The human eye has about 13 million photo-receptor cells sensitive to light. Cones are sensitive to color and come in 3 types (L, M, S). Rods are sensitive only to brightness.
Each cell produces an electro-chemical signal when exposed to light. Signals are sent to the brain’s visual cortex for processing. The image is perceived through a complex network of neurons in the visual cortex.
Mechanical Properties
- Focusing
The eye focuses on objects from nearby to infinity by changing the shape of the biological lens. - Light adaptation
Each photo-receptor is sensitive only to a narrow intensity region. The pupil dilates or contracts to admit the amount of light the photo-receptors are most sensitive to.
Perceived Brightness
Perceived brightness is not a simple function of light intensity.
- Simultaneous contrast
A region’s perceived brightness depends on the background intensity, not only its own intensity. - Edge sensitivity
The eye is more sensitive to edges, so perceived brightness tends to undershoot or overshoot at sharp brightness differences.