Gamma Correction the Second – Compensating the Monitor Properties

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The second reason for gamma correction lies in the behavior of the cathode ray tubes in our monitors and televisions. These convert the input voltage values non-linearly into brightness values. Instead, they naturally follow a power function with gamma as an exponent, as expressed by the mathematical function below:

Formula 18

L = Luminance (brightness)
V = Voltage
ε = Coefficient for the black component
γ = Gamma

Depending on the device, the gamma value is between 2.3 and 2.6. Higher values are usually due to an incorrect brightness setting.

If we represent this behavior graphically, we get the curve A in figure 57 (Characteristic curves monitor). By sheer coincidence, it is the almost perfect mirror image of the curve in figure 20 (Estimation of the brightness on a linear scale) in the section „The minimum size of brightness differences„, thus giving the almost exact inverse function of our contrast perception. By exponentiating the input voltage value V with the function 1/gamma, we correct this nonlinearity and obtain the curve B. Combining these two curves yields a torn straight line at a 45° angle, thereby establishing the desired linear relationship between the input voltage and the perceived brightness. As we will see in the next section, this behavior is extremely useful. To ensure compatibility with the prevailing standard, more linear LCD monitors and plasma TVs emulate it.

Now, when we install Photoshop and launch the small Adobe Gamma utility, it instructs us to adjust the monitor’s contrast and brightness to specific levels and evaluate various stripe patterns and color charts. For motivation, we are fobbed off with the explanation that this is necessary to calibrate the monitor and create a profile. All well and good and useful, but actually we do nothing more than adjust the monitor settings to the gamma value inherent in the device. This has nothing to do with gamma correction! When Adobe Gamma asks us to adjust the screen brightness so that we can just make out the dark square used for matching against the even darker background, we are merely bringing the black and white scale of the signal into line with the black point of the curve in order to make the best use of the available brightness range. Because if the brightness is set too high, as shown in figure 58, no signal can produce a true black and the displayed image appears without contrast. Conversely, if the brightness is set too low, as in figure 59, there is too much black in which a lot of actual useful signals are lost. In this case, the image lacks abundant details in the shadows.

Next Gamma correction the third – distribution of brightness values on 8 bits

Main Contrast

Previous Gamma correction the first – distorting linearity

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