Visible light is any electromagnetic wave that falls into the wavelength range extending from 380 nanometers (frequency = 7.9 x 1014 Hz, adjacent to ultraviolet), to 825 nm (f = 3.6 x 1014 Hz, adjacent to infrared). When we look at an object, the lens of the eye intercepts a cone of light from the object and focuses it on the retina. As the object being viewed moves closer to the eye, accommodation muscles change the shape of the lens (within limits) so as to maintain sharp focus.
At the center of the eye, the foveola, cone cell sensory receptors are about 3 ?m in diameter. This corresponds to a visual angle of 0.7?> . Including a factor of safety, it is customary to assume that the human eye cannot resolve objects that subtend an angle of less than 1?>.
Cone photoreceptors handle daylight from the maximum tolerable illuminance level to ?>black,?> an amplitude range of approximately 4500 to 1, with 7 minutes required for dark adaptation. The visible range is extended an additional factor of around 22 by rod photoreceptors, with 30 minutes required for dark adaptation.
It is conjectured that the photoreceptors supply graded potentials proportional to the logarithm of visual stimulus rather than proportional to the stimulus itself.
The retina contains five layers: the photoreceptors feed a receptor potential to bipolar and horizontal cells. The latter gather in the receptor potentials over a local area and, in effect, measure the average local value. A fract ion of the average receptor potential is subtracted from the bipolar cell input to yield an output that is fed to ganglion cells. All of the potentials leading up to the ganglion cells are graded; the ganglion cells translate the graded potentials into action potentials (APs). A layer of amacrine cells is involved with the detection of illuminance change.
In the process of subtracting local receptor potential from bipolar cells, the horizontal cells enhance contrast and improve the visibility of small objects. Several simple examples are given, including that of Mach bands.
Many of the ganglion cells are of a transient variety: they and amacrine cells respond to change (flicker) in the bipolar cell output. The amacrine cells gather in the flicker over a local area and, in effect, measure the average local flicker. The average flicker is subtracted from the transient ganglion cell input. This suppresses transient ganglion cell output due to massive flicker and improves the detection of flicker (movement) of small objects.
About 65% of the cones respond maximally to yellow wavelengths and are called red; 33% respond maximally to green and are called green; 2% respond maximally to indigo and are called blue. The subjective sensation of color (hue and saturation) depends on the red: green: blue (C
The standard chromaticity diagram issued by the International Commission on Illumination is discussed. It turns out that if we plot 0.9 log C