Color Display Calibration Basics

BY THOMAS SCHULTE, CET

Understanding

how we perceive

Figure 1. The human color, and how eye sees energy in a small slice of the electromagnetic electronics spectrum as visible light. create color,

helps produce

consistent

displays.

All color video display devices share one different frequency, or wavelength, of light critical calibration: color balance. This is energy is perceived by the human eye/ brain true whether the device displays standard as a different, fully saturated, color. In a NTSC signals, standard computer-format certain respect, the eye/brain acts like a signals, proprietary high resolution signals tuned, high frequency radio receiver (see or HDTV signals. To display the most ac- Figure 1). curate colors, every video display’s color Three characteristics define the way the balance must be adjusted to match the color human eye/brain sees light that is either balance of the signal source. This exact radiated from or reflected off an object. balance of red, green, and blue to make • Brightness: The eye sees the total exactly the right color of white and neutral amount of light energy radiated or reflected grays in the picture (often called White by an object, in comparison to other light Balance) must be correct in both low sources or lighted objects in the field of brightness and high brightness parts of the view, as the brightness of the object. picture. To understand how we see, specify • Saturation: A single frequency of light, and measure this color balance, let’s review at any energy level, is seen as a fully some basics of light and color. saturated, vivid color. The saturation of that Thomas Schulte, an color decreases as any other light frequency, application engineer at Human Sight Characteristics or any combination of light frequencies, is Sencore, Inc., for the past Light is electromagnetic energy within a added to the original light. If equal energy of 16 years, has taught narrow range of frequencies that are higher all visible light frequencies is added and worked in the video frequency than microwaves, but lower together, the result is zero saturation, pure industry for many years. frequency than x-rays. If seen by itself, each white light.

Figure 2. • Hue: For any combination of light en- The 1931 ergy other than pure white, the dominant C.I.E. perceived frequency of the combined light Chromatic energy is known as the hue of the light. ity Diagram provides a Light Measurement convenien To specify different amounts and combi- t tool for easily nations of light energy, we would like to specifying measure light in a way that corresponds the hue closely to the way we see light. In the video and display industry, two types of mea- saturation of any surement units are used to measure light visible and relate it to the human sight character- color. istics. These two measurement units are luminance and chromaticity. • Luminance is a type of light measure- ment closely related to the human sight characteristic of brightness. The U.S. mea- surement unit of luminance-foot-lambert- specifies the number of lumens of light energy emitted from each square foot of a light source, such as a video display, or reflected off a solid object. (Footcandles, on the other hand, are a measurement unit of illuminance, the light falling on an object.) • Chromaticity is a type of light mea-

surement related to the human sight characteristics of hue and saturation. This combined method of chromaticity measurement was developed by the C.I.E. (International Commission on Illumination) in 1931 and is used by the video industry for all display color measurements. The C.I.E. developed a chromaticity diagram which graphically depicts the relationship between the hue

Figure 3. A triangle formed by and saturation of light sources. Figure 4. Three types of cone receptors in our any three colors encloses all The C.I.E. Chromaticity Diagram eye provide response to light in the red, green the colors able to be formed (Figure 2) plots the pure spectral and blue portions of the visible spectrum. by mixing the three colors. colors (hues) around the curved border (380-780 nm). The results of produced by mixing equal light energy C.I.E. coordinate pair of x = 0.313, y = mixing any of these fully saturated of all wavelengths (zero saturation). 0.329(C.I.E. standard illuminant D65) spectral colors are shown at the base The color of any point immediately is the white “color” which was chosen and interior of the diagram. Any surrounding the equal energy white as the standard white reference for all visible color can be specified by the x point would also appear white, if seen video and computer display systems. coordinate and the y coordinate by itself with no other color reference. This allows displays to appear position of that color on the diagram. It would seem logical that equal-en- brighter without producing addition- ergy white (E) would be used as the al light energy output, yet doesn’t shift White Reference standard color of white for video dis- the color of white enough to be The C.I.E. coordinate pair of x = 0.333, play systems. However, since a more detrimental to color accuracy. y = 0.333 (E) specifies the white light bluish white appears brighter, the

White from Red, Green, Blue On the C.I.E. chromaticity diagram, if any three color points are chosen, the area included by the connecting triangle represents the range of colors that are able to be produced by mixing the three chosen colors. The three points are known as the primary colors. The connecting triangle encloses the full range of colors (gamut) which is able to be produced by a display that produces mixtures of those colors of red, green, and blue light.

The Eye: A Tristimulus Device The human eye sees light through rod- and cone-type light receptors (Figure 4). Red, green and blue cone type receptors that give us color Figure 5. The C.I.E. Standard Observer Response graph shows the relative sensitivity of each type cone at different light frequencies. vision. The rod receptors give us black-and- white vision, especially in small detail and low Response graph, developed by the International Commission on light. Illumination (C.I.E.) (Figure 5). Each of the red, green, and blue cone receptors We call this tristimulus vision because there are three types of has a different response to different colors (fre- receptors that individually send information to our brain and allow us quencies) of light. The average response of the to perceive different colors for the different mixtures of light energy human eye receptors to light across the visible within the visible spectrum. spectrum is shown by the Standard Observer

Figure 6. The three light Tristimulus sensors in a colorimeter receive filtered light with Measuring Devices responses equal Tristimulus color measurement to that of the devices are called colorimeters human eye. (Figure 6). This type of device works similar to the human eye. Three filtered light sensors receive light from the source to be measured. The filter for each light sensor allows only a certain amount of each color of light to reach the sensor. The response of each of the three filters is designed to mimic the response of one of the types of cones in the average human eye. The measurement information from each of the three light sensors is the same as the information from accurately predict the response of the Display Technology each of the three types of cones in the human eye to a combination of light eye, only in electronic-signal form. energy at different frequencies, a Challenges This information allows us to compute tristimulus color measurement device Each of today’s new display types - a different measurement result for the must “see” light exactly the same way plasma, LCD, DLP, LCOS, D-ILA, etc. different mixtures of light energy the human eye sees light, as - produces light energy with a spectral within the visible spectrum, in a way documented by the C.I.E. Standard power distribution (SPD) that is that duplicates the response of the Observer Response graph. usually quite different from the human eye/brain combination. To average spectrum produced by CRTs.

This is a change from working only If a colorimeter uses optical filters Figure 7. Different display with direct-view CRTs, because all that are accurate to the human-eye technologies each have a unique CRT phosphor sets produced strong response at all frequencies of light, not spectral power distribution (SPD). (Courtesy Joe Kane Productions) peaks of light and low levels of light at just high output CRT frequencies, the pretty much the same color instrument will accurately measure all frequencies. Some of the new display displays of the past, present, and types produce strong peaks of light at future, no matter what their SPD. color frequencies where CRTs produce Displays can be made to have ac- very little light. Each of the new display curate colors. Also, adjacent displays types can still produce a standard color can be made to produce the same ac- of white by adjusting the relative curate colors, whether they are mul- balance of colors in the red, green, and tiple projection displays in the same blue portions of the spectrum (Figure facility, or the individual cubes of a 7). video wall. Accurately measuring and Because the new technology displays calibrating the color of each display is may produce strong peaks of light at the key to producing beautiful end just about any color frequency, it is results. now critical that a colorimeter’s optical filters must accurately duplicate the CIE standard observer response at all color frequencies, not just at the particular frequencies at which direct-view CRTs produce high light output. Its three color sensors must see light over the entire visible spectrum with the identical amplitude Figure 8. The response of a response as the three color sensors of colorimeter’s optical filters (white) our eye (Figure 8). needs to be accurate at all color frequencies to accurately measure light from any type of display.