Gray Scale Calibration
understand the importance of a properly calibrated gray scale, we first need to
delve into how the NTSC and HD television systems works.
TV picture is derived from two separate components, luminance and color.
Luminance was all we had back in the old days of black and white television. TV sets back then had white phosphor
hit by a cathode ray.
The more powerful the ray, the brighter the point in the picture. We called this
the "cathode ray tube" or CRT.
advent of color television, we simplicity of the white phosphor got lost, since
we now had to reproduce colors. Today's televisions has three different
"primary colors", red, blue and green. Since there is no longer white available, a color television has to "mix" the three primary colors into
something that looks white. This is done by lighting up three separate
points (one of each color) very close to each other to give the illusion on one
"white" point. If you stand close to a TV you can see the different
introduces one problem: How much of each color should we light up to give an
appearance of "white"? How about different white levels, light gray vs.
dark gray vs. bright white? What is the correct "mix" for each possible
level of white?
you may ask, we only watch color television anyway. Why do we care how the
television makes black and white images? The answer is that color
information is added "on top of" the white (luminance) information. Think
of the white as a canvas, and the color as the paint. If the canvas
doesn't have the correct color, any paint you apply to it would be tinted by the
United States, the NTSC television standard specifies that the color of white is
D65, or 6500 Kelvins, approximately the color of snow on a overcast day. A
lower Kelvin number (like 5000) would make the white more "reddish" and a higher
Kelvin number would make the whites more "bluish".
would happen if we calibrate our TVs to have white information displayed with
9000 Kelvins? Since the underlying "canvas" would now be tinted blue, we
would not be able to accurately reproduce red or green information. No
matter how much red or green we add, there would always be a element of blue in
Gray Scale calibration. By measuring, with
a color analyzer, the "mix" of the three primary colors for any possible level
of white, and adjusting the "mix" where needed, we can make sure that all levels
of white correspond to the D65 standard. This will improve color fidelity,
and the display will be able to reproduce the colors correctly. You will
probably be able to see green and yellow for the first time like they were
intended to be seen.
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