Vision researchers, however, are not the only ones who need well-calibrated CRT (cathode-ray tube) screens. Users in graphic arts, TV broadcasting and other professions also require accurate measurement of color coordinates, correlated color temperature and the luminance of television and computer monitors.

Color-matching test
The best device for quantifying and calibrating the color and brightness of a CRT screen is a colorimeter (Figure 1), an instrument that determines the amounts of red, green, and blue light that, when additively mixed, match a test color in International Commission on Illumination chromaticity coordinates. There are many different types of colorimeters, most of which employ three color filters and three detectors.

Colorimeter performance can degrade if excessive CRT light is lost as it’s transmitted to the detectors, and spatial nonuniformity (the variation in brightness and chromaticity between two points on the screen) can produce measurement discrepancies.

To overcome these potential problems in its SLS 9400 handheld colorimeter, Graseby Optronics of Orlando, Fla., studied several options. Product engineers first considered traditional diffusing materials, such as ground glass and opal glass. Opal glass spreads the light uniformly through the color filters and onto the detectors, but it attenuated the light emitted from the CRT screen, causing the loss of too much signal. Without adequate signal to process, Graseby’s handheld colorimeter would not be sensitive enough to measure very low light levels, a customer requirement.

Ground glass proved even less effective. Mechanically etched so that it essentially had a frosted surface, the ground glass failed to produce the even field of light necessary to overcome spatial nonuniformity, resulting in erroneous color measurements.

Finally, the engineers turned to a new optical solution, a holographic optical element known as a light-shaping-diffuser. These surface-relief holographic elements shape light by precisely controlling the energy distribution along the horizontal and vertical axes, spreading light evenly.

Located in a measurement probe that attaches to the display via a vacuum-seal suction cup, the diffuser homogenizes and shapes the light emitted from the CRT screen and transmits it through the instrument's color filters and evenly onto all of the photodiode detectors (Figure 2). Thus, the instrument can manage spatial nonuniformity so effectively that the measurement probe can be rotated in the same position on the screen and still render the same precise X and Y chromaticity values.

Uniform light distribution
Manufactured by Physical Optics Corp. of Torrance, Calif., the diffusers distribute a uniform field of light across the detectors in a circular pattern 1.7 in. in diameter. This is important because each of the detectors, which are positioned adjacent to one another, must "see" the identical sampling of light to process accurate color information. The transmissive diffusers employed in the SLS 9400 have an angular divergence of 65° and homogenize and transmit more than 90 percent of the light emitted from CRT screens. Laminated on a glass BK7 substrate, the diffuser is protected from dirt and scratches by a clear piece of glass on the outside of the measurement probe’s suction cup.

First introduced in late 1994, the Graseby SLS 9400 has four detector/filter combinations and achieves X and Y chromaticity measurement accuracy of 0.002 over the full color range of a CRT. The software has a data logger that programs the instrument to take automatic CRT color measurements at regular intervals.

Meet the author
Scott Giancola is sales manager of Graseby Optronics in Orlando, FLA. He has a bachelor’s degree in imaging science from the Center for Imaging Science at the Rochester Institute of Technology.