This is the eighth in a series of posts on color reproduction. The series starts here.
I said in the last post that I’d get to 3D LUT based camera profiles in this one. I lied. Sorry, but the story of this series on color reproduction, which started out as an attempt to explain how to measure the color accuracy of cameras, has been like peeling an onion. Every time I think I’m ready to explain something, I notice that there’s something else that needs to be explained first.
Today, it’s metameric failure.
I’ve talked about it in passing before, but I think I should address it head on before I get to LUT based camera profiling, since it can affect that process so significantly.
Before we get to metameric error, let’s talk about metamerism in general. Metamerism is a property of the human vision system, and occurs because the color-normal (insert caveats here) human reduces spectra falling on the retina to three coordinates, and thus there are an infinite number to spectra that can produce colors that look the same to our putatively color-normal person. That phenomenon is called metamerism, and spectra that resolve to the same color are called metamers.
What’s metameric failure? It’s the lack of occurrence of a metameric match when one might be expected. It can occur for a variety of reasons. Let’s take a look at some.
Let’s imagine that we’re engineers working for an automobile manufacturer, and developing a vehicle with a carbon-fiber roof, aluminum fenders, and plastic bumper covers. The marketing folks have picked out ten paint colors. They don’t care that we meet their color specs exactly, but they do care that the roof matches the fenders and the bumpers match both. We meet with our paint suppliers, and find out that the paint formulations have to be slightly different so that they’ll adhere properly to the three different substrates, and so the bumpers can survive small impacts without the paint flaking off, and so the weave of the carbon fiber won’t show through and wreck the infinitely-deep pool high gloss effect that we’re going for.
Turns out that the reflectance spectra of all three paints are somewhat different, and we mix the paints so that we get a metameric match for a color normal person for each of the ten colors when the car is parked in bright Detroit sunshine.
What can go wrong?
We have prototypes painted in each of the colors, park them in front of the development building on a sunny day, and call in the brass. Two of them, both men are what we used to unfeelingly call color blind. One suffers from red-green colorblindness protanopia (no rho cell contribution), and one the other red-green colorblindness, deuteranopia (no gamma cell contribution). About one percent of males suffer from each.
Each of the color blind persons says that some of the paints don’t match, but they identify different paint pairs as being the problem ones. This is called observer metameric failure. Everybody else says that the colors match just fine, and can’t figure out what’s wrong with the two guys who are colorblind. There’s one woman who has a rare condition called tetrachromacy (four kinds of cone cells), and none of the color pairs match for her. That’s another kind of observer metameric failure.
Now we call in the photographers, and have them take pictures of the cars in bright sunshine. In some of the pictures, the color pairs don’t match. This is called capture metameric failure. The odd thing is that the colors that don’t match for the Phase One shooter are different from the guy with the Nikon.
We bring the cars indoors for a focus group. We carefully screen the group to eliminate all colorblind people and tetrachromats. The indoor lighting is a mixture of halogen and fluorescent lighting. Several people complain that the colors on many of the cars don’t match each other. When this is pointed out to the focus group as a potential problem, all agree that some colors don’t match, and they all agree on which colors they are. This is called illuminant metameric failure.
The photographers take pictures of the cars in the studio using high CRI 3200K LED lighting, and a bunch of colors don’t match, but they’re not all the same colors that didn’t match when the same photographers used the same cameras to take pictures of the cars outside. This is a combination of illuminant metameric failure and capture metameric failure.
We find a set of pictures where the colors match, and the photographer prints them out on an inkjet printer. We look at the prints in the 5000K proofing booth, and the silver car looks neutral. We take the prints into a meeting with the ad agency in a fluorescent-lit conference room, and the silver car looks yellowish. All the observers are color normal. This is a combination of one or more instances of illuminant metameric failure. In the viewing booth, the observer is adapting to the white surround, and the spectrum of the inks depicting the silver car resolves to a color with a chromaticity similar to the surround. In the conference room, the observer is adapting to the white surround, and the spectrum of the inks depicting the silver car resolves to a color with a chromaticity different from the surround. The fact that the printer uses fluorescent yellow ink and the paper has optical brighteners doesn’t help matters.