Sagredo: Because billions of humans utilize vision as the primary means to gather information about our world, it would seem that humans as intelligent beings must have long ago figured out the mechanism of vision.
Simplicio: Humans have proposed a variety of explanations for vision over the ages. But certainly the correct explanation is widely given in science textbooks all over the world.
Sagredo: Yes, there is an abundance of well accepted explanations. But the explanations of color vision seems confusing.
Salviati: Perhaps the confusion is due to too many explanations that are not logically consistent?
Simplicio: Please allow me to explain. A convex lens at the front of the eye focuses light upon an array of rod and cone sensors covering the back of the eye ball.
Sagredo: So far, so good. But I've heard there are two different sets of primary colors. How can this be?
Simplicio: There is really only one set of primary colors of light: Light is known to be a combination of three primary colors: red, green, and blue. The three colors of light add together to make white light. The confusion comes when chemical substances called pigments absorb the primary colors. Colored Pigments such as those used for printing are three other colors: yellow which removes blue light, cyan which removes red light, and magenta which removes green light. These pigments each subtract light so a combination of the three pigments remove all the light forming black.
Sagredo: So why do some printing process use four colors?
Simplicio: That is a matter of economics. The colored pigments are expensive. There are cheaper materials such as lamp black, a form of carbon, that absorb all light and therefore appear black. Because black pigment is cheaper, four color printing uses a black ink as a substitute for a combination of the more expensive three colored pigments.
Sagredo: That is helpful. There are only three primary colors of light. The second set of colors are actually substances that absorb the primary colors of light. But that being so, I don't understand why a rainbow is not three bands of color. If raindrops, like glass prisms, merely separate the light, why do we not see just the primary colors?
Salviati: The three primary colors seems to be a simple and clear explanation. The explanation helps us to reproduce color by printing and to make televisions, and computer monitors. But that explanation seems inadequate, failing to explain other observations known since the age of Isaac Newton. Clearly a rainbow is not three stripes of primary colors but a continuous spectrum gradually progressing from the deepest crimson, through orange, yellow, green to violet. Indeed educated observers with large vocabularies can identify a great many colors in a rainbow. And similarly, the artist's pallet, just like modern paint mixing machines, often contains over a dozen pigments of different colors to create the range of shades on the canvas.
Simplicio: Yes, yes. The real world is a bit more complicated. Issac Newton did demonstrate that the rainbow is composed of a spectrum of colors gradually progressing like the infinite gradation of real numbers of a number line. However the three primary colors of light are special, being the simplest and most basic way to create any color of the rainbow. The variety of pigments has a slightly different explanation. Real pigments do not perfectly absorb a single primary color. So it is less expensive and indeed necessary to use other combinations of less than ideal pigments to create the thousands of advertised paint colors.
Sagredo: This does seem to complicate the explanation. Perhaps it is the workings of our eyes that causes the confusion between three primary colors and the continuous spectrum?
Salviati: I think you are close to the truth. It seems clear that the world contains a continuous range of colors, each carried by a different length of light wave. The sizes of the waves extends from very long radio signals way too large for our eyes can detect, through infrared light which we feel as heat, through the visible rainbow, to ultraviolet which tans our skin and finally to the extremely short length gamma and X-rays. If our eyes detected every kind of light wave, they would need to contain an infinite variety of detectors. Our eyes, but an inch or two large, must contain only a small collection of detectors which serve to detect the whole range of visible colors.
Simplicio: You are right! There are several kinds of light detectors on the back side of the eyeball. Faint light is detected by rod-shaped detectors and brighter light of various colors is detected by three types of cone-shaped detectors, each sensitive to the different range of color with sensitivities centering on blue, green, and red light. The cones must contain different organic molecules with conjugated double bonds that absorb the light that matches the resonance frequency of the electrons in the conjugated double bonds. The energy from the absorbed light briefly breaks one of the rigid double bonds, allowing the molecule (called retinal) to rotate to a new shape. The newly shaped molecule interacts with a much larger protein (named opsin), setting off a series of chemical reactions which that causes ion channel proteins imbedded in photoreceptor membrane to open and for change in polarization, This starts an electrical signal passing along the cell membrane surface of the optic nerve to the brain. The chemistry of the retinal seems well understood and the successive protein and nerve reactions fit with many other similar enzyme reactions that occur in living creatures.
Sagredo: I see that many branches of science are contributing to our understanding of vision. Surely the explanations of the physicists must be consistent with the biologists findings by dissection and the chemicals and their reactions isolated by chemists. If our world were not logically consistent, we could not find explanations that would match the real world.
Salviati: But small problems remain to be addressed. The measured nerve impulses coming from the eye don't relate well to the colors viewed. The chemistry that distinguishes the three distinct kinds of cones and the connection to the retinal photo-reaction remains unclear. Some sources mention colored pigments that appear to distinguish the three types of cone cells but remain vague about the necessary chemistry.
Simplicio: You are of course right. There are still matters to be discovered. But in time scientists will find any missing molecules, distinguish the required three types of cones, and determine how nerve signals are processed. The three primary color theory and the biology of rods and cones is in a great many textbooks. Surely if the theory were even slightly challenged it would be well known to all.
Salviati: Actually Edwin Land, a famous chemist and inventor, performed a large number of experiments in the mid-twentieth century finding that full colored images can be produced without three primary colors (as created on a screen by overlaying black/white and red projector images: see May 1959 Scientific American cover to right). Land concluded that the determination of color is spacial and that lightness and color depends on the remainder of the content in the field of vision. (See Matthew's excellent Wendy Carlos site. While there, spend time with the 9/11 WTC section.)
Sagredo: Are you saying there is evidence that the well accepted theory of colored vision is in error? Can not Mr. Land's finding's be explained by the universally accepted theory?
Simplicio: Well, Mr. Land's results do seem strange. But when you look at his actual results, the colors do seem pale. Perhaps they are not really inconsistent with what we all accept as true? The accepted theory suggests that three variations in the cone's retinal pigment make some cones more sensitive to red light, others more sensitive to green light, and the remainder to blue light. The retina has five layers of cells. Between each layer there are extensive interconnections in which processing of the visual image takes place. The signals from groups of cones are combined with their signals inhibited by more distant neighbors. Unfortunately what is currently known about this nerve signal processing doesn't correspond well to our conscious experience of color.
Salviati: If we are to believe the current evidence, there may not be three primary colors of light. Human eyes do transmit three distinct signals that correspond to three different ranges of color. All three are equally sensitive to blue light, two have expanded ranges that include green and yellow light, and the third signal includes sensitivity to red light. (View sensitivities of the human retina are presented in graphs plotting measured % sensitivities and log of sensitivities) One proposal by G. C. Huth is that the rods and cones comprise an array of detectors, the shape and spacing of which determines the absorption of light rather than each rod and cone individually detecting color and other information. The spacing of the rods specifies the blue range they detect, the spacing of the cones specifies the red range, and the spacing between the rods and the cones specifies the green range. This explanation explains the missing chemical distinctions between cones and the three or four different light detecting reactions required by the 3-cone theory.
Sagredo: Do you mean to say that the detection of color might not be due to different kinds of detectors in our eyes, but due to the spacing and arrangement of the detectors? Perhaps the rods and cones don't really detect the light after all?
Salviati: No. Even with Huth's theory each rod and cone probably does detect light. And the rods and cones may have different sensitivities. But the back side of the eye is like a forest of detectors. Because the spacing of the detectors is about the same of as the length of the light waves, the detectors probably work as a team with the light scattered and shared. If Huth is right, the spacing of a particular set of detectors matches that particular color's wavelength, the results of each detector combine to produce a strong nerve signal sent to the brain. For a color with different wavelength, the signals from this set of detectors would, because of the mismatch of distances, be out of coordination resulting in a much weaker nerve signal. But even if Huth is incorrect about the importance of spacing, each array of detectors would give its strongest nerve signal for a particular color range. In all the latest explanations there must be three different arrays, each sending different strengths of nerve signals to the brain. The brain would then compare the three nerve signals to try to infer the original light color.
Sagredo: So if Huth is correct, all the cones would be identical and not need different chemicals and chemical reactions. This would be consistent with the findings of the biologists and chemists. Would this also explain how human eyes and brains can be faked into seeing a rainbow of colors by combinations of just three primary colors of light?
Salviati: Yes. For example orange light would result in the red and green detector arrays sending nearly equal strength signals to the brain. But printed documents or televisions using equal amounts of red and green (but perhaps not a bit of orange wavelength light) would also results in sending nearly equal strength red and green signals to the brain therefore also be perceived to be orange.
Sagredo: So the three primary colors of light and the success of three and four color printing are merely effects of the mechanism our eyes use to detect color. The real world does not have primary colors!
Simplicio: It is probably too soon to tell for sure. But if a continued search for the necessary chemicals and distinct cones fails, the world may be forced to abandon our current understanding of colored vision.
Salviati: Yes, for the world to accept a change in explanation, more research will be needed. Rods and cones are not evenly distributed over the back side of the eye (retina). The sensitive of different parts of the eye to different colors should correspond to the distribution of the array spacing in different regions of the eye.
Simplicio: Some people are color blind. Do differences in array spacings account for all eight known kinds of color blindness?
Sagredo: Like any new theory, it looks like there are lot of matters that need further investigation and clarification.
Sagredo: So let me collect all these ideas: The convex lens at the front of the eye focuses light upon an array of rod and cone sensors covering the back of the eye ball (retina). If Huth is right, the rods and cones comprise a forest of detectors, the shape and spacing of which determines the wavelengths of light which can be absorbed. Perhaps the spacing between the rods specifies the blue range they detect, the spacing of the cones specifies the red range, and the spacing between the rods and the cones specifies the green range. Alternately, different pigmented cones would work in groups to be most sensitive to one of three ranges of color. The rod and cones contain an organic molecule (such as retinal) with conjugated double bonds. Light that matches the resonance frequency of the electrons in the conjugated double bonds is absorbed, briefly breaking one of the rigid double bonds, allowing the molecule to rotate to a new shape. The newly shaped molecule interacts with a much larger protein (called opsin), setting off a series of chemical reactions and signal processing which transmit an electrical polarity signal along the surface of an optic nerve to the brain. (View sensitivities of the human retina are presented in graphs plotting measured % sensitivities and log of sensitivities) Wether Huth is right or wrong, three nerve signals are sent to the brain, one for each range of color detected. The brain attempts to infer from the strength of the three nerve signals what the original light color might have been. Because the brain only receives three signals, it is possible to easily create the impression of a rainbow of colors using only three pigments or three colors of light. And because the eye and brain compare neighboring parts of an image to determine color, impressions of color can be created even when that color of light is absent.
Investigate your eye's detection of color at different locations: Fixing your stare at one object, determine if you can distinguish various colors at various angles, say >45° from the direction of your stare. Can you identify red at large angles? Can you identify blue at large angles?
At what angle do detailed objects become blurred at the side of your vision?
Biography of Edwin Land at Rowland Institute.
RIT Munsell Color Science Laboratory's Color and Vision Questions and Answers.
A New Model for Light Interaction with the Retina of the Eye and the Vision Process, G. C. Huth, Ojai CA
Cone sensitivity data was obtained from the UCSD Color and Vision database which is apparently no longer available online.