Vergence and accommodation responses of human vision are very important factors when a 3D image is observed, and a vergence-accommodation conflict (VAC) causes perceptual distortion, visual discomfort, and fatigue for an observer. Theoretically, a hologram is expected to provide a 3D image without such a conflict. In this article, natural focusing was verified by human accommodation response (A-R) measurement during on-axis analog reflection Denisyuk hologram observation. The A-R of a group of participants were measured for a real marker and its Denisyuk hologram at various visualization distances using an Nvision K5001 autorefractor. The experimental results statistically confirmed the equivalence of the responses to the Denisyuk hologram and its real counterpart, as well as the absence of a VAC.
Most of the current approaches to creating three-dimensional (3D) images use a single-eye mechanism, the binocular-stereoscopic-depth perception principle that shows the plasticity of objects well but dissociates the natural coupling between vergence and accommodation . Some well-known examples of 3D displays using this principle are polarized 3D glasses to watch 3D films on a computer or movie screen, virtual reality (VR) headsets, or Microsoft Hololens smart glasses (Figure 1). This vergence-accommodation conflict causes perceptual distortions, and visual discomfort and fatigue when these stereoscopic displays are used for too long [19,20].
Accommodation refers to the muscle tension used to adjust the eye’s focal distance. The convergence is the angular difference in the viewing directions between a person’s left and right eye when they look at the same fixing point. Figure 2(a) shows the change in the thickness of a crystalline lens to focus the image on the retina when a human observes an object at either a far or near distance. When looking at the object at a near distance, the thickness of the crystalline lens becomes thicker because of the contraction of the ciliary muscle and relaxation of the ciliary zonule. When observing an object at a far distance, the thickness of the crystalline lens becomes thin as a result of the relaxation of the ciliary muscle and contraction of the ciliary zonule. When accommodation occurs, an adjustment of the thickness of the crystalline lens occurs. And, when the accommodation contracts, the radius of curvature decreases and the refractive power increases. On the other hand, when the accommodation is relaxed, the radius of curvature increases and the refractive power decreases. Figure 2(b) shows the change in convergence angle that occurs when a human observes an object at either a far or near distance. The convergence angle is the intersection of the left and right eye when a human looks at the convergence point.
Accommodation and convergence function together. When we look at an object at a near distance, the crystalline lens becomes thick. At the same time, the convergence angle becomes large. Conversely, when we look at an object at a far distance, the crystalline lens becomes thin, while the convergence angle becomes small.
Additionally, because both eyes are separated by approximately 60 mm, a difference occurs in the relative positional relationship between images formed on the right and the left retina. This is called binocular disparity . Accommodation, convergence, binocular disparity, and other factors enable humans to see objects in three dimensions.
A hologram-theorized in 1948 by the British-Hungarian physicist Gabor  – is a recording in a medium of the interference pattern formed when a light reference beam of fixed wavelength encounters light arriving from the object beam. When the hologram is illuminated by the reference beam alone, the diffraction pattern recreates the wave fronts of light from the original object and a viewer sees a 3D image indistinguishable from the original object. For this reason, a hologram is expected to provide a 3D image that does not cause visual fatigue, which is a result of the conflict between accommodation and vergence.
Studies related to the visual perception of holograms have suggested that holography provides natural parallax and focusing, but these studies have been conducted mostly on the view angle of observers [2,3,4]. The study of the accommodation of the human eye using some types of holograms has been an area of research for the past 10 years. Several studies with quantitative measurements of the accommodation of the human eye using computer-generated holograms (CGHs) [5,6] and off-axis Leith and Upatnieks holograms [7,8] have been conducted. CGHs of these studies use electronic devices such as liquid crystal displays (LCDs) and spatial lighting modulator (SLM) instead of silver halide plates to display holograms, while off-axis Leith and Upatnieks holograms are monochrome analog transmission holograms  visible only with laser light illumination. These studies confirmed that there was no problem of accommodation-vergence for these two types of holograms.
There is another kind of hologram, introduced in 1965 by the Russian scientist Yuri Denisyuk  with the single-beam reflection holographic technique, for which the accommodation of the human eye has not been verified. The most recent generation of full-color on-axis analog reflection Denisyuk holograms is categorized as ultra-realistic because it is difficult for a spectator to discriminate between these holograms and their real counterparts [11,12]; furthermore, unlike transmission holograms, Denisyuk holograms can be observed using a white light source on the same side of the hologram as the spectator. Quantitative measurement of the accommodation reaction of eyes has been conducted in this article to verify the natural parallax and focusing when observing a Denisyuk hologram of a marker (a cardboard card representing a drawing), with a group of participants and an autorefractor, and a statistical test has been performed to analyze the results.
The hologram was recorded on iso-panchromatic silver-halide holographic material Ultimate 04 (U04)  glass plates. U04 is an emulsion specifically designed for recording full-color analog holograms without diffusion. To record a two-color hologram, the red wavelength was provided by a helium-neon 633 nm, 20 mW laser, and the green was provided by a diode-pumped solid-state 532 nm, 100 mW laser. The hologram was then developed with two baths of chemicals recommended by the manufacturer. After drying, the hologram was sealed with optical glue to prevent emulsion thickness variations and color changes and protect the gelatin from scratches.
A two-color reflection analog hologram of the marker was recorded with the on-axis single-beam Denisyuk technique. The recording setup used two different laser beamsŰred and greenŰcombined with an X-cube prism to give a yellow laser beam. As the marker is yellow and black, it was not necessary to add a blue laser to obtain a realistic hologram of the original. After passing through a spatial filter, the divergent yellow beam illuminated both the holographic plate and the marker (Figure 3) at an angle of 45 degrees. The marker was positioned exactly 67 mm below the holographic plate.
To illuminate and reconstruct the hologram of the marker, the lamp has to be placed facing the holographic plate at the correct angle and position. The choice of the illumination source is important in full-color reflection holography because the light must be a source point and match the wavelengths of the original recording lasers. RGB LEDs currently offer the best solution because their wavelengths are centered on the lasers’ wavelengths, with no unwanted colors, which usually create diffusion . To illuminate the hologram, a 3 W RGB LED was chosen and placed 50 cm from the center of the hologram at a 45-degree angle. The spectrum and the color gamut in CIE 1931 color space of our RGB LED are shown in Figure 4(a) and (b).
In this article, an open automatic refractometer was used as an accommodative response measurement method. The accommodative response under binocular viewing conditions was measured using an N-vision K5001  autorefractor from Shin-Nippon. An autorefractor  is a computer-controlled machine commonly used during an eye examination to provide an objective measurement of a patient’s refractive error. This experiment is to observe the accommodative response of the crystalline lens when the ocular is subjected to an external near stimulus. The N-vision K5001 wide-view window allows the subject to naturally look with both eyes and relax during measurement (Figure 5(a)). Tests can be performed with an attached near marker (Figure 5(b)).
The accommodative response of the eye was measured with an N-vision K5001 for a group of participants with a real marker and its hologram placed at various positions.
The hypothesis is that the accommodative response for the holographic marker was not made on the plane of the holographic plate but, on the contrary, 67 mm behind this position, where the virtual holographic image of the marker is reconstructed by the RGB illumination.
A total of 30 participants ranging from 20 to 30 years old (mean: years old) who did not have ophthalmologic disease, mental illness, or systemic disease were enrolled for this study. They had a near or far corrected visual acuity of 0.8 or more.
Three experimental setups were used to verify the hypothesis. In the first setup, a real marker was attached in front of the N-vision K5001 autorefractor at a distance of 3.00 diopters (D), as shown in Figure 6(a). The accommodative response of the eye was then measured for each participant. In the second setup, the same real marker was this time attached at a distance of 2.50 D, as shown in Figure 6(b). At 2.50 D parallel rays of light focus at 40.0 cm. The accommodative response of the eye was then measured for each participant. In the third setup, the hologram of the marker-illuminated with a RGB LED lamp at a 45-degree angle-was attached at a distance of 3.00 D (Figure 6(c)). The accommodative response of the eye was then measured for each participant. In the experiment, we measured by setting Auto-Refractor (NVision-K5001, Shin-Nippon) in 0.125 D units. In optometry, it was determined that the difference in the diopter (0.25 D) was in the range of the error of measurement. In this experiment, the measurement distances were set to 2.50 and 3.00 D to set the difference (0.50 D) between the two-stage diopters. Accordingly, when the hologram target was set at a distance of 3.00 D, the hologram image was reconstructed at 2.50 D. In this experiment, the illumination conditions were set to maintain brightness that does not interfere with the measurement of refractive power. The wall was set to be a uniform white color, and there were no objects obstructing the visual field around the target. The subjects measured three times at each location (real marker 3.00 D, real marker 2.50 D, and hologram marker 3.00 D) and used the average value for statistics.
For data analysis, a paired sample t-test was performed using SPSS software, Ver. 18.0 for Windows (SPSS Inc., Chicago, IL, USA). The paired sample t-test is a statistical procedure used to determine whether the mean difference between two sets of observations is zero. In a t-test, each entity is measured twice, resulting in pairs of observations. Statistical significance is determined by looking at the -value which gives the probability of observing the test results under the null hypothesis. Statistical data analysis of this article was performed with the corresponding samplesŠ t-test, and the normality test was performed using the Kolmogorov–Smirnov test. The null hypothesis assumes that the true mean difference between the paired samples is zero. Statistical significance was set at with a 95% confidence interval. This corresponds to a 5% or less chance of obtaining a result like the one that was observed if the null hypothesis was true.
A Denisyuk hologram of the marker was recorded on a U04 glass plate according to the Denisyuk method (Figure 7(a)) and developed in two chemical baths. After processing, a blur-free, two-color, transparent 1:1 scale image was obtained. When illuminated at the proper distance with an RGB LED lamp, a sharp holographic reconstruction of the marker appeared completely behind the glass plate surface with a large parallax, both horizontally and vertically (Figure 7(b)). The hologram was then sealed, and a black adhesive was laminated on the back to increase the contrast.
The accommodative response of the eye was measured for each participant, with a real marker attached at a distance of 2.50 and 3.00 D and a holographic marker attached at 3.00 D. The experimental results found with the N-vision K5001 autorefractor are shown in Figure 8.
A t-test was performed to compare the accommodative response in the observation of a real marker between a 3.00 D stimulus and a 2.50 D stimulus. The accommodative response to the 3.00 D stimulus was significantly higher than that to the 2.50 D stimulus, as shown in Table 1.
|3.00 D||2.50 D||−0.49||−11.188|
|( )||( )|
SD: standard deviation.
MD: mean difference.
These results confirm that the accommodative response of the eyes of an observer measured with the N-vision K5001 is different, if the real marker is placed at 3.00 or 2.50 D.
Table 2 presents a comparison of the accommodative response between a 3.00 D stimulus of a real marker and a 3.00 D stimulus of a holographic marker.
|( )||( )|
SD: standard deviation.
MD: mean difference.
The results show that the accommodative response to a stimulus of a real marker was significantly higher than that of the stimulus of a holographic marker placed at the same distance. The accommodative response of the eyes of the viewer did not take place at the same level when the hologram glass plate was placed at 3.00 D.
Table 3 shows results of comparison of accommodative response of the viewers eyes between a 2.50 D stimulus of a real marker and a 3.00 D stimulus of an holographic marker.
|( )||( )|
SD: standard deviation.
MD: mean difference.
The accommodative response to the 3.00 D holographic marker stimulus was slightly higher than that to a 2.50 D stimulus of a real marker, but the difference between the two was not statistically significant . This experimental result shows that the accommodative response for the holographic marker was not made on the plane of the holographic plate, but rather 67 mm behind this position, where the virtual holographic image of the marker is reconstructed by the RGB illumination. The slight difference between the holographic marker and the real marker can be explained by an incorrect reconstruction of the hologram. Indeed, if the RGB lamp is not positioned exactly at the same distance and angle as when recording, the reconstructed image may show distortions in size and position.
Experimental results verify that an on-axis analog reflection Denisyuk hologram provides a complete 3D image and can be naturally observed because the accommodative response and the convergence response are generated in accordance with the position where the image is reconstructed. So there is no vergence-accommodation conflict in a Denisyuk hologram. This study can be applied to human ocular physiological viewpoints of quantitative evaluation indices of analog reflection holographic images. In the case of a subject wearing spectacles, measurement with an Auto-Refractor may cause an error in the measured value due to light reflex. As a result, it is judged that the standard deviation was relatively high. However, the final mean value of the experimental result data is calculated by including the accommodative lag in ocular physiology.
The work reported in this article was conducted during the sabbatical year of Kwangwoon University in 2017. This research was supported by the MSIT (Ministry of Science and ICT), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2021-0-01846) supervised by the IITP (Institute of Information & Communications Technology Planning & Evaluation). This research was supported by Institute of Information communications Technology Planning Evaluation (IITP) grant funded by the Korea government (MSIT) (No. 2021-0-00922, Development of holographic stereogram printing technology based on multi-view imaging).
Conflict of interest: Authors state no conflict of interest.
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© 2021 Seunghyun Lee et al., published by De Gruyter
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