The human eye has about 13 million photo-receptor cells sensitive to light. Cones are sensitive to color and categorized into 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.
Depth Cues
The visual system infers depth from image properties even when only 1 eye views the scene.
- Relative size
Similar objects that project to a smaller image are inferred to be farther away. - Texture gradient
Surface texture becomes denser and less distinct with distance. - Linear perspective
Parallel lines in a scene converge toward a vanishing point in the image.
