Principles of Stereo Fundus Photography


Fig. 1.1

Reflecting mirror stereoscope invented by Wheatstone


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Fig. 1.2

Refraction stereoscope invented by Brewster



Early 3D movies came out at the end of the nineteenth century. With the emergence of filming, scientists used two cameras to simulate human eyes and projected the film through polarized filters, while the audience watched the movie wearing polarized glasses. Stereoscopic glasses first appeared in the Hollywood movies on May 24, 1953, which marks the beginning of a new era of stereoscopic films. Then, the “stereoscopic TV adopting dual channel polarized imaging technology” and the “complementary color stereoscopic imaging technology” made black & white and color TV three-dimensional, respectively. Currently, the most advanced 3D TV is fractional liquid crystal glasses stereo TV, which can provide vivid stereoscopic color image. When the TV frequency becomes higher, the image is stable without flicker. It is compatible with current color TV system and computer screen, and can easily be transformed to digital TV system.


Ophthalmic operating microscope is also one form of stereovision. As early as in 1590, the Dutch Hans Jansen invented the first compound microscope composed by multiple prisms. OPMI-1 produced by Zeiss corp. in the 1950s was the first operating microscope of modern sense. The current ophthalmic operating microscope integrates illumiation, suspension, multi-optical path, synchronization, built-in inverter, and HD camera in one, which can greatly improve the quality of ophthalmic surgery and expand the scope of surgical indications.


1.2 Stereopsis


Worth proposed that binocular vision was composed of three levels in 1921, namely, simultaneous perception, fusion function, and stereopsis from low to high, respectively. Simultaneous perception is the basis for binocular vision, which requires that both eyes must have the ability to percept simultaneously, and the images from each eye must have corresponding points on both retinas. Fusion function is a level II binocular vision function, including sensory fusion and motor fusion. Sensory fusion is the ability to integrate two images from two retinas with tiny disparities into one complete image through the analysis of the brain. Stereopsis is a level III binocular vision function. True stereopsis is dependent on disparities between the two images received by each eye, and therefore, a certain number of points must fall on disparate points on the retina, which is the physiological parallax (Fig. 1.3), or the intersection angle of the line of sight is different (Fig. 1.4). The brain then analyzes the depth of the image, i.e., the stereopsis. Although one eye can also make a rough judgement of the distance through perspective, shadow, parallax, and shield, it is not accurate enough to make fine judgement and operation. Only the depth perceived through stereopsis is sufficiently accurate [3–5].

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Fig. 1.3

Physiological parallax


O1, O2, the lens plane for right and left eyes


A, F, B, the distant, middle, and near observing points


a1, b1, c1, a2, b2, c2, the corresponding points of A, F, B on the retina of two separate eyes


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Fig. 1.4

Intersection angle α


F the focus


O1, O2, the lens plane for right and left eyes


B, the basic plane crossing the two lenses


D, the vertical distance between the focus and basic plane


f1, f2, the corresponding points of F on the retina


α, intersection angle


Physiological parallax (η) is the distance between the corresponding images on both retinas. The physiological parallax of objects with different distances is not the same, and this disparity then is transmitted to the cerebral cortex center to create the sense of distance. Physiological parallax is the basis of stereopsis.






$$ \eta a={f}_1{a}_1-{f}_2{a}_2\kern0.875em \eta b={f}_1{b}_1-{f}_2{b}_2 $$

The physiological parallax of fixation point F is zero, for the points further than F, η < 0, like point A, on the contrary, for the points nearer than F, η > 0, like point B.


The distance of objects is determined by the intersection angle α of the line of sight, the object with larger α is nearer, and the object with smaller α is more far away.


The relationship between D and α is below:



$$ tg\frac{\alpha }{2}=\frac{ba}{2D}\kern0.875em \alpha = ba/D $$

In general, the eye feels most comfortable when D is 25 cm, which is the distance of distinct vision. The eye loses the ability of discerning depth when α is less than 30′, equivalent to the distance of 450 m, which is the observation radius of human stereopsis.


The fovea is responsible for fine stereopsis and could detect the parallax ranging from 2″ to 1200″, suitable for high spatial frequency, static and colored objects. The peripheral retina is responsible for coarse stereopsis and could only detect the parallax ranging from 0.1° to 10°, suitable for low spatial frequency, dynamic and colorless objects. Stereoscopic vision is the resolution of the stereovision, that is, the minimal depth and diameter difference that can be detected. The stereoscopic vision is described in arc seconds.


Object method and picture method have been used to measure stereoscopic vision. Picture method uses a pair of stereo images simulating what both eyes would see. Stereo images can be divided into three types. The first type of stereo image is line stereogram, the individual elements seen by each eye such as edges and lines are matched but with horizontal parallax to make stereovision. The second type of stereo image is random-dot stereogram made by black and white dot matrix, which is disordered lattice seen by one eye and parallax image fused by both eyes. The third type is auto stereogram, a single image stereogram with repeated 2D images. The vergence function of eye will match and fuse similar objects with parallax to create stereoscopic (3D phosphenes).


1.3 Artificial Stereopsis and Its Observation


The object produces images on the photosensitive materials, which can be observed through human eyes to form physiological parallax to reconstruct stereovision. This kind of stereovision is called artificial stereopsis (Fig. 1.5).

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Mar 22, 2020 | Posted by in OPHTHALMOLOGY | Comments Off on Principles of Stereo Fundus Photography

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