Color vision 35-1 The human eye The phenomenon of colors depends partly on the physical world. We discuss 5-1 The human eye the colors of soap films and so on as being produced by interference. But also of course, it depends on the eye, or what happens behind the eye, in the brain 35-2 Color depends on intensity Physics characterizes the light that enters the eye, but after that, our sensations are 35-3 Measuring the color sensation the result of photochemical-neural processes and psychological responses 35-4 The chromaticity diagram There are many interesting phenomena associated with vision which involve a mixture of physical phenomena and physiological processes, and the full appreci- 35-5 The mechanism of color vision ation of natural phenomena, as we see them, must go beyond physics in the usual sense. We make no apologies for making these excursions into other fields, because 35-6 Physiochemistry of color vision the separation of fields, as we have emphasized, is merely a human convenience, and an unnatural thing. Nature is not interested in our separations, and many of the interesting phenomena bridge the gaps between fields In Chapter 3 we have already discussed the relation of physics to the other ornea sciences in general terms, but now we are going to look in some detail at a specific ris field in which physics and other sciences are very, very closely interrelated. That area is vision. In particular, we shall discuss color vision. In ent chapter we shall discuss mainly the observable phenomena of human and in the next chapter we shall consider the physiological aspects of vision, both in man and gament It all begins with the eye; so, in order to understand what phenomena we see, Vitreousiihumor some knowledge of the eye is required. In the next chapter we shall discuss in Choroid Reting ome detail how the various parts of the eye work, and how they are interconnected with the nervous system. For the present, we shall describe only briefly how the eye functions(Fig. 35-1) t enters the eye through the cornea; we have already discussed how it is Macula lutea bent and is imaged on a layer called the retina in the back of the eye, so that different optic nerve parts of the retina receive light from different parts of the visual field outside. The retina is not absolutely uniform: there is a place, a spot, in the center of our field 35-1.The of view which we use when we are trying to see things very carefully, and at which we have the greatest acuity of vision; it is called the fovea or macula. The side parts of the eye, as we can immediately appreciate from our experience in looking at things, are not as effective for seeing detail as is the center of the eye. There is also a spot in the retina where the nerves carrying all the information run out; that is a blind spot. There is no sensitive part of the retina here, and it is possible to demonstrate that if we close, say, the left eye and look straight at something, and then move a ger or another small object slowly out of the field of view it suddenly disappears omewhere. The only practical use of this fact that we know of is that some physiol ogist became quite a favorite in the court of a king of france by pointing this out to him; in the boring sessions that he had with his courtiers, the king could amuse himself by"cutting off their heads" by looking at one and watching another's head disappear Figure 35-2 shows a magnified view of the inside of the retina in somewhat schematic form. In different parts of the retina there are different kinds of struc tures. The objects that occur more densely near the periphery of the retina are called rods. Closer to the fovea, we find, besides these rod cells, also cone cells We shall describe the structure of these cells later. As we get close to the fovea, the number of comes increases, and in the fovea itself there are in fact nothing but cone Fig. 35-2. the structure of the retin cells, packed very tightly, so tightly that the cone cells are much finer, or narrower ( Light enters from below 35
here than anywhere else. So we must appreciate that we see with the cones right in the middle of the field of view, but as we go to the periphery we have the other cells, the rods, Now the interesting thing is that in the retina each of the cells which is sensitive to light is not connected by a fiber directly to the optic nerve, but is connected to many other cells, which are themselves connected to each other There are several kinds of cells: there are cells that carry the information toward the optic nerve, but there are others that are mainly interconnected"horizontally There are essentially four kinds of cells, but we shall not go into these details now The main thing we emphasize is that the light signal is already being"thought about. "That is to say, the information from the various cells does not immediately go to the brain, spot for spot, but in the retina a certain amount of the informatio s already been digested, by a combining of the information from several visual receptors. It is important to understand that some brain-function phenomena occur in the eye itself. One of the most striking phenomena of vision is the dark adaptation of eye. If we go into the dark from a brightly lighted room, we cannot see very we ell for a while, but gradually things become more and more apparent, and eventually we can see something where we could see nothing before. If the intensity of the light is very low, the things that we see have no color. It is known that this dark adapted vision is amost entirely due to the rods, while the vision in bright light is ult, there are a number of phe that we can easily appreciate because of this transfer of function from the cones and rods together There are many situations in which, if the light intensity were stronger, we could see color, and we would find these things quite beautiful. One example is that through a telescope we nearly always see"black and white""images of faint nebulae, but w. C. Miller of the Mt. wilson and Palomar Observatories had the patience to make color pictures of some of these objects. Nobody has ever really seen these colors with the eye, but they are not artificial colors, it is merely that the light intensity is not strong enough for the cones in our eye to see them. Among the more spectacular such objects are the ring nebula and the Crab nebula.The former shows a beautiful blue inner part, with a bright red outer halo, and the latter shows a general bluish haze permeated by bright red-orange filaments In the bright light, apparently, the rods are at very low sensitivity but, in the dark, as time goes on they pick up their ability to see light. The variations in light intensity for which one can adapt is over a million to one. Nature does not do all this with just one kind of cell, but she passes her job from bright-light-seeing ells, the color-seeing Among the interesting consequences of this shift is, first, that there is no color, and econd, that there is a difference in the relative brightness of differently colored objects. It turns out that the rods see better toward the blue than the cones do and the cones can see, for example, deep red light, while the rods find that absolutely impossible to see. So red light is black so far as the rods are concerned. Thus two pieces of colored paper, say blue and red, in which the red might be even brighter than the blue in good light, will, in the dark, appear completely reversed. It is a very striking effect. If we are in the dark and can find a magazine or something that has colors and, befo know for sure what the colo dge the lighter and darker areas, and if we then carry the magazine into the light, we may see this ble shift between which was the brightest color and which was not The phenomenon is called the Purkinje effect. dashed curve represents the sensitivity of dark i.e., using the rods, while the solid curve represents it in the light. We see that the k sensitivity of the rods is in the green region and that of the cones is more in the yellow region. If there is a red-colored page(red is about 650 mu)we can see it if it is brightly lighted, but in the dark it is almost invisible
Fig. 35-3. The spectral sensitivity of the eye. Dashed curve, rods; solid curve cones Another effect of the fact that rods take over in the dark, and that there are lo rods in the fovea, is that when we look straight at something in the dark,our vision is not quite as acute as when we look to one side , a faint star or nebula can sometimes be seen better by looking a little to one side than directly at it, because we do not have sensitive rods in the middle of the fovea Another interesting effect of the fact that the number of cones decreases as we go farther to the side of the field of view is that even in a bright light color disappears as the object goes far to one side. The way to test that is to look in some particular fixed direction, let a friend walk in from one side with colored cards, and try to decide what color they are before they are right in front of you One finds that he can see that the cards are there long before he can determine the olor. When doing this, it is advisable to come in from the side opposite the blind spot, because it is otherwise rather confusing to almost see the color, then not see anything, then to see the color again Another interesting phenomenon is that the periphery of the retina is very sensitive to motion. Although we cannot see very well from the corner of our eye if a little bug moves and we do not expect anything to be moving over there e are immediately sensitive to it. We are all "wired up"to look for something jiggling to the side of the field 35-3 Measuring the color sensation Now we go to the cone vision, to the brighter vision, and we come to the question which is rhost characteristic of cone vision and that is color. As we know, white light can be split by a prism into a whole spectrum of wavelengths which appear to us to have different colors that is what colors are, of course appearances. Any source of light can be analyzed by a grating or a prism,and one can determine the spectral distribution, i.e, the"amount"of each wavelength A certain light may have a lot of blue, considerable red, very little yellow, and so on. That is all very precise in the sense of physics, but the question is, what color will it appear to be? It is evident that the different colors depend somehow the spectral distribution of the light, but the problem is to find what characteristics of the spectral distribution produce the various sensations. For example, what do we have to do to get a green color? We all know that we can simply take a piece of the spectrum which is green. But is that the only way to get green, or orange, or any other color? Is there more than one spectral distribution which produces the same apparent visual effect? The answer is, definitely yes. There is a very limited number of visual effects, in fact just a three-dimensional manifold of them, as we shall shortly see, but there is an infinite number of different curves that we can draw for the light that comes from different sources. Now the question we have to discuss is, under what conditions do different distributions of light appear as exactly the same color 35-3
The most powerful psycho-physical technique in color judgment is to use the eye as a null instrument. That is, we do not try to define what consititutes a green sensation, or to measure in what circumstances we get a green sensation because it turns out that this is extremely complicated. Instead, we study the conditions under which two stimuli are indistinguishable. Then we do not have to decide whether two people see the same sensation in different cir cumstances, but only whether, if for one person two sensations are the same, they are also the sanhe for another. We do not have to decide whether, when one sees something green, what it feels like inside is the same as what it feels like inside someone else when he sees something green; we do not know anything about that. To illustrate the possibilities, we may use a series of four projector lamps which ave filters on them, and whose brightnesses are continuously adjustable over vide range: one has a red filter and makes a spot of red light on the screen, the next one has a green filter and makes a green spot, the third one has a blue filter, and the fourth one is a white circle with a black spot in the middle of it. Now if we turn on some red light, and next to it put some green, we see that in the area of overlap it produces a sensation which is not what we call reddish green, but a new color, yellow in this particular case. By changing the proportions of the red and the green, we can go through various shades of orange and so forth. If we have set it for a certain yellow, we can also obtain that same yellow, not by mixing these two colors but by mixing some other ones, perhaps a yellow filter with white ight, or something like that, to get the same sensation. In other words, it is possible to make various colors in more than one way by mixing the lights from various filters What we have just discovered may be expressed analytically as follows. A particular yellow, for example, can be represented by a certain symbol Y, which is the"sum""of certain amounts of red-filtered light(R)and green-filtered light(G) By using two numbers, say r and g, to describe how bright the(r)and(G)are,we can write a formula for this yellow: Y=rR+gG. (351) The question is, can we make all the diferent colors by adding together two or hree lights of different, fixed colors? Let us see what can be done in that connec tion. We certainly cannot get all the different colors by mixing only red and green, because,for instance blue never appears in such a mixture. However, by putting some blue the central reg appear to be a fairly nice white. By mixing the various colors and looking at this central region, we find that we can get a considerable range of colors in that region by changing the proportions, and so it is not impossible that all the colors can be made by mixing these three co lights. We shall discuss to what extent this is true; it is in fact essentially correct, and we shall shortly see how to define the proposition better In order to illustrate our point, we move the spots on the screen so that they all fall on top of each other, and then we try to match a particular color which appears in the annular ring made by the fourth lamp. What we once thought was white"coming from the fourth lamp now appears yellowish. We may try to match nd blue as be by a kind of trial a error, and we find that we can approach rather closely this particular shade of " cream"color. So it is not hard to believe that we can make all colors. We shall try to make yellow in a moment, but before we do that, there is one color that might be very hard to make. People who give lectures on color make all the"bright colors, but they never make brown, and it is hard to recall ever having seen brown light. As a matter of fact, this color is never used for any stage effect, one never sees a spotlight with brown light; so we think it might be impossible to make brown In order to find out whether it is possible to make brown, we point out that brown light is merely something that we are not As a matter of fact, ke it by red and yellow. To prov that we are looking at brown light, we merely increase the brightness of the annular
background against which we see the very same light, and we see that that is, in fact, what we call brown! Brown is always a dark color next to a lighter back ground. We can easily change the character of the brown. For example, if we take ut we get a reddish brown, apparently a chocolatey reddish and if we put more green into it, in proportion, we get that horrible color which all the uniforms of the Army are made of, but the light from that color is not so horrible by itself; it is of yellowish green, but seen against a light background Now we put a yellow filter in front of the fourth light and try to match that (The intensity must of course be within the range of the various lamps; we cannot match something which is too bright, because we do not have enough power in the lamp. But we can match the yellow; we use a green and red mixture, and put in a touch of blue to make it even more perfect. Perhaps we are ready to believe that, under good conditions, we can make a perfect match of any given color. Now let us discuss the laws of color mixture. In the first place we found that diferent spectral distributions can produce the same color; next, we saw that any"color can be made by adding together three special colors, red, blue, and green. The most interesting feature of color mixing is this: if we have a certain light, which we may call X, and if it appears indistinguishable from Y, to the eye (t may ectral distrib but it appears indistinguishable), call these colors"equal, " in the sense that the eye sees them as equal, and we write X= Y Here is one of the great laws of color: if two spectral distributions are indistinguish able, and we add to each one a certain light, say Z (if we write X+ Z, this means that we shine both lights on the same patch), and then we take y and add the same amount of the same other light, Z, the new mixtures are also indistinguishable X+Z=Y+Z (353) We have just matched our yellow; if we now shine pink light on the whole thing, it will still match. So adding any other light to the matched lights leaves a match In other words, we can summarize all these color phenomena by saying that once ye have a match between two colored lights, seen next to each other in the same circumstances, then this match will remain . and one light can be substituted for the other light in any other color mixing situation. In fact, it turns out, and it is very important and interesting, that this matching of the color of lights is not dependent upon the characteristics of the eye at the moment of observation: we know that if we look for a long time at a bright red surface or a bright red light, and then look at a white paper, it looks greenish, and other colors are also dis- torted by our having looked so long at the bi d if we now have a match between, say, two yellows, and we look at them and make them match, then we look at a bright red surface for a long time and then turn back to the yellow, it may not look yellow any more; i do not know what color it will look, but it will not look yellow. Nevertheless the yellows will still look matched, and so, as the eye adapts to various levels of intensity, the color match still works, with the obvious exception of when we go into the region where the intensity of the light gets so low that we have shifted from cones to rods then the color match is no longer a color match, because we are different system The second principle of color mixing of lights is this: any color at all can be made from three different colors, in our case, red, green, and blue lights. By suitably mixing the three together we can make anything at all, as we demonstrated with our two examples. Further, these laws are very interesting mathematically. For those who are interested in the mathematics of the thing, it turns out as follows Suppose that we take our three colors, which were red, green, and blue, but label them A, B, and C, and call them our primary colors. Then any color could be made by certain amounts of these three: say an amount a of color A, an amount b of color B, and an amount c of color C makes x: X=aA+ bb+ cC