SEEING' LIGHT

AND COLOR One of our most familiar sensory experiences is perhaps

the most difficult to define. The study of it requires

the application of physics, physiology and psychology

MOST of us feel quite sure that we know what is meant by the term color. We could all give some sort

of definition of it, although we would not find it easy. We are likely to wind up with a pretty vague description of some­thing having to do with light and ob­jects.

The reason for the difficulty is not far to seek. What we call color is a purely mental effect. It takes place only in our minds, and we have no words to describe anything rurely mental. It is the same with all a the other senses. A pain can­not be described except to say that it hurts, nor a sound except that it is loud or soft, high or low, complex or simple. To try to get deeper and describe what a sound is results in vagueness, again be­cause there are no words.

In the 500 most frequently used words in the English language, according to the Thorndike-Lorge word counts, there are only 15 related to vision in any way. Of these only five are concerned directly with color. These are the words "color," "light," "white," "green" and "red."

Accordingly we are considering a sub­ject which is not easy to put into words and which contains many philosophical problems too complex to attempt to pre­sent. Fortunately it is possible to illus­trate some of the examples.

Statistics tell us that in this country less than half of one per cent of the men and practically no women are seriously color blind. A few people with anatomi­cal difficulties may never see any colors at all, but this is rare. Some eight to 10 per cent of men, however, have com­paratively poor ability to distinguish colors, and among persons with good color perception not all see colors alike, even in the pure colors.

Some of the apparent mysteries of color are fundamental to the way we see everything, not just color. Vision is per­haps the most wonderful fact in our lives. Color cannot be understood with-

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by Ralph M. Evans

out some basic knowledge of its charac­teristics.

Let us first consider form vision as distinct from color vision. When we look at a natural scene we see certain objects spread out in a definite arrangement in space. For each of these objects an ap­proximate size, distance, shape and color can be given. If we analyze our thoughts carefully, we find that around each thing we call an object, there is an abrupt change in the nature of the light that reaches our eyes. The closed or partially closed outline formed by this more or less abrupt change in light is considered as meaning that this is the edge of some actual thing in the scene. We recognize this outline and other visible character­istics and give the thing a name-say we recognize it as a "tree." This recognition of the shape of objects is called form vision. In a sense it is a more important type of vision than seeing the color red or green, since complex seeing is based on form recognition. So it is worth while to spend a little time on analyzing how you decide that this is a tree.

Consider the tree drawing on the op­posite page. Here it is fairly obvious that the concept of tree develops entirely through the recognition of the outline: there is nothing else present in the pic­ture. A little thought will show, how­ever, that an outline is not sufficient to make us see a tree. We have to know what a tree is first.

If we know what the object is, only a suggestion or two is necessary to make it possible for us to see it. In fact it is not necessary that even the outline as such be shown. The shadow alone, as in the drawing showing the shadows of block letters on the opposite page, is usually enough to evoke at least some concept of the whole figure.

Seeing form, therefore, is partly a matter of outline but also, to a great extent, a matter of imagination. The same is true, to a lesser extent, of seeing

color. When we associate a color very strongly with a particular object, we tend to assume that the object has the same color over its whole area, even though it may not have, and this is the way we see it, unless there is definite evi­dence that the color is not the same all over. This is partly because we don't look carefully and partly because we be­lieve it so strongly.

IN COLOR the nearest parallel case to form vision consists in seeing an ob­

ject color in spite of conflicting evidence from reflections, transmitted colors, or sometimes even of colored illumination.

The light, our eyes and our mind are the three phases of color. Each of them enters into every ordinary situation in which color is seen. The three sciences to which these three phases relate are: physics, which deals with the nature of the light reaching the eye; physiology, which deals with the response of the eye and its transmission to the brain; and psychology, which deals with the mental image so produced. There is a fourth science known as psychophysics dealing with a combination of the three. This science concerns itself with laws that re­late the physical nature of the light to the repeatable and measurable effects produced. In practice the boundaries be­tween these phases are about as sharp as usual in related sciences.

The problem is to explain how matter and 'energy, the "real things" in the do­main of phYSics, produce different effects on people. Because of differing past ex­periences, if hvo people do not look for the same thing they do not see the same thing. The science of physics, however, states flatly that the light striking the eyes of two people looking at the same scene from the same position is the same in quality.

What is meant by "the same quality"? Suppose we consider one small beam of light striking the eye. Physics tells us

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FORM VISION is bas�d on outline and experience. The drawing at the top of this page is obviously a tree. Yet to a person who had never seen a tree it would be mean­ingless. Even outline is not essential to form vision. The

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block letters in the center of the page are not strictly outlined. Shadows are sufficient to make them appar­ent. The ink blot at bottom represents only an ink blot, but experience causes us to see many other things in it.

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CUBE seen from one corner through one eye will appear to many as a two­dimensional drawing with six sides.

TILTED forward, the same trans­parent cube will represent to more people a three-dimensional object.

ROTATED, the two-dimensional out­line will appear three-dimensional to even larger number of people.

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that if this light is passed through a glass prism it is spread out into a color spec­trum. This may be taken to mean that the beam of light is made up of a mixture of different kinds of light, which are separated in the prism by being bent in slightly different directions. The eye sees these differences as colors. Physicists identify them as differing wavelengths of energy, and measure them in terms of Angsh'om units. The visible wavelengths range from about 4,000 Angstrom units, at the blue or violet end of the spectrum, to about 7,000, which is seen as red. These numbers describe the kind of light independently of how it looks to an ob­server, and we need them as a reference point because of differences in seeing. A given wavelength, for example, may be seen under certain conditions as green and under other conditions as yellow or blue.

For the physics of color this is prac­tically the end of the story. Light from different sources and objects differs in the amount of each kind of wa.velength present, and this may all be described by a curve indicating the amounts of the various components. The only other way in which two light beams may differ as far as the eye is concerned is in the direc­tion from which they arrive. Direction, quality and quantity, therefore, are the characteristics of light before it has en­tered our eyes. The physiology of. vision begins where physics ends-at the paint where light enters the eye.

THE EYE, like a camera, has a lens at the front which bends the rays arriv­

ing at its surface so that light from each point of the object is focused at a point at the back of the eye. All the points taken together form an image of the scene. It is often assumed that in effect the mind looks at this image in the back of the eye and thus sees the scene out front, but this is a perfectly ridiculous notion. What actually happens is that the image affects some six or seven million nerve fibers that are sensitive to light. Each fiber gives off an electric current through a process which is perhaps pho­tochemical in nature. The currents travel through a nerve cable to the brain and there produce in a fantastically complex manner some sort of replica of the image. That this brain image has any direct re­lationship to what we think of as a photo­graphic image, however, is exceedingly unlikely.

There is no simple way of describing the action of the eye in terms of its re­action to various kinds of light under various circumstances. A statement of the mere physical characteristics of a scene does not describe its appearance to the eye with any degree of accuracy. The image is influenced by what the eye has seen just previously, by the effects of parts of the image upon one another and by the effect of the total image upon the

appearance of its parts. For example, a patch of color with approximately the same spectral distribution as noon sun­light is seen as blue when it is sur­rounded by yellow. vVhen the surround­ing area is purple, the central patch appears yellowish green. Again, if it is surrounded by green, the center will be seen as pink. This effect is known as si­multaneous color' contrast. The sur­rounding areas also affect the brightness as well as the hue of the central area. Such effects are not restricted to directly neighboring areas; they may occur over all parts of the field of vision.

The carryover from aIle image to an­other is illustrated by this experiment: If you stare for 15 or 20 seconds at any clearly defined small area of color and then suddenly shift your gaze to a me­dium gray card, you see an "afterimage" of the first area. The form of the after­image is the same as the original, but its colors and brightnesses are different. The lighter parts are now the darker and vice versa, and the colors are complementary -for example, blue becomes yellow and green becomes pink.

There is another phenomenon, the ex­act opposite of simultaneous contrast, known as the "spreading effect." Ordi­narily, simultaneous conh'ast should lighten the colors next to black. But if small, repeated areas in a design are sur­rounded by narrow black lines, they look darker. Conversely, small white lines around the same areas make them w hi ter. This effect is seen quite 0 hen in daily life if one looks for it. Any small area of color such as a block letter imme­diately becomes darker if a black line is drawn around it; its color will mix with whatever color is used to draw the line. The effect, which has not yet been satis­factorily explained, is purely visual.

NOW ALL these effects are what you might call properties of the eye it­

self. They occur whenever these condi­tions are present, and are effectively the same for all people. They lie in the field of physiology; they do not depend on individual experience in any way.

In looking around us, we see not patches of light but real objects in a real world. And this brings us to the psycho­logical aspects of vision.

Let's start with the flat statement that when you "see" an object in front of you at any distance beyond your reach, you actually see not what is there but what you think is there. And this is not meant in any abstract philosophical sense. Con­sider the series of three drawings at the left. The first shows a six-sided figure with radial lines from the center to the corners. This is a perfect drawing of a transparent cube as seen with one eye looking directly at one corner. Many peo­ple, however, will see it only as the two­dimensional drawing which it really is. If we tip the cube, as in the second draw-

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SHOE standing by a cube makes it possible for the viewer roughly to es­timate the apparent size of the cube.

ing, more people will tend to see it as a cube, or at least as a drawing of a cube; and if we rotate it slightly, as in the third drawing, most people will see it as a cube built up out of 12 white wires rather than as a drawing at all.

Many reasons can be given for this change in attitude. All three of the draw­ings are two-dimensional, and yet you change from seeing the drawing as a design to seeing it as a reproduced ob­ject. You see what you consider the most likely meaning of the drawing. If you believe it is intended as design, you see it t!lat way; and vice versa, if you feel that it represents a cube you change to that way of seeing it. And you see the lines as a cube not so much because they are like a cube but because a cube is the nearest thing in your experience to what you see in the diagram.

Our vision is so highly trained as to make all our other skills comparatively crude. Practically every waking mo­ment, since we first tried to reach out and touch a bright something in front of us, has been devoted in part to learning to see. The net result, by the time we reach maturity, is amazing, but few of us ever have occasion to consider it objectively. Indeed, the very fact that most of us are unconscious of this skill makes its proper­ties surprising and not easy to under­stand.

As Adelbert Ames of the Dartmouth Eye Institute puts it, "The things we see are the mind's best bet as to what is out front." They represent the assumption we should have to make if we were to take some action toward the objects.

A more dramatic illustration than the drawings is the series of three photo­graphs at the top of this page. In the first you see a familiar object, a shoe, along with a not so familiar cube. Although you do not know the exact size of the shoe, you have a fair concept of its length, and accordingly, you have an idea about the size of the cube. In the second photograph you note that this is a different shoe and a ,much smaller cube. There is nothing unusual to you about either picture because you have assumed them as usual. But when you

ANOTHER shoe provides basis for estimating another cube to be small­er than the one that appears at left.

look at the third photograph you imme­diately readjust your thinking drastical­ly. You now realize that you chose the wrong class of objects as the standard of measurement. You measured the cubes by the shoes, instead of vice versa, sim­ply because your assumption about the size of the shoes was the safest you could make. There was nothing in your expe­rience to suggest that one of the shoes might be size 52!

Seeing, therefore, is as much a matter of experience as of physics or physiology. It is always based in part on assumptions. We see what we believe we are looking at. Our mental pictures are our own. They are not necessarily shared either by others or by the objects themselves.

SUPPOSE you were suddenly con­fronted by an entirely unfamiliar ob­

ject, what would you see? The answer is quite simple: you would see it as similar to a possible something you had en­countered before. Given a particular scene, you do your best to see the objects as you really believe them to be. On the whole you do a very creditable job. Oc­caSionally, however, the situation is too much for you and the result does not jibe with the facts. Or conversely, and more importantly, as the result of your experi­ence you may see something correctly even when the physical facts of the situ­ation are misleading or confUSing.

It is interesting to review a few cases where the mind sees correctly and in­stantly things which from the physical standpoint are quite complex. If a clear colorless glass object is placed in front of a colored surface, you see the color of the background through it. The glass seems to have the color of the back­ground. If the situation is apparent enough, however, you may distinguish the clear colorless glass from the color of the light reaching your eyes. You see this even though the situation is obscure. But in order that anything be "seen" in the ordinary sense of the word, it must tie in with an experience in some way.

Once you do see a particular property of an object, however, you then proceed to make a fairly obvious but usually un-

COMPARISON of shoes shows that one is size 52. Cube was thus judged by experience rather than by reality.

conscious assumption about it. You as­sume that, in the absence of other indi­cations, these properties hold over the whole of the object. If they do not, you may get rather badly fooled, but in everyday experience it is a fairly safe assumption.

One of the most remarkable abilities the mind possesses is that of seeing two colors at the same place simultaneously. The image of a red traffic light, for ex­ample, may be seen reflected from a glossy blue surface in such a way that the patch appears both red and blue at the same time. There has been much controversy over this phenomenon from the time of Hermann von Helmholtz. Three possible explanations have been proposed. You may see simply the mix­ture of the two colors according to the simple laws followed by color. This is the result which would be predicted by the psychophysical approach. Or you. may actually see both colors simultane­ously and separately. This is the result suggested by the work of some psycholo­gists. Or you may just imagine that one color continues across under the other without looking at the color at all. This was Helmholtz' suggestion; he called it "unconscious inference." There are ex­amples which appear to support each of the three cases. Reflections are of almost constant occurrence in the world around us. It is their very familiarity which is probably the key to the whole matter. We are so accustomed to seeing the phenomenon and knowin� the facts it presents that we have simply learned to recognize the true meaning.

"The eye is blind to what the mind does not see." This old Arabian proverb could have been written by any modern worker in the field of vision. Seeing is not simply a matter of the physics of the light reaching the eye, nor of the proper­ties of the eye which it strikes. It depends on the mind and the experiences that have shaped it.

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Ralph M. Evans is head of the Color Control Depm·tment of the Eastman Kodak Company.

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