Barco RHDM-2301 User Manual page 96

Video signal processing
Figure 51: The inverse of the Rec.709 encoding function fitted with a power function
But a gamma of 2.0 sounds very strange in the broadcast world! All these years, video
looked good on CRTs, where people are used to values of gamma of 2.2, 2.5, or even 2.8,
but never 2.0. Thus this mean that we have been looking at non-linear end-to-end
transfer?
Indeed, according to CRT physics, the cathode ray tube transfer characteristic has a
gamma of 2.5. An early EBU specification actually uses a number of 2.8. However, a later
in-depth characterization of the gamma of a CRT (Alan Roberts: Methods of measuring
and calculating display transfer characteristics (gamma); BBC RD 1991/6) yields values
closer to 2.2-2.3.
So yes, in fact the scene-to-screen transfer has not been linear from the start. With an
effective encoding gamma of 0.52, and an effective display gamma of 2.2-2.5, the
system transfer is somewhere in between 1.1-1.3. A recent EBU recommendation
(EBU.Tech.3320) actually specifies the value of 2.35 for new reference (grade 1)
monitors.
Why does this non-linear scene-to-screen transfer give such good results?
The scene and the display have very different viewing conditions. Video monitors are
typically calibrated at a luminance of about 100 cd/m
A scene can have a luminance which is 100-1000 times larger (the sun!) than the
luminance which can be produced by the display. These different conditions give rise to
perceptual differences: a video shown on a display with a linear end-to-end transfer
function will look very pale and unsaturated. Therefore a "transfer gamma" (end-to-
end) with an exponent higher than 1 is desired (around 1.2). This situation is referred to
as "rendering intent": the video is encoded in such a way (exponent 0.45, effective
gamma 0.52), that when a CRT decodes it (with a gamma of 2.2-2.5), the end result has
an end-to-end transfer gamma of around 1.2.
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, and viewed in dim environments.