The aim of this article is to give a brief introduction to the human visual system and to explain how light is transformed into visual perception.
Visual perception starts with the light emitted from or reflected from an object or a scene entering our eyes through the cornea, pupil and lens. The cornea and the lens help to concentrate and project the light onto a photosensitive layer of cells located at the back of the eyeball —the retina. The lens has the additional function of regulating the focus on objects at different distances by making the necessary adjustments. The amount of light that reaches the retina is regulated by changing the size of the pupil, which is an opening between the cornea and the lens delimited by the iris. The retina is responsible for translating the differences in light wavelength (color), contrast and luminance into a biological signal. This signal is transmitted through the optic nerve and neuronal pathways to the visual processing areas of the brain.
The binocular human visual field spans about 220 degrees horizontally and 135 degrees vertically. However, the level of detail is not homogeneous across the visual field. The quality of this information depends on where it falls on the retina. About 94% of the photosensitive cells in the eye are rods and about 6% are cones. Rods do not require much light to function, but do, on the other hand, only provide a blurred and less colorful image of our surroundings. For more detailed and clear vision, our eyes are also equipped with photoreceptor cells called cones. Cones are present in three different varieties; ones that register short (blue), medium (green) or long (red) wavelengths. While being efficient in providing a clear picture, the cones require more light to function. Hence, when we look at things when it’s dark around us, we lose the ability to see color and use mainly information registered by rods, providing us with a grey scale image. Cones are mostly found within the fovea where they are tightly packed providing a clear, bright image of the environment in that region.
The figure on the left is a schematic representation of how the photosensitive cells process the visual field. The area that is in focus and in full color represents the part of the visual field that is covered by the fovea (high density of cone cells). This area has a slightly irregular shape and is about half a millimeter in diameter (about 1–2 visual degrees). Within the rest of the visual field (the parafoveal and peripheral areas) the image we perceive becomes more blurry and thus harder to interpret and discriminate in high detail.
Snowden R.J, Thompson P. and Troscianko T. 2012. Basic Vision: An Introduction to Visual Perception. Oxford University Press, Revised Edition, pp. 424.