Calibration Versus Profiling

Colour management works by two key process - calibration and profiling.

Unfortunately the two processes are often confused with each other, and consequently many people have a poor understanding of what they're all about. It's quite simple really, and this page aims to explain the difference and give you an idea of what's going on, at a basic level, in colour management.

Put simply:

Colour Management works best when both processes are performed - first a calibration is performed, then a profile is made of the device in its calibrated state.

More basic colour calibration systems often offer only one of the two options - calibration (eg a printer may offer a 'linearisation' function that performs a basic calibration), or profiling (eg cheaper monitor colour management systems which simply profile the monitor in whatever state it happens to be in when the process is started, but do not guide you through calibration).

Calibration

Calibration is using hardware adjustments on a device to set the device into a known, repeatable state (ideally close to some absolute benchmark for that device's behaviour).

An example is adjusting the brightness control on your monitor to a specific point - say 120 candellas.

Most CRT monitors have extensive hardware calibration controls for brightness, contrast, and colour (on good quality monitors, individual control over the Red, Green and Blue guns is available, on cheaper monitors there are usually colour presets to choose from).

Most cheaper to mid level LCD monitors have only one true hardware control - this is for brightness. While your monitor may offer other controls, unless they are specifically advertised as true hardware adjustments, these adjustments are in fact occuring in your video card rather than the monitor. For the best quality from your screen, you should in general ignore these controls and leave them set to factory defaults. In general during colour managment of these screens you should only adjust the brightness, and skip all the other steps (contrast/colour etc).

High level LCD monitors, such as those made by Eizo, offer true hardware adjustments for brightness and red, blue and green gain, much like good quality CRTs. These monitors essentially have an internal video processor (usually using 10, 14 or 16 bit mathematics, better than your computer's 8 bit video card LUT) where the adjustments are made at much higher quality. On these monitors, you should adjust both brightness and the colour controls during the first stage of monitor colour control (calibration).

NB No LCDs monitors have a true contrast control (LCDs have fixed contrast ratios). You should always use LCD monitors with digital connections rather than analogue connections for better results across the board, most noticeably in sharpness and colour accuracy.

Profiling

Profiling is the process of measuring a device's colour output, and tweaking the signal going into that device to achieve more accurate colour.

It's easiest to understand by an example. Say we have a particular shade of red we want to achieve on a device, let's call it Red 50. What we do is we send the Red 50 signal to the device and see what actual colour the device produces. In this example, lets say it produces Red 55. So we know the device is outputting a little too much red. Next, we try and send Red 45 to the device, and see what happens. This time the device successfully produces Red 50. So from now on, whenever we want Red 50 displayed by the device, we just send it Red 45 instead.

Repeat for a large number of colours. The more the better.

Pretty soon, you have a big table of input signals and the resulting colours:

This is what a profile is - a table the maps input signals to the real colours produced. Of course we can look at this another way - we know, when we want a particular colour, which signal we have to send. I.e. from the table above, if we want Blue 17, we know we need to send the device the Blue 23 signal.

Thus a profile is simply a very accurate description of a device, usually in a table form.

Profiling works best when the device behaves well - that is, the better the device inherently is, the easier it is for the profile to produce really accurate colour across the full gamut of the device. This is why it's important to calibrate devices BEFORE they are profiled, and it is the key flaw in cheaper colour management systems - they simply profile the device in whatever state it is currently in, and this means the profiles often have to have very big corrections in them - which leads to poorer quality results overall.