What the Heck is Metamerism?
If you've gotten serious about printing your images, especially with pigment based inks, you've probably come across the term "metamerism." You probably also know that ink manufacturers do their best to minimize the problem, yet everyone still talks about it. But do you really understand what the term even means?
To clarify this commonly used but rarely understood term, it's necessary to first look at how we perceive color. The electromagnetic spectrum covers an extremely broad range with the visible light portion sitting in the middle. On one side of the visible spectrum lies infrared, microwaves and radio waves, and on the other side you'll find ultraviolet, x-rays and gamma rays. It's all the same stuff, but we can see but a small slice of the electromagnetic spectrum. Within this slice, light of differing wavelengths appears to us as differing colors. This is what makes the colors of the rainbow as white light is refracted into its varying constituents.
When light shines on an object, it generally absorbs some wavelengths and reflects others. The wavelengths reflected give an object its perceived color. An object that absorbs all or most of the light that falls on it will appear black since very little gets reflected. Because of this, black objects tend to heat up in the sun more quickly than objects that reflect all or part of this energy rather than absorbing everything.
A theoretically idealized object that reflects just a single wavelength would appear to us as a pure color from somewhere within the spectrum of the rainbow. But real world objects are more fickle. From across the visible spectrum, they generally absorb varying wavelengths to varying degrees, neither entirely absorbing nor entirely reflecting any. The color we end up seeing is based on the weighted average of all the light that object reflects. Think of this somewhat as the result of mixing paints — ten percent of one color plus thirty percent of another plus just a dash of something else. You get the idea. Sometimes when mixing paint you'll end up with a brown muddy mess, just as the real world does which is, in fact, why mud appears "brown" to us in the first place.
If we could actually see the entire distribution spectrum of wavelength reflectivity when we looked around us, it would be a confusing world indeed with an overwhelming amount of information to process. Thankfully, we see colors as averages derived from the spectrum of colors reflected by real world objects. But since we do, we lose a lot of information in the process. Two objects can look to us as if they have the same color when in reality those average colors come from entirely different spectrums of reflected light. One object might reflect nothing but pure green light while another reflects no actual green but does reflect both blue and yellow equally. Adding those blue and yellow reflections together appears green to us, just as green as does the object that reflects nothing but pure green light. Generally the percentage of each wavelength constituent creates a more complex response curve, but the concept is the same.
Just as real world objects tend not to reflect but a single, pure wavelength of some color, real world light sources tend to have apparent colors that are in fact composed of a more complex distribution of different light intensities at various wavelengths. Natural daylight when the sun is directly overhead appears to us not to impart any color tint since this is the light we are used to and use as a standard. But in fact, it has a complex spectral distribution of various wavelengths present in various strengths that together appears to us as midday daylight.
Sunlight at midday doesn't appear the same color to us as it does right at sunrise or sunset. This is what drives nature photographers like me to get up before dawn so they can be where they want to be as the "magic hour" of "golden light" commences. Incandescent lights indoors aren't the same color as halogen or fluorescent lighting which leads to the fascinating topic of white balance and the frustration of getting the color of indoor shots to look right. As such, the color of light can be both a blessing and a curse.
All of this might be relegated to nothing but fodder for dinner party trivia to impress your friends if it weren't for metamerism, or more correctly "metameric failure."
Two different spectral distributions that appear to us the same color are known as "metamers" of each other. Whether these spectral distributions come from two light sources or are created from light reflecting off two seemingly same colored objects, the two spectra are said to be metameric so long as they look the same to us but are created by averaging out different spectral curves. Metamerism is simply a fact of existence, a description of one aspect of how we see color. It's neither good nor bad. It just is. That's the way our vision works.
Metamerism isn't the problem, metameric failure is. When we judge two colors to be the same as each other under one light source but different than each other under another light source, metameric failure is said to occur. Let's look at why this happens sometimes.
Suppose we shine a light with a completely uniform spectral pattern on something. The light reflected will be based solely on the spectral reflectivity of that object. But if the light we use isn't itself spectrally uniform, the light reflected will be a combination of the spectral pattern of the light together with that of the object. The only wavelengths even possible to be reflected are those that are present in the light source, and only to the degree they are present. Wavelengths not present can't possibly be reflected, and you can't get reflected any more of those wavelengths that are present than are contained within the light source. If we shine a red light on a "green" object we won't see much. Since that object (on average) reflects green light, it will appear black, or nearly black under a red light.
Suppose we have two objects that appear to have the same color under some arbitrary light source. The two objects may actually share the same spectral distribution curve, but chances are they merely average out to the same color. Now suppose we replace that light source with a different one that appears to have the same color as the first but in fact differs in the composition of its actual wavelength constituents. If this new light source shares wavelengths more strongly with one of our objects than with the other, the relative color of each will shift in different ways. One will now appear tinted slightly differently compared to the other. If the apparent color of the light source itself were to change, we expect our objects to shift colors, but we would expect them to do so equally. But when the apparent color of the light source appears to stay the same yet the colors of the objects appear now to differ, something seems amiss. This is metameric failure. Our objects look the same color under one light source but different colors under another, with no apparent explanation.
As I say, metamerism isn't always a bad thing. Take a close look at the paint color on most new cars for example. Car manufacturers have learned to take advantage of metamerism to create paint colors that seem to shift slightly under different light sources. These paint colors attract our eye and seem hard to nail down, and apparently sell more cars. But in most cases, metameric shifts can lead to frustration and disappointment. Someone trying to buy a sweater that matches the color of the pants they have on might carefully compare the two before buying the sweater, only to get them home and find out they maddeningly now look quite different.
In terms of printing photographic images, color shifts due to metamerism can cause images that look right in one light to look strange in another. For example, while skin color does vary quite widely from one individual to the next, tints of random colors can still look very wrong, especially if only present in the highlights perhaps.
Pigment inks suffer from metameric shift more often than dye based inks, but even they have indeed been getting much better over the years. Improvements generally come from ink manufacturers formulating inks with fewer "spikes" in their spectral response curves. Chemistry to the rescue, and all that. I remember trying to get useable prints from my early Epson 2000P, the first commonly used, affordable pigment printer — much more painful than with my current Epson 4900. By the way, this is also a good reason to stick with name brand ink for your printer. Cheaper ink will likely suffer from all sorts of problems, including metameric failure.
So, as I mentioned at the outset, even while metamerism is less of a problem than it once was, photographers who print their work should still be familiar with the problem and what causes it. Who knows, if you're lucky, all you'll ever need this information for is to impress your dinner companions with your vast but esoteric wealth of knowledge. You're welcome.