| Color Theory for the Desktop |
| The Physiology of Human Vision |
The third part of the color triad is human vision. After all consideration has been made to the nature of the light and the spectral reflectance of the object being viewed, how you see color depends on the combination of three distinct stimuli of the retina. For this reason, human vision is often referred to as a tristimulus response.
This aspect of seeing color was well described by British physicist James Clerk Maxwell who wrote in 1872,
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We are capable of feeling three different color sensations. Light of different kinds excites these sensations in different proportions, and it is by the different combinations of these three primary sensations that all the varieties of visible color are produced. |
Maxwell's studies, along with those of Thomas Young and Hermann von Helmholtz, form the basis for all currently held views on human color vision.
Vision Basics
The simple mechanics of human vision are as follows:
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The cornea draws light and focuses it on the lens, which adjusts for distance. As it travels from the cornea to the lens, the light passes through an aperture called the pupil. This aperture narrows and widens in response to the brightness or dimness of the surrounding light by the action of the iris (the colored part of the eye). |
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The lens then passes the light through a transparent gel called the vitreous humor and focuses an inverted image of the object being viewed on the retina at the back of the eyeball. |
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The retina is the light-sensitive part of the eye and its surface is composed of photoreceptors or nerve endings. These receive the light and pass it along through the optic nerve as a stimulus to the brain. The photoreceptors are of two types, rods and cones. The greatest concentration of rods and cones is in an area of the retina called the fovea. In the very center of the fovea is an area called the foveola composed entirely of cones. The area of the fovea/foveola is the most light- and color-sensitive part of the retina. |
There are two distinct axes by which light travels through the mechanics of the eye, the optical axis and the visual axis:
The optical axis is the most direct line through the center of cornea to the pupil, the lens, and the retina. This is the line that draws sharpest focus when we look at an object. However, this line intersects the retina below the fovea and is not the most light and color sensitive.
The visual axis draws a line from the center of the pupil to the fovea. This axis gives the best color vision, but, because it doesn't intersect the cornea and lens at their exact centers, is not as optically clear as light passing on the optical axis.
Photoreceptors
As mentioned above, light and color are sensed by the rods and cones in the retina. They are structurally similar in most respects; the rod is mostly cylindrical along its length, while the cone is tapered (hence their names). Each rod or cone is roughly 1/500th of a millimeter in diameter and 1/25th of a millimeter in length.
The visual process begins at the outer segment of the rods and cones. This is where light and the pigments in the photoreceptors interact. The light is further absorbed by the inner segment, made up of the ellipsoid and myoid, and passed into the nucleus. From there the stimulus goes through the synaptic body to form nerve fibers that connect to the optic nerve and then to the brain where the stimulus is interpreted as the light, color, and shapes we see.
Exactly where the functions of the rods and cones differ from each other is unclear. What is known is that the rods contain a pigment called rhodopsin and are light sensitive but not color sensitive (that is, they're monochromatic), while the cones contain the pigments erythrolabe, chlorolabe, and rhodopsin, which are sensitive to wavelengths in the red, green, and blue parts of the visible spectrum. These three sensitivities are most commonly signified by the Greek letters r (rho), g (gamma), and b (beta) for red, green, and blue, respectively.
Moreover, the rods are more acutely sensitive to light, while the cones are insensitive to light below a certain level of luminance. When we see in dim light, rods receive the light and relay it to our brains—but because the rods are monochromatic, we see only shades of gray.
Spectral Sensitivity
Similar to the spectral power distributions and spectral reflectance curves we discussed in the preceeding sections, visual sensitivity to colored light is also characterized by a graph called a spectral response or sensitivity curve.
We mentioned above that certain cones are sensitive to red, green, or blue light. However, the sensitivities don't actually peak at these wavelengths; instead, the curves cover portions of the spectrum, which could be called reddish, greenish, and bluish. For example, the r sensitivity curve covers the wavelengths from 475nm to about 700nm and peaks at roughly 590nm which is yellow light.
Below are the sensitivity curves for the r, g, and b cones as well as the curve for the scotopic vision of the rods:
Stimulus
The stimulus received by the brain is what we see as color. This is a combination of all the aspects of seeing color discussed in this and the preceding sections. The spectral power distribution of the light source, times the spectral reflectance of the colored object, times the spectral sensitivity of the cones in the human eye equals the stimulus of color that we see:
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