Lighting, Viewing, and Metamerism

Background

Metamerism. It might sound like a word that can only be understood by techno geeks, but it affects nearly everything you print. Have you ever noticed that the colors in your printed photos look good under daylight from a window but those colors change under fluorescent or incandescent lighting? Blue skies may turn purple under certain lighting, skin tones may look more yellow/orange, and grays may take on a color cast. B/W prints often suffer from metamerism more than color prints and they can look neutral under some lighting only to take on a red or green color cast as you move the prints around your office or home. Metamerism is often an issue when buying carpet, trying to match clothing color, drapes, and other items. Colors may appear to match nicely in the store but when you get home, the colors look completely different in the lighting in your house. This is metamerism in action. In this article we'll take a look at metamerism, what causes it, and look at our options for controlling it.

Understanding how we perceive colors

The first thing we need to understand is how people perceive color. Our eyes, like most photographic equipment, see color by using three primary color receptors: red, green, and blue. By "sampling" the amount of red, green, and blue light present, our eyes can determine the color of an object. Equal amounts of red and green let us perceive the color yellow. Equal amounts of red and blue allow us to perceive magenta. By varying the amounts of red, green, and blue light, we can see any color in the visible spectrum. Image capture devices (cameras, scanners) record RGB intensities similar to the way our eyes record them and we use output devices (monitors, printers) to put these primaries back together so that our eyes see the same RGB intensities that were present in the original scene. While printers use different primaries (a form of cyan, magenta, and yellow) the end result is the same: the devices try to put the data back together so that the red, green, and blue sensors in our eyes see the same intensities (or very close) as those in the original image. Doing this "record and playback" successfully gives you an accurate representation of color in a printed (or displayed) photograph.

Primaries versus the color spectrum

It all sounds simple so far. Unfortunately, while any color can be "simulated" by using primaries like red, green, and blue, spectral distribution is also important. Think about a rainbow. In every rainbow, we can see all the visible colors from red, orange, yellow, green, blue, indigo, and violet and all colors in between. As the wavelength of the light changes from red to violet through the range of colors in the rainbow, the wavelength of light changes from say 700 nm (red) to 400 nm (violet). If we had a light bulb that could reproduce any wavelength we desire by just turning a knob, we could turn the knob to about 580 nm and we would see yellow. If we look at the spectral distribution of color for this light, we'd see a single spike of color at the 580 mark on the rainbow-colored graph. The graph would show the rainbow of colors from violet to red across the bottom with a single vertical spike in the yellow location.

Now instead of a single yellow bulb, consider two bulbs: a red bulb and a green bulb. The red and green bulbs will produce two spikes on the spectral distribution graph: a spike at red and a spike at green. The graphs look completely different, but if we set the intensities just right and mix the red/blue light together, our eyes will perceive the two colors as the same because both "excite" the red and green cones in our eyes to the same degree. So we can arrive at the same perceived color with very different lighting. At this point, you might be tempted to say "who cares", but this is the first step to understanding metamerism.

How lighting affects prints

We all know that different lighting can affect the way you perceive color. People are starting to buy bulbs that are advertised as more "natural" to improve the appearance of people (or other objects) in the home. What makes different bulbs and different lighting technology more desirable? First we have to consider the light spectrum from above. Lets start with the sun. Sunlight or "daylight" is considered full spectrum. Full spectrum simply means that you have a relatively even distribution of every color in the spectrum from violet through red. If you look at the spectral distribution of sunlight, you'd see a relatively straight line across the graph indicating that every color from violet, blue, cyan, green, yellow, orange, red are all present at nearly the same intensity. This is the very definition of "white": the presence of all colors at once.

In contrast to daylight, most man-made lighting such as fluorescent and incandescent lighting has a very "spiky" distribution of wavelengths. Our eyes may be able to adjust to any of these light sources, but each color in the spectrum may not be represented equally. Here is a page showing the spectral distribution of different light sources. As you can see, fluorescent lighting has a large green component but is deficient in red. This can make fluorescent lighting look a bit green. Conversely, incandescent lighting has a larger red component and is deficient in green and blue, making objects (or the light itself) look orange.

When we print photos, our printers distribute ink/dye with the assumption that we have full spectrum lighting as our viewing source. We really have to assume full spectrum lighting because there is so much variation in lighting of even the same type (incandescent, fluorescent) that to do otherwise would only make the problem worse. Because our printers reproduce color using primaries like cyan, magenta, and yellow, uneven distribution of light from a non full spectrum light source can shift colors due to the way the "spikes" created by our printer's cyan, magenta, and yellow inks happen to align with the spikes in our light source. If our light source has a "valley" in the spectrum in the blue area and a "peak" in the red area, this might have the affect of enhancing the yellow ink while subduing the cyan ink. Move to a different light source, and the opposite may be true, forcing the perceived color to change. It's sort of like watching a runner who is pacing at a steady rate. As long as the runner keeps pace, it doesn't matter whether the ground between his steps is solid or he is running on the tops of well placed poles because he doesn't use the ground that he isn't stepping on. Place poles where he is stepping and he'll do fine. As soon as he changes his pace, however, and his stride no longer matches the placement of the poles, he falls. Same idea with matching your lighting with the color distribution of your inks.

So how do we deal with metamerism?

We have to consider the spectral distribution of light plus the spectral distribution of our inks to understand and control metamerism. Sound complicated? You bet! So complicated in fact that there really is no simple answer. Even if you use ICC profiles to get the best color for your printed photos, almost all ICC profiles are designed to produce accurate color under full spectrum lighting (D50). View your prints under most indoor lighting, and your results may vary. Some printers have inks that are more or less prone to metamerism, and some individual inks can cause more of a problem than others. For example, the yellow ink in some printers has been found to be a major contributor to metamerism, so some specialized (mostly B/W related) software is designed to try to use less yellow ink so that your prints don't shift color as much from room to room. It really is a complicated issue, but there are things that you can do to take control of metamerism.

The best (and relatively inexpensive) way to deal with metamerism in a controlled environment is to buy better lighting. While Solux bulbs probably produce the best metamerism-free lighting, there are other options available that do a relatively good job. For example, most home and garden centers now carry "daylight" or "natural" fluorescent bulbs for your home or office. They are more expensive than regular office fluorescents but you can still probably replace all of the 48 inch fluorescent bulbs in a small office for less than the cost of a few packs of photo paper! Most of these bulbs are labeled with a CRI (Color Rendering Index). The closer the number to 100 (perfect daylight), the better. A CRI of 90 would be considered good, 95-98 very good, and anything above 98 exceptional. Not only will these bulbs help prevent color surprises in your prints, they also brighten the room and often give more of a revived feeling to the surroundings.

If you are printing B/W photos, you may want to invest in specialized software that produces B/W prints that are less prone to metamerism. Most inkjet printers do not just use black ink to produce B/W prints: they still use a mix of black plus some of the color inks. To understand why printers still need to place color ink on the paper when printing B/W photos is another article in itself, but most print drivers don't offer a "black ink only" option. To make matters worse, most black inks aren't truly neutral anyway! Our eyes can be more sensitive to color casts in B/W prints because we are very sensitive to slight color casts when we know the entire photo should be neutral. If you find that your B/W prints shift color when moving to different lighting, you might want to consider a RIP (Raster Image Processor) designed for your printer such as QuadTone RIP. In most cases though, an accurate ICC profile for your printer will do a decent job.

Also be aware that different printing technologies suffer from metamerism to different degrees. Pigment based printers usually suffer from metamerism more than dye based printers, however, the latest pigment based printers have far fewer problems than the earlier models such as the Epson 2000P and 2200 which are known to produce prints more prone to metamerism.

Summary

Most people will probably go through life unaware of the issue of metamerism and to be honest, the problem is rarely so severe that people complain about it. For those who are concerned with color accuracy and producing the best photos, however, there may be some situations where colors are difficult to match and knowing that metamerism can be an issue can at least allow you to deal with the problem. As a general rule of thumb, metamerism is most evident in printed photos, so if you are having trouble with color matching in your printed photos, always take the photo to a window or take it outside to see if the problem persists under full spectrum lighting. You might just be dealing with a particular color that is affected by metamerism. It can be a tedious and losing battle to try to tweak colors under difficult lighting, so it's always a good idea to try daylight when dealing with color issues in printed photos just to identify where the problem is coming from. This might not make the fix any easier if you must deal with difficult lighting or your photos must be displayed under that lighting, but at least if we can see and identify our enemy, we have more options in how to deal with it.

-- Mike Chaney