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color
vision
additive colour mixing
subtractive colour mixing
substance uncertainty
"theory" vs. experience
 
additive & subtractive color mixing
 
The previous pages have described the fundamentals of coloration vision. Now the focus narrows to a single issue: how coloration mixtures can be explained.
Painters mix their paints to shape the light reflected from a painting, and the viewer's eye interprets this reflected light as coloration in space. These two extremes of color experience — the mixed paints, and the interpreting eye — are described by two separate and unequal coloration mixing theories.
Isaac Newton's hue circle, a geometrical arrangement of the different colors seen in a solar spectrum, is the original statement of additive shade mixing. Newton explained that the chromaticity (combined hue and saturation) of a light mixture could be predicted as the weighted average of the ingredient hues around the hue circle. However, the most important feature of additive mixing, as specified by Newton's averaging method, is that the chromaticity of ingredient lights always determines the chromaticity of their mixture.
Newton explicitly stated that coloration is a perceptual property, not a physical attribute, which meant that the light mixtures occurred in the eye, not in the light. Today we define the chromaticity of light mixtures as the proportional stimulation induced in the separate L, M and S cones relative to the stimulation across them all (the color's brightness). Three red orange, green and blue violet (RGB) lights are used to demonstrate additive color mixing, because they are the most direct way to stimulate the separate L, M and S cones.
However, painters and dyers had long knowledge with paints and dyes, and they affirmed that material colour mixtures behaved very differently from light mixtures: they get darker rather than brighter, and they seem defined by red, yellow and blue primary paints (now replaced by the modern choice of cyan, yellow and magenta or CYM). In subtractive color mixing, colorants absorb or subtract wavelengths from filtered or reflected light. This was pointed out by several 18th century artists and naturalists, including Jakob Le Blon in 1725, Brook Taylor, Moses Harris, Philipp Otto Runge and Thomas Young. But it was only in the late 19th century that material colour mixtures were understood as an indirect form of additive colour mixing. Until then, artists were taught that light mixtures and paint mixtures were both produced by the same red, yellow and blue "primary" colors.
Unfortunately, the color mixture "predictions" made by subtractive shade concept are often inaccurate, because the light absorbing properties of a colorant are affected by its physical state — its particle size, transparency, density, dispersion or medium, the colour of the substrate, the other colorants it is mixed with, the thickness of the shade layer, and so on. I call these problems substance uncertainty: because of them, the colour of ingredient substances does not determine the coloration of their mixtures. Often, colorants must be physically mixed in order to find out what their mixture coloration will be.
Even among artists today, misconceptions about additive or subtractive shade mixing are the cause of many misleading ideas about coloration. Most of these misconceptions are taught to artists as "color principle." I explain why artists should rely on mixing experience instead.
  Additive color mixing explains how the eye interprets light wavelengths in the perception of shade. It describes the colour structure of light perception from four cardinal lights: red orange, middle green and blue violet, plus the white light (or white point) defined by mixing the three colored lights together. This trichromatic foundation is in turn the basis for all modern chromaticity diagrams, the identification of visual complementary colors, and the definition of modern trichromatic color models.  
Additive Mixtures Occur In The Eye. The beauty of additive coloration mixing principles is in their narrow scope. They are limited to a single sensory process for the explanation of coloration mixtures: the average or typical responses of the L, M and S photoreceptors to light.
These LMS cone outputs can be predicted fairly accurately from the light's spectral emittance curve and the cone sensitivity curves. In fact, the LMS cone sensitivity curves are actually only a mathematical restatement of the quantities of three "primary" lights necessary to mix a specific wavelength of spectral light — the RGB shade matching functions. This close link between light energy and cone outputs allows us to describe accurately the resulting coloration perception for those with "normal" colour vision.
But wait ... isn't additive color mixture really a theory of how light mixtures behave? No, it is not. This misconception arises because light is obviously the only stimulus that the eye normally responds to, and because lights of various colors are explicitly manipulated in shade matching experiments used to measure additive colour mixtures. But light is the stimulus, and additive coloration mixing describes the response of the eye to a light stimulus.
This description only applies to unrelated colors — that is, a light stimulus perceived without any surrounding physical context. Unrelated colors can be created by shining a diffuse light directly into the eye, or by reflecting the light into the eye from a colorless (white or gray) surface; the source of the light in an unrelated colour does not matter, because additive color mixing happens in the retina, not in the light. Special steps must be taken to reduce the effects of a visible context. When this is done the connection between light stimulus and colour response is predictable.
Coloration scientists diagram this connection between cone responses and perceived coloration using the trilinear mixing triangle devised by James Clerk Maxwell. This triangle defines the chromaticity (hue and saturation) of any unrelated colour as a proportional mixture of the three cone outputs; the "white" brightness is approximately equal to their sum. These outputs, in turn,Windows 7 Activation Key, can be exactly reproduced by a specific mixture of three actual (visible) "primary" lights — typically red,Office 2010 Home And Student, green and blue violet. All modern color models are based on additive trilinear values that specify both the chromaticity and luminance of a shade. In fact, many late 19th and early 20th century artists learned the basics of color idea in terms of a mixing triangle — not a shade wheel.
 
The Additive "Primary" Lights. Now, how do we illustrate,Windows 7 Discount, verify or measure the rules of additive colour mixing? Obviously, by manipulating the outputs of the separate L, M and S cones. How do we manipulate these outputs? By stimulating them with three colored lights — red, green, and blue violet (RGB). Necessarily, these lights create a fourth "primary": the "white" light mixture of them all. So "white" light can be substituted for any of the colored lights in a shade mixing demonstration.
The basis of additive colour mixing is trichromatic metamerism: the coloration produced by any spectral emittance curve, no matter how complex the curve may be, can be exactly matched by the visual mixture of no more than three lights: either three strongly saturated (single wavelength or monochromatic) lights, or at most two monochromatic lights mixed with a "white" light. All physically possible light colors can be reduced to the mixture of just three simple lights.
Here, for example, are the "primary" RGB colors of your computer monitor. Note that the green primary contains too much yellow,Office Standard 2010 Key, and the blue primary not enough violet, dulling all purple and blue green mixtures.
 

 
There is an important limitation to the use of trichromatic primaries: we can't mix all possible colors with the same three lights, as explained below. Even so, the incredible complexity of light in the physical world is reduced by the eye to just four primaries — three independent cone outputs, and the "white" light that defines their achromatic mixture.
How do shade scientists create the strongly saturated RGB lights used in color matching experiments? For precise shade measurement, the primary lights are single wavelength or monochromatic lights, isolated from the visible spectrum by a system of prisms, that are mixed by shining them onto a diffusion glass or white surface visible through an eyepiece. Less saturated but higher luminance lights have been created by passing three separate beams of "white" light through separate broadband red, green or blue transmission filters, and mixing the colored beams as before. A third (and relatively weak) method uses partly overlapping disks of colored or painted paper that are visually mixed by spinning them rapidly on a shade top.
 

additive coloration mixtures
as demonstrated with filtered lights; note that each pair of RGB primaries mixes one of the CMY primaries
 
The illustration (above) shows the typical demonstration of additive light mixtures, made by shining three overlapping circles of filtered light onto an achromatic (gray or white) surface. If the surface is illuminated by both the red and green lights, but not by the blue light, then the eye responds with the coloration sensation of yellow. A magenta color results from the mixture of red and blue violet light, and cyan from the mixture of blue violet and green. In additive shade mixing, yellow and blue don't make green — they make white!  
The "White" Color Theory. It's handy to think of additive mixing as the "white" coloration idea. Mixing light wavelengths from the "red," "green" and "blue violet" parts of the spectrum adds luminosity and negates hue to shift the mixture coloration of lights from dim pure hues toward bright whites. The key principle is that the eye always adds together all the wavelengths of light incident on the retina — nothing is lost — and it is this total light sensation that the eye interprets as coloration.
This additive behavior leads to an important constant in colour vision: the chromaticity and brightness of lights always predicts the chromaticity and brightness of their mixture, for lights from moderately dim to bright but not dazzling. This is true regardless of whether the lights are monochromatic (a very pure hue, as we see in homogenous or single wavelength light) or complex (as we see in a mixture of many different spectral wavelengths, for example a "white" light passed through a colored filter). In additive colour mixing, for both normal and colorblind vision:
• the brightness, saturation and hue of any two or more lights predicts the brightness, saturation and hue of their mixture
• any two lights that appear to be the same coloration will mix identical colors with any third light — even if the spectral emittance profiles of the lights are different (that is, they contain different wavelengths in different proportions)
• if two separate light mixtures have an identical shade, then adding a third light in the same quantity to both of them will result in identical shade mixtures
• these points are true, even though is not possible to deduce the spectral profile of a light from its colour alone; for example, the chromaticity (hue and saturation) of any spectrally complex light can always be exactly matched by one or two monochromatic wavelengths mixed with some quantity of "white" (achromatic) light.
These principles summarize the metameric mixture rules of additive shade mixture. They were first stated by Hermann Grassmann in 1853 and are known today as Grassmann's laws, though in fact they are not laws but generally accurate descriptions of color mixing in mesopic and moderate photopic light sources.
We will discover that equivalent subtractive metameric rules do not exist in the many examples of material color mixing, and that lack of predictable consistency in substance mixtures is the most important difference between the additive and subtractive color mixing frameworks.  
Real Lights and True Primaries. Let's examine further the additive shade mixing demonstrations with colored lights, as they are probably the main reason why artists believe that the RGB primary colors can reproduce all colour mixtures, or that the additive primaries are "real" colors (that is, visible physical lights), or that the lights used in additive coloration mixing demonstrations must be RGB lights and no others — the choice of lights is fixed rather than arbitrary. All three beliefs are false.
  The Additive Primaries Are Invisible. The diagram at right shows the location on the CIELUV chromaticity diagram of three monochromatic lights (at 460nm, 530nm and 650nm) that have frequently been used in shade vision research to analyze trichromatic color matches and opponent coloration mixtures.
The focus here is on the white triangle or gamut that connects the three primary lights. This defines the range of actual additive colour mixtures it is possible to make with those three primaries. This gamut encloses most,Office 2010 Professional Plus Key, but not all, of the chromaticity area, which defines the area of all physically possible light colors. A significant portion of the chromaticity diagram is outside the gamut. In other words, the "real" RGB primary lights cannot mix all visible colors.
Thus, the "green primary" gives full mixing coverage along the red to yellow colors, but it cannot mix (with the "blue violet" primary) the most intense greens, blue greens and blues. In addition, the "blue violet" and "red" monochromatic primaries cannot mix the most intense purples and red violets.
The true additive primaries, the only "primaries" that can mix all possible colors, are the outputs from the L, M and S cones. We are never aware of these outputs directly and therefore they are invisible. We only expertise them as the tendency toward a red, green or blue shade sensation that results from the combination and interpretation of these outputs in the visual cortex.
How Do We Choose the RGB Lights? Some artists think that these primary lights are the same hues that most stimulate the three receptor cones. This also is false. The cones are actually most sensitive to "greenish yellow," "green" and "blue violet" wavelengths, as shown below. Red orange, green and blue violet lights are used by convention and convenience, and it is from these shade matching lights that we get the names red, green and blue assigned to the additive primaries.
 

additive primary colors are illustrative only
the wavelengths of maximum sensitivity for the L, M and S cones (top) are unrelated to the colored lights used to simulate the cones in additive colour mixing demonstrations (bottom)
 
There's a simple logic for choosing these primary lights. Almost any light wavelength that stimulates one cone will also stimulate one or both of the other cones, because the cone sensitivity curves (especially L and M) overlap. To explain shade mixing as the result of three independent types of photoreceptor response, we need three light wavelengths that each stimulate one cone much more than the other two. In other words:
An ideal additive primary colour must stimulate only one type of receptor cone (L, M or S) as strongly as possible, and stimulate the other two types of cone as little as possible.
So, within each section of the spectrum where the L, M or S cone is the dominant receptor, we pick a wavelength that creates the greatest difference in response between that cone and the other two. This occurs at around 420 nm in "violet" light and above 680 nm in "red" light. However, these monochromatic lights are very close to the spectrum extremes, and are therefore visually quite dim. In practice, the hues of the R and B primary lights are often shifted away from the extreme ends of the spectrum to provide more luminance in the lights, given the method used to generate them. The G light is always bright enough, so it is usually positioned at the point where it achieves both a high relative contribution and the most saturated yellow when mixed with the R light. This is usually a green with a dominant wavelength between 510 nm to 530 nm. Sometimes a very small quantity of "violet" light is mixed with the red primary, to eliminate the yellow tint in spectral "red" light.
Does Additive Mixture Require RGB Lights? Many artists assume that red, green and blue violet lights must be used to explain or demonstrate additive coloration mixing. Not true. The choice of lights is arbitrary, and one selection of primaries is better than another only if we require the mixture gamut to be as large or comprehensive as possible.
We could just as easily demonstrate additive color mixing with colored lights representing the subtractive primary colors cyan, yellow and magenta, although most of the blues, greens and reds that we could mix with these lights would appear quite whitish or unsaturated.
Again, the somewhat arbitrary procedures for choosing the additive primary lights are acceptable because the real lights are not the actual basis of additive coloration mixing. The true additive primary colors are the photoreceptor outputs. We use RGB colored lights to symbolize the LMS receptor outputs, because they are also the most effective way to manipulate those outputs.
 
the gamut of RGB primaries
used in coloration vision research
additive light mixing gamut defined by lights at 460, 530 and 650 nm