TV Tweaks - Part Two: Behind the Numbers Page 2
Color TemperatureColor-temperature readings are usually taken with a TV's Low or Warm preset selected, since that's often the one that measures closest to 6,500 K. Using a Photo Research PR-650 spectroradiometer, we take readings from both high- and low-level window test patterns in the "Grayscale and Levels" section of Ovation Software's Avia Guide to Home Theater test DVD (also see "House Calls"). The IRE scale (named for the Institute of Radio Engineers) is used to specify the light intensity of a TV picture, from black at 7.5 IRE to peak white at 100 IRE. In general, a 20-IRE pattern is used for the low-window measurement and a 100-IRE pattern for the high-window measurement, although the exact patterns used will vary with the type of set being tested. For example, an 80-IRE pattern is used for high-window measurements on rear-projection TVs, while a 30-IRE pattern may be required for the low-window measurement on a set with weak light output.
"Before calibration" readings indicate the set's color temperature after its user-accessible picture controls (brightness, contrast, color, tint, and sharpness) have been optimally adjusted. Since the numbers obtained at this point rarely come close to 6,500 K, we usually have to make additional adjustments in the set's service menu using manufacturer-supplied service codes. These adjustments are based on readings by the radiometer, which provides both x and y values on the CIE chromaticity diagram and correlated color-temperature readings in kelvins.
The "after calibration" readings indicate the set's color temperature after service-menu adjustments have been performed. In most cases, the high- and low-window numbers will now be within +/-100 K of the NTSC standard. For a really accurate calibration, however, the set must yield similar readings at other points of the grayscale, from 20 to 100 IRE. Although we take a full set of measurements at 10-IRE increments as part of our test procedure, we only print the high- and low-window numbers, but the others also figure in the overall assessment of the set's "grayscale tracking" given in the notes part of the lab box.
BrightnessBrightness readings, taken from the high-level window pattern from the Avia DVD, are given in footlamberts (ftL), a unit representing the amount of light emitted by a source in an area of 1 square foot. TV manufacturers tend to set brightness and contrast levels extremely high at the factory, so a set's "before calibration" brightness measurement will often be higher than its "after calibration" measurement.
A properly calibrated direct-view TV in a dimly lit environment should deliver about 30 ftL of brightness. Acceptable levels for projection TVs may be even lower - around 20 to 30 ftL for a rear-projection model and 10 to 15 ftL for a front projector. To put those numbers in perspective, the THX Theater Alignment Program (TAP) guidelines for certifying movie theaters call for 16 ftL +/-2 ftL measured at center screen. Most theaters fail to meet the TAP spec, however, measuring only 8 to 9 ftL on average.