollects thethe raysrays fromfrom thethe LEDLED sourcesource andand deflectsdeflects themthem The collimator is an optical element that collects the rays from the LED source and deflects them toto becomebecome parallel parallel rays. rays. AA typicaltypical collimatorcollimator maymay bebe constructedconstructed simplysimply byby aa parabolicparabolic mirrormirror oror lens,lens, to become parallel rays. A typical collimator may be constructed simply by a parabolic or lens, andand thethe LEDLEDsource source isis positionedpositionedat at itsits focalfocalpoint. point. TheseThese typestypes ofof collimatorscollimators onlyonly workwork withwith aa lightlight and the LED source is positioned at its focal point. These types of collimators only work with a light sourcesource that that has has a a small small view view angle. angle. AA typicaltypical LEDLED withwith aa largelarge viewview angleangle ofof 120120°◦ cannotcannot bebe collimatedcollimated source that has a small view angle. A typical LED with a large view angle of 120° cannot be collimated efficientlyefficiently by by these these conventionalconventional collimators. collimators. Therefore, Therefore, wewe propose propose the the technique technique of of designing designing aa efficiently by these conventional collimators. Therefore, we propose the technique of designing a plano-convexplano-convex lens lens using using freeform freeform optics. optics. The The design design methodology methodology of of the the freeform freeform lens lens is describedis described in plano-convex lens using freeform optics. The design methodology of the freeform lens is described Figurein Figure3. 3. in Figure 3.

Energies 2017, 10, 2091 4 of 15 Energies 2017, 10, 2091 4 of 15

FigureFigure 3.3. Design principle of the plano-convexplano-convex lenslens basedbased onon freeformfreeform optics.optics.

AsAs shown in in Figure Figure 3,3, the the plano plano-convex-convex lens lens is iscomposed composed of two of two surfaces: surfaces: a plano a plano-surface-surface and a andfreeform a freeform surface. surface. The plano The plano-convex-convex lens lens in our in our study study is is rotational rotationallyly symmetric symmetric.. In In a a rotationally-symmetricrotationally-symmetric system, only only 2D 2D coordinates coordinates are are discussed. discussed. Suppose Suppose that that an an LED LED chip chip is isa apoint point sour source.ce. The The LED LED is is in in the the air air with with the the refractive n0,, and and the collimation lenslens mademade ofof plasticplastic hashas thethe refractiverefractive indexindex nn11.. TheThe LEDLED sourcesource isis locatedlocated inin thethe originaloriginal pointpoint O.O. RayRay OAOAnn isis anan incidentincident rayray toto thethe plano-surfaceplano-surface with with an an incident incident angle angle of ofα nα.n.OA OAnnis is refracted refracted to to ray rayA AnnBBnn atat pointpointA Ann onon thethe flatflat surfacesurface ofof thethe lens.lens. AccordingAccording toto Snell’sSnell’s lawlaw ofof thethe followingfollowing EquationEquation (1),(1),

nn01sin(nn ) sin( ). (1) n0 sin(αn) = n1 sin(βn). (1) The LED source emits a spherical wavefront. After the spherical wavefront is refracted by the The LED source emits a spherical wavefront. After the spherical wavefront is refracted by the freeform surface, it becomes a plane wavefront. All the light rays that emit from the LED have the freeform surface, it becomes a plane wavefront. All the light rays that emit from the LED have the same optical path from the source to the wavefront. According to the conservation of optical path same optical path from the source to the wavefront. According to the conservation of optical path obtained by Equation (2), obtained by Equation (2), nOA nAB  nBP  nOA  nAB 0n 1 n n 0 n n 0 0 1 0 0 , (2) n0|OAn| + n1|AnBn| + n0|BnPn| = n0|OA0| + n1|A0B0|, (2)

where OA0 is the initial incident ray along the x axis and A0B0 is the refractive ray by the plano-surface. where OA0 is the initial incident ray along the x axis and A0B0 is the refractive ray by the plano-surface. As the incident angle is zero degrees, it goes straight. OA0 = d0 is the distance from the LED to the As the incident angle is zero degrees, it goes straight. OA0 = d0 is the distance from the LED to the plano-surface, and A0B0 = d1 is the length of the plano-convex lens. All the points on the freeform plano-surface, and A B = d is the length of the plano-convex lens. All the points on the freeform curve can be calculated0 0 by solving1 Equations (1) and (2). Therefore, the entire freeform curve can be curve can be calculated by solving Equations (1) and (2). Therefore, the entire freeform curve can be obtained by collecting the coordinates of these points. The whole lens surface was built by revolving obtained by collecting the coordinates of these points. The whole lens surface was built by revolving the 2D curved profile in the 3D modeling software (LightToolsTM). Figure 4 shows a solid model of the 2D curved profile in the 3D modeling software (LightToolsTM). Figure4 shows a solid model this freeform plano-convex lens with ray tracing. Table 1 listed all the design parameters for a of this freeform plano-convex lens with ray tracing. Table1 listed all the design parameters for a collimator. If the light source is a point source, the proposed collimator can collimate the beam collimator. If the light source is a point source, the proposed collimator can collimate the beam perfectly. perfectly. However, in this study, the LED chip (1 mm × 1 mm) cannot be considered a perfect point However, in this study, the LED chip (1 mm × 1 mm) cannot be considered a perfect point source. source. Therefore, the output beam from the collimator is not a parallel beam; it has a small divergent Therefore, the output beam from the collimator is not a parallel beam; it has a small divergent angle. angle. The travelling of the beam, which is emitted from a small source, passing through the The travelling of the beam, which is emitted from a small source, passing through the collimator is very collimator is very complicated and cannot be calculated by theory. For this situation, the simulation complicated and cannot be calculated by theory. For this situation, the simulation using LightTools is using LightTools is conducted to calculate the divergence of the output beam. We obtained an output conducted to calculate the divergence of the output beam. We obtained an output beam divergence of beam divergence of 7.5°. The effect of the non-ideal collimated beam on the efficiency and uniformity 7.5◦. The effect of the non-ideal collimated beam on the efficiency and uniformity of the illumination of the illumination system is discussed in Section 3. system is discussed in Section3. A collimating lens for an LED light source is an essential device that is A collimating lens for an LED light source is an essential device that is widely used in lighting widely used in lighting engineering. Many methods to design and fabricate the collimator can be found engineering. Many methods to design and fabricate the collimator can be found in the literature on in the literature on LED lighting engineering, such as in [6,12]. However, for illumination purposes, LED lighting engineering, such as in [6,12]. However, for illumination purposes, the collimation the collimation requirement is not high, and therefore, a simple structure and a low fabrication cost requirement is not high, and therefore, a simple structure and a low fabrication cost are preferred. are preferred. The proposed plano-convex lens design using the freeform optics technique satisfies The proposed plano-convex lens design using the freeform optics technique satisfies all these all these requirements. The fact that some similar products are available on the market with a low requirements. The fact that some similar products are available on the market with a low price [13] shows that our proposed plano-convex lens design is commercially viable. One hundred of the

Energies 2017, 10, 2091 5 of 15

Energies 2017, 10, 2091 5 of 15 price [13] shows that our proposed plano-convex lens design is commercially viable. One hundred of the designeddesignedEnergies 2017 plano plano-convex, 10, 2091-convex lenses lenses are arearranged arranged in a 10 in × a10 10 array× 10 as arraya collimation as a collimation layer for the layer LED forarray,5 of the 15 LED array,as asshown shown in Figure in Figure 5. 5. designed plano-convex lenses are arranged in a 10 × 10 array as a collimation layer for the LED array, as shown in Figure 5.

Figure 4. Solid model of the plano-convex lens for LED collimation. Figure 4. Solid model of the plano-convex lens for LED collimation.

FigureTableTable 4. Solid 1. 1.Design modelDesign of parametersparameter the planos- forconvex for the the plano lens plano-convex for-convex LED collimation.lens lens.. Design Parameters Value Table 1. DesignDesign parameter Parameterss for the plano-convex lens. Value View angle of the LED (collection angle of the plano-convex lens) 120° ◦ View angleRefractive of the LED index (collectionDesign of the plano Parameters angle-convex of the lens plano-convex (PMMA) lens)Value1.49 120 ViewRefractive angleDistance of the index of LED the ofsource(collection the plano-convexto the angle plano of- theconvex lens plano (PMMA)lens,-convex d0 lens) 0.5120 mm° 1.49 DistanceRefractiveThickness of theindex source of the to plano the plano-convex-convex lens,lens (PMMA)d1 lens, d 0 101.49 mm0.5 mm DistanceThicknessDiameter of the ofsource of the the plano-convex planoto the- planoconvex-convex lens, lens, D lens,d 1 d0 0.510 mmmm10 mm DiameterThickness of of the the plano-convex plano-convex lens, dD1 10 mm10 mm Diameter of the plano-convex lens, D 10 mm

Figure 5. An array of plano-convex lenses is placed on top of the LED array to collimate the LED light.

2.Figure4. LinearFigure 5. AnFresnel 5. An array array Lens of of plano-convexDesign plano -convex lenseslenses is placed placed on on top top of ofthe the LED LED array array to collimate to collimate the LED the light. LED light. The high-intensity collimated LED beam needs to be distributed uniformly in the receiver for 2.4. Linear Fresnel Lens Design 2.3. Linearillumination Fresnel purposes. Lens Design The uniform distribution of light is a critical issue in solid state lighting. In our case,The thehigh light-intensity source collimated is small in sizeLED (100 beam mm needs × 100 to mm) be distributedwith high intensity, uniformly and in it the is areceiver collimated for The high-intensity collimated LED beam needs to be distributed uniformly in the receiver for beamillumination of LED purposes.light. Most The of typical uniform light distribution distributers of thatlight are is useda critical in LED issue lighting in solid engineering state lighting. do not In illuminationsupportour case, a the purposes.redistribut light sourceion The ofis uniform smallthe collimated in distributionsize (100 beam mm to of× 100the light mm)illumination is awith critical high area issueintensity, uniformly. in solid and itIn state is this a collimated lighting. research, In our case,abeam theuniform light of LED illumination source light. is Most small distribution of intypical size (100light in the mmdistributers illumination× 100 mm) that plane are with used ishigh desired. in intensity,LED The lighting special and engineering itdesign is a collimated of a do linear not beam of LEDFresnelsupport light. lens a Mostredistribut that of can typical spreadion oflight the the collimated collimated distributers beam beam that touniformly are the used illumination is in proposed LED lightingarea here. uniformly. engineeringThe linear In this Fresnel doresearch, not lens support a redistributioncana uniform redirect illumination collimatedof the collimated distribution rays to beamthe in expected the to theillumination illumination position splane alongarea is onedesired. uniformly. dimension The special In. The this designproposed research, of a linear a uniform illuminationFresnel lens distribution thatinclude cans spread many in the thesegments illumination collimated, and beam planeeach uniformlysegment is desired. is is a Theproposed tiny special convex here. design lens. The Thelinear of aparallel linear Fresnel Fresnelbeam lens lens thatpasscan can ing redirect spread through thecollimated collimated each segment rays beamto the is uniformly expected focused at position isa proposedverys along short here. one focal dimension The length linear and. The Fresnel spread proposed lens over canlinear the redirect collimatedoneFresnel-dimensional rayslens include to the receiver.s expected many Twosegments positions linear, and Fresnel along each segment lenses one dimension. placed is a tiny perpendicularly convex The proposed lens. The can linearparallel redirect Fresnel beam the lens collimatedpassing through light uniformly each segment on a two is- dimensional focused at illuminationa very short area focal. Similar length to Section and spread 2.2, we over applied the includes many segments, and each segment is a tiny convex lens. The parallel beam passing through one-dimensional receiver. Two linear Fresnel lenses placed perpendicularly can redirect the each collimated segment light is focused uniformly at aon very a two short-dimensional illumination and spread area. Similar over the to Section one-dimensional 2.2, we applied receiver. Two linear Fresnel lenses placed perpendicularly can redirect the collimated light uniformly on a

Energies 2017, 10, 2091 6 of 15 Energies 2017, 10, 2091 6 of 15 thetwo-dimensional SMS method to illumination calculate the area. curve Similar of the to Fresnel Section lens’s2.2, we segment. applied The the SMSSMS methodmethod to for calculate designing the nonimagingcurve of the optical Fresnel components lens’s segment. was Thedeveloped SMS method several for years designing ago and nonimaging is widely used optical in solar components energy andwas lighting developed applications several years [14]. agoThe and design is widely process used for inone solar segment energy is anddescribed lighting in applicationsFigure 6a. Some [14]. initialThe design conditions, process including for one segmentleft edge isA describedLBL, right edge in Figure ARBR6 ofa. Somethe segment initial conditions,and the receiver including size RR’ left are given. The left ray ALBL is deflected to the rightmost of the receiver, R’. Similarly, the right ray edge ALBL, right edge ARBR of the segment and the receiver size RR’ are given. The left ray ALBL is ARBR is deflected to leftmost of the receiver, R. Two rays ALR’ and ARR intersect at point F, which is deflected to the rightmost of the receiver, R’. Similarly, the right ray ARBR is deflected to leftmost of the the focal point of a segment. Every ray between ALBL and ARBR must be concentrated at F after they receiver, R. Two rays ALR’ and ARR intersect at point F, which is the focal point of a segment. Every are refracted on the surface of the segment. These rays reach the receiver after passing though F. ray between ALBL and ARBR must be concentrated at F after they are refracted on the surface of the Therefore,segment. Theseall rays rays are reachredistributed the receiver over after the receiver passing RR’ though afterF .passing Therefore, through all rays the are segment. redistributed As all raysover are the receiver focused RR’ on afterF, they passing must through satisfy the segment. law of optical As all rays path are length focused (OPL) on F conservation, they must satisfy; see Equationthe law of (3 optical): path length (OPL) conservation; see Equation (3):

nAB1L L nBF 0 L  nAB 1 R R  nBF 0 R  nAB 1 n n  nBF 0 n , (3) n1|ALBL| + n0|BLF| = n1|ARBR| + n0|BRF| = n1|AnBn| + n0|BnF|, (3) where n0, n1 are the refractive indices of air and the Fresnel lens material, respectively. Based on these equationswhere n0,, nall1 are coordinates the refractive of the indices segment of air surface and the curve Fresnel are lenscalculated material,. Profiles respectively. of all other Based segments on these ofequations, the linear all Fresnel coordinates lens are of thecalculated segment using surface the curvesame areprocedure. calculated. Figure Profiles 6b shows of all otherthe performance segments of ofthe a linearsingle Fresnellinear Fresnel lens are lens calculated in a 1D usingreceiver. the same procedure. Figure6b shows the performance of a single linear Fresnel lens in a 1D receiver.

Figure 6. (a) Design method for a segment of a linear Fresnel lens; (b) A single linear Fresnel lens Figure 6. (a)Design method for a segment of a linear Fresnel lens. (b) A single linear Fresnel lens redistributes light to a 1D receiver; (c) A light distributor including two linear Fresnel lenses can redistributes light to a 1D receiver. (c) A light distributor including two linear Fresnel lenses can redistribute the light uniformly over a plane. redistribute the light uniformly over a plane.

WeWe placed placed two two linear linear Fresnel Fresnel lenses lenses perpendicularly perpendicularly to to spread spread the the light light in in two two-dimensional-dimensional illilluminationumination areas areas,, as as shown shown in in Figure Figure 66cc.. The exaggeration of F Figureigure 66cc isis thethe detaildetail ofof thethe doubledouble linearlinear Fresnel lens.lens. OnOn the the basis basis of of this this design design method, method, we builtwe built Fresnel Fresnel design design software software that can that specify can specifythe characteristics the characteristics of double of Fresnel double lenses Fresnel by lenses controlling by controlling some input some conditions, input conditions, such as positions such as of positionsthe Fresnel of lensthe Fresnel and illumination lens and illumination plane (receiver), plane the (receiver), size of the the Fresnel size of lens,the Fresnel the number lens, the of segments number ofin segments the lens and in the the lens size ofand the the illumination size of the plane.illumination This method plane. This is flexible method because is flexible two linearbecause Fresnel two linear Fresnel lenses are utilized to redistribute light along the horizontal and vertical directions. The

Energies 2017, 10, 2091 7 of 15 Energies 2017, 10, 2091 7 of 15 Energiessize (width 2017, 10 and, 2091 length) of the illumination plane can be changed freely by changing the initial7 of 15 conditions. One other interesting aspect of our design is that it can illuminate the target not only lenses are utilized to redistribute light along the horizontal and vertical directions. The size (width and sizewhen (width the light and source length) lies of on the the ill symmetricalumination planeaxis, but can also be changedwhen it is freely out of by the changing symmetrica thel axis initial of length) of the illumination plane can be changed freely by changing the initial conditions. One other conditions.the illumination One otherplane, interesting as shown inaspect Figure of 7a,b our. design is that it can illuminate the target not only interesting aspect of our design is that it can illuminate the target not only when the light source lies when the light source lies on the symmetrical axis, but also when it is out of the symmetrical axis of on the symmetrical axis, but also when it is out of the symmetrical axis of the illumination plane, the illumination plane, as shown in Figure 7a,b. as shown in Figure7a,b.

Figure 7. (a) The light source is on the symmetrical axis of the illumination plane and (b) out of the illumination plane. Figure 7. (a) The light source is on the symmetrical axis of the illumination plane and (b) out of the Figure 7. (a) The light source is on the symmetrical axis of the illumination plane and (b) out of the illumination plane. 3. Simulationillumination Results plane. and Comparison

3. SimulationThe commercial Results optical and Comparison simulation software LightToolsTM is utilized to design and simulate the 3. Simulation Results and Comparison geometrical structure of the proposed LED lighting system. The structure of the system for simulation The commercial optical simulation software LightToolsTM is utilized to design and simulate the is shownThe commercial in Figure optical 8. A square simulation shape software of lighting LightTools is chosenTM is utilized as an example to design to and investigate simulate the the geometrical structure of the proposed LED lighting system. The structure of the system for simulation geometricalperformance structure of the designedof the proposed lighting LED system. lighting As system. described The above,structure the of LEDthe system illumination for simulation system is shown in Figure8. A square shape of lighting is chosen as an example to investigate the performance isincludes shown an in array Figure of 8.LED A chips, square an shape array ofof lightingplano-convex is chosen lens as as a an collimator example and to investigatea double linear the of the designed lighting system. As described above, the LED illumination system includes an array of performanceFresnel lens asof athe light designed distributor. lighti ng Lambertian system. As LED described chips are above, used inthe the LED simulations. illumination Each system LED LED chips, an array of plano-convex lens as a collimator and a double linear Fresnel lens as a light includesgenerates an 50 array lm; therefore,of LED chips, the total an array flux ofof theplano LED-convex array lens is 5000 as a lm collimator. In the designed and a double system, linear the distributor. Lambertian LED chips are used in the simulations. Each LED generates 50 lm; therefore, FresnelFresnel lens lens as and a light collimated distributor. source Lambertian are made of LED poly chips methyl are used methacrylate in the simulations. (PMMA). The Each double LED the total flux of the LED array is 5000 lm. In the designed system, the Fresnel lens and collimated genFresnelerates lens 50 forlm; producing therefore, uniformthe total illumination flux of the LEDdistribution array is in 5000 a 3 mlm ×. 3In m the receiver designed is designed. system, Thethe source are made of poly methyl methacrylate (PMMA). The double Fresnel lens for producing uniform Fresnelreceiver lens is located and collimated 3 m away source from are the made light of source. poly methyl Efficiency methacrylate and uniformity (PMMA). are The important double illumination distribution in a 3 m × 3 m receiver is designed. The receiver is located 3 m away from Fresnelcharacteristics lens for of producing the lighting uniform effect. illumination distribution in a 3 m × 3 m receiver is designed. The the light source. Efficiency and uniformity are important characteristics of the lighting effect. receiver is located 3 m away from the light source. Efficiency and uniformity are important characteristics of the lighting effect.

Figure 8. Illustration of the simulation structure for efficiency analysis. Figure 8. Illustration of the simulation structure for efficiency analysis.

InIn thethe linea linearr FresnelFigure Fresnel 8. lens Illustration lens design, design, of the the the mostsimulation most important important structure parameter parameterfor efficiency is the is analysis. thesize size of the of segment. the segment. The Thesegment segment size dominates size dominates both optical both opticalefficiency efficiency and uniformity and uniformity at the illumination at the illumination target. To quantify target. Tothe quantify effectIn the of linea the segment effectr Fresnel ofsizesegment lens on design,the sizeefficiency the on themost and efficiency important uniformity and parameter uniformityof the sys is tem,the of the sizea parametric system, of the segment. a parametric analysis The is segment size dominates both optical efficiency and uniformity at the illumination target. To quantify the effect of segment size on the efficiency and uniformity of the system, a parametric analysis is

Energies 2017, 10, 2091 8 of 15 Energies 2017, 10, 2091 8 of 15 performed while varying the number of segments of the linear Fresnel lens. Note that the linear analysisFresnel lens is performed size is 100 while mm × varying 100 mm; the therefore, number ofchanging segments the of number the linear of Fresnelsegments lens. is equivalent Note that the to linearchanging Fresnel the segment lens size size. is 100 Based mm × on100 the mm; design therefore, principle changing for the the linear number Fresnel of segmentslens in Section is equivalent 2.3, we tobuilt changing a calculation the segment program size. using Based MATLAB on the design, which principlecan provide for the linearFresnel Fresnel lens surface lens in curve Section based 2.3, weon the built initial a calculation conditions program such as usingFresnel MATLAB, lens size, whichnumber can of providesegments, the illumination Fresnel lens target surface size curve and basedits coordinates. on the initial Then conditions, the Fresnel such lens as Fresnel parameters lens size,and numbercoordinates of segments, were imported illumination into LightTools target size andsoftware its coordinates. to analyze Then, the optical the Fresnel performance. lens parameters In LightTools and coordinates software, were the imported analysis intoof illuminance LightTools softwaredisplay is to based analyze on the a measur opticalement performance. gird. The In LightToolshigh resolution software, of 200 the analysis× 200 of ofthe illuminance mesh grid display in the isillumination based on a measurement receiver was gird. chosen The in high our resolution simulation. of 200We× generated200 of the differentmesh grid linear in the Fresnelillumination lens receiverstructures was with chosen several in our different simulation. segment We numbers generated N different ranging linear from Fresnel 10–60 in lens increments structures of with 10. severalFigure 9a different–c shows segment the effects numbers of theN illuminationranging from performances 10–60 in increments for segment of 10. numbers Figure9a–c of shows10, 20 and the effects30, respectively. of the illumination performances for segment numbers of 10, 20 and 30, respectively.

Figure 9. a N b N c N Figure 9. LightLight distributiondistribution onon aa receiverreceiver withwith ((a)) N == 10;10, (b)) N = 20 and ( c) N = 30. 30.

Energies 2017, 10, 2091 9 of 15

EnergiesThe2017 uniformity, 10, 2091 equation (the following equation uniformity, Equation (4)) is utilized to calculate9 of 15 the uniformity U of the light on the receiver. Figure 10 shows the dependence of U on the segment number N. The higher the number of segments (or the smaller the size of the segments), the higher The uniformity equation (the following equation uniformity, Equation (4)) is utilized to calculate the optical uniformity. This can be explained by the methodology of the Fresnel design. The the uniformity U of the light on the receiver. Figure 10 shows the dependence of U on the segment collimated beam output from the collimator is divided into many pieces, and each piece is number N. The higher the number of segments (or the smaller the size of the segments), the higher the redistributed and spreads over the receiver. Clearly, a smaller segment size can provide higher optical uniformity. This can be explained by the methodology of the Fresnel design. The collimated uniformity in the receiver. beam output from the collimator is divided into many pieces, and each piece is redistributed and spreads over the receiver. Clearly, a smallerMinimum segment Irradiation size can provide higher uniformity in the receiver. U 100% (4) AverageMinimum Irradiat Irradiationi on U =× 100% (4) Average Irradiation Optical efficiency, which is defined as the ratio of the luminous flux in the receiver to the total luminousOptical flux efficiency, of the light which source, is defined is also as a the function ratio of of the segment luminous number flux in N the. Figure receiver 10 to shows the total the dependenceluminous flux of optical of the efficiency light source, η on is N alsotogether a function with uniformity. of segment Efficiency number decreasesN. Figure when 10 shows segment the sizedependence decreases. of Optical optical efficiencyefficiency ηhason twoN together types of with loss effects: uniformity. Fresnel Efficiency loss and decreases geometrical when loss segment of the segment.size decreases. In this Opticalproposed efficiency system, hasthe Fresnel two types losses of loss occur effects: on the Fresnel surface loss of the and plano geometrical-convex losslenses of andthe segment.linear Fresnel In this lenses. proposed Another system, loss is the the Fresnelgeometrical losses loss occur of the on Fresnel the surface lens. ofIn thean ideal plano-convex case, no geometricallenses and linear loss occurs Fresnel when lenses. a Anotherperfect collimated loss is the beamgeometrical reaches loss the of Fresnel the Fresnel lens. lens. However, In an ideal our designedcase, no geometricalcollimator based loss occurs on plano when-convex a perfect lenses collimated provides beam a beam reaches with thea divergence Fresnel lens. of 7.5°. However, As a ◦ largerour designed segment collimator has a higher based acceptance on plano-convex angle than lenses a smaller provides one, a beamthe geometrical with a divergence loss is smaller. of 7.5 . FigureAs a larger 10 shows segment a trade has-off a higher between acceptance optical efficiency angle than and a uniformity. smaller one, For the different geometrical specific loss purposes, is smaller. theFigure parameters 10 shows of a the trade-off Fresnel between lens should optical be efficiencydesigned andappropriately. uniformity. For different specific purposes, the parameters of the Fresnel lens should be designed appropriately.

FigureFigure 10. 10. EfficiencyEfficiency and and uniformity uniformity depend depend on on the the number number of of segments segments NN..

ToTo evaluate evaluate the the advantages advantages of of our our proposed proposed lightin lightingg system, system, the the applications applications for for indoor indoor lighting lighting andand street street lighting lighting are are simulated, simulated, and and the the performances performances of of the the systems systems are are compared compared with with those those of of traditionaltraditional fluorescent fluorescent lamps lamps and and LED LED panel panel ceiling ceiling light light.. For For a a typical typical office office building, building, the the minimum minimum illuminanilluminancece required required is is about about 500 500 lux, lux, and and the the requirement requirement for for uniformity uniformity is is 60% 60% [15] [15.]. In In this this study, study, wewe perform perform a a simulation simulation to to evaluate evaluate the the performance performance of of the the designed designed LED LED lighting lighting system system in in a a virtual virtual room.room. The The size size o off the the room room is 3 m × 33 m, m, and and the the interior interior is is designed designed using using LightTools LightTools TM.. The The distance distance fromfrom the the ceiling ceiling to to the the working working plane plane (illumination (illumination target) target) is is 2.7 2.7 m. m. The The 3D 3D view view of of the the room’s room’s interior interior isis illustrated illustrated in in Figure Figure 11a,b 11a,b.. The The Fresnel Fresnel lens lens parameters parameters are are calculated calculated according according to to these these conditions. conditions. AsAs shown inin FigureFigure 10 10,, we we select select the the Fresnel Fresnel lens lens segment segment number number of 30. of Uniformity30. Uniformity is achieved is achieved at 76.9% at 7and6.9% optical and optical efficiency efficiency at 82%. at Other82%. O simulationsther simulation usings using the traditional the traditional fluorescent fluorescent lamp lamp and theand LED the LEDpanel panel ceiling ceil lighting light for interior for interior lighting lighting were were also performedalso performed for comparison. for comparison. A typical A typical fluorescent fluorescent light lightfixture fixture consist consist of a fluorescent of a fluorescent tube and tube louver and louver as shown as shown in Figure in 12Figurea. In our12a. simulation, In our simulation, a compact a compactfluorescent fluorescent lamp (fabricated lamp (fabricated of V-shape of reflector,V-shape steelreflector, material steel and material three and fluorescent three fluorescent tubes) was tubes) taken wasinto taken consideration. into consideration. Figure 12 bFigure is a fluorescent 12b is a fluorescent lamp model lamp built model in LightTools. built in LightTools. Recently, Recently, the LED thepanel LED ceiling panel light ceiling has beenlight anhas innovative been an innovative lighting product lighting which product can directlywhich can replace directly the conventional replace the conventionalfluorescent lamps fluores orcent light lamps bulbs. or We light built bulbs. a simulation We built model a simulation of a commercial model of LED a commercial flat panel light LED based flat

Energies 2017, 10, 2091 10 of 15 Energies 2017, 10, 2091 10 of 15 panel light based on some parameters of the LED panel light model TLP-XX-6060 from SGSlight Energiescompanypanel2017 light, 10 (Shenzhen,, 2091based on some China) parameter [16]. Figures of the 13a ,LEDb is panel an LED light panel model light TLP and-XX the-6060 dissection from SGSlight of10 its of 15 Energies 2017, 10, 2091 10 of 15 components.company (Shenzhen, China) [16]. Figure 13a,b is an LED panel light and the dissection of its components.panel light based on some parameters of the LED panel light model TLP-XX-6060 from SGSlight on some parameters of the LED panel light model TLP-XX-6060 from SGSlight company (Shenzhen, company (Shenzhen, China) [16]. Figure 13a,b is an LED panel light and the dissection of its China) [components.16]. Figure 13 a,b is an LED panel light and the dissection of its components.

Figure 11. (a) Simulation configuration for indoor lighting; (b) visual image of the test room. Figure 11. (a) Simulation configuration for indoor lighting; (b) visual image of the test room. FigureFigure 11. (a )11. Simulation (a) Simulation configuration configuration for for indoorindoor lighting; lighting; (b) ( bvisual) visual image image of the of test the room test. room.

FigureFigure 12.Figure 12.(a )( a12. A) A compact(a compact) A compact fluorescent fluorescent fluorescent lamplamp lamp withwith with aa V-shapedV--shapeshapedd louver louver louver and and and (b )( ba (b )fluorescent )a afluorescent fluorescent light light model light model model builtbuiltFigure with builtwith 12. LightTools with LightTools(a) ALightTools compact software. software. software. fluorescent lamp with a V-shaped louver and (b) a fluorescent light model built with LightTools software.

Figure 13. (a) Illustration of an LED panel light and (b) the dissection of its components Figure 13. (a) Illustration of an LED panel light and (b) the dissection of its components. FigureFigure 14a 13.– c( ashow) Illustrations the light of distributionan LED panel on light the working and (b) theplane dissection by using of our its proposedcomponents method, the fluorescentFigure 13. lamp(a) Illustration and the ofLED an LED panel panel light. light Table and 2(b ) shows the dissection the comparison of its components of the optical FigureFigureperformances 14 a–c14a– showsc show between thes the lightthree light distribution types distribution of lighting on on the methods. the working working Using plane plane LED by lightingby using using for our our the proposed proposed building method, alsomethod, the fluorescentthe fluorescentFigureovercomes lamp 14a inherent and–c lamp show the andproblemss LED the the light panel suchLED distribution light. as panel mercury Table light. on pollution2 showsthe Table working or the the 2 comparison shows short plane lifetime theby using comparison ofof the our fluorescent optical proposed of performancesthe lamp. method, optical betweenperformancesthe fluorescentAlthough three types LED between lamp offlat lighting panels and three thehave methods.types LED been of commercialized panel lighting Using light. LED methods. Table lighting widely Using 2 in showsfor recent the LED years buildingthe lighting comparison, current also for flat overcomes the panel of building the optical inherent also problemsovercomesperformancesstill such have inherent assome between mercury big problems drawbacks. three pollution typessuch As as orshown of mercury the lighting in short Figure pollution methods. lifetime 14c, an or ofLED Using the the flat short fluorescentLED pane lifetime lightingprovided lamp.of forthea narrow thefluorescent Although building lighting LEDlamp. also flat panelsAlthoughovercomes havearea and LED been inherent a hotflat commercialized spot panels problems [17] .have Simulation such been widely as commercialized resultsmercury in recentalso pollution show years, thatwidely or the current the uniformityin shortrecent flat lifetime panelyears of 39%, lightscurrentof is themuch fluorescent still flat lower have panel than some lamp.lights big drawbacks.stillAlthough have some LED As shown bigflat drawbacks.panels in Figure have A been 14s shownc, commercialized an LEDin Figure flat pane14c, widely an provided LED in recentflat apane narrow years provided, current lighting a narrowflat area panel lighting and lights a hot spotareastill [17 haveand]. Simulation asome hot spot big drawbacks.[17] results. Simulation also A shows shown results that in also theFigure uniformityshow 14c, that an theLED of uniformity 39% flat pane is much providedof 39% lower is amuch than narrow thelower lighting standard than area and a hot spot [17]. Simulation results also show that the uniformity of 39% is much lower than requirement. In practice, the uniformity can be improved by increasing the number of LED panels; however, the material and installation costs also increase. Our proposed design can achieve higher lighting area and good uniformity. As shown in Table2, the delivery efficiency, which is defined by the ratio of luminous flux on the working plane and the light source, is 76.9%, 24% and 45.5%, Energies 2017, 10, 2091 11 of 15 the standard requirement. In practice, the uniformity can be improved by increasing the number of LED panels; however, the material and installation costs also increase. Our proposed design can Energiesachieve2017 higher, 10, 2091 lighting area and good uniformity. As shown in Table 2, the delivery efficiency, 11which of 15 is defined by the ratio of luminous flux on the working plane and the light source, is 76.9%, 24% and 45.5%, corresponding to our method, the fluorescent lamp and the LED panel, respectively. In our corresponding to our method, the fluorescent lamp and the LED panel, respectively. In our design design method, all light output from the source is delivered and distributed uniformly to the method, all light output from the source is delivered and distributed uniformly to the illumination illumination target directly so that the achieved delivery efficiencies can be very high in comparison target directly so that the achieved delivery efficiencies can be very high in comparison to existing to existing methods. Our proposed system requires only 60-W LEDs with 8000 lm to delivery 6150 methods. Our proposed system requires only 60-W LEDs with 8000 lm to delivery 6150 lm to the lm to the working plane. The fluorescent lamps, which have a very large output angle, require 300 W working plane. The fluorescent lamps, which have a very large output angle, require 300 W of electrical of electrical consumption corresponding to 21,000 lm to illuminate the working area with consumption corresponding to 21,000 lm to illuminate the working area with illumination of 500 lux. illumination of 500 lux. Following the Table 2, only 5109 lm is delivered to the working plane by Following the Table2, only 5109 lm is delivered to the working plane by using fluorescent lamps with using fluorescent lamps with 21,000 lm, and the other part is delivered and absorbed by the room’s 21,000 lm, and the other part is delivered and absorbed by the room’s walls. An economic comparison walls. An economic comparison is also shown in Table 2. Using our proposed design can save is also shown in Table2. Using our proposed design can save five-times the electric power consumption five-times the electric power consumption for illumination in comparison with using of traditional for illumination in comparison with using of traditional fluorescent lamps. The physical dimension fluorescent lamps. The physical dimension of our design is much smaller than others, so this may of our design is much smaller than others, so this may contribute to the reduction of material and contribute to the reduction of material and installation cost. installation cost.

Figure 14. Light distribution on the working plane using: (a) our proposed lighting method; Figure 14. Light distribution on the working plane using: (a) our proposed lighting method; (b) fluorescent lamp; (c) commercial LED panel light. (b) fluorescent lamp; (c) commercial LED panel light.

TableTable 2.2. ComparisonComparison ofof thethe performancesperformances betweenbetween thethe threethree typestypes ofof lightlight source:source: our proposed LED source, fluorescentfluorescent lamplamp andand commercialcommercial LEDLED panelpanel light.light.

Comparison Categories Proposed LED Lighting Fluorescent Lamp LED Panel Light Comparison Categories Proposed LED Lighting Fluorescent Lamp LED Panel Light Initial conditions Room dimensions (m)Initial 3 × 3 conditions× 3 3 × 3 × 3 3 × 3 × 3 Lamp dimensionsRoom dimensions (mm) (m) 100 ×3100 × 3 ×× 320 610 ×3 ×400 3 × ×3 12203 600× 3 × 3600 × 11 LumenLamp efficacy dimensions (lm/W) (mm) 100 130 × 100 × 20 610 × 400 70 × 1220 600 × 600 × 130 11 Luminous fluxLumen of lightefficacy source (lm/W) (lm) 8000130 21,00070 130 10,000 LuminousElectric flux power of light source (lm) 608000 21,000 300 10,000 77 Electric power 60 300 77 Optical performances Optical performances LuminousLuminous flux on flux working on working plane plane (lm) (lm) 61506150 5109 5109 4547 4547 IlluminanceIlluminance Min/Max Min/Max (lux) (lux) 313/680313/680 360/728 360/728 201/860 201/860 AverageAverage illuminance illuminance 514514 568 568 506 506 UniformityUniformity (%) (%) 7474 62 62 39 39 EnergyEnergy savingssavings WorkingWorking hours hours (hours/day) (hours/day) 1010 10 10 10 10 WorkingWorking days (days/year)days (days/year) 320320 320 320 320 320 kWh perkWh year per year 192192 960 960 246.4246.4 KoreanKorean average average cost per cost kWh per kWh ($/kWh) ($/kWh) 0.40.4 0.4 0.4 0.4 0.4 Total costTotal for cost lighting for lighting per year per year ($) ($) 76.876.8 384 384 98.4 98.4

Optical solutions for conventional street lights rarely work well with LEDs. Many newcomers, Optical solutions for conventional street lights rarely work well with LEDs. Many newcomers, and even some big players in the street lighting business, are having difficulties finding the best and even some big players in the street lighting business, are having difficulties finding the best solutions to meet the relevant regulations. Getting the most out of streetlights is easy, but doing it uniformly is a challenge. We conduct simulations for street lighting using our proposed method and Energies 2017, 10, 2091 12 of 15 Energies 2017, 10, 2091 12 of 15 Energies 2017, 10, 2091 12 of 15 solutions to meet the relevant regulations. Getting the most out of streetlights is easy, but doing it uniformly is a challenge. We conduct simulations for street lighting using our proposed method and comparesolutions the to performance meet the relevant of our regulations. system with Getting that of the typical most LED out of street streetlights lamps. Figureis easy, 15 buta showsdoing theit compareuniformly the is performance a challenge. ofWe our conduct system simulations with that of for typical street LEDlighting street using lamps. our Figureproposed 15a method shows the and visual image of a street light, and Figure 15c illustrates the structure of a conventional street lamp. visualcompare image the of performance a street light, of andour systemFigure 1with5c illustrates that of typical the structure LED street of alamps. conventional Figure 1 street5a shows lamp. the Most lighting technologies work with the light source on a symmetrical axis, and thus, conventional Mostvisual lighting image technologies of a street light, work and with Figure the light 15c sourceillustrates on athe symmetrical structure of axis, a conventional and thus, conventional street lamp. street lamps use an arm to direct the light source to the center of the road. This arm makes the lamp streetMost lamps lighting use technologies an arm to direct work the with light the source light source to the oncenter a symmetrical of the road. axis, This and arm thus makes, conventional the lamp structure bulky and hinders traffic. As mentioned above, our proposed lighting method can support structurestreet lamps bulky use and an hinders arm to directtraffic. the As lightmentioned source above, to the centerour proposed of the road. lighting This method arm makes can supportthe lamp off-symmetricaloffstructure-symmetrical bulky axis axis and lighting, lighting, hinders andtraffic.and thus,thus As, thismentioned arm can can above, be be eliminated, eliminated, our proposed as as shown shown lighting inin Figure method Figure 15b 15can. b. support off-symmetrical axis lighting, and thus, this arm can be eliminated, as shown in Figure 15b.

FigureFigure 15. 15. ((aa)) Visual Visual image image of street lighting; lighting; (b (b) )o ourur proposed proposed street street lamp; lamp; (c) (ca) conventional a conventional streetstreetFigure lamp. lamp. 15. (a) Visual image of street lighting; (b) our proposed street lamp; (c) a conventional street lamp. We carried out two simulations on our proposed design and a typical LED street lamp for We carried out two simulations on our proposed design and a typical LED street lamp for comparisonWe carried and evaluation. out two simulations Assume that on the our distance proposed from design light source and a to typical the illuminati LED streeton surface lamp is for comparison and evaluation. Assume that the distance from light source to the illumination surface 10comparison m and the illuminationand evaluation. area Assume is a rectangular that the distance, 10 m × 20from m. lightThe main source goal to thewas illuminati to illuminateon surface a road is is 10 m and the illumination area is a rectangular, 10 m × 20 m. The main goal was to illuminate surface10 m and area the of illumination10 m × 20 m witharea isa minimuma rectangular illuminance, 10 m × 20 of m. more The than main 20 goal lux was, which to illuminate is the standard a road a road surface area of 10 m × 20 m with a minimum illuminance of more than 20 lux, which is the requirementsurface area for of regular10 m × 20 vehicle m with traffic. a minimum Based on illuminance this initial ofcondition more than, we 20 calculated lux, which the is structurethe standard of standard requirement for regular vehicle traffic. Based on this initial condition, we calculated the therequirement linear Fresnel for regularlenses as vehicle shown traffic. in Figure Based 16a. on The this upper initial Fresnel condition lens, we in Figurecalculated 16a thewas structure designed of structureforthe on linear-axis of theFresnel illumination linear lenses Fresnel, asand shown lenses the lower in as Figure shown Fresnel 16a. in lensThe Figure upper is for 16 a.Fresneloff The-axis upper lens illumination. in FresnelFigure 16a The lens was ray in Figuredesigned tracing 16 a wasperformancefor designed on-axis for illuminationof on-axis one proposed illumination,, and street the lower lamp and F theresnel module lower lens is Fresnel shown is for lens off in -axis Figure is for illumination. off-axis 16b. For illumination. comparison, The ray tracing The we ray tracingconductedperformance performance another of one simulation of proposed one proposed for street a typical street lamp LED lamp module street module is lamp. shown is We shown in used Figure in the Figure 16b.design For16 b.of comparison, an For LED comparison, street we welampconducted conducted using a another anotherfreeform simulationsimulation optics lens for, forwhich a atypical typical is available LED LED street streetfrom lamp. the lamp. LightTools We We used used thesoftware the design design library ofof an an, LEDas LED shown street street lampinlamp the using exaggeration using a freeform a freefor ofm opticsFigure optics lens, 17. lens Figure which, which 18a is is available isavailable the light fromfrom distribution the LightTools LightTools on the software softwareillumination library library, area, as asof shown shownour inproposed thein the exaggeration exaggeration design. The of of Figurelight Figure distribution 17 17.. Figure Figure on 18 18a theais isroadthe the surface lightlight distributiondistribution which utilized on on the thetypical illumination illumination LED street area area lamp of of ouris our proposedshowproposedn in design. Figure design The18b. The. light light distribution distribution on on the the road road surfacesurface whichwhich utilized typical typical LED LED street street lamp lamp is is shownshow inn Figurein Figure 18 b.18b.

Figure 16. (a) The designed linear Fresnel lenses for street lighting purposes and (b) raytracing FigureanalysFigure 16.is of 1(a6 our). The(a )proposed The designed designed street linear lamp. linear Fresnel Fresnel lenses lenses for street for street lighting lighting purposes purpose and (sb and) raytracing (b) raytracing analysis of ouranalys proposedis of our street proposed lamp. street lamp.

Energies 2017, 10, 2091 13 of 15 Energies 2017 10 Energies 2017, 10, 2 2091091 1313 of 15 15

Figure 17. The structure of an LED street light based on freeform optics design and its Figure 17. raytracingFigure 17. analysisThe The structure structure. of an of LED an street LED light street based light on freeform based optics on freef designorm andoptics its raytracing design analysis.and its raytracing analysis.

FigureFigure 18. 18. LigLightht distribution distribution of of the the street street lamp lamp on on the the road road surface surface using: using: ( (aa)) our our proposed proposed design design and and (Figure(bb)) typical typical 18. LigLED LEDht street streetdistribution lamp lamp based based of the onstreet freeform freeform lamp optics. on the road surface using: (a) our proposed design and (b) typical LED street lamp based on freeform optics. AsAs shown shown in in Figure Figure 1188bb,, conventional conventional LED LED street street lights lights have have a a high high-intensity-intensity area area right right under under thethe lamp, lamp,As shown and and the thein intensityFigure intensity 18 awayb away, conventional from from the the centercenter LED streetdecreases decreases lights rapidly, rapidly, have a thereby therebyhigh-intensity decreasing decreasing area one’s one’sright ability abilityunder totheto observe observe lamp, and light light the contrasts. contrasts. intensity Ideally, Ideally,away from street street the lighting lighting center decreasesshould should be be rapidly, as as uniform uniform thereby and and decreasingas as visually visually one’s pleasing pleasing ability as as daylighttodaylight observe [[18]18 light].. InIn our contrasts. our proposed proposed Ideally, system, system, street the thelightlighting light was should distributedwas distributed be as uniformly uniform uniformly onand the as road on visually the surface; road pleasing therefore surface as; therefordaylightit coulde increase [18]it could. In one’s our increase proposed ability one’s of observation. system, ability the of Tablelight observation. 3was shows distributed the Table comparison 3 uniformly shows of the the on comparison performances the road surface of andthe; performancesthereforenergy savingse it could and between energy increase two saving one’s typess between of ability street oftwo lamps. observation. types Our of proposedstreet Table lamps. design 3 showsOur canproposed the save comparison 50% design of the can energy of save the 50%performancesconsumption of the ene inrgyand comparison consumptionenergy saving with ins using betweencomparison a typical two with types LED using of street street a typical lamp. lamps. LED Our street proposed lamp. design can save 50% TheofThe the uniform uniform energy distribution consumption distribution over in over acomparison road a road surface surfacewith using using our using proposeda typical our proposed LED design street can design leadlamp. to cana breakthrough lead to a breakthroughin solutionsThe uniform for in LEDsolutions distribution street for lighting. LED over street In a terms lighting. road of surface mass In term production, usings of mass our the production, proposed linear Fresnel design the linear lens can structure Fresnel lead tolens can a structurebreakthroughallow using can the allowin extrusionsolutions using moldingforthe LED extrusion street method, lighting.molding which Inmethod is term the leasts ,of which mass costly production,is manufacturingthe least costlythe linear process. manufacturing Fresnel The lens cost process.structureefficiency The can of the callowost proposed efficiency using LEDthe of extrusion thelighting proposed systemmolding LED can method lighting be realized, which system because is canthe of beleast the realized simplicitycostly becausemanufacturing in material, of the simplicityprocess.shape, form, The in function,material, cost efficiency mannershape, of ofform, the operation, proposedfunction, assembly manner LED lighting and of usage,op eration, system which assembly can all be present realized and obvious usage, because capabilities which of theall presentsimplicityfor commercialization. obvious in material, capabilities shape, for form, commercialization. function, manner of operation, assembly and usage, which all present obvious capabilities for commercialization. Table 3. Comparison between our proposed LED street lamp and a typical LED street lamp. Table 3. Comparison between our proposed LED street lamp and a typical LED street lamp. Comparison Categories Proposed LED Street Lamp Typical LED Street Lamp Comparison Categories InitialProposed conditions LED Street Lamp Typical LED Street Lamp Height of post (m) Initial conditions10 10 IlluminationHeight of post area (m) (m) 10 10× 20 10 10× 20 LumenIllumination efficacy area (lm/W) (m) 10130 × 20 10130 × 20 LuminousLumen flux efficacy of light (lm/W) source (lm) 12000130 1800130 Luminous flux of light source (lm) 12000 1800

Energies 2017, 10, 2091 14 of 15

Table 3. Comparison between our proposed LED street lamp and a typical LED street lamp.

Comparison Categories Proposed LED Street Lamp Typical LED Street Lamp Initial conditions Height of post (m) 10 10 Illumination area (m) 10 × 20 10 × 20 Lumen efficacy (lm/W) 130 130 Luminous flux of light source (lm) 12,000 1800 Electric power (W) 92.3 140 Optical performance Illuminance Min/Max (lux) 20/70 18/130 Average illuminance 45 70 Uniformity (%) 44 25.7 Energy savings Working hours (6 pm–6 am) 12 12 Working days 365 365 kWh per year 404.3 613.2 Korean average cost per kWh ($/kWh) 0.4 0.4 Total cost for lighting per year ($) 161.7 245.3

4. Conclusions A uniform LED lighting system using nonimaging linear Fresnel lenses has been proposed and analyzed with the purpose of uniform illumination and energy savings. The design of the Fresnel lens based on the SMS method was discussed in detail. The designed system was modeled and simulated with LightTools software to explore the optical performance. The simulation results indicate that 82% optical efficiency was achieved at a uniformity of 76.9% for the proposed system. The simulation of the performance of our design for practical purposes, such as indoor and street lighting, and the comparison with a conventional light source were conducted. They show that our proposed design exhibits great potential for energy savings and commercialization.

Acknowledgments: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03031338) and the 2017 Research Fund of Myongji University. Author Contributions: Ngoc Hai Vu conceived of and developed the original ideas. Ngoc Hai Vu carried out the performance analysis and simulations and wrote the paper. Thanh Tuan Pham supported the theoretical calculation process. Seoyong Shin supervised the research and finalized the paper. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature and Symbology l Lattice of LED array mm d0 Distance from LED chip to collimator mm d1 Thickness of collimator mm D Diameter of the plano-convex lens mm n0 Refractive index of the air - n1 Refractive index of PMMA - ◦ αn Incident angle to plano-surface of collimator ◦ βn Refracted angle U Uniformity % N Number of grooves - η Optical efficiency % Energies 2017, 10, 2091 15 of 15

References

1. Ding, Y.; Liu, X.; Zheng, Z.; Gu, P. Freeform LED lens for uniform illumination. Opt. Express 2008, 16, 12958–12966. [CrossRef][PubMed] 2. Su, Z.; Xue, D.; Ji, Z. Designing LED array for uniform illumination distribution by simulated annealing algorithm. Opt. Express 2012, 20, A843–A855. [CrossRef][PubMed] 3. Lumex. Is a High-Power LED the Best Choice for Your Application? 2017. Available online: http://www. lumex.com/is-a-high-power-led-the-best-choice-for-your-application (accessed on 23 August 2017). 4. Shi, Y.; Li, B.; Zhao, M.; Zhou, Y.; Zhang, D. The design of LED rectangular uniform illumination lens system. Opt. Int. J. Light Opt. 2017, 144, 251–256. [CrossRef] 5. Gordon, J.M.; Rabl, A. Reflectors for uniform far-field irradiance: Fundamental limits and example of an axisymmetric solution. Appl. Opt. 1998, 37, 44–47. [CrossRef][PubMed] 6. Wang, G.; Wang, L.; Li, F.; Kong, D. Design of optical element combining fresnel lens with microlens array for uniform light-emitting diode lighting. J. Opt. Soc. Am. A 2012, 29, 1877–1884. [CrossRef][PubMed] 7. Kim, B.; Choi, M.; Kim, H.; Lim, J.; Kang, S. Elimination of flux loss by optimizing the groove angle in modified fresnel lens to increase illuminance uniformity, color uniformity and flux efficiency in LED illumination. Opt. Express 2009, 17, 17916–17927. [CrossRef][PubMed] 8. Moreno, I.; Tzonchev, R.I. Designing light-emitting diode arrays for uniform near-field irradiance. Appl. Opt. 2006, 45, 2265–2272. [CrossRef][PubMed] 9. Moreno, I.; Munoz, J.; Ivanov, R. Uniform illumination of distant targets using a spherical light-emitting diode array. Opt. Eng. 2007, 46, 1–8. 10. Whang, A.J.W.; Chen, Y.Y.; Teng, Y.T. Designing uniform illumination systems by surface-tailored lens and configurations of LED arrays. IEEE/OSA J. Disp. Technol. 2009, 5, 94–103. [CrossRef] 11. Qin, Z.; Wang, K.; Chen, F.; Luo, X.; Liu, S. Analysis of condition for uniform lighting generated by array of light emitting diodes with large view angle. Opt. Express 2010, 18, 17460–17476. [CrossRef][PubMed] 12. Wang, G.; Wang, L.; Li, F.; Zhang, G. Collimating lens for light-emitting-diode light source based on non-imaging optics. Appl. Opt. 2012, 51, 1654–1659. [CrossRef][PubMed] 13. IODA. N-Series Collimator Led Lens. Available online: http://ioda-it.com/product/collimators-led-light- lens/ (accessed on 1 November 2017). 14. Miñano, J.C.; Benitez, P.; Liu, J.; Infante, J.; Chaves, J.; Wang, L. Applications of the SMS method to the design of compact optics. Proc. SPIE 2010, 7717.[CrossRef] 15. Gary, S.; David, D.; Kevin, H.; Richard, M. Lighting Handbook, 4th ed.; Illumination Engineering Society: New York, NY, USA, 1966. 16. SGSlight Lighting Solution. Available online: http://sgslight.com/products/led-panel/596x596/ (accessed on 27 October 2017). 17. Wen, H.; Lin, L.B.-S. Improvement of illumination uniformity for LED flat panel light by using micro-secondary lens array. Opt. Express 2012, 20, A788–A798. [CrossRef][PubMed] 18. Laakkio, O. Winning the Optical Challenges in LED Street Lighting. Digi-Key Electronics. Available online: https://www.digikey.com.au/en/articles/techzone/2011/may/winning-the-optical-challenges- in-led-street-lighting (accessed on 23 August 2017).

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).