Vision works differently than a camera converting a scene into a picture. We see because of the photochemical reaction that occurs in the eye when light stimulates the retinal receptors. These receptors then supply electrical impulse patterns to the mind that forms understandable information. The images are formed in the mind and have meaning based upon the observer’s prior experiences.
Color Vision Process
Perception is the understanding of what we see. Combined data from direct sensory signals and knowledge derived from previous experience form perception. Color perception is central to visual aesthetics and profoundly affects our emotional state.
Human vision uses a dual system to form the images humans perceive: an optical system, the eye, and a neural system, the mind.
The neural system responsible for vision starts with a specialized extension of the cerebral cortex, the retina. The retina is a thin multi-cell layer of interconnected nerve cells that convert light into electrical impulses.
The two kinds of light-receptors that create the electrical impulses are the cones and the rods. The cones function in high illumination, and give chromatic (color) vision. The rods function under low illumination, and give achromatic (gray) vision. The retina’s central region, the fovea centralis, distributes the cones more densely compared to rods.
Color is a sensation, and we sense color in relative terms composed of simultaneous and inseparable sensory data. We receive these primary data with color vision: the light wave frequency, its intensity, and its saturation. We also recognize the color area quantity, its shape, and light temperature.
Each spectral color is a different light frequency. The problem for our vision, then, is how to get a different neural response for different frequencies.
The answer is in the human retina and not in light. The retina uses three kinds of color-sensitive receptors (cones) that respond respectively to red, green, and blue light frequencies, the additive primary colors. All colors are mixtures of signals from these three cone receptors.
The vision system’s color-mixing ability enables full color vision from the three color primaries (RGB). The eye behaves differently in this respect from the ear. Two sounds cannot mix to produce a different, pure, third sound, but two colors create a third color that hides constituent identities. As an example, despite yellow’s pure appearance, it is not a color. There is no retinal receptor sensitive to the yellow frequency. The perception of yellow is arrived at through the combined activity of the red-sensitive and green-sensitive cones. The electronic signals are sent to the mind simultaneously creating the sensation of yellow.
Brightness and Intensity
The retinal receptors are extremely sensitive light detectors. The smallest amount of radiant energy stimulates them. The intensity of light entering the eye causes the sensation brightness.
Bright light bleaches the photo pigments of the retina. It is this bleaching that stimulates the nerves, and it takes time for the photochemical activity of the eye to return to normal. It is during the time photo pigments bleach a region of the retina that the region is less sensitive than surrounding regions. This causes afterimages.
A ‘positive’ afterimage indicates continued electrical impulses of the retina and optic nerve after the stimulation. An example of this is when a camera flash momentarily blinds us after a picture is taken. A ‘negative’ after-image indicates the reduced sensitivity of the stimulated part of the retina due to photo pigment bleaching.
The adjustment our eyes make to light conditions is adaptation. If our eyes experience low light for some time they grow more sensitive, and a given light appears brighter. This is ‘dark’ adaptation. The cones and rods adapt at different rates: cone adaptation completes in about seven minutes, while rod adaptation continues for an hour or more.
As a photographer increases exposure in dim light, dark adaptation increases vision’s ‘exposure-time’ to increase its sensitivity. However, with decreased light and increased visual sensitivity, we lose the ability to make out fine detail.
Surrounding areas also affect color vision. A given color area generally looks brighter if the surroundings are dark, and a given color looks more saturated if surrounded by complementary color. This contrast enhancement relates to the general importance of borders in perception. It is primarily the color area borders that signal the mind.
The tendency to perceive inaccurate nature-specific and object-specific colors as true is color constancy. Some examples of these ‘memory’ colors are: eggshell white, fire-engine red, lemon yellow, and sky blue. Skin tones also come under color constancy. When a printed image is viewed individually the color looks correct, when we compare the accurate flesh tone to the inaccurate color then the color difference becomes evident.
In addition to these other aspects, the eye is most sensitive to colors in the middle of the spectrum. Given equally intense colors, midspectrum colors such as green appear brighter than the endspectrum colors of red and blue.
Vision is highly sensitive and accurate, however, it is also subjective. Vision and perception vary among individuals, and even change with individual emotional and physical states. Therefore, accurate, consistent color evaluation must account for vision deficiencies resulting from the interaction of the optical system and the mind.
Color vision is part of being human. It is almost certain that no mammals up to primates possess color vision, although it occurs highly developed in birds, fish, reptiles, and insects.
Color blindness occurs in some people, while others see only a limited number of colors. It is estimated that approximately 10% of all males experience some degree of color blindness while females are substantially less susceptible with only about 1% experiencing the condition.
Random retinal impulses and involuntary rapid eye movements (saccadic eye movements) keep the vision system perpetually active. Both are essential to vision. Vision soon fades when an image becomes optically fixed on the retina. Part of the eye movement’s function is to sweep the light pattern over the retina to continually signal the mind the presence of the image. However, overuse fatigues the system, and vision fatigue impairs color judgment. For example, viewing a saturated color for a time causes a second color to appear different as vision fatigue subtracts some of the first color from the image. The reduced sensitivity is called negative after image, and is due to retinal photo pigment bleaching.
Even with the complete absence of light there are random retinal impulses reaching the mind. This continuous background of random activity sets a continuous problem for the mind. The mind must ‘decide’ whether this neural activity is ‘noise’ or information. This internal visual noise increases with age and is partly responsible for the gradual loss of visual discrimination with aging. Visual accuracy and adaptation also decline with aging.
Along with vision deficiencies and variation, external conditions also affect color judgment. Some external variables are: diverse lighting types; different substrates; disparate viewing angles; multiple observers; unconventional illumination angle; and the size and shape of color area.
Because color vision requires sensory data, it is impossible to remember color. We can only compare color. However, visual color comparison is almost impossible unless under exactly the same viewing conditions. For this reason, the graphic arts industry has established the standard for color comparison as a neutral environment with 5000-degree Kelvin illumination. This light temperature appears clear and color-balanced, and is ideal for color comparison.
Quantification and Measurement
Precise standardized measurement is also part of color evaluation. The major color measurement instruments are: Densitometers that compute density, the amount of light absorption of a surface or material. Densitometers precisely measure standard colors used in graphic arts and photography. Colorimeters that measure and compute XYZ color values in a way that models vision, and usually report results in a CIE color space. Colorimeters record all visible colors, but generally not as precisely as densitometers. Spectrophotometers that measure and covert spectral data to a CIE color space. Spectrodensitometers that serve all the functions of a densitometer, colorimeter, and spectrophotometer in one device.