Color Management: Calibrating versus Profiling
Do you calibrate your monitor, or do you profile it? In discussions on color management, the terms "calibrating" and "profiling" are often used somewhat interchangeably. In point of fact though, they refer to quite distinct processes.
And you need to do both of them — first you calibrate, then you profile.
"Calibration" means to set a device to a known, repeatable state. Before you can begin to consider profiling your monitor, you need to set it in a way that you are happy with. And if you want to get the best performance out of it, what should make you happy is a monitor set to maximize its range. You want the greatest range practical between black and white, and if possible, between desaturated and fully saturated for each color channel.
The first thing you want to do is turn it on. This seems an obvious place to start since it makes the display show an image rather than sit there blankly dark and lifeless. No doubt you didn't need me to point that out of course, but what may be less obvious is that it also allows it to reach normal operating temperature. Even when using a solid state CRT monitor, the display will tend to drift a bit as it warms up. You should allow it to do this any time you need to rely on it to make critical color decisions so if we are talking about a known, repeatable state, you also need to do so before getting out your colorimeter or spyder in order to calibrate and profile your monitor.
Now for the next step. A traditional CRT monitor that most of us used up until recently (and some of us still do) came with a range of knobs along its front edge or on the back that one could twiddle to change the appearance of what the display looked like. You could make things brighter or darker, more or less saturated, horribly purple or ghastly green. It was your choice. Not all monitors provide the same range of calibration settings but most offer at least some. A profile is only valid when the monitor's knobs and dials and sliders are all set the way they were when you built that profile. The question of course is, how do you know where to set them? When you plug in your colorimeter and fire up the software, it will offer to help you calibrate your monitor. It will guide you through setting the contrast and brightness in order to maximize the available range. If your monitor supports it, many will also help you adjust all three individual channels to maximize the range of each color. Once you finish calibrating your monitor, you want to leave the controls alone. If the settings rely on actual knobs, put a piece of tape next to each and mark it with a pen so you won't accidentally mess it up later. If your monitor uses an on-screen display during the calibration process, take note of where each adjustment ends up after calibrating so you have a record for future reference. In short, set it, but don't forget it.
Now we can move on to profiling.
"Profiling" means to measure the response of a device (in an set, unchanging state) to various inputs. The results of these measurements can later be used by the color management system built into your operating system and imaging software to compensate for the particular quirks of your monitor. As mentioned, profiling is useless unless your monitor is in the same state when you use it as it was when you profiled it so be sure you have calibrated it and that it is in that known state first.
The software that came with your monitor colorimeter or spyder will help greatly with the profiling process. With the device stuck to the face of your monitor or hanging in front of it, the program will produce a series of color patches. Each one is supposed to be a particular color, but since no monitor is perfect what actually gets displayed may vary slightly from what was intended. No matter, since the result will get measured by the colorimeter and compared against what was supposed to show in order to determine how far off each is.
Obviously this doesn't inherently fix any accuracy problems your monitor may have, but it does form the basis for compensating for such deficiencies later on. That's what the resulting profile is for in fact. Most monitor profiling software will automatically tell your operating system about the new profile and install it in the correct folder, so once you finish profiling you truly are done with things. The use of the profile is automatic. Don't even think about setting your Photoshop working space to this new profile. Just let your color management system do its thing and all will be well.