70 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 2, FEBRUARY 2012 Accommodative Response of Integral in Near Distance Youngmin Kim, Jongshin Kim, Keehoon Hong, Hee Kyung Yang, Jae-Hyun Jung, Heejin Choi, Sung-Wook Min, Jong-Mo Seo, Jeong-Min Hwang, and Byoungho Lee

Abstract—Objective evaluation results using optometric device 3D image so that it appears as if the regenerated object is in the to measure accommodative responses in viewing a real object real world. However, the coherent light source is necessary in and are presented. From the empirical results between the real object and an integrated three-dimensional (3D) general, and it is still hard to reproduce full- image, we find that over 73% of participants keep eyes on the real [5]. Integral imaging is a potential alternative of stereoscopy integrated 3D image instead of a display panel. The results also and holography. It uses an array of small lenses that are spher- show the participants do not recognize a mismatch between the ac- commodative response and the convergence of the eye, which used ical, square, or hexagonal to produce the 3D images which can to be believed as one of the major factors to cause visual fatigue provide both horizontal and vertical , resulting from a in viewing near-distance integral imaging. Seventy-one normal two-dimensional (2D) lens array [3], [6]. It provides different adult subjects (23 38 years old) participated in the experiment, and accommodative response measurement results of the real perspectives according to view directions although the spatial integrated image show a statistically significant concordance with and angular resolutions are in a tradeoff relation. real objects. With the progress of the 3D displays, visual fatigue related to Index Terms—Display human factors, geometrical , three- the 3D display has recently emerged again as an important issue. dimensional (3D) displays. Many stereoscopic films were produced in the early 1950s; how- ever, the public interests rapidly declined. One of the main rea- sons was associated with the visual fatigue caused by poor 3D I. INTRODUCTION effects. Since then, great attention has been paid to clarify the cause of visual fatigue related to the 3D images. Researches con- HE three-dimensional (3D) display technique is one of centrated on the 3D images, especially stereoscopic displays, T the most promising ways to express real images. Unlike revealed that causes of visual fatigue included anomalies of the other next-generation display techniques such as an organic , distortion error of the eyes, conflict between light-emitting diode (OLED), flexible display, and ultrathin and convergence of the eye, and an excessive display, only the 3D display technique can depict an exquisite binocular parallax. Among them, conflict between accommo- image of natural world [1]–[4]. Naturalness of the reproduced dation and convergence of the eye has been mentioned as one 3D image varies with the 3D display methods. In an autostereo- of the controversial issues because of the unnatural combina- scopic display method based on either a or a tion of stimuli. The conflict occurs because the accommodative , there is a tradeoff between the number of views response of the eye remains fixed on the display panel, and the and resolution of the reconstructed 3D image. Therefore, the convergence response varies in depth depending on the disparity limitation of the number of views makes the autostereoscopic degrees of display panel. display unnatural, while it is easy to fabricate the lenticular lens Numerous research has attempted to expound an eye fatigue or parallax barrier. , which is known as one mechanism caused by mismatch of accommodation and of the most progressive 3D display methods, can reconstruct a convergence in the stereoscopic display [7]–[13]. Despite many attempts to explain the mechanism of eye fatigue in Manuscript received March 15, 2011; revised July 20, 2011; accepted July 26, stereoscopic displays, little research has been conducted both 2011. Date of current version January 25, 2012. This work was supported by the subjectively and objectively in the field of integral imaging. Brain Korea 21 Program (Information Technology of National University) of the Ministry of Education, Science and Technology of Korea. Recently, it was confirmed that there was no significant visual Y. Kim, K. Hong, J.-H. Jung, J.-M. Seo, and B. Lee are with the School fatigue difference between integral imaging method and con- of Electrical Engineering, Seoul National University, Seoul 151-744, Korea ventional binocular method by ten observers [14]. It was an (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). objective evaluation based on biological reactions, such as the J. Kim is with the Graduate School of Medical Science and Engineering, cerebral blood flow observation, the autonomic nervous system, KAIST, Daejeon 305-701, Korea (e-mail: [email protected]). and an endocrine system [14]. These methods were based on H. K. Yang, and J.-M. Hwang are with Department of , Seoul National University College of Medicine, Seoul National University indirect measurement of the eye fatigue which was obtained Bundang Hospital, Gyeonggi-do 463-707, Korea (e-mail: [email protected]; from a biological reaction instead of the direct measurement. [email protected]). It is almost impossible to measure the visual fatigue, therefore H. Choi is with Department of Physics, Sejong University, Seoul 143-747, Korea (e-mail: [email protected]). no wonder they approached a way of indirect measurement. S.-W. Min is with Department of Information Display, , Optometric devices, such as autorefractor for measurement Seoul 130-701, Korea (e-mail: [email protected]). of mismatch between accommodation and convergence, have Color versions of one or more of the figures are available online at http:// ieeexplore.ieee.org. been applied in comparisons to determine the amount of change Digital Object Identifier 10.1109/JDT.2011.2163701 of accommodation in viewing integral imaging [15], [16]. This

1551-319X/$31.00 © 2012 IEEE KIM et al.: ACCOMMODATIVE RESPONSE OF INTEGRAL IMAGING IN NEAR DISTANCE 71 approach can objectively analyze the visual fatigue caused by mismatch between accommodation and convergence. However, so far, measurements of an accommodative response of the eye fatigue in integral imaging have never been examined for large sample size of subjects. In this paper, we present objective evaluation results using an optometric device to measure accommodative responses in viewing a real object and integral imaging. Measurement data for the accommodative response of both the real object and the reconstructed image from integral imaging were obtained from 71 volunteers (30.9 years of average age, ranging from 23 to 38 years, 18% of eyeglasses wearers). The predefined interval of the accommodative response was subdivided into 10 cm in near Fig. 1. Mismatch between accommodation and convergence in viewing 3D dis- distance. We performed statistical analysis to compare distinctly play based on autostereoscopic display. accommodative response of both cases. By the comparison, we could verify that more than 73% of participants stared at the real integrated 3D image from integral imaging instead of the left and right images. The causes have been elusive; this is at- display panel. The data clearly show that accommodation-con- tributed to intricate complex of visual nerve system and it does mismatch, which is mentioned as a possible cause for not appear as a form of resultant visual symptom. visual fatigue when watching 3D displays, did not coincide with Literature mentions that the mismatch between accommoda- integral imaging in near-distance observation. tion and convergence was known as the possible factor for the eye fatigue [18]–[20], but it needs a great deal of debate and research to investigate the eye fatigue mechanism. In viewing II. ASSESSMENT OF THE EYE FATIGUE real objects, both accommodation and convergence response are coupled reflexively. It is referred to as a Donder’s line, which A. and the Causes Leading to the shows the range that is effective in terms of coupled mechanism Eye Fatigue between two stimuli in viewing real scene [21]. On the other When fixed objects are observed by human eyes, the observer hand, the accommodative stimuli are said to be fixed on - uses both psychological and physiological cues to recognize scopic display, while the convergence stimuli vary depending depth information of the objects. The psychological cues in- on the screen disparity as shown in Fig. 1. The mismatch can clude a linear , an overlapping, shading, and texture induce visual fatigue; in particular, the situation becomes se- gradient, which could be generated by . The rious in near-range observation of 3D images. Depth perception physiological cues could be classified into an accommodation, in near distance could be affected by binocular disparity, accom- a binocular disparity, a convergence, and a motion parallax. Be- modation, and convergence. The observation of 3D images can cause the two eyes of human are separated horizontally by about be mostly done within 2 3 m. Therefore, the depth percep- 6.5 cm and each eye has its own perspective, slightly different tion in the stereoscopic displays could be strongly affected by images are projected to the of each eye. The images are the mismatch inducing the visual fatigue. combined by brain, and the depth information is obtained by Despite relatively vigorous research about visual fatigue in both psychological and physiological cues. The difference be- the field of stereoscopic displays, few have attempted to address tween a real object and a 3D image based on stereoscopic dis- a visual fatigue mechanism in integral imaging. It attracted only plays came from artificial depth information. Although a 3D limited attention; however, it has come to the forefront again in image combining the above depth cues is plausible artifacts, it recent years because of recent progress of the high-resolution does not provide all of depth cues, like real objects. Therefore, it and spatial light modulator (SLM). Integral imaging is might give rise to serious visual fatigue such as eyestrain, feeling a potential candidate to realize 3D displays; however, it is not of pressure in the eyes, an eye ache, a difficulty in focusing, and free from the visual fatigue as well. Therefore, it is important to headaches [17]. investigate causes and effects of the visual fatigue in viewing in- The visual fatigue could be defined as decline of a vision tegral imaging. Effective ways to assess the visual fatigue have system. The causes of visual fatigue are diverse and still highly been widely examined in the stereoscopic displays; it could also controversial issues for debate. In this paper, we shall focus on be applied in integral imaging. The following two sections dis- not whole visual fatigue system but the visual fatigue related cuss both subjective and objective methods to measure the vi- to the 3D display. Visual fatigue when 3D images are observed sual fatigue. could be categorized into many types, such as dry eyes, a diffi- B. Subjective Methods to Assess Eye Fatigue culty in focusing objects, headaches, shoulder pain, nausea, and so on. However, to date, no definitive answer has been given to A classical subjective assessment for visual fatigue is to use the causes leading to these symptoms. Many investigations and questionnaires as an evaluation indicator. Questionnaires have researches concerning causes of the symptoms refer to the mis- been widely applied to determine the degree of visual fatigue. A match between accommodation and convergence, an excessive simulator sickness questionnaire (SSQ), which was devised by binocular parallax, and a geometrical distortion came from the Kennedy et al., is a well-established tool for the investigation of 72 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 2, FEBRUARY 2012

TABLE I TABLE II QUESTIONNAIRES IN ACCORDANCE WITH EXPERIMENTAL CONDITIONS AND CLASSIFICATION OF OBJECTIVE METHODS TO ASSESS VISUAL FATIGUE EVALUATION ITEMS

be used in ‘before and after viewing stereoscopic displays’ test to determine the degree of visual fatigue. The optometric devices could be classified into two groups depending on analytical methods; one is to estimate the visual fatigue using analysis of comprehensive biological reactions. For example, there are twelve cranial nerves, which connect the brain with the eyes, ears, nose, tongue, etc. When the stimuli that can cause the visual fatigue are perceived, brain activities have different perspectives than usual. However, these biolog- ical signals do not indicate visual fatigue directly and have com- plex patterns, therefore it is critical for better understanding vi- sual fatigue to analyze the captured biological reactions. These biological signals can be caught by electrode labeling devices, such as electroencephalography (EEG), magneto-encephalog- raphy (MEG), electromyography (EMG), and electrocardiog- raphy (ECG) [28]–[31]. Numerous researches have been done in a brain activity research field about depth perception based on stereoscopic displays, and ongoing research has been per- formed. However, it was not exactly revealed that biological sig- nals are associated with causes of the visual fatigue. Another ex- ample to assess the visual fatigue is monitoring certain proteins in body as a marker for the visual fatigue. Certain proteins, such as Chromogranin A (CgA) in the saliva, have been reported to the visual fatigue [22]. It has been used extensively in studies increase with mental or physical stress [32]. of simulator sickness and has 26 possible symptoms and pro- The other method to figure out objectively the visual fatigue vides scores in four categories: nausea, oculomotor discomfort, is to assess direct indicators which appeared in fatigued eye. disorientation, and general factors. Since then, many modified The optometric devices, such as refractometer and eye tracker, versions of the SSQ have been provided by researchers. How- are typical examples of measurement tool. Those devices ever, these questionnaires covered a wide scope because they would be categorized as indirect instruments because they do incorporated the fields of medical, mental, physiological, and not measure the visual fatigue directly; however, they enable social sciences. None of them ever came to standard protocols. us to assess directly accommodation and eye movements. Therefore, they are usually modified in accordance with exper- Hence, they could be classified as a direct assessment method. imental conditions and evaluation items. Because the visual fa- For pupillary activities, the fatigue or tiredness after viewing tigue is based on subjective experience, these methods can be stereoscopic images can affect autonomic nervous systems. used in combination with objective assessment. Table I shows The assessment of the degree of the visual fatigue has been subjective methods using questionnaire to assess the visual fa- conducted on measuring pupil size which is affected by mental tigue [9], [23]–[27]. conditions. A refractometer is the most classical method to measure accommodative responses of the eye. The device has C. Objective Methods to Assess Eye Fatigue been used for verifying a hypothesis that predicts difference The most effective way to assess the visual fatigue is to find between the before- and after-state in viewing stereoscopic out biological signals indicating the visual fatigue. Because the displays. Several studies have reported accommodative re- visual fatigue is influenced by subjective experiences and it is sponses of the eye in viewing stereoscopic displays; however hard to assess the degree of the visual fatigue, implementing op- the experiments using optometric devices are time consuming tometric devices or medical instruments is one of the potential and performed with small number of subjects as shown in candidates for assessing visual fatigue objectively. These de- Table II [8], [28]–[35]. vices are applied in measuring characteristic decline of the eye Recently, the optometric device using open view window of physical changes caused by visual fatigue. They could also method, which adopted the half mirror, made it possible to KIM et al.: ACCOMMODATIVE RESPONSE OF INTEGRAL IMAGING IN NEAR DISTANCE 73 take a measurement without the subject being aware as there is no distraction of stereoscopic images. In this paper, using the optometric device, we conducted measurement of accom- modative response while each participant is observing integral imaging. The number of participants in the experiment was more than 70. Because it never has been clearly proven that the accommodative responses have a causal relationship with visual fatigue, here we assumed that the visual fatigue will be relieved by reducing the mismatch between accommodation and convergence. Although integral imaging can display 3D images with full parallax, resulting from a 2D lens array, it has never been clarified whether the viewer looks at 3D images or a flat panel display in integral imaging. The purpose of this paper is to verify that the observers’ eyes see the 3D images at desired positions as if they saw the real objects in near distance.

III. EXPERIMENTS Using a modified optometric device (WAM-5500, Grand Seiko Company), we measured the accommodative response of the eye in the case of both a real object and a reconstructed image from integral imaging. The validity and repeatability of the optometric device was confirmed by several researches [36]. The modified optometric device can make us select any target, so it can be a real object or a 3D image based on integral Fig. 2. Experimental setup. imaging. Additionally, since we can put a target at any distance, we can measure near and far targets as well. A patch was worn over the left or right eye during the experiment. A measurement a real object. The experiment was repeated five times, attaining of accommodative responses of the eye was performed on 71 the mean difference, standard deviation, and 95% confidence staff and students of Seoul National University Hospital (33 intervals. Due to the inherent problems of analyzing cylindrical male and 38 female), with a mean age of 30.9 years (range components in the eyeball’s conventional form, sphere was con- from 23 to 38 years, 18% of eyeglasses wearers). The eyes verted into a vector representation: a spherical lens of power of subjective (left or right) are randomly selected to measure means spherical equivalence (equal to sphere [cylinder/2]). refractive power of the eye. The collected participants had little Total time to perform the measurement was not carried over experience of the 3D display observation. Exclusion criteria 5 min to avoid distraction of the observers. The measurement included: 1) best-corrected visual acuity of 20/40 or worse in was taken under an ambient illumination of 250 lux for focusing either eye; 2) myopia or hyperopia of more than 6.00 diopters them straight, which ensured that no readings were rejected be- in either eye; 3) anisometropia of more than 2.00 diopters; 4) cause the pupil size was too small to measure the accommoda- or nystagmus; and 5) history or presence of other tive response [37]. The ophthalmologist reminded the partici- ocular diseases. The experiment followed the tenets of the pants explaining what they have to focus on in every step of the Declaration of Helsinki and was approved by the Institutional experiment. We used a lozenge pattern that is made of electro- Review Board of Seoul National University Hospital. Informed luminescent film (EL film) as shown in Fig. 2. We could assure consent was obtained from all participants after the details of the participants saw the target without any hindrance because the study were explained. Each subject who participated in the EL film is a self-light-emitting device and the measurement was experiment received comprehensive optometric evaluations of significantly affected when the eccentricity of gaze was over the accommodative response. All subjects rested in a comfort- 10 from the visual axis. Therefore, an examined eye, modified able condition for 3 min before and between experiments. optometric device, and a real target object were aligned on a Before measuring accommodative response in viewing inte- straight line to ensure that convergence of the eye was not con- gral imaging, calibrations are necessary to get accurate data. tributing to the measurement. The data were arranged to dif- Generally, the diopter is equal to the reciprocal of the focal ferent individuals for comparison with reconstructed 3D image length measured in meters. However, refractive power of the case. eye with emmetropia does not show 1.00 diopter when they see The reconstructed image from integral imaging was produced any real target at 1 m because refractive power of the eye is with following procedures [15], [16]. For of the ex- not the same for everybody. To normalize the diopter unit, we periment, we used an overhead projector (OHP) film and printed adopted following empirical experiment. The subjects were re- elemental images on it using a printer for elemental im- quired to focus on real objects that were placed at intervals of ages. The printed elemental image resolution is 1200 dpi. The 10 cm from 50 cm to 110 cm, and the accommodative response EL film is behind of the elemental image on OHP film, and of the eye was assessed according to the step-wise movement of lens array is in front of the OHP film. Although the OHP film 74 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 2, FEBRUARY 2012

shown in Fig. 4 and Table IV, each subject had different ac- commodative responses when the observers saw a real object because each subject had different refractive power of the eye despite of strict experimental conditions. For example, when the participants were asked to see a real object at a distance of 0.5 m (equal to 2.00 diopter), the accommodative response varied from 1.80 diopter to 0.60 diopter. This variation re- sulted from different refractive power of the eye of each par- ticipant. Note that it had a tendency of decreasing the absolute value of accommodative response as accommodation stimulus increases. However, absolute value of accommodative response did not show exactly the reciprocal of located distance of the real object. In this case, we cannot figure out where the partici- pants see when they were asked to see the 3D image. Therefore, we need to obtain the raw data of accommodative response when each subject sees a real object. At first, accom- Fig. 3. Experimental schematic. modative responses of the real object were measured from 50 to 110 cm at intervals of 10 cm. After five times repetition of TABLE III each subject, we can obtain accommodative response raw data EXPERIMENTAL CONDITIONS of 71 participants when they see the real object. This makes us estimate consecutive accommodative responses of the real ob- ject from 50 to 110 cm per participant, as shown in Fig. 4. Then, we also measured accommodative response of every participant in case of integrated 3D image. The was fixed at 80 cm from the participant, while the reconstructed 3D image by integral imaging was formed at 60, 70, 90, and 100 cm. To probe on the question whether the participants look at the display panel or not when they were forced to see integral cannot display a moving image or we cannot change the ele- imaging, accommodative responses of the real object and the mental image, we can control brightness by applying variable reconstructed 3D image were compared. Because the display voltage supply to remove the luminance difference between a panel was fixed at 80 cm when integral imaging was displayed, real object and a reconstructed 3D image from integral imaging. we compared the accommodative response between 80 cm of a We called this a display unit. As shown in Fig. 3, the display unit real object case and 60, 70, 90, and 100 cm of the 3D image case. was fixed 80 cm away from the observer, while altering the el- The accommodative response was compared by T-test [38]. The emental image in order to vary the depth of the reconstructed T-test was executed independently four times per each partici- 3D image. The elemental image was displayed so that the re- pant ([60 cm 3D image, 80 cm real object], [70 cm 3D image, 80 constructed 3D image is integrated as a real integrated image cm real object], [90 cm 3D image, 80 cm real object], and [100 located 60 and 70 cm from observer. In the case of a virtual in- cm 3D image, 80 cm real object]), and we performed a T-test tegrated image located at 90 and 100 cm, the elemental image per participant independently (Results were interpreted sepa- was adjusted to display the virtual integrated image behind the rately as statistically significant when values were 0.05). display unit. Therefore, a real integrated image and a virtual in- However, because it was not confirmed whether measurement tegrated image represent reconstructed 3D images in front and samples have same standard deviation or not, we carried out behind of the display unit by using integral imaging method. Ac- F-test before independent T-test and Welch’s corrected T-test commodative response of the eye was measured five times also, were conducted [39]. When two groups have the same standard attaining the mean difference, standard deviation, and 95% con- deviation distribution, independent T-test was carried out after fidence intervals. In the 3D image case, the measurement was the F-test was conducted to each sample. The Welch’s corrected not carried over 5 min; the experiment was performed on equal T-test was conducted when they have a different standard devi- terms as the real object case. A detailed description of the ex- ation distribution. perimental setup is shown in Table III. Fig. 5 shows the comparison of accommodative responses when they saw a real object at the position of 80 cm and a 3D image of each case. If the accommodative response of the partic- IV. EXPERIMENTAL RESULTS AND DISCUSSION ipant in viewing a real object was different from that in viewing Complete recordings for all participants were obtained for an integrated 3D image, we marked it “different.” It represented each accommodation stimulus, and accommodative responses they did not see the display unit. As shown in Fig. 5, more than were averaged into five measurements for each accommodation 62% of the participants (44 participants) did not focus on the stimulus. Fig. 4 shows sample results of the accommodative re- display unit when they saw the integrated 3D image (both a sponses for real objects. The real target was located at 50, 60, real integrated 3D image and a virtual integrated 3D image). In 70, 80, 90, 100, and 110 cm, moved with a step response. As other words, the participants did not keep eyes on display unit KIM et al.: ACCOMMODATIVE RESPONSE OF INTEGRAL IMAGING IN NEAR DISTANCE 75

Fig. 4. Accommodative response measurements in viewing a real object (sample of six participants).

TABLE IV ACCOMMODATIVE RESPONSE MEASUREMENTS IN VIEWING A REAL OBJECT FOR SIX SUBJECTS IN OPTOMETRIC EVALUATION TESTS (STD: STANDARD DEVIATION, CL: CONTROL LIMIT)

Fig. 5. Participant numbers that have different accommodative response from a real object at the position of 80 cm when they saw a 3D image (integrated 3D images at the position of 60, 70, 90, and 100 cm).

saw the real object. In order to estimate the concordance of the real object and the integrated 3D image, we compared accom- modative responses at each step in the integrated 3D image and accommodation curve of the real object. Fig. 6 shows the re- sults. When the participants saw the real integrated 3D image at 60 cm, more than 65% of participants recognized it as seeing a real object at a range of 50 70 cm (50 60 cm: about 30%, 60 70 cm: about 35% of concordance). Also, the re- sults show a similar tendency at the 70 cm of a real integrated image (50 60 cm: about 25%, 60 70 cm: about 35% of con- cordance). On the contrary, when the virtual integrated image in near-distance (within 1 m) when they saw an integrated 3D was displayed behind of the display unit at 90 and 100 cm, they image. Moreover, the result indicates that participants had more recognized it as if it were located at the whole range of exper- distinctive accommodative response (over 73%, 52 participants) iment although more than 62% of participants did not see the in viewing real integrated 3D images (integrated at the position display panel itself as shown in Fig. 5. of 60 and 70 cm) than in viewing virtual integrated 3D images Given these statistical results we could take into account two (integrated at the position of 90 and 100 cm). important things. The result reflects that the participants kept However, in the above analysis, it was not clarified where the their eyes on the real integrated 3D image when the recon- participants exactly see in viewing integral imaging. Because we structed 3D image was displayed in front of the display unit. measured accommodative responses of each participant from 50 Secondly, when the reconstructed virtual 3D image was dis- to 110 cm of a real object at intervals of 10 cm, the accommoda- played behind of the display unit, the proportion distribution tive response curve in near distance could be acquired when they of accommodative response of participants was concentrated 76 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 2, FEBRUARY 2012

normal adult subjects participated in the experiments. The ex- periment was performed within 1 m, and the interval was subdi- vided into 10 cm. The accommodative response was measured in both real and virtual integrated images. The presented results showed that over 73% of participants kept eyes on the real in- tegrated 3D image instead of display panel, while the propor- tion distribution of accommodative response of participants was widely distributed on the central range in the case of virtual in- tegrated 3D image. Based on the assumption that participants suffered less visual fatigue by reducing the mismatch between accommodation and convergence, we concluded that visual fa- tigue will be relieved when they accommodated at the position Fig. 6. Accommodative response distribution of participants when they see the where the 3D integrated image was displayed in front of the dis- integrated 3D image (real and virtual case). play panel. 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[20] M. Emoto, T. Niida, and F. Okano, “Repeated vergence adaptation Jongshin Kim received the M.D. degree from the causes the decline of visual functions in watching stereoscopic tele- College of Medicine, Seoul National University, vision,” J. Display Technol., vol. 1, pp. 328–340, 2005. Seoul, Korea, in 2006. He is currently working [21] I. M. Irvin, Clinical Refraction, 3rd. ed. New York: Professional toward the Ph.D. degree in mechanical engineering Press, 1970. from KAIST, Seoul, Korea. [22] R. S. Kennedy, N. E. Lane, K. S. Berbaum, and M. G. Lilienthal, “Sim- After a general internship, he completed his resi- ulator sickness questionnaire: An enhanced method for quantifying dency in Ophthalmology at Seoul National Univer- simulator sickness,” Int. J. Aviat. Psychol., vol. 3, pp. 203–220, 1993. sity Hospital in February 2011. His research interests [23] J. Kuze and K. Ukai, “Subjective evaluation of visual fatigue caused focus on development of new biomedical optical in- by motion images,” Displays, vol. 29, pp. 159–166, 2008. strumentation for high-resolution in vivo retina and [24] S. L. Ames, J. S. Wolffsohn, and N. A. Mcbrien, “The development choroid imaging (allowing of individual of a symptom questionnaire for assessing viewing using cellular structures). a head-mounted display,” Vis. Sci., vol. 82, pp. 168–176, 2005. [25] M. Ogata, K. Ukai, and T. Kawai, “Visual fatigue in congenital nys- tagmus caused by viewing images of color sequential projectors,” J. Display Technol., vol. 1, pp. 314–320, 2005. [26] K. Murata, S. Araki, N. Kawakami, Y. Saito, and E. Hino, “Central Keehoon Hong received the B.S. degree in electrical nervous system effects and visual fatigue in VDT workers,” Int. Arch. engineering from , Seoul, Korea, in Occup. Environ. Health, vol. 63, pp. 109–113, 1991. 2008. He is currently working toward the Ph.D. de- [27] J. 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Bekiaris, “Using EEG spectral components to assess algorithms for detecting fatigue,” Expert Syst. Hee Kyung Yang received the M.D. degree from Applic., vol. 36, pp. 2352–2359, 2009. the College of Medicine, Seoul National University, [31] A. Zhang, Z. Zhao, and Y. Wang, “Estimating VDT visual fatigue Seoul, Korea, in 2004. based on the features of ECG waveform,” in Proc. Int. Workshop Inf. She is a Clinical and Research Fellow with the De- Security Applic., F. Gao and X. Zhu, Eds., Oulu, 2009, pp. 446–449. partment of Ophthalmology, Seoul National Univer- [32] S. Fujimoto, M. Nomura, M. Niki, H. Motoba, K. Ieishi, T. Mori, H. sity Bundang Hospital, Seoul, Korea. She completed Ikefuji, and S. Ito, “Evaluation of stress reactions during upper gas- an internship and residency in Seoul National Univer- trointestinal endoscopy in elderly patients: Assessment of mental stress sity Hospital in 2009. Her current research of inter- using chromogranin A,” J. Med. Invest, vol. 54, pp. 140–145, 2007. ests includes fields of pediatric ophthalmology, stra- [33] T. Shibata, T. Kawai, K. Ohta, M. Otsuki, N. Miyake, Y. Yoshihara, bismus and neuro-ophthalmology. and T. Iwasaki, “Stereoscopic 3-D display with optical correction for the reduction of the discrepancy between accommodation and conver- gence,” J. Soc. Inf. Disp., vol. 13, pp. 665–671, 2005. [34] M. Miyao, S. S. Hacisalihzade, J. S. Allen, and L. W. Stark, “Effects of VDT resolution on visual fatigue and readability: An eye movement approach,” Ergonomics, vol. 32, pp. 603–614, 1989. Jae-Hyun Jung received the B.S. degree in elec- [35] H. T. Nguyen, D. M. Isaacowitz, and P. A. D. Rubin, “Age- and fa- tronics engineering from Pusan National University, tigue-related markers of human faces: An eye-tracking study,” Oph- Pusan, Korea, in 2007. He is currently working thalmology, vol. 116, pp. 355–360, 2009. toward the Ph.D. degree in electrical engineering at [36] A. L. Sheppard and L. N. Davies, “Clinical evaluation of the Grand Seoul National University, Seoul, Korea. Seiko auto ref/keratometer WAM-5500,” Ophthalmic Physiol. Opt., His primary research interests are in the areas of vol. 30, pp. 143–151, 2010. 3D display, optical information processing, and depth [37] M. S. Livingstone and D. H. Hubel, “Stereopsis and positional acuity perception in human visual system. under dark adaptation,” Vision Res., vol. 34, pp. 799–802, 1994. [38] B. L. welch, “The significance of the difference between two means when the population variances are unequal,” Biometrika, vol. 29, pp. 350–362, 1937. [39] D. J. Best and J. C. W. Raynor, “Welch’s approximate solution for the Behrens-Fisher problem,” Technometrics, vol. 29, pp. 205–210, 1987. [40] O. Franzén, H. Richter, and L. Stark, Accommodation and Vergence Mechanisms in the Visual System. New York: Birkhäuser Verlag, 2000. Heejin Choi received the Ph.D. degree in electrical engineering from Seoul National University, Seoul, Youngmin Kim received the B.S. degree and Ph.D. Korea, in 2008. degree in electrical engineering from Seoul National After working for Samsung Electronics, in 2010, University, Seoul, Korea, in 2005 and 2011, respec- he joined the Department of Physics, Sejong Univer- tively. sity, Seoul, Korea. His primary research interest is in He is currently with Seoul National University as the areas of 3D displays, 3D/2D conversion, and 3D a Postdoctoral Researcher. His current research inter- human factor. ests focus on the 3-D display, image processing in- cluding multiview 3D display, and visual fatigue of human vision related on 3D display. 78 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 2, FEBRUARY 2012

Sung-Wook Min received the B.S. and M.S. degrees Jeoung-Min Hwang received the M.D. degree from in electrical engineering and the Ph.D. degree from Seoul National University College of Medicine, Seoul National University, Seoul, Korea, in 1995, Seoul National University, Seoul, Korea, in 1985. 1997, and 2004, respectively. She is a Professor with the Department of Oph- Presently, he is a faculty member with the Depart- thalmology, Seoul National University Bundang ment of Information Display, Kyung Hee University, Hospital, Seoul, Korea. Her current research of Seoul, Korea, which he joined in 2007. He is in- interests includes fields of pediatric ophthalmology, terested in 3D imaging and the advanced display strabismus, and neuro-ophthalmology. system, especially based on the integral imaging technique.

Jong-Mo Seo received the M.D. degree and M.S. and Byoungho Lee received the Ph.D. degree in electrical Ph.D. degrees in biomedical engineering from Seoul engineering and computer science from the Univer- National University (SNU) School of Medicine, sity of California, Berkeley, in 1993. Seoul, Korea, in 1996, 2002, and 2005 respectively. Since 1994, he has been with the School of Elec- He completed the ophthalmologic residency in trical Engineering, Seoul National University, Seoul, 2002 and the retina fellowship in 2005 at SNU Korea, as a faculty member, where he is currently Hospital (SNUH). He served as a Clinical Instructor a Full Professor. His research group has published with the Department of Ophthalmology, SNUH, and, more than 280 international journal papers and pre- since 2008, he has been an Assistant Professor with sented more than 490 international conference papers the Department of Electrical Engineering at SNU including more than 90 invited papers. His research School of Engineering. His research interest is the fields are 3D display, diffractive optics, fiber devices, artificial retina for the blind, neural prosthesis, mechanism, and surface plasmon–polariton applications. and medical information technologies. Dr. Lee is a Fellow of the Optical Society of America and the International Society for Optical Engineers. He received the 5th Young Presidential Scien- tist Award of Korea in 2002 and the Scientist of the Month Award of Korea in September 2009.