Regular Papers

Evaluation of Visual Fatigue in Viewing a Depthfused 3-D Display in Comparison with a 2-D Display
Shiro Suyama†, Yasuo Ishigure, Hideaki Takada,
Kenji Nakazawa, Jun Hosohata, Yasuko Takao,
and Takashi Fujikado
Abstract
The depth-fused 3-D (DFD) display developed in NTT has been found to have the same level of visual fatigue as a conventional 2-D display. The DFD display presents depth information using a stack of
two LCDs. We subjectively and objectively examined the visual fatigue it induces compared with 2-D
displays. We found that (1) there were no statistically significant differences between 2-D images and
DFD images in terms of impairments of visual functions and that (2) subjects responded more quickly
when viewing DFD images than viewing 2-D images.

1. Introduction
Developing a three-dimensional (3-D) imaging
method for a simple and practical 3-D display isimportant for producing a realistic virtual world.
Many display methods including integral photographic, holographic, volumetric, and stereoscopic
methods have been investigated, but the first three are
not yet practical because they need a lot of complicated image data. Only the stereoscopic display using
two stereoscopic images has reached a practicalstage. However, it is well known that stereoscopic 3D displays based on binocular disparity*1 lead tovisual fatigue [1]-[4] as compared with conventional
2-D displays or the real 3-D world. This is because
the lens accommodation*2 does not match the vergence*3 point, which moves freely in 3-D space, as
shown in Fig. 1 [1], [3].
We have developed a new 3-D display using thedepth-fused 3-D (DFD) visual illusion in which twooverlapped images with many edges displayed at different depths can be perceived as a continuous 3-D† NTT Cyber Space Laboratories
Musashino-shi, 180-8585 Japan
E-mail: suyama.shiro@lab.ntt.co.jp

82

image [5]-[9]. A compact DFD display, which simply
consists of stacked LCD displays, can be developedby using the summation of polarization changes in
two LCDs [10]-[12].
With respect to visual fatigue, we can assume thatthe DFD display may have an advantage because wearrange it to have multiple images with comparatively narrow gaps (for example, 3 min. of arc) in thedepth direction. In this study, we tested this assumption by evaluating visual fatigue in terms of subjective and objective effects on visual functions when
*1 Binocular disparity: The positional
Fixation point
difference between the two retinal
projections of a given point inspace. This positional difference
results from the fact that the twoeyes are laterally separated andtherefore see the world from twoslightly different vantage points.
*2 Accommodation: The ability of theRight eyeeye to adjust its focal length by Left eyechanging the shape of the lens.
*3 Vergence: Vergence eye movements cause the foveae of both eyes
to point at a near object. (The fovea is the center part of the retina.
This tiny area is responsible for our central, sharpest vision.) Thismeans that each eye rotates in the opposite direction (i.e., the righteye to the left and the left eye to the right) when converging on anear object.

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Regular Papers

DFD images and 2-D images were observed.
2. Concept of DFD display
The fundamental perceptual phenomenon involvedin the DFD display [5] is reviewed in Fig. 2. It is composed of two processes: (i) depth-fusing to an apparent image with one depth by overlapping front andrear images from the midpoint of the observer’s eyesand (ii) continuous-depth perception by changing theluminance ratio between front and rear images.
The concept of the DFD display [6] is reviewed inFig. 3. When the luminance ratio between front andVergence point

Stereoscopic
display plane(accommodation point)
Vergence point

Eyes

Fig. 1. Vergence point and lens accommodation withstereoscopic 3-D display.

rear images is changed at each pixel according to thedepth (b) and the front and rear images are overlapped (a), an apparent image with continuous depth
can be observed (c). This was described in detail in
our previous report in NTT Technical Review [11].
3. Experimental method
3.1 Experimental apparatus and task
The DFD display system that we used for evaluating visual fatigue is shown in Fig. 4. It consists of a
DFD display, an ordinary PC with a video card that
outputs two signals for the two planes of the DFD display, a 2-D display for operation, and a speaker to
audibly indicate correct or incorrect answers. The
DFD display has two LCDs as two planes with a gapof 5 mm. When we tested the visual fatigue before
and after viewing the 2-D display, only the front LCD
was activated in the DFD display. The distance
between the subject and the display was 40 mm.
The experimental task is illustrated in Fig. 5. The
subjects were ten healthy people, none of whom had
any eye problems other than ametropia*4, 24–48
years old (average: 29.4). Front, middle, and rearstimuli consisting of three white squares were presented at random depth positions on a DFD display
with a 4-inch diagonal. As the cue to judge which

Depth-fusing process
Overlapping 2-D images

Front
plane

Perceived 3-D image

Rear
plane

Continuous-depth perception process
Luminance ratio change

Perceived 3-D image

Fig. 2. Fundamental perceptual phenomenon in DFD.
*4 Ametropia: An eye abnormality, such as nearsightedness, farsightedness, or astigmatism, resulting from faulty refractive ability of the eye.

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(a)

(b)

Rear image

Rear plane
Original image

Front plane
Front image(c)

Schematic illustration of perceived 3-D image

ObserverObserverFig. 3. Concept of DFD display.

LCD display
for operator

4-inch DFD(gap: 5 mm)Speaker

Keyboard
for answering

Personal computer

Fig. 4. DFD evaluation system.

stimulus was front, middle, or rear in testing the 2-D
display, the sizes of stimuli were changed in three levels. When the DFD display was tested, the judgment
cues were the size and perceived depth in three ranks.
In the task, subjects had to indicate the position of the
middle stimulus by pushing one of three keys on the
keyboard. After the key was pressed, another set ofstimuli was immediately presented. Subjects were
instructed to do as many sets as possible in a given
time. As we wanted to evaluate the visual fatigue for
a comparatively long time, the experiment time was
set to 20 minutes. To keep subject motivation high84

during testing and to minimize the resting time, wetold the subjects that they would be ranked by the
total number of answers and the correct answer ratio.
These instructions to subjects were also useful forevaluating how quickly a subject could do the task inviewing a 2-D and DFD display.
3.2 Subjective tests
Subjective tests based on a questionnaire (10 items)
provided various indices of visual fatigue after theabove task. Each question had five ranks, for example, from feeling no fatigue “1” to feeling very strong
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Regular Papers

Stimuli

5 times

Measurement

Practice start
Task: Indicate the position of the middle depth target
on a key of the keyboard.
Measurement

20
minutes
Experiment start

FinishFig. 5. Experimental task (DFD or 2D).

fatigue “5” when fatigue was asked about. The conventional indices of visual fatigue were selected as
fatigued, bleary, hazy, painful, dull, heavy, vomitous,
nauseated, dizzy, and sleepy, which have conventionally been used for subjective tests of visual fatigue
after viewing a 2-D visual display terminal (VDT) or
stereoscopic 3-D display.
3.3 Objective tests
The visual fatigue after viewing a 2-D VDT (suchas a cathode ray tube or projection display) or stereoscopic 3-D display (such as parallax barrier, lenticular lens, polarization glass, or liquid crystal shuttertype) has been reported to affect visual functions,producing for example a decrease in visual acuity,
myopia of the refractive state, decrease in the amountof accommodative response, decrease in pupil diameter, decrease in pupil-constriction ratio, decrease inflicker-free frequency, increase in accommodative
deviation, increase in high-frequency component ofaccommodation fluctuation, and increase in delay
time of accommodation change [1]-[4], [13]-[20]. As
indices of our objective tests for visual fatigue in 2-D
and DFD displays, we selected low-contrast acuity,
refractive state, accommodative response, pupil-constriction ratio, and lens accommodation fluctuation.
The low-contrast acuity was visual acuity in viewing
a low-contrast “Landolt C”*5 using a 10% ETDRS(early treatment of diabetic retinopathy study) chart.
The refractive state means how far a subject’s eye
focused on when the stimulus was set to infinity, evalVol. 3 No. 12 Dec. 2005

uated by ARK-730A (NIDEK). The accommodative
response was the maximum change in lens accommodation, evaluated by ARK-730A (NIDEK). The
pupil-constriction ratio indicated how much the pupil
constricted due to a change in accommodation to thenear stimulus, evaluated by TriIRIS (Hamamatsu
Photonics). The lens accommodation fluctuation was
the amount of high-frequency component of accommodation fluctuation when the accommodation was
set to a low diopter*6 value (= 1/m), for example, 0 to
0.75 diopters, which was evaluated by AA-1
(NIDEK).
Objective tests were measured before and after thetask. To judge whether the results before and after thetask were different, we statistically calculated thepaired t-test*7.

*5 Landolt C: A broken circle used as
the test object in distinguishing visual acuity. The width of the gap in the
5SScircle is equal to the breadth of theblack line and one-fifth the circle
S
diameter.
*6 Diopter: A unit of measurement of the refractive power of lenses
equal to the reciprocal of the focal length measured in meters.
*7 Paired t-test: The two-sample t-test is used to determine whether
two population means are equal. For a paired t-test, the data isdependent, i.e., there is a one-to-one correspondence between thevalues in the two samples, such as the same subject measured
before and after a process change or the same subject measured at
different times.

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4. Results
The results of subjective testing are shown in Fig. 6.
The horizontal axis indicates evaluation items and thevertical axis indicates the scores, each of which is anaverage calculated from the answers of all subjects
because all subjects had the same tendency for each
evaluation item. The item “fatigued” had the highest
score (2-D: 3.4, DFD: 3.3), and the high-scoring
items were “sleepy” (2-D: 2.9, DFD: 3.0), “bleary”(2-D: 2.6, DFD: 2.6), “dull” (2-D: 2.4, DFD: 2.1), and“hazy” (2-D: 2.3, DFD: 2.2). The results indicate thatviewing was fatiguing, but the results for both 2-D
and DFD displays were much the same; that is, there
were no significant differences between 2-D and
5-level evaluation

Higher 5
level

DFD displays.
Figure 7 shows an example of the objective test
results before and after trials with either the 2-D display or DFD display. The horizontal axis indicates the
subjects by their initials and the vertical axis showsthat subject’s refractive state. These results also
demonstrate that the 2-D and DFD displays both produced the same objective levels of fatigue under theexperimental conditions we were using.
Table 1 show the results of all objective tests before
and after trials with 2-D and 3-D displays. “P” is thestatistical result of a paired t-test, which means theerror probability if two results before and after thetask are judged to be different. That is, when “P” issufficiently small, two results before and after thetask are different.
We found no significant differences between theobjective-test results before and after the 2-D task,
except for the accommodative response result

2-D
DFD

3

SleepyDizzy

Nauseated

Vomitous

Heavy

Dull

Painful

Hazy

BlearyLowerlevel 1

Fatigued2Hyperopia 3
2
1
0
–1
–2
–3
–4
–5
–6
Myopia –7
Accommodation (diopters)

Score

4

Before and after: 2D
Before and after: DFD

YT

HT

TT

HF

JC

EM KM THNT

MT

Subjects

Evaluation indicesFig. 6. Results of subjective testing.

Fig. 7. Results for refractive state.

Table 1. Results of objective tests before and after 2-D and DFD trials.
DFD trial

2-D trial
Before

After

Error
probability (P)Before

After

Error
probability (P)Low-contrast acuity(10%ETDRS chart)(Log MAR unit)

0.01 ± 0.11

0.02 ± 0.08

0.460

0.01 ± 0.08

0.03 ± 0.10

0.182

Refractive state(diopters)–1.50 ± 2.82

–1.30 ± 3.08

0.269

–1.50 ± 2.57

–1.53 ± 2.73

0.811

Lens accommodation(high-frequency component)

47.29 ± 6.12

47.28 ± 5.86

0.989

47.32 ± 7.80

44.85 ± 6.06

0.153

Accommodative response(diopters)

2.12 ± 0.55

1.74 ± 0.67

0.021

2.05 ± 0.78

1.81 ± 0.65

0.181

Pupil-constriction ratio(%)

55.90 ± 10.36

56.16 ± 14.48

0.923

52.48 ± 10.10

53.67 ± 12.17

0.495(“P” is the statistical result of a paired t-test, which means the error probability
if two results before and after the objective tests are judged to be different.)

86

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Regular Papers(P<0.05 is sufficiently small), which means that adecrease in accommodation response occurred in the
2-D task. Results for the DFD task were the same as
those for the 2-D task, except for the accommodative
response result (P>>0.05), which indicates that adecrease in accommodative response did not occur in
the DFD task.
As shown in Table 2, there were no significant differences between the results of changes before andafter the 2-D and DFD trials (P>>0.05). Thus, subjects were affected to the same degree by 2-D and
DFD images both subjectively and objectively.
In addition to subjective and objective evaluation,
we examined the total number of answers and the correct answer ratio for 2-D and DFD displays in our
experiments (Table 3). The total number of answers
indicates how quickly a subject could respond. There
were no significant differences between 2-D and
DFD displays in terms of correct answer ratio. Nevertheless, the total number of answers obtained with
the DFD display was considerably more (1.23 times)
than with the 2-D one. This is because the depth judg-

ment cues were both stimulus size and apparent depth
in the DFD display in contrast to the single cue of
stimulus size in the 2-D display.
The results demonstrate that the DFD is no more
fatiguing than a comparable 2-D display. Furthermore, while it is no less fatiguing, it does enable subjects to judge depth information more quickly using
the DFD display.
5. Conclusion
We evaluated visual fatigue in subjects using 2-D
and DFD displays. The results show that: (1) visual
fatigue from DFD display is at the same level as thatfrom a 2-D display and (2) subjects responded more
quickly when viewing DFD images than when viewing 2-D images.
References[1]

S. Yano, M. Emoto, and T. Mitsuhashi, “Two Factors in Visual
Fatigue Caused from Stereoscopic Images,” ITE, Vol. 57, No. 9, pp.

Table 2. Changes before and after 2-D and DFD trials.
Average change
Average change
Error
before and after 2-D trial before and after DFD trial probability (P)Low-contrast acuity(10%ETDRS chart)(Log MAR unit)–0.01 ± 0.06

–0.02 ± 0.04

0.856

Refractive state(diopters)–0.20 ± 0.54

0.025 ± 0.32

0.225

Lens accommodation(high-frequency component)

0.02 ± 3.34

2.43 ± 4.98

0.103

Accommodative response(diopters)

0.38 ± 0.43

0.24 ± 0.53

0.361

Pupil-constriction ratio(%)–0.26 ± 8.29

–1.19 ± 5.29

0.779

Table 3. Total number of answers and correct answer ratio for 2-D and DFD.
2-D total number

DFD total number

MK
TO
TH
TM
ME
FH
HT
TY
TT

1553
1250
1098
1293
1278
1205
1375
1633
1632

1633
1643
1169
1659
2027
1497
1611
2099
1754

99.42
99.28
99.09
97.68
96.87
96.35
96.00
95.53
94.12

99.76
97.57
99.91
95.12
94.23
98.06
96.59
96.38
98.52

Average1368.51676.9

97.15

97.35

Standard deviation

194.1

274.9

1.86

1.95

Vol. 3 No. 12 Dec. 2005

2-D correct ratio (%) DFD correct ratio (%)

87Regular Papers

1187-1193, 2003.
T. Fujikado, “Effects of Stereoscopic Devices on Visual Function,”
Japanese Journal of Visual Science, Vol. 18, No. 1, pp. 2-8, 1997 (inJapanese).
[3] T. Hatada, Nikkei Electronics, No. 444, pp. 205-223, 1988 (in Japanese).
[4] H. Ohzu, “Human Factors in Stereoscopic 3-D Displays,” Ergonomics, Vol. 39, No. 11, pp. 1294-1309, pp. 1310-1314, pp. 1344-1356,
pp. 1364-1369, 1996.
[5] S. Suyama, S. Ohtsuka, H. Takada, K. Uehira, and S. Sakai, “Apparent 3-D image perceived from luminance-modulated two 2-D images
displayed at different depths,” Vision Research, Vol. 44, No. 8, pp.
785-793, 2004.
[6] S. Suyama, H. Takada, and S. Ohtsuka, “A Direct-Vision 3-D Display
Using a New Depth-fusing Perceptual Phenomenon in 2-D Displayswith Different Depths,” IEICE Trans. Electron., Vol. E85-C, No. 11,
pp. 1911-1915, 2002.
[7] S. Suyama, H. Takada, K. Uehira, and S. Sakai, “A Novel DirectVision 3-D Display Using Luminance-Modulated Two 2-D Images
Displayed at Different Depths,” SID’00 Digest of Technical Papers,
pp. 1208-1211, 2000.
[8] S. Suyama, H. Takada, K. Uehira, and S. Sakai, “A New Method forProtruding Apparent 3-D Images in the DFD (Depth-Fused 3-D) Display,” SID’01 Digest of Technical Papers (53.3), pp. 1300-1303,
2001.
[9] H. Takada, S. Suyama, and K. Nakazawa, “A new 3-D display method
using 3-D visual illusion produced by overlapping two luminancedivision displays,” IEICE Trans. Electron., Vol. E88-C, No. 3, pp.
445-449, 2005.
[10] H. Takada, S. Suyama, M. Date, K. Hiruma, and K. Nakazawa, “A
compact depth-fused 3D display using a stack of two LCDs,” Journalof the Institute of Image Information and Television Engineers, Vol.
58, No. 6, pp. 807-810, 2004 (in Japanese).
[11] H. Takada, S. Suyama, M. Date, and K. Nakazawa, “A Compact[2]88

Depth-fused 3-D Display using a Stack of Two LCDs,” NTT Technical Review, Vol. 2, No. 8, pp. 35-40, 2004.
[12] M. Date, T. Hisaki, H. Takada, S. Suyama, and K. Nakazawa, “Luminance Addition of a Stack of Multi-Domain LCDs and Capability for
Depth-Fused-3D (DFD) Display Application,” Applied Optics, Vol.
44, No. 6, pp. 898-905, 2005.
[13] S. Saito, M. Sotoyama, S. Saito, and S. Taptagaporn, “Physiologicalindices of visual fatigue due to VDT operation—Pupillary reflexes
and accommodative responses,” Industrial Health, Vol. 32, No. 2, pp.
57-66, 1994.
[14] O. Ostberg, “Accommodation and visual fatigue in display work.
Ergonomic Aspect of Visual Display Terminals,” pp. 41-52, Taylor &Francis, London, 1990.
[15] S. Ishikawa, “Examination of the near triad in VDT operators,”
Ergonomics, Vol. 33, No. 3, pp. 787-798, 1990.
[16] F. Okuyama, “Human visual accommodation and vergence eye movement while viewing stereoscopic display and actual target,” Proceedings of the 20th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Vol. 20, Biomedical Engineering Towards the Year 2000 and Beyond, No. 2, pp. 552-555,
1998.
[17] F. Okuyama, “Evaluation of stereoscopic display with visual function
and interview,” Proceedings of the SPIE, Vol. 3639, pp. 28-35, 1999.
[18] S. Yano, M. Emoto, and T. Mitsuhashi, “Two factors in visual fatigue
caused by stereoscopic HDTV images,” Displays, Vol. 25, No. 4, pp.
141-150, 2004.
[19] S. Yano, S. Ide, T. Mitsuhashi, and H. Thwaites, “A study of visual
fatigue and visual comfort for 3D HDTV/HDTV images,” Displays,Vol. 23, No. 4, pp. 191-201, 2002.
[20] M. Emoto and S. Yano, “The influence of the dissociation of vergence
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images,” Journal of ITE, Vol. 56, No. 3, pp. 447-54, 2002 (in Japanese).

NTT Technical Review

Regular Papers

Shiro Suyama

Jun Hosohata

Senior Research Engineer, Visual Media Communications Project, NTT Cyber Space Laboratories.
He received the B.E., M.E., and Ph.D. degrees
from Kyushu University, Fukuoka in 1979, 1981,
and 1990, respectively. Since joining Musashino
Electrical Communication Laboratories of Nippon Telegraph and Telephone Public Corporation (now NTT) in 1981, he has been engaged inresearch on transistor devices and liquid-crystal
devices. Currently, he is also engaged in the
R&D of 3-D display systems. He received theAchievement Award from the Institute of Electronics, Information and Communication Engineers of Japan (IEICE) in 2003.

Research Associate, Department of Ophthalmology, Osaka University, Graduate School of
Medicine.
He received the M.D. degree from Osaka University, Osaka in 1993. He is a member of theJapanese Association of Strabismus and Amblyopia (JASA), the Japanese Society of Ophthalmic Surgeons, and the Japanese Ophthalmological Society.

Yasuo Ishigure

Yasuko Takao

Deputy Manager, Life Science Business Planning Office, NTT DATA Corporation.
He received the B.E. and M.E. degrees inknowledge-based information engineering and
the Ph.D. degree in electronic and informationengineering from Toyohashi University of Technology, Aichi in 1995, 1997, and 2000, respectively. Since joining NTT Cyber Space Laboratories in 2000, he has been engaged in researchon super-high-resolution displays and 3-D displays and their human factors. He participated in
the visual media communications project at NTT
Cyber Space Laboratories. He is a member of theInstitute of Image Information and Television
Engineers of Japan (ITE) and the Vision Societyof Japan.

Orthoptist, Department of Ophthalmology,
Yukioka Hospital, Osaka.
She is a certified orthoptist and a member of
JASA and the Japanese Association of Certified
Orthoptists.

Hideaki Takada

Professor, Applied Visual Science, Osaka University Medical School.
He received the B.S. and M.E. degrees fromthe University of Tokyo, Tokyo in 1976 and1978, respectively. He received the M.D. and
Ph.D. degrees from Osaka University, Osaka in
1982 and 1989, respectively. Since joining the
Department of Ophthalmology, Osaka University Medical School in 1985, he has been engaged
in research and clinical studies on pediatric ophthalmology and neuro-ophthalmology. Currently, he is also engaged in the evaluation of stereopsis using 3-D display systems. He received the
Yuge Award from JASA in 1995.

Visual Media Communications Project, NTT
Cyber Space Laboratories.
He received the B.E. degree in information andcommunication technology from Tokai University, Shizuoka in 1995 and the M.E. degree in
information systems from the University of Electro-Communications, Tokyo in 1997. Since joining NTT Integrated Information & Energy Systems Laboratories, Tokyo in 1997, he has been
engaged in the R&D of 3-D display systems. He
is a member of the Virtual Reality Society ofJapan (VRSJ). He received the Best Paper Award
from the 3-D image conference in 2001, theYoung Researcher’s Award from VRSJ in 2002,
and the Achievement Award from IEICE in
2003.

Takashi Fujikado

Kenji Nakazawa
Senior Research Engineer, Supervisor, Visual
Media Communications Project, NTT CyberSpace Laboratories.
He received the B.E. and M.E. degrees in electronics, and the Ph.D. degree in materials scienceand engineering from Kanazawa University,
Ishikawa in 1981, 1983, and 1991, respectively.
He joined the Musashino Electrical Communications Laboratories of Nippon Telegraph andTelephone Public Corporation (now NTT) in
1983 and engaged in research on amorphous andpolycrystalline Si thin-film materials and TFTs
using these films. He also engaged in R&D oflarge-screen-size super-high-resolution display
systems. He is currently engaged in R&D of 3D
display and image systems. He is a member of
the IEICE, ITE, and the Society for Information
Display.

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