by Steve Upton
Metamerism - or Things That Go Weird in the Light.
Have you ever compared two garments in a store and decided they matched, only to find that when you left the store and went out into daylight they no longer matched and instead looked quite different?
If so, you have seen an optical phenomenon called metamerism (me 'ta merizm). Strictly speaking, metamerism occurs when you see two samples match under one light source (illuminant) and not match under another.
How can this be?
Well, it comes down to the difference between how an object affects light, and the color it appears to our eyes. Objects affect light by selectively reflecting or absorbing light of different wavelengths. So an object that absorbs most blue wavelengths and reflects most red wavelengths will usually appear red to our eyes. The actual color it appears to us is dependent on the spectral composition of the light reflecting off the object.
Let's say, for example, we have two objects that each reflect red light in approximately the same way but one reflects blue light while the other absorbs it. If you put both objects under reddish lighting (and most indoor tungsten lighting falls into this range) then they may appear to be very close to the same color. As there is very little blue light falling on our objects, the difference between their blue reflectiveness is almost invisible. The red reflection is about the same so they both reflect similar wavelengths and our eyes see them as the same color!
This would not be a problem if we didn't have many different colors of lighting in everyday life.
So let's take our objects outside into mid-afternoon daylight. Sunlight at that time of day contains considerably more blue light than indoor lighting. As before, our pair of objects will reflect red light similarly but one of them will reflect a significant amount of blue light while the other absorbs it. Our eyes will see the blue light from one object combined with the red light and we would probably call the result magenta. Suddenly what we thought were two reddish objects no longer match at all!
Is this a problem?
That depends. In many ways this very phenomenon is essential to color reproduction (see "Metamerism as Friend" below), but when colors "shift" from our expectations, clients stop paying bills, and that is a problem.
Where will we see this problem in the business of digital imaging?
- Proofs and press jobs failing to match under different lighting.
- Color builds chosen for normal printing failing to match under unusual lighting. A good example of this is trade show booths and how they are lit with unusual lights in exhibit halls.
- Two prints using different technologies - such as inkjet vs photographic print - failing to match under certain lighting.
- A product shot failing to match the product in all lighting conditions.
There are other situations as well.
Is there anything we can do about it?
- Be aware - this isn't really a solution but it is always the first step toward one.
- Choose pigments carefully. Beware of pigmented inks for inkjet printers (see Pseudo Metamerism below)
- Control your lighting - both for producing prints and for final viewing, if possible.
- Profile carefully
Can color management using ICC profiles correct for this problem?
No... and yes. ICC profiles are typically built using readings referenced to D50 (5000K) lighting. That means that prints created using these profiles will look best under D50 lighting. Viewing them under any other lighting can give unpredictable results. Most printing pigments and dyes have been carefully chosen to not conflict with each other or other pigment sets. One exception that is appearing more and more is pigmented inks for inkjet printers (again, see Pseudo Metamerism below). Sometimes you can measure printed or scan/camera targets with a different light source such as D65 in the calculations. This should make the print viewable optimally under D65 lighting. This is not always successful and requires the appropriate settings to be available both on the instrument and in the software.
We are continually experimenting with such techniques to find solutions for our clients.
One closely-related problem cropping up more and more often in the inkjet printing world is often (incorrectly) called metamerism.
When colorants are mixed carefully in a printer, you can achieve a smooth, neutral gray gradient from black to white. With most inkjet printers, the ink combination will include Cyan, Magenta, and Yellow inks in varying amounts along with Black ink. When properly balanced, pleasing black and white images can be printed. Many users are also experimenting with near-neutral imaging such as adding a slightly blue or sepia tone for effect.
With the fugitive nature of dye-based inks, many users are switching to pigment-based inks for the vastly improved permanence. After all, if you are printing and selling works for display, your customers tend to have the expectation that the work will last beyond 2-3 years. Pigmented inks however, can suffer from a pigment balance problem which rears its head in a similar manner to the two-sample metamerism problem.
When a quality ICC profile is created for a pigmented ink set, you will typically see a good neutral gray gradient. As mentioned above, ICC profiling assumes the print is viewed under D50 lighting. With some pigment sets (ie CMYK inks sold and/or used together) the gray gradient color shifts considerably when viewed under different illuminants. It is important to note that this is not an expected color shift but rather a shift that appears strange to the eye. One would expect that a gray tone viewed under D50 lighting would appear to be a warmer gray when viewed under warmer, tungsten lighting. The color balance failure we are referring to here shows up as a green or magenta cast and is noticeably different than a shift normally attributed to warmer or cooler light.
Many people incorrectly refer to this phenomenon as metamerism. Metamerism, however, is specifically defined as a phenomenon that occurs between two samples. The ink balancing situation does not involve two samples but rather a balance of pigments in one sample. Strictly speaking, then, it is not metamerism and we (at CHROMiX) often refer to the problem as Gray Balance Failure or Color Balance Failure.
What can I do to avoid Gray Balance Failure (GBF)?
As with the suggestions above there are several things to do to minimize the effects of GBF.
- Control your lighting - not always possible, but if you can it is often the easiest solution.
- Choose your pigmented ink set carefully. View them under different lighting conditions to see if they shift considerably. Remember, it is the combination of pigments that is causing the problem. Some users have found success in mixing inks from different vendors such as the black from one set in combination with the colored inks from another. Needless to say, this route can require extensive testing.
- Profile for different lighting conditions - as mentioned before, this is not often very easy to do.
Metamerism as Friend
After all is said and done, it is fair to say that metamerism is our enemy, right?
Not so fast...
Metamerism, remember is when an object matches another under a certain illuminant even though the spectral characteristics of the two objects differ. The act of balancing three or four colorants (such as CMYK inks) so they appear to be the same color as an original object is also based on metamerism. Because of the 3-channel nature of our eyes, we can get 4 inks to appear to match a real-world object like a person's face without the spectral characteristics of the inks resembling the face much at all. This means that the print and the face affect light differently but appear to be the same color to our eyes!
This is the basis of digital imaging and printing today. It is fair to say, then, that without metamerism we would not be able to do ANY of the imaging we do today! It is only when the balance fails that we call it a problem. Perhaps a match-failure problem should be called metamerism "failure" rather than metamerism, but this term does not seem to be used at all.
As with anything in the color management world, being aware of the problem is half the battle. Now that you know about metamerism and GBF you can consider it as a contributing factor when things don't look right.
Also, if you have no D50 lighting under which to view your prints it is possible they will never look quite right. Invest in controlled lighting for print viewing. With the many variables in digital color work that can give you problems, nailing down lighting is considered a basic requirement for print viewing as well as monitor to print matching (see ColorNews issue #2).