Color is an experience enjoyed by almost everyone from a very early age. As we grow, we learn to recognize and often name specific colors, such as sky blue and grass green, and yet we can never be sure that all persons derive precisely the same sensation from a given stimulus. Our paintings reflect our love of color and contain an infinite variety of hues and shades. While we enjoy the variety, it also condemns us to be forever vigilant to its appearance in different ambient conditions.
When we look at the physical causes of color, it is apparent that they may be easily defined, and therefore any personal differences in sensation are a problem of perception. Paint pigments are much more complex today than in history. They are mixed with other materials or coated to give visual shifts or other active color effects. To gain an appreciation of the color theory and the problems of color matching, it is essential to consider the physics of sight in some detail. Before continuing, though, some background knowledge is required.
The sensation of color is achieved when electromagnetic waves between the limits of approximately 380 and 760 nm are incident upon the eye. These particular electromagnetic waves are commonly referred to as light waves. The shorter wavelengths produce the sensation of violet, and as the wavelength increases, the sensation changes to blue, green, yellow, orange, and red successively. Thus whenever a visual sensation is received, which is red, for example, it means that the light incident upon the eye has a predominance of long waves.
The Nature of Color
Light that appears red contains a predominance of long waves begs the question. In the real world, the objects around us appear to have a variety of colors, and yet the sources of light energy, such as daylight, frequently appear to have little or no color. What is it, then, that gives rise to the appearance of color in these objects? The answer lies in the demonstrations of Newton that "white" light contains all the wavelengths within the visible spectrum. When light is incident upon objects in the real world, they attenuate it.
For an observer to perceive an object, they must receive a complex pattern of light rays that have emanated from that object and contain the information required. Yet the majority of objects do not emit light in themselves, as can be readily shown by removing any light energy sources. Therefore, it is apparent that the light necessary for perception generally comes from an outside source of energy, and how this is attenuated by the object that defines the perception.
When electromagnetic waves are incident upon any object, three possibilities may be transmitted, reflected, absorbed, or combined. An object that appears red when viewed in daylight has generally absorbed a predominance of the shorter wavelengths of the visible spectrum and reflected or transmitted the remaining light into the eye of the observer. The absorbed light energy is then usually re-emitted as heat. Thus it is apparent that the perception of color in objects is dependent upon how that object attenuates the light waves incident upon it. The relative absorption of the various wavelengths determines its color and the total absorption of the light of that color. How the light is reflected or transmitted is essential in determining gloss, texture, and transparency.
It is necessary to define some of the terminology used in color science to discuss the perception of color and its measurement. It is generally agreed that one can define an isolated color stimulus by three attributes. Hue is the phenomenon that has given rise to color names and specifies whether a color appears red, orange or blue, etc. Saturation is a phenomenon that denotes to what degree the color is pure or is mixed with white, gray, or black and hence all other wavelengths of light. Lightness is a phenomenon that specifies the extent to which the color appears to emit light and will range from black to white (or colorless for transmitting samples) since black denotes no light emission and a perfect white reflects all light incident upon it. (The terms luminosity or brightness are used for self-luminous objects such as light sources.)
The three attributes of hue, saturation, and lightness (or luminosity) are subjective and cannot be readily determined by measurement. Furthermore, they change with the state of adaptation of the observer, as will be discussed later. Since the measurement of color is an essential observer-independent control procedure, it is necessary to have equivalent terms which are objective, and these are dominant wavelength, purity, and luminance, respectively. For many purposes, the difference between subjective and objective attributes is unimportant. Since the subjective terms are by far the more common and widely understood (although perhaps inaccurately), these will be used in this article except where necessity demands.
One other term worthy of mention at this time is chromaticity. This is again an objective term and relates to the effect of combining hue and saturation.
The subject of color perception is an area of science that is still far from completely understood. Apart from the lack of complete understanding of the mechanics of the visual process, there are also the problems of adaptation and contrast effects. For many purposes, the mechanism of the visual process or the influence of adaptation effects may be ignored. Then the color measurement will offer a perfect definition of color, but as we shall see, this is all too frequently impossible.
For many years, the theory of color vision, most widely accepted by color scientists, was first postulated by Young (1807) and later developed by Helmholtz (1852, 1866). It was suggested that in the retina of the eye, there exist three receptors, one principally responsive to short wavelengths of the visible electromagnetic spectrum, one to middle wavelengths, and one to long wavelengths. These receptors, known as cones, are connected to the brain's visual cortex by a series of neural networks, and together these produce color perception.
This theory received strong supporting evidence over many years, mainly because it shows that all colors may be matched with suitable mixtures of three widely separated monochromatic radiations. There is little doubt today that it is the accepted hypothesis of the mechanism of the eye's response to color despite the fact that no cone pigments have yet been isolated. Nevertheless, it has certain shortcomings as a complete theory of color vision, partly in classifying those with defective color vision. Still, perhaps more interesting is the peculiarity of yellow.
Suitable mixtures of red and green light may produce yellow light; at the same time, it is a color with no apparent redness or greenness. This property type is shared by only three other colors red, green, and blue. These four colors are known as the psychological primaries and have unitary hues; each is perceived as being totally independent of the other three primaries. Based on this aspect of color vision, Hering (1878) developed an opponent response theory. He postulated the existence of three pairs of response processes, white-black, yellow-blue, and red-green, which take place in the visual mechanism, suggesting the existence of the psychological primaries.
As a complete theory of color vision, the Hering hypothesis is invalid because of the color-matching evidence described earlier. Nevertheless, it can be developed into a form that thoroughly explains the phenomenon of defective color vision. Any hypothesis of color vision that is finally proved correct will likely contain both the Young-Helmholtz and Hering concepts within it and define some complex neural connections bridging the gap between them. One such theory, postulated by Muller (1930), has attracted much interest.
Having discussed the mechanics of color vision, let's look at the influences of extraneous factors on the perception of color. To do this, it is necessary to describe one of the most critical single phenomena occurring in color perception: metamerism.
Metamerism may be defined as the property of the eye and brain to receive the same color sensation from two objects with different spectral energy distributions. In other words, even though at each wavelength in the visible spectrum, the visible light energy emitted by the two stimuli may differ, both appear the same to the observer. Two objects matching in this way are known as metamers. The reason this occurs is that the eye has three receptors that are color sensitive. Extending the thought means that the sole requirement for two colors to match is that the total light energy, with respect to the sensitivity of the receptors, should be the same for both objects.
Metamerism may be necessary for a variety of ways. There are three conditions to consider:
- observer metamerism;
- illuminant metamerism; and
- object metamerism.
Observer metamerism occurs because of individual differences in sensitivity to color. We know that minorities of the population have defective color vision (primarily men), and even among the remaining, differences in sensitivity occur. Thus, if two colors are a metameric match for one person, it is likely that they may not match precisely for anyone else. Alternatively, they may match two individuals with different sensitivities but not a third. However, while individual sensitivity differences may be pretty significant unless the degree of metamerism is very high, we can ignore it.
With illuminant metamerism, we consider the effects of varying the spectral power distribution of the illumination in a unique way. Imagine a standard observer and two objects with the same spectral reflectance. If the objects match under illumination with different spectral power distributions, we have an example of illuminant metamerism. If the spectral power distribution of either illuminant is then changed such that the two are still not equal, the match will likely break down despite the equivalence of the spectral reflectance of the objects. Once again, for most practical situations, this metamerism aspect is not crucial. While we are concerned with changes in illumination, we cannot use the illumination to obtain a color match. For our purposes, object metamerism is the most important.
Object metamerism is concerned with differences between the spectral reflectance of the objects. Consider an example where we have two objects with different spectral reflectances, yet for a given observer under controlled illumination, they match for color. The match will likely break down if we change the observer or the illumination.
Here, the most significant difference is the spectral reflectance at the long wavelengths. Later we shall see the relative light output at the longer wavelengths of tungsten light is very high compared to daylight's. Furthermore, the sensitivity of the eye at long wavelengths is relatively low, and the combination of these effects is sufficient to ensure an approximate match under daylight, despite the significant difference in spectral reflectance. As an aside, I own a red car that I often lose at night in parking garages since the bright red of the car in sunlight transforms to a muted brown under sodium lighting. I must examine the car's contents to be convinced it is mine.
A great deal of emphasis has been placed upon metamerism, and the reason for this is its fundamental importance to color science, matching, and reproduction. It is the foundation stone of colorimetry, which frequently arises when undertaking a color match; without it, photographic, printing, and television color reproduction would be impossible. For these reasons, the concept of metamerism must be understood. The importance of ensuring that standard viewing conditions are used for color matching and assessing applied coatings becomes apparent, as do the significant pitfalls to look for in color matching.
Viewing Colors in the Real World
Let's briefly look at the problems of color perception in the real world. To a large extent, the discussion thus far has been for single isolated stimuli, or in the case of metameric colors, a pair of isolated stimuli that match for color. Generally, however, we do not see color in this way but are surrounded by other stimuli, each influencing our perception of that color. This is shown in the following illustrations. They may not work as well on a backlit monitor screen (depending upon its calibration) as they do on a printed page, but the metameric effects are still visible.
Adaptation is an important phenomenon to consider in any discussion on the perception of color. Light and dark adaptation we are all familiar with; a car headlamp that can be dazzling at night appears quite dim during daylight, suggesting that the eye has the capability of increasing and reducing its sensitivity to suit the ambient conditions. The fact that a bright light can be dazzling for quite a few minutes at night when the eye is dark-adapted demonstrates that these adaptation effects are not instantaneous.
Brightness adaptation is only one aspect of such sensitivity changes. However, color adaptation also occurs and can be seen as quickly. A yellow patch seen with an eye adapted to green light will appear red; it seems that the adaptation to green light has decreased the eye's sensitivity to green, thereby allowing the red content of yellow to predominate. Such effects may be total in that the whole eye is adapted or local, as seen in illustration 1. When you looked away, you saw a temporary reduction in sensitivity of only part of the retina, causing an after-image of a complementary color to the area viewed.
Color adaptation produces one significant visual phenomenon known as color constancy, with which we are all familiar, perhaps without realizing it. Objects seen in a room illuminated by daylight do not dramatically change their color appearance at night when illuminated by artificial light. Nevertheless, the spectral emission from those objects will be quite different in both situations, and only rarely could the effect be explained by illuminant metamerism. Under special conditions, it is possible to demonstrate that this color constancy can be complete so far that neutral colors can appear neutral under a wide range of colored lights. Still, under normal conditions, the adaptation is rarely complete. Fluorescent sources further complicate it because most have strong line spectra and can make some of the color distortions appear very large. When such a situation exists, a lamp is said to have poor color rendering.
Simultaneous contrast is another significant phenomenon in that the appearance of a color is highly dependent upon any surrounding colors. A color that is moderately bright when seen against a gray background can be made to appear gray if viewed against a background of the color of the same hue but very much brighter. This is very extreme, but under many practical conditions, quite significant changes in appearance can be observed by moving color from one part of a room to another to change the surrounding conditions. Unlike adaptation, contrast effects appear to be instantaneous.
Suppose we return to the single isolated color stimulus with no other object in the field of view. In that case, the effects of simultaneous and successive contrast when viewing this are insignificant, at least if a short time has elapsed. Pre-adaptation effects may be necessary for some minutes, but after that, the eye can be assumed to be adapted to the stimulus. Under these conditions, all color-matching data used for most color-measuring systems has been obtained. It has been shown that a metameric color match produced under one set of conditions does not break down when the conditions of adaptation change. The implication is that color measurement cannot specify the change in the appearance of that color due to adaptation effects; it can only specify the physical stimulus itself. For this reason, there is no complete correlation between hue, saturation, lightness and dominant wavelength, purity, and luminance factor.
With the light effects of pigments commonly used in coatings becoming more complex, utilizing pigments that change color with changing light wave incidence, metamerism becomes very important. It is hoped that this brief insight into a few aspects of perception will help to emphasize the importance of being very careful when describing the appearance characteristics of colors.