Metamerism

Metamerism

Metamerism (mə ta' mə ri' zəm) is the phenomenon which occurs due to the ability of human eyes to see two colors as being the same even when the spectral power distribution of the two colors is different. In other words, even though the spectral components of two lights are obviously different, to our eyes they may appear as the same color. These two matching colors are called metamers.

A spectral distribution defines the light from any physical stimulus, whether the light is reflected, emmitted or transmitted. It describes the proportion of total light of a color sample at every visible wavelength.

This means that the same perceived color can be produced by different combinations of various colors.

Perhaps the easiest way to understand this principal is to use the 1931 CIE xyY gamut chromicity chart. Any color that lies along the line between two colored source lights can be made by adjusting the ratio of those two lights. In the illustration below, if we want to make the color found where the two lines intersect, we can make it two ways. We can mix the colors found at points A and B in the correct proportion. Or we can mix the colors found at points C and D in the correct proportion. Both will produce a color that we will perceive as the same, even though the spectral power distribution of the two colors is vastly different.

You can use different source colors to
effectively create the same color.

Working with Three Colors

This effect also extends to three or more colors. In the following diagram, you can see that any color within the triangle can be created by using just the three source colors of red, green and blue. This, by the way, is how your TV, computer monitor and even magazines produce color. If we plot the values of the primary colors of red, green and blue on the xyY gamut chart, all colors which are found inside the triangle can be produced by mixing the three primary colors in different amounts.

Metamerism applies to combinations
of three colors as well.

As you can see, the white spot appears in the chart near the middle. This means, and it is true, that the white you are seeing in the background of this page (if you are reading this on a computer screen) is being generated by a combination of three red, green and blue dots very close together. You can test this out by using a very strong magnifying glass or an eye loop.

Metamerism is a good thing. Since our eyes can be fooled to see almost any color from only three light sources, is was possible to create affordable color televisions. The current color displays are only required to control the brightness of three colors at each pixel. Without metamerism, color TV's would have needed to control the brightness at a large number of different colors. If that were the case, color television of any significant resolution may not have been invented, yet, because of how difficult it would be to control the complete color spectrum distribution at each pixel. Even if that control were possible, it would be impossible to send color TV signals via radiowaves since each channel would need far more bandwidth because so much more color information would have been needed.

How Is This So?

The fact that we perceive different colors as being the same is not all that surprising once you realize that we only have four different light sensitive cells in our eyes - rods, which only see in black and white, and three types of color sensing sells, Long (L), Medium (M) and Short (S). Since there are only three color sensitive cells in our eyes, we only have to illuminate those three cells correctly to simulate any color.

The process of thinking about color from the stimulus effects of color on the eye is called Tristimulus. Tri, or three, and stimulus, from the process of stimulating the three cone cells.

Metameric Failure

There are certain situations where metameric matches fail in the case of printing and coloring objects. When using glossy printing paper, the colors can look fine under some lighting conditions but they can look horrible in others. This is due to the properties of the paper interacting with the light sources. For example, the reflectance of the glossy material may vary or the light source may not emmit some colors as strongly as others. This is common in mercury fluorecscent bulbs where the mercury emmissions can be pronounced in the resulting light.