Wednesday, January 24, 2007

The colors are wrong

When I meet new people, after they read my business card they often exclaim "Oh, nice to meet you, I have one of your printers and the colors are not the same as on the screen; what should I do?"

Unfortunately, there is no answer that fits on the back of a business card, except maybe a URL which you could also find by using your favorite search engine. In the following I will try to explain briefly why the answer does not fit on the back of a business card.

Color does not exist in nature; it is an illusion that is elicited in our visual system. Therefore, color reproduction is the art and science of predicting an illusion by doing physical experiments, and this is what we color scientists here at HP Labs do for a living.

Our life is made easier by the fact that people do not want us to produce the same color of their original, they just want a color match. This is where it starts to get tricky, because colors match only under certain reference conditions.

Let me make a comparison with my car. Under reference conditions, it makes 38 miles per gallon, but in reality I never reach 38 miles on a gallon. The reason is that I do not drive under the reference conditions, because most of the time in my 2.4 mile commute is spent idling with a cold engine on red lights and stop signs; moreover I drive uphill to work.

In our industry we have it a little easier than in the car industry, because our reference conditions are more realistic in modeling what the actual user has (or at least this was so when the sRGB standard was written). Today, all vendors do an excellent job at matching colors under the reference conditions, and they even do a very good job at preventing things from falling apart when the actual viewing conditions are somewhat off from the reference conditions.

Let us do another comparison with cars. Given my driving habits, I bought a basic car with a small engine and hand-crank windows. Every September I drop it off at my mechanic for a service, which essentially consists in changing the oil, tuning up the engine, and setting the tire pressure.

In the case of displays and printers, the tune-up is called calibration, and for devices in the equivalent range of my car the service interval is so long that you actually forget about it.

Motoring enthusiasts do not buy cars like mine, they buy high performance cars; often they even race. In a race, they do not set the tire pressure; they determine the temperature and road conditions and then choose the appropriate tire. They are also very picky on the oil they use and constantly tune up their cars.

Similarly, if you are a serious photographer, you select the best printing paper for the mood of your image and you keep recalibrating your devices.

Depending on your needs, your calibration efforts will be different. At the "lower end" you can get by just adjusting your viewing conditions so they are close to the sRGB conditions, which are:

Reference Display Conditions

  • luminance level: 80 candela per square meter
  • white point: D65
  • gamma: 2.2

Reference Viewing Conditions

  • screen background: 20% of reference display area
  • surround: 20% of reference ambient illuminance level
  • proximal field: 20% of reference display luminance level
  • ambient luminance level: 64 lux
  • ambient white point: D50
  • veiling glare: 1%

Your device manual may have instructions on how to set it up, or your imaging software may have instructions, including software tools that allow you to do the calibration visually, without having to buy color measurement instruments. You can also search the Web with phrases like "color calibration" and "color workflow".

At the "high end" you will have to bite the bullet and learn all about device calibration, ICC profiles, work spaces, and spectrophotometers. The fastest way is to take one of the many classes. You may also want to follow a blog addressing professional photographers, such as HP's

If you found a Web site that was particularly helpful for you, use the comment feature of this blog to share its URL and your experience.

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