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Gamma Curve Correction

In each test of any LCD monitor in our reviews you can see one or more diagrams of the so-called gamma curves. These curves show the dependence of the digital signal from the graphics card and the effective brightness of the pixel set by that signal. Ideally, the curve must comply with the equation y=x^γ where γ=2.2. This ideal curve is colored black in the diagram.

In the ideal case, the curves for red, blue and green must coincide with the theoretical curve, merging into a single line in the diagram. It means the monitor is set up perfectly. Alas, there are very few such monitors in reality.

A more frequent case is illustrated by the diagram above: green and red are more or less the same as the theoretical curve but blue sags below them. It means that if you try to display a blue halftone on the screen, it will be darker than you could expect basing on the standard gamma of 2.2. A halftone (including a gray), which is a mix of the three basic colors, will prove to be shifted into the area of reds and greens due to the same lack of blue.

This resembles the color gamut problem described above: applications take it for granted that the monitor meets certain standard requirements (sRGB color space, gamma value of 2.2) but it is not so in reality. The method of solving the problem is somewhat different. A large color gamut should not be corrected. It just must be accounted for when displaying images. Inaccuracies of the gamma curves are, on the contrary, undesired, and it is better to correct them right away on the level of the graphics card or monitor. And applications must still assume that the monitor has a gamma of 2.2.

Gamma curves are corrected with a calibrator. A sequence of halftones, from darkest to lightest, is displayed under the calibrator’s sensor. For each halftone the necessary correction is calculated. Thus, the gamma curve is verified and corrected in a certain number of points. A correction table is created basing on the results of the measurement. The table is loaded into the graphics card and the graphics subsystem gets a guaranteed gamma of 2.2 and neat curves. User’s applications don’t have to do anything. The data is obtained and loaded into the graphics card by the calibrator’s software. Of course, the table must be loaded into the graphics card anew after each reboot, so the calibrator’s software must be installed in the system permanently although the calibration process proper can be performed only once every few weeks or even months.

In fact, monitor’s gamma curves can be corrected without a calibrator, but this is a tedious and daunting task and the result may prove to be not very accurate.

As I wrote above, the correction table is loaded into the graphics card. This is the most frequent case, but some professional monitors allow to load the correction table into themselves. This approach improves calibration accuracy and, accordingly, the color accuracy of the monitor, and you don’t have to run the software for loading the table into the graphics card on each reboot. Once loaded, the table will be stored in the monitor until the next calibration.

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