Frequently Asked Questions about Gamma
Charles Poynton
www.poynton.com poynton@poynton.com
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In video, computer graphics and image processing the gamma symbol γ represents a numerical parameter that describes the nonlinearity of inten- sity reproduction. Having a good understanding of the theory and prac- tice of gamma will enable you to get good results when you create, process and display pictures.
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Table of Contents
What is intensity? 3
What is brightness? 3
What is luminance? 3
What is lightness? 3
What is gamma? 4
What is gamma correction? 5 Does NTSC use a gamma of 2.2? Does PAL use a gamma of 2.8?
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I pulled an image off the net and it looks murky. 6
I pulled an image off the net and it looks a little too contrasty. 6
What is luma? 7
What is contrast ratio? 7
How many bits do I need to smoothly shade from black to white? 7
How is gamma handled in video, computer graphics, and desktop computing? 8
Voltage relates to intensity through a 2.5-power function, but how do RGB code values relate to voltage? 9
1998-01-04a Charles Poynton. All rights reserved. 1 of 12
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Frequently Asked Questions About Gamma
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What is the gamma of a Macintosh? 9
Does the gamma of CRTs vary wildly? 9
How should I adjust my monitor’s BLACK LEVEL and PICTURE CONTROLS? 10
Should I do image processing operations on linear or nonlinear image data? 10
What are the I, B, L, and V components in HSI, HSB, HLS, and HSV? 10 What’s the transfer function of offset printing? 11
References 12
1 What is intensity?
Intensity is the rate at which radiant energy is transferred per unit area. In image science, we measure power over some interval of the electromag- netic spectrum, and we’re usually interested in power radiating from or incident on a surface. Intensity is what I call a linear-light measure, expressed in units such as watts per square meter.
The voltages presented to a CRT monitor control the intensities of the color components, but in a nonlinear manner. CRT voltages are not proportional to intensity.
Image data stored in a file (TIFF, JFIF, PPM, etc.) may or may not repre- sent intensity, even if it is so described.
Brightness is defined by the Commission Internationale de L’Éclairage (CIE) as the attribute of a visual sensation according to which an area appears to emit more or less light. Brightness is a perceptual quantity; it has no firm objective measure.
The CIE defines luminance, denoted Y, as radiant power weighted by a spectral sensitivity function that is characteristic of vision. The magnitude of luminance is proportional to physical power. In that sense it is like intensity. But the spectral composition of luminance is related to the brightness sensitivity of human vision.
Luminance can be computed as a properly-weighted sum of linear-light red, green, and blue primary components. For contemporary video cameras, the coefficients are these:
Y709 =0.2125R+0.7154G+0.0721B
In video it is standard to compute a luma component Y’ as a weighted sum of nonlinear R’G’B’ primary components. Although this quantity is often carelessly referred to as luminance, it is not luminance.
To learn about the relationship between physical spectra and perceived brightness, and other color issues, refer to the companion Frequently Asked Questions about Color.
Human vision has a nonlinear perceptual response to brightness: a source having a luminance only 18% of a reference luminance appears about half as bright. The perceptual response to luminance is called Lightness and is defined by the CIE [1] as a modified cube root of luminance:
Yn is the luminance of the white reference. If you normalize luminance to reference white then you need not compute the quotient. The CIE defini- tion applies a linear segment with a slope of 903.3 near black, for
(Y/Yn) < 0.008856. The linear segment is unimportant for practical purposes but if you don’t use it, make sure that you limit L* at zero. L* has a range of 0 to 100, and a delta L-star of unity is taken to be roughly the threshold of visibility.
2 What is brightness?
3 What is luminance?
Frequently Asked Questions About Gamma 3
4 What is lightness?
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Y3 Y
L* =116Y −16; 0.008856
(Acrobat PDF format).
[4] Charles Poynton, “Gamma on the Apple Macintosh,” available on the Internet as
[5] Alan Roberts, “Measurement of display transfer characteristic (gamma, γ),” in EBU Technical Review 257, Autumn 1993, 32–40.
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