To specify precise color measurements of light energy from a source or reflecting surface requires light measurement units. Many light measurement units have been developed, but only a few are important in video system calibration and servicing.

The footlambert is the U.S. measurement unit of luminance. It specifies the amount of light energy, per square foot, emitted from a light source (such as a CRT) or reflected from a lighted surface.

Luminance is a light measurement term closely related to the human sight characteristic of brightness. The luminance of monitor CRTs, at full brightness and contrast settings, is typically in the range of 30 to 100 footlamberts.

A more familiar light measurement unit, the footcandle, is very similar to the footlambert. The footcandle is the U.S. measurement unit of illuminance. It specifies the amount of light energy, per square foot, falling on a lighted surface.

The specification of light measurements relating to hue and saturation has proven a bit more complex. In 1931 an international commission on illumination (the C.I.E.) developed a chromaticity diagram which graphically depicts the relationship between hue (light wavelengths) and purity and allows the specification of individual colors with an x-y grid system.

This C.I.E. Chromaticity Diagram (also known later as the Kelly Chart for improvements Mr. Kelly made to it) also shows the affect of mixing two or more colored lights to produce another color or white.

The tongue-shaped diagram shows the pure spectral colors around the curved border of the tongue, plus the results of mixing any of these spectral colors at the base and center of the tongue.

If any two color points are chosen on the diagram, a line drawn between the points passes through the range of colors produced by mixing various proportions of the two original colors. Colors shown around the outside of the diagram are highly saturated, progressing to zero saturation white at the diagram's center.

Note that the perception of purple and other colors at the base of the diagram cannot be produced by a single wavelength, but require a mixture of short and long wavelengths.

If any three color points are chosen, the area included by the connecting triangle represents the range of colors able to be produced by mixing the three chosen colors. The three points are the colors of the CRT phosphors primaries specified by the NTSC for the U.S. television system (but seldom precisely the actual phosphor colors used in TVs). The connecting triangle includes the full range of colors able to be produced by a CRT using these color phosphors.

Any three colors not lying on a straight line with one another are a set of color primaries.

The C.I.E. diagram provides a grid coordinate system to specify a precise measurement of any spectral color or color combination. A C.I.E. coordinate of x=0.333 and y=0.333, for example, specifies the white light perception produced by equal light energy of all wavelengths.

The color of any point immediately surrounding the equal energy white point would, if seen by itself, be perceived as white. This range of nearly white colors surrounding the equal energy white point is known as the "near white" region of the C.I.E. diagram.

We sometimes use yet another method of measurement known as color temperature, to specify the differences in the hue of near-white light sources. The unit of measurement for color temperature is degrees Kelvin (°Kelvin = °Celsius + 273). The reference for this measurement is the color of light produced when carbon is burned at different temperatures.

Common light sources range from a color temperature of around 2500° K (more red) to over 9000° K (more blue). Equal energy white is approximately 6000° K.

An easy comparison to think of for color temperature is a black iron bar heated to different temperatures in a blacksmith's forge. As the bar begins heating, it glows an orangish-red, while at hotter temperatures the bar becomes a bluish-white.

Below are color temperatures of some common light sources and white reference standards. Incandescent light contains more red light, while fluorescent light or sunlight tends to be more blue. Any object reflects light of slightly different hues under each of these different lighting conditions.

In order for a CRT to produce an image on the screen, the intensity of the electron beam from the cathode to the phosphor screen must be made to vary. Changing the bias voltage in step with the video image to be produced varies the beam current. White and bright colors require high beam current, while grays and dark colors require a small beam current.

A video signal containing voltage variations that correspond to the picture is applied to the monitor. The video signal controls the three separate CRT electron guns. To produce various shades of gray, all three guns are turned equally more on or more off.

To produce color the three video signals change differently from each other. One or two guns are turned on more and the other gun(s) are turned more off.

Before being applied to the CRT, the signal is amplified by the monitor’s video display circuits. Depending on the type of monitor, video input signals are typically 1 VPP or 5VPP. A signal of 100 or 200 VPP is needed to drive most CRTs.

Video display circuits include two sets of adjustments to produce neutral white and proper grays and colors over the entire brightness range. The first set of adjustments are called the CRT cutoff or bias controls. They control the DC bias applied to the CRT guns to obtain color balance (neutral gray) at low luminance level just above cutoff.

The second set of adjustments is called the CRT drive or gain controls. They control the maximum amplitude of the video signal sent to each CRT gun to obtain balance (neutral white) at a high luminance level near maximum gun conduction.

Adjusting the bias and drive controls in a video display is called the White Balance or Gray Scale adjustment.

A color analyzer measures the x and y chromaticity and Y luminance or brightness parameters required for making the important computer monitor color and brightness adjustments. A color analyzer makes the x and y and luminance parameter measurements in the following way:

The red, green, and blue light emitted by the CRT phosphors enters the receptor area of the measuring probe. The light passes through the spectral-response correction filters and strikes the light sensors. Each light sensor outputs a voltage that is proportional to the intensity of the light striking it. These voltages correspond to both the intensity and color temperature of the light applied to the probe.

 

Analog-to-digital converters change the voltages into digital values that are displayed in either a graphical or a numerical format.