Light & Color
Rods are sensitive to dim light but cannot detect sharp edges. Thus, rods provide scotopic (dark-adapted) vision.
Cones require more light to operate in most animals, including humans. Hence, color washes out in low light. But cones detect details in brighter light that rods can. The 6–7 million cones that a human eye has may be divided by their spectral sensitivity: red (64%), green (32%), and blue (2%).
The additive color model is based upon colored light mixtures. Red, green, and blue mixed create white light. More precisely, light activating the 3 different cone types in the proper proportion gives the mind-brain the impression of white light.
Scottish physicist James Clerk Maxwell is credited with experimentally discovering additive color, though the concept was articulated earlier by German physician and physicist Hermann von Helmholtz ~1850. Maxwell is best known for formulating classical electromagnetic theory, demonstrating that electric and magnetic fields travel as waves.
Calling the long-wave cone receptors “red” is something of a misnomer. The normalized wavelength peak of red cone response is around 580 nm, which is yellow light, not red. Red light kicks in at over 600 nm. The mind reddens sensed yellow light. While hominids evolved red color reception for social signaling, most other mammals do not see red.
The light response of rods peaks sharply at higher wavelengths: bluish light. Rods scarcely respond to slower wavelength red light.
With only 2% of the cone population, blue cones are grossly outnumbered, even though they are much more light-sensitive than red or green cones. This numerical disadvantage is overcome by mental image processing, which provides a compensatory blue amplifier.
The retina has a static contrast ratio of 100:1. By contrast, a typical clean film print at a movie theater may be 500:1, and flat-panel television displays are commonly 1000:1 on up.
While high contrast is desired for any display, it creates a biological trade-off for vision between image clarity and motion adjustment. The need for adjustment wins, hence low contrast reception.
As the eye constantly moves (saccades), it adjusts light exposure both chemically and geometrically via the iris, which regulates pupil size. Low contrast reception is mentally compensated for during visual processing: heightening contrast by successive image comparison at slightly different light levels and focal planes.
It takes 4 seconds of utter darkness for the eyes to initially adjust. Full adaptation by alterations in retinal chemistry (the Purkinje effect) takes a half hour or more.
Czech anatomist Jan Evangelista Purkynĕ (aka Purkinje) often meditated at dawn during long walks in blossoming Bohemian fields. He noticed that his favorite flowers appeared vivid red in bright sunlight, but dark at dawn. He correctly surmised in 1819 that the eye has 2 adaptive techniques to detect colors: one for sunshine, and the other for crepuscular light.
The Purkinje effect occurs at the transition between primary use of cones (photopic) to rods (scotopic). This is the mesopic state: as luminance dims, the rods take over; but before color disappears completely, sight shifts toward the rods’ top sensitivity. The upshot is that human eye color reception shifts toward blue at lower light level, because rods, which are more light-sensitive but color-insensitive, respond best to green-blue light.
Red lighting is the answer to having both the photopic and scotopic systems working. Submarines are dimly lit to preserve the night vision of the crew; so, for example, to be able to see through a periscope at night. But the control room must be sufficiently lit to afford reading instrument panels. By employing red lights, cones receive enough light to provide photopic vision: the high-acuity vision requisite for reading, while not saturating the rods, which are not sensitive to long-wavelength red light.
Eye adaptation results in a dynamic contrast ratio range of about 1,000,000:1 (about 20 ƒ-stops). Adaptation, like most biological mechanisms, is nonlinear and multifaceted.
Full adaptation depends on good blood flow. Dark adaptation may be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco.
Physiological activity related to post-reception processing transpires in the visual cortex. Glia are processor and memory cells, as well as the maker and keeper of nerve cells which serve as signal transceivers.
The 3 cone types have some response wavelength overlap. Given various levels of input intensity, owing to discrepancies in population and distribution by cone type, it is more mathematically efficient to process the differences in cone responsiveness rather than direct intensities.
Cone wavelength overlap provides sampled comparative baselines of light level. This sampling is supplemented by secondary light level input information from bipolar and ganglion cells. The resultant data matrix provides relative input upon which to apply an inscrutably sophisticated, near-instantaneous algorithmic process that approximates the scene in view within the physical tolerances of photoreceptors. The acuity of vision operates at the outermost bounds of the known laws of physics. Just as rods have been honed by evolution to detect a single photon, so biological vision processing is a mathematical wonder beyond comprehension.
For decades it was thought that cones in the retina send their respective blue, green, and red color signals to the mind-brain, whereupon the mind combines colors, like a color printer (albeit using additive color). Instead, color vision processing works much like filling in a coloring book or colorizing a black-and-white film.
Prior knowledge, such as object coloration, is mentally projected in the earliest stages of visual processing. ~ German vision zoologist Andreas Bartels
Each type of cone comes in 1 of 2 varieties: color and value (light level or whiteness). Most cones detect value, which is employed to create a high-resolution picture defined by edges. Meanwhile, the color signal that the mind-brain receives is low resolution. These color splotches are combined and applied to edge-delineated areas to render an image.
Color perception is learned, not congenital, though the faculty to learn and apply colorization is innate. The mind takes color information from sensory inputs and colorizes according to heuristics acquired through practice.
Infant monkeys were reared for nearly a year in a room where the illumination came from only monochromatic lights. After extensive training, they were able to perform color matching. But their judgment of color similarity was quite different from normal animals. Furthermore, they had severe deficits in color constancy. Their color vision was very much wavelength dominated, so they could not compensate for changes in wavelength composition. Early visual experience is indispensable for normal color perception. ~ Japanese psychologist Yoichi Sugita