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A SIMPLE MONITOR TEST To see whether your computer monitor is properly adjusted and can display shadow detail properly, you should see the left half of the circle in the large black square very faintly (or not at all), but the right half should be clearly visible. If not, you need to adjust your monitor's contrast and/or brightness setting. This alters the monitor's perceived gamma. POWER LAW FOR VIDEO DISPLAY A gamma characteristic is a Power-law relationship that approximates the relationship between the encoded Luminance in a Television system and the actual desired image brightness. With this nonlinear relationship, equal steps in encoded luminance correspond to subjectively approximately equal steps in brightness. Computer Graphics systems that require a linear relationship between these quantities use gamma correction. The following illustration shows the difference between a scale with linearly-increasing intensity (i.e., gamma-corrected) scale and a scale with linearly-increasing encoded luminance signal.
On most displays (i.e., those with a standard gamma of 2.5), one can observe that the linear-intensity scale has a large jump in perceived brightness between the intensity values 0.0 and 0.1, while the steps at the higher end of the scale are hardly perceptible. The linearly-encoded scale, that has a nonlinearly-increasing intensity, will show much more even steps in perceived brightness. On a monitor with an analogue input, the limited signal bandwidth may cause vertical black and white stripes to have a different brightness than horizontal black and white stripes. This problem will cause the squares on the image on the left to appear at different brightnesses. If your browser does not gamma correct images, then you can read your combined video card and monitor gamma on the image at the right, at the point where the stripes match in brightness. A Cathode Ray Tube (CRT), for example, converts a video signal to light in a nonlinear way, because the electron gun it contains is a nonlinear device. The light intensity ''I'' is related to the source Voltage VS according to : where γ is the Greek letter Gamma . For a CRT, γ is about 2.5. By coincidence, this results in the perceptually homogeneous scale as shown in the diagram on the top of this page. For simplicity, consider the example of a monochrome CRT. In this case, when a video signal of 0.5 (representing mid-grey) is fed to the display, the intensity or brightness is about 0.21 (resulting in a dark grey). Pure black (0.0) and pure white (1.0) are the only shades that are unaffected by gamma. To compensate for this effect, the inverse transfer function (gamma correction) is sometimes applied to the video signal so that the end-to-end response is linear. In other words, the transmitted signal is deliberately distorted so that, after it has been distorted again by the display device, the viewer sees the correct brightness. The inverse of the function above is: : where ''V''C is the corrected voltage and ''V''S is the source voltage (e.g. from a camera or VCR). In our CRT example 1/γ is 1/2.5 or 0.4. A colour CRT receives three video signals (red, green and blue) and in general each colour has its own value of gamma, denoted γR, γG or γB. However, in simple display systems, a single value of γ is used for all three colours. Other display devices have different values of gammas: for example, a Game Boy Advance display has a gamma between 3 and 4 depending on lighting conditions. In LCD displays such as those on laptop computers, the relation between the signal voltage ''V''S and the intensity ''I'' is very nonlinear and cannot be described with gamma value. However, such displays apply a correction onto the signal voltage in order to approximately get a standard γ=2.5 behaviour. In NTSC Television recording, γ is 2.2. The gamma function, or its inverse, has a slope of infinity at zero. This leads to problems in converting from and to a gamma colorspace. For this reason most formally defined colorspaces such as SRGB will define a straight-line segment near zero and add raising x+K (where K is a constant) to a power so the curve has continuous slope. This straight line does not represent what the CRT does, but does make the rest of the curve more closely match the effect of ambient light on the CRT. In such expressions the exponent is ''not'' the gamma, for instance the sRGB function uses a power of 2.4 in it, but more closely resembles the 2.2 gamma function. PHOTOGRAPHY The same term — gamma correction — was previously used in Photography to describe an analogous problem. The Photographic term refers to the straight-line region of the Hurter-Driffield Curve , which is a plot of density (or the Logarithm of opacity) of the film image versus the Logarithm of the film's exposure to light. Photographic Film has a much greater ability to record fine differences in shade than can be reproduced on Photographic Paper . Similarly, a video screen is not as capable of displaying the range of brightness which can be captured by electronic cameras. For this reason, a considerable amount of artistic effort in photography must be invested choosing what reduced form of the original image should be presented. The gamma correction is part of the Photographic repertoire used to adjust the recorded image. CONFUSING TERMINOLOGY The names of the various quantities are somewhat confusing. The term Intensity refers strictly to the amount of light that is emitted per unit of time and per unit of surface, in units of Lux . Note, however, that in many fields of science this quantity is called Luminous Emittance , as opposed to Luminous Intensity , which is a different quantity. ''Luminance'', can mean several things even within the context of video:
Likewise, ''brightness'' can refer to the "amount of light" either before or after application of the gamma power law. The ''gamma function'' described above, is completely unrelated to the mathematical Gamma Function . The two are merely represented using the same Greek Letter , Γ Or γ (gamma) . SEE ALSO EXTERNAL LINKS
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