This is meant to be a simple tutorial, for fast but usefull results. If you need more acurate results you need specialized hardware and software.
Everithing can be uncalibrated, even our own eyes. I'm sure you have been in a movie theater... after 2 hours of being in a dark room our eyes become used to that environment, and when we go out to the mall, we just see things too bright. They were "calibrated" to the darkness.
We can do a little experiment right now.
In ideal conditions we should be able to see our prints just as we prepare them on the monitor... but if we turn lights off it becomes evident that the viewing conditions are critical to this. What happens for example if we use a tungsten lamp?. It is obvious that our colors won't match, just because of this environmental variable.
So the recommendation is that we have an uniform illumination with white balanced lamps. Not too bright, not too dim. Avoid direct sun light.
After controlling the environment it is time to configure our monitor. First we have to adjust controls directly on the monitor. (You need to have your monitor turned on for some minutes before doing this)
We will start with contrast. This configures the bightest point. If the contrast is too low we won't have white but gray areas.
It's recomended that you maximize contrast, maybe at 90%-95%. If you have to calibrate 2 diferent monitors maximize the less bright and adjust the second one. But if you feel it too bright you can dim it a little.
The second part is the color temperature. The "normal" color temperature is 6500°K. If you see a simple white background with a yellow-orange tint the temperature is too low, maybe it is at 5000K. If it has a blue tint the temperature is too high, maybe at 9300K.
On some monitors their RGB values can be adjusted independently to achieve a neutral white, if the standard presets are not good enough, you can try to adjust them, but don't move these unless you know what you are doing.
Previously we controlled the brighter values, the white point. With the Brightness button on the monitor we will control the darker points, but we need some images.
In this test image you need to adjust your monitor's brightness so the numbers become very very dark, almost not viewable as follows:
Now we need to adjust our monitor profile, for this we need some software to adjust the middle tones.
Your computer comes with a video configuration panel, from your manufacturer's video card.
I have made some test patterns, the idea is to adjust the values on our calibration software so we can match the background of column 2 with column 1. The numbers on column 1 also must "disolve" with the background.
There is a separate R-G-B version on the bottom of the page, so that they can be adjusted individually.Gamma Test Patterns