White Balance Calibration
To understand the importance of a properly calibrated gray scale, we first need to delve into how the NTSC and HD television systems works.
Any color 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.
With the 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 colored points.
This 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?
"So what?", 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 underlying canvas.
In 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".
What 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 an element of blue in there.
Solution: White Balance 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.