Color Management: The Eyeglasses Analogy
Since writing extensively about color management this past spring, I've continued to ruminate on the subject, trying to come up with even better ways to explain how things work.
Enter the eyeglasses analogy.
I don't wear glasses, but allow me to go on for a minute anyway about what a trip to the eye doctor entails. Our starting premise is that someone's eyesight is deficient in one way or another and they want to correct it. They can see, but they suspect that what they see isn't an accurate representation of what things really look like. Not everyone who needs glasses has the same problem with their vision either. Some are near sighted, some far; others have more complicated vision problems. A person's vision can be at least somewhat deficient and they may not even realize it. After all, we can only be aware of the outside world through our own senses and there is no easy, absolute frame of reference to detect problems. Eventually though, through one means or another, people who need glasses realize things aren't quite right and they need to get their vision checked. And off to the eye doctor they go.
The optometrist will then subject them to a battery of tests designed to measure how well they do see. Apart from a procedure such as Lasik which would probably be done by an ophthalmologist instead of an optometrist, the doctor does not actually change their patient's eyesight; his mission is limited to accurately describing their vision in a quantitative, parameterized manner. This description then becomes a prescription that is used to create eyeglasses which will counteract the measured vision deficiencies. In the end, their weak eyes coupled with new eyeglasses provide them with corrected vision.
Now, how does all this relate to color management? Allow me to explain....
Without help, no computer monitor can accurately reproduce color, although manufacturers try to make them come as close as they can when new. As monitors age though, it becomes harder and harder for them to approach the accuracy they had when young. Much the same as our own vision tends to get worse as we age. To correct our own vision, we go to the eye doctor; to correct our monitor's vision, we get a colorimeter and put it through its paces.
Before actually profiling, we put a monitor in a known state by adjusting the brightness, contrast and color temperature, but this merely makes profiling easier by providing the maximum range of potential to the software. The profiling process itself does not change the physical characteristics of a monitor at all. It merely creates an accurate description of its capabilities in that known state. That description then gets saved as your monitor profile which will be valid as long as the controls on the front of the monitor are kept as they were when the profile was created. The Color Management System built into your computer's operating system then uses this "prescription" to counteract the problems in your monitor in much the same way that eyeglasses do for human vision. As the monitor continues to age, it become increasingly less able to live up to its former potential and should be re-profiled, much as people often need stronger prescriptions as their vision deteriorates further with age.
Printer profiles work much the same way. They are merely an accurate quantitative description of what it is capable of when printing on a specific kind of paper. Photoshop (or your printer driver) then uses the profile to counteract what your printer isn't capable of doing accurately on its own. The capabilities of printers don't generally deteriorate as much with age as do monitors, but driver changes, paper manufacturing changes and other variables may necessitate obtaining new profiles to restore optimal accuracy.
There is a common misunderstanding that the objective of profiling is to "calibrate your monitor to your printer," but this is no more true that it would be to say that an eye doctor works his magic on one patient by calibrating their eyesight to that of another patient. Instead, just as every eyeglass prescription is created independently, with reference only to how things are supposed to look and what is needed to correct a given patient's vision, every device profile is created specifically for that device (and device configuration such as monitor brightness and contrast or printer paper choice). Each prescription/profile is then used to compensate for the flaws in the capabilities of whom or what it was made for.
And by the way, when your monitor starts to behave like it needs bifocals, it's time to buy a new one. Lasik surgery really isn't an option.