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The process of human vision

Light passes through the concave-convex cornea and biconvex lens of each eye and produces a rough real image of the visual scene on the retina. This image is called rough because it is still a very poor optical element in terms of spherical aberration and chromatic aberration by instrument standards. The smaller the aperture of the optical device, the clearer the image. This phenomenon is automatically formed in the eye by brighter scenes in the human eye. The clearest part of the image is the center of gaze, because the lens in the eyeball is adjusted for this part. This part of the image is on the fovea, where the obstruction to light is the smallest and the structure of the receptor cells is the finest.

Stereoscopy The image is produced on the retinas of the two eyes. Because the angles of the two eyes looking at the target are slightly different, the images produced are not completely consistent. The brain can judge the visual depth perception (i.e., distance) from the difference in the image and from the angle of intersection of the two eyes' sights, at least within a range of six meters from the observer. The field of vision seen by both eyes at the same time does not include the entire field of vision, but is limited to the middle 120° range of the 190° total field of vision on the horizontal plane. Monocular vision will not completely lose the sense of depth, because the brain can also get a lot of hints from perspective, shadows and parallax, and the brain has already accumulated some experience in estimating the size of general targets.

Photochemistry of receptor cells (Receptor potochemistry) The projection of visual images on the retina means that the receptor cells have received the radiation power. The black color of the pupil of the eye indicates that the light has been absorbed, actually absorbed by the photochemical pigments in the receptor cells. When these pigments receive light, they undergo chemical changes and will produce a certain amount of "whitening products" depending on factors such as the wavelength of the light, the illuminance and the duration of exposure. When there is no light stimulation, the chemical changes will proceed in the opposite direction, so at any time the receptor cells contain a mixture of the original pigments and whitening products.

Retinal potential wave (Retinal potential wave) The formation of retinal potential is related to photochemistry and may be the result of photochemistry. The retinal electric wave recording diagram shown in Figure 2-3 shows that: constant illumination cannot cause any change, but can only produce a constant potential, while the large signal is generally related to the change of light irradiating the sensory cells. Now it is possible to analyze this potential wave into components related to retinal illumination, changes in retinal illumination, and secondary feedback signals from optic nerve impulses.

Optical nerve discharge The third stage of the visual process is the discharge of optic nerve fibers, which is presumably caused by retinal potential waves. The form of discharge is a pulse discharge with constant amplitude but variable frequency. The discharge record can be decomposed into three signals: "on", "off" and "continuous". It is interesting and clear from the figure that the nerve impulse is like a coded form of retinal potential waves, and the pulse frequency corresponds to the size of the potential.

Involuntary eye movement Since the image is not static on the retina, the nerve impulse generated by the retinal image is further complicated. Even when the observer's eyes are fixed on one place, the eyes are in a state of constant movement. Three types of motion are now thought to occur:

(1) tremors with a frequency of 30 to 80 Hz and an amplitude equal to one or two receptor cell diameters; (2) irregular excursions lasting less than a second and with an amplitude equal to 40 receptor cell diameters; and (3) brief bounces that return the image of the fixation point to the fovea at the end of each excursion. These movements reinforce the idea that changes in the illumination of the receptor cells play a greater role in vision than the illumination itself.

Visual cortexVision ultimately resides in the visual cortex at the back of the brain. Here, about two million electrical signals from the eye are sorted and organized. It has been shown that point-by-point projections on the retina are cast onto the special surface of each visual cortical cell. However, in contrast to explaining vision, in explaining perception, the visual cortex and other cortices must be explained as having a large integrative capacity. Therefore, many theoretical processes related to the interpretation of images and the recognition of colors remain to be studied.

Retinal illuminance: Vision scientists often use the unit of retinal illuminance, Troland, to express the stimulating effect of external light passing through the pupil (whether it is the pupil of the eye or an artificial pupil). In practice, it is also necessary to consider the loss of illuminance in the eye and the fact that light passes more efficiently through the center of the pupil than through its edge. However, for a large uniform field of view, the retinal illuminance in Troland is equal to the product of the external surface brightness in candela/square meter and the pupil area in square millimeters.

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