If you don't calibrate your computer screen, there's a risk that the images you produce appear very different on your screen and on the screen of your viewers, or on printed paper. There are two facts to remember:

• You are never in control of the whole graphics chain.

• You should learn to accept when a result is "good enough".

Of course, this "good enough" result will already be much better than the defaults used by most people. That's why you can't control exactly how your images appear on those people screens.

Therefore, your best friend will be a colorimeter. Different manufacturers offer models more or less sophisticated but the principle remains the same. The goal is to match the visual outcome data to digital color data given by the computer, taking into account the physical environment of the screen. The black should be black and the white, white (without glare) and the gray, gray, devoid of tint. These non-color (whose composition is still the same amount of red, green and blue, in an additive color model) must be displayed as neutral as possible.

Before getting to the calibration itself let's describe the problem a bit. The eye is a formidable tool for comparison but not for absolute measurement. On the other hand, the human brain "corrects" a lot of the data, gets tired, and is sensitive to interference from the environment... To maximize the benefit of screen calibration, you should try to control many parameters of your working environment. Avoid direct light on the screen, if possible the windows should be occulted (at least partially) to fully compensate for differences in brightness and changes in light quality (this is important because it is not always possible to work in the dark)...

Calibrate your screen by following the recommendations of the manufacturer of your colorimeter. Once the calibration performed, try not to change your screen's surroundings.

If you do not have a colorimeter, you can still get good results! In software, you can use test patterns ranging from black 0 to white 255 and continuous color test patterns to check that color shades of reference differ from each other. This technique will get you good color and tints rendition on your screen. Feel free to create your own patterns with your favorite software. It is important to start by adjusting the brightness and contrast with black and white patterns. Because flat screens are very bright, do not not hesitate to drop the brightness to 50% so that the white stops glaring. Just make sure you can still distinguish subtle levels of black ... it is a matter of compromise. This step is very important for the perception of pure colors. In a second step, with the color patterns, adjust the balance  between red, green and blue. You must reach a point where grays are neutral and reference color are displayed with good fidelity. Again, it will sometimes involve subtle compromises. 

You shouldn't set your standards too high: Once the calibration done, you're a part of this higher class of people whose screen displays color about right ... but don't be too hard on those who are not. Indeed, you should repeat calibration regularly because your screen ages and you too. Your working conditions may also change. So screen calibration is never over. In the old times of analog photography, printers had to be calibrated at least once a day.

Remember, you are not alone in this chain. If you care about web design, chances are that you intend to display a web site to readers. Now they have the screen that they have, and it is certainly not yours! You will have to deal with it and remember that their viewing conditions and screen settings will probably not be perfect. You who have recently calibrated your screen, probably because of your professional ethics, keep in mind that many have given up on this idea before you and many will give up after. When you use your super light gray font over a background a slightly darker shade or when you design a dark gray menu over a black background, remember to ask yourself how it will be displayed on other monitors. Using your screen as an efficient tool implies, of course, to seek colors as accurate as possible but also to take into account the screens of your viewers.

It would be great if screens could be factory-calibrated to give perfect rendition when purchased, but sadly that is not the case. Screen calibration is needed to check what your subcontractors do and facilitate joint work. The user typically does not care about that. Therefore, after having checked that your work is as you want, check it will display correctly on your users screens.