User Manual

5. Color Temperature and Grayscale Calibration
Accurate color depends on calibrating the display for the proper grayscale color temperature of 6500 degrees
Kelvin, which is called D65 or D6500. The color temperature must remain constant over the entire range of the
grayscale from near black to peak-white (100 IRE). It is somewhat ironic that accurate color is dependent on
accurate shades of gray. If the color temperature of gray is too high, the picture will have a blue tint. If the color
temperature is too low, the picture will have a red tint. If the color temperature varies from dark to bright the color
tint will also vary throughout the picture. An electronic instrument called a Color Analyzer is required to measure the
color temperature over the range of grayscale values from about 10 IRE to 100 IRE to ensure it is accurate and
remains constant.
Direct-view CRT monitors tend to have the best grayscale tracking behavior when properly calibrated. The color
temperature can usually be adjusted to remain within a few hundred degrees Kelvin of D65 over the entire
grayscale range. Projectors tend to have a wider variation in color temperature even with best possible calibration.
A total variation of about 1000 degrees Kelvin is not uncommon. CRT projectors tend to become more red at
brightness levels above 75 IRE because the beam current of the blue CRT is stressed the most.
Display products provide two (red and blue) or three (red, blue, and green) controls to adjust the color temperature
at the dark end of the grayscale. These are usually called the Bias or Cutoff controls and they adjust the offset level
of the appropriate video amplifiers. They have the largest effect at the darker end of the grayscale between 10 and
50 IRE. Two (red, blue) or three (red, blue, green) Gain or Drive controls are provided to adjust the gain of the
video amplifiers. They affect the color temperature over the entire grayscale so the Bias and Gain controls interact
over the grayscale range. The Bias controls also directly affect the black-Level, which must be readjusted as the
Bias controls are adjusted. The Gain controls affect the 100 IRE brightness, so the Contrast control will also have to
be re-adjusted as the Gain controls are changed (particularly the green gain).
BK-1253 Grayscale Calibration Patterns:
The BK-1253 includes a Window patterns that consist of a gray rectangle adjustable between 0 and 100 IRE
against a black background. This ranging is convenient to use for adjusting the grayscale color-temperature
controls. As these controls are adjusted the black level may shift. The left side of the Window pattern has PLUGE
stripes so that the Black-Level (Brightness control) can be adjusted if necessary as the grayscale adjustments are
made. There is also an ANSI Grey pattern with full screen 100 IRE with 90 IRE and 95 IRE boxes. This can be
used with DLP and LCD projectors to verify that the peak-white level is not clipped while adjusting the Gain
controls. If that happens use the Contrast control to reduce the peak-white level so that both boxes are distinctly
visible again.
The BK-1253 also provides a Horizontal Luminance Linearity Staircase pattern with 10-IRE steps from 0 to 100 IRE
to verify the color temperature at other luminance values. These patterns should appear to have a constant color
temperature. Displays can also have hot spots that might affect the color temperature in some physical portions of
the screen.
Grayscale Adjustment Procedure:
The Gain and Bias controls will interact when used to set the color temperature over the entire grayscale range. A
Color Analyzer must be used to ideally set the color temperature to D65 (x=0.3127, y=0.3290) at 25, 50, 75, and
100 IRE using the Grayscale Window calibration patterns. Be sure to adjust Black-Level (Brightness control) using
the PLUGE patterns and the 100-IRE Peak-White brightness using the 100 IRE Grayscale Window pattern before
beginning the grayscale adjustment.