'Calibration' and 'profiling' are often used interchangeably in connection with getting accurate on-screen colour for photography, image editing and graphic design. They mean different things, though, and you need understand how calibration is done if you're to get the best results from your monitor. This article explains software and hardware calibration, and what to look for in quality software.
Setting up a monitor to display accurate color requires two distinct processes, calibration and profiling. The phrase 'profiling a monitor' is generally taken to mean performing both operations, though the distinction isn't always made clear, especially in entry-level products.
Calibration means measuring and adjusting the monitor to be as near as possible to a specified condition in terms of its white and black luminance, color temperature (what color 'white' is) and gamma (the relationship between input and output tonal levels).
Profiling then measures the way it behaves in that calibrated state as a range of colors and tones are displayed and measured. The ICC profile that is made maps this behavior and allows the color values sent to the screen to be adjusted on-the-fly by color management-aware applications so that the correct colors are displayed. The calibration data is also saved within the ICC profile.
There's a further distinction that's important to understand, especially if you need to change monitors and profiles: that of 'software' and 'hardware' calibration.
Although both types use a measuring instrument, a 'software' calibration is one in which all the changes necessary to meet your calibration requirements are made via a look-up table (LUT) loaded into your PC's video card from the display's ICC profile. The display is usually set to factory default values for brightness, contrast, color temperature and so on, but if the necessary corrections are extreme, this can potentially lead to inferior display accuracy (see 'manual hardware calibration of luminance' later on in this article).
'Hardware' calibration strictly means making physical adjustments to the behavior of the electronics inside the monitor itself to meet the target values. In CRT screens, that meant adjusting the respective strengths of the red, green and blue electron guns to achieve the desired white point and luminance. This could be done manually, via an iterative process of adjustment and re-measurement, or in some high-end models, it could be done automatically via calibration software and a digital link between computer and screen.
Many LCD displays have controls which appear to offer individual adjustment of the red, green and blue channels, color temperature, contrast, hue and saturation. However, you can't control the individual behavior of the red, green and blue filters in an LCD panel - all you can vary is the brightness of the backlight. Anything else is done in software within the display's electronics via further layers of tonal and color adjustment via the software controlling the LCD filters; the fewer of these the better.
In high-end 'hardware calibration' LCD displays such as NEC SpectraView, Eizo ColorEgde or the BenQ models, luminance adjustment is made automatically by the calibration software. Adjustments to meet the target white point and tonal response are then implemented within the displays' internal electronics.
'Visual' calibration tools: There are software tools that aim to provide monitor 'calibration' purely by visual means, usually requiring you to make a series of judgments of tone or color equivalence in on-screen graphics. From these, the software attempts to deduce the gamma and color values of your monitor and build an ICC profile. The problem with this is that nothing is actually measured and different users tend to end up with different results, which defeats the object.
Manual hardware calibration of luminance: Although software calibration can be applied via a video card look-up table (LUT) to any monitor running at factory default settings, the downside of this approach is that you may sacrifice tonal accuracy if the monitor's native behavior is too far from the settings you want. For example, many current LCD panels have a maximum luminance in excess of 300 cd/m2 (candelas per square meter), but manufacturers' recommended values for are usually well under half this.
Limiting the display to around half its native luminance purely via a calibration LUT in the video card means effectively throwing away most of the top half of the 256 levels per channel available, because using them would produce unacceptably bright colors. The remaining 128 or so levels would then have to cover the entire tonal range in each channel, with greatly increased chances of banding or incorrect display of colors due to the limited set of remaining values available to represent adjacent tones.
The solution is to adjust your monitor controls manually, guided by measurement, in order to get the luminance as close to the desired levels as possible before applying any corrections in software, so this is a feature to look for when choosing calibration and profiling software. If it's not offered, the software probably does everything via the LUT as described, and won't be a high quality solution.
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