The Retina

The retina is a complex, incompletely understood processing membrane. Its layered-cell construction contains 300 million highly intertwined cells, some of which comprise a brain surface extension tethered to same-tissue counterparts via the optic nerve. These light-sensitive estranged brain cells perform extensive image processing within the eye. Some image processing techniques discussed later can exploit their processing sensitivities.

Treviranus discovered retinal sensors in 1835 but did not notice they faced away from the lens, like film installed backwards in a camera. Because of this, other dispersion, and absorption elsewhere, retinal sensors receive a meager 10 percent of all light entering the eye.

Light strikes retinal sensors, causing a poorly understood receptor photo-pigment chemical bleaching. This triggers a receptor electrical property change and subsequent nerve stimulation within 0.001 of a second. Before reuse, the sensor must return to its initial state. The retina contains two light sensor types: rods and cones.

• The retina’s 75 to 150 million rods provide our principal night (scotopic) vision that only produces light level intensity differences. Rods provide achromatic gray scale (black and white TV) sensations. Because rods have only one spectral sensitivity function, moon-lit colored objects appear colorless.

  Rod vision is relatively primitive but excellent for detecting contrast changes such as peripheral vision shadow movements. Note that high-contrast stationary objects are invisible at the retina’s edge. Because several rods connect to a given nerve end, rods provide general field-of-view perceptions but cannot provide detail information.

• The retina’s eight million cones provide daylight color (photopic) vision. Cones concentrate in the fovea which contains some 300,000 cones packed together at a 150,000 per mm² density. The retina’s edge contains few cones. Hence, peripheral vision lacks color and detail. Because cones connect to unique nerve ends, they provide detail information, called acuity, as well as color.

The Fovea

The fovea is a small, 0.75 mm radius, circular retinal area that senses our “direct line of sight.” Though the fovea is the retina’s thinnest part, it is the retina’s “sweet spot” for color differentiation and detail because it contains the retina’s highest cone concentration (300,000 tightly packed cones). Foveae are also important for precise depth perception. As you might expect, hawks have them but cats do not.

We typically focus conscious attention on fovea cone stimulation and process peripheral vision as a background task based on rod sensor input. When the rod “early warning” system signals alert, we pivot the vision system to bathe our foveae with pertinent quanta (glance at the danger) for assessment.

On the retina, both the rod and cone distributions are nonuniform, radially symmetric distributions around the fovea. The fovea contains the greatest cone concentration level which tapers precipitously as measured away from the fovea. The largest rod concentration surrounds the fovea and tapers off to the retina’s edge to provide unconscious vision. Because rods are more sensitive to faint intensity levels than cones at night, it is helpful to look slightly away from stars, allowing their faint twinkles to fall on rod-rich areas, not on the cone-rich fovea. Conversely, if you look at intensely lit color objects, turning your head prevents discerning the object’s color and detail because rods cannot match cones in these respects.

We now turn our attention to a human perception overview discussion that emphasizes important concepts for digital image processing.