Metamerism and Metameric Pairs

Metamerism -- with two objects matching in color under some lighting conditions but not others -- is easy to see, but hard to understand. It's best known as the jacket and pants problem, and you may have run into the issue in precisely that way.

If you look around in most department stores, you will often find a rack of jackets and a rack of similar pants (or skirts) that you can put together to make an outfit. Typically, the two pieces will come in a small assortment of colors, for you to mix and match. You can often get both pieces in the same color, and if you put them next to each other in the store, there's no question that they match exactly. When you get them home however, you may find that they don't match anymore.

Look into the issue more closely, and you'll find that the objects match in color under whatever light source the store offers -- typically fluorescent lights -- but they don't match under incandescent light or daylight. The phenomenon is metamerism, and the two colors are called a metameric pair.

The fundamental reason for metamerism is that color is a sensation rather than a property of an object. As a result, the cones in your eyes can register the same sensation from an essentially infinite variety of combinations of different light frequencies.

Probably the easiest way to approach this idea is to consider a similar, but much simpler, example. Rest your hand on a flat surface, and then rest a 10-pound weight on your hand. The sensation of pressure against your hand is identical to the sensation you'll feel if you use stack two 5-pound weights with the same cross section on your hand, or a 4-pound weight and a 6-pound weight, or five 2-pound weights, or an infinite number of other combinations.

With color, the situation is more complicated, but the concept is the same. The human eye detects color through light-sensitive cells called cones. As mentioned in the associated sidebar "Lighting Affects Color" , there are actually three different types of cones, named red, green, and blue for the portion of the spectrum each type is best at absorbing. Yet just these three types of cones let you see all the colors you can see -- roughly 7 million colors. (That's somewhat less than half the number of colors computers offer for "true color." Computer systems need the much larger number of colors, because the eye is more sensitive to changes in some parts of the spectrum than in others, but computers distribute the steps evenly across the spectrum.)

In essence, when you see a color, it's because the three kinds of cones contribute to a sensation that your brain recognizes as a particular color. When you see a frequency of yellow light, say, (as measured by its wavelength), your red, green and blue cones absorb the light in particular proportions. When you see a combination of red and green (as measured by the wavelengths of the light once again), your red, green, and blue cones absorb light in the same proportion, and your visual system once again senses yellow. In fact, there are an infinite number of combinations of light frequencies that will register the same way on your visual system to produce the same sensation -- which is to say, the same color.

All of which brings us back to the jacket and pants problem.

If the jacket and pants you look at in the store are made from the same bolt of cloth -- which means they are part of the same dye lot -- they will almost certainly both reflect each wavelength of light the same way. Regardless of the light source, then, they will always reflect the same combination of wavelengths, and you'll see them as being the same color regardless of the light source.

If they're made from cloth that came from different dye lots, however, odds are that they won't reflect each wavelength of light the same way. The mix of wavelengths they reflect from one light source -- the light source in the store -- will produce the same color, because the dyes were mixed to look the same under a light source that matches the light source in the store. But because different dye lots usually reflect and absorb different wavelengths differently, it's highly unlikely that the mix of wavelengths they reflect from a different light source will also look the same. Move to a different light source, in short, and the colors won't match.

As you might guess, you can run across problems with metameric pairs in all sorts of situations -- a set of furniture with fabric from different dye lots, wall paper produced in different runs, paint mixed to match a color from a different brand of paint, and so on.

More important, metamerism is something you need to keep very much in mind when dealing with color on a computer system. When you're working hard to match colors of a scanned photo to printed output for example, you need to think in terms of matching the colors in a given light. What matches at home under incandescent light may not match at the office under fluorescent light.

Copyright © 2004 Ziff Davis Media Inc. All Rights Reserved. Originally appearing in ExtremeTech.