Sony G90 manual White Reference Color Temperature, Color Measurement Color Bars

V I D E O

		Table 1		SMPTE C Color Bars
	White	Yellow	Cyan		Green	Magenta	Red	Blue
x	0.3127	0.421	0.231		0.310	0.314	0.630	0.155
y	0.3290	0.507	0.326		0.595	0.161	0.340	0.070

is that colors created by following the video signal’s “recipe”

for mixing light from the red, green, and blue primaries will

result in wrong colors. Hence, the colors from consumer

monitors must be wrong! How wrong is a function of their

deviation from the standard, and I’ll look at how to measure

it below. But that isn’t the only error that plagues color accu-

racy for consumer monitors and projectors. Another error is

usually much larger when consumer TVs are purchased.

White Reference Color Temperature

It is necessary, but not sufficient, that the chromaticity

coordinates of the primaries match the SMPTE C standard

to accurately reproduce color. It is also necessary that the

relative brightness of light from the three primaries be cali-

brated to produce the standard D65 white reference color,

otherwise the contribution of light from each primary will

not be correct when creating any other colors. White is

defined to be the color represented by equal red, green, and

blue values of an RGB signal. The color produced by any

other combination of signal values depends on the initial

calibration of the relative primary light outputs for the

white signal values. But what brightness of white should be

used when calibrating the color temperature?

dards, and no one has ever built a display that can be cali-

brated for perfect grayscale tracking. It is interesting to note

that direct-view CRT monitors can usually be calibrated for

significantly better grayscale tracking than CRT projectors,

but CRT projectors can have primary colors that more close-

ly match the standard because each primary color is gener-

ated by a separate CRT. This allows more specialization of

phosphor selection and possible color filtering of the light

output from the CRTs.

Now that we know that no consumer display will be per-

fect, let’s discuss what sort of technique we can use to mea-

sure color accuracy.

6. Color Measurement

Color Bars

It is helpful to have some standard video test signals that

can be used to calibrate a display, or to measure the accu-

racy of the display’s color performance. The easiest signals

to generate are the common color bars that almost every-

one has seen at one point or another. (Figure 5)

Grayscale Color Temperature

The answer is that all brightness levels of white should pro-

duce the same standard D65 color temperature. That means

the relative light outputs from the three primaries must

track together as the total brightness changes. As a result,

any color generated by another mix of the primaries will

also stay at the same CIE (x,y) location regardless of the

brightness of the color. Grayscale is the term used to

describe the color temperature of the reference white over

the range from dark gray to peak white. The closer the

grayscale color temperature can be held to D65, the more

accurate the colors will be at all brightness levels within the

picture. For instance, if the color temperature increases to

the more blue-white of 7500K in the middle of the brightness

range, then colors will be seen with a bluer hue than desired

when they appear at that brightness level.

So we have seen that two conditions are necessary to

achieve perfect display color accuracy. The grayscale must

maintain a perfect D65 color temperature across the entire

brightness range of the display, and the CRT phosphors

must match the SMPTE C standards. It’s just that simple.

Unfortunately, we can’t seem to get any consumer products

that exactly match the SMPTE phosphor chromaticity stan-

Fig. 5

Color bar signals are generated by creating all possible

combinations of the three primary colors with each of the

RGB signals set to the same value. The resulting colors are

red, green, and blue, their complementary colors, cyan

(blue + green), yellow (green + red) and magenta (red +

blue), and white, which is always defined as equal RGB sig-

nal levels. The most common color bars are those that use

75 percent of the maximum signal value. We rather obvi-

ously refer to those as 75 percent bars, but 100 percent bars