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Gradients

The lack of banding in smooth color gradients is a parameter describing the monitor’s quality of color reproduction. This is evaluated subjectively. A few horizontal gradients (from black to red, from black to blue, etc) are displayed on the monitor and the reviewer makes sure they don’t look striped at any values of brightness and contrast.

The banding is indicative of inaccurate processing of image data by the monitor’s electronics. Some say it is an indication of the monitor’s using an 18-bit matrix, but this is not so. The banding disappears on some monitors at the factory settings, so it is important to check it out at different values of brightness and contrast.

Gamma Curves

Gamma curves are graphs that show the dependence of the signal coming from the graphics card and the brightness of the pixel on the screen. This is an exponential dependence and the exponent is referred to as gamma . According to the sRGB standard, modern home monitors must have a gamma of 2.2.

The measurements are made with a ColorVision Spyder calibrator. The gamma curves are measured individually for red, blue and green and are drawn in the diagram with the corresponding color. The black curve is the ideal curve for gamma 2.2.

Below is an almost ideal result: the curves are all rather smooth and lie in a dense group. You don’t often see such an idyllic picture, though.

For example, in this diagram the curves lie close to each other but go much lower than the ideal theoretical curve. It means that the halftones in the sagging section of the curves will look much darker on the screen than they should be (the eye will perceive this as a higher-contrast image). Conversely, when the real curves go higher than the theoretical one, you’ve got a whitish picture. When the curves don’t go close to each other, it’s a problem, too.

An even more critical problem you can identify by means of the gamma curve diagram is a loss of details in lights or darks. It’s when the monitor reproduces all light halftones as pure white or all dark halftones as pure black, respectively. You can identify this in the diagram if the gamma curves coincide with the X-axis or have a characteristic bend as shown in the picture below (in the top right part of the diagram):

Such image defects usually occur when the brightness and contrast settings are reduced (loss of darks) or increased (loss of lights) beyond a certain value.

Color Temperature

Color temperature is measured with the Spyder calibrator at the default brightness and contrast and on four levels of gray, from a dark gray to pure white.

We have to do this because many LCD monitors have very different temperatures of white and gray due to their gamma curves having inaccurate shapes. It means that if you set your monitor up so that white looks really white on its screen, you suddenly see that gray looks bluish. You try to set the monitor up once again, this time basing on gray, but now you see that white has acquired a red-yellow hue. Alas, it is impossible to correct this without a hardware calibrator like our Spyder.

The gamma curves do not give you enough information about color temperature (the curves for different colors are normalized to be displayed in one diagram so that their bottom left and top right points coincided), and we measure the color temperature separately and publish the results as a small table whose rows are levels of gray we measure the temperature of and whose columns are the color temperature modes offered by the monitor.

Two things are important here: the average temperature for each mode (e.g. if the monitor is set to yield a temperature of 8000K, you are likely to think its image too cold) and the above-mentioned difference of temperatures of different levels of gray. For the latter parameter, a difference of 100K and lower is ideal. A difference of 1000K is acceptable and a difference of over 1000K is bad.

 
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