Published in March 2003

Color Display Calibration Basics
By Thomas Schulte, CET

Understanding how we perceive color, and how electronics create color, helps produce consistent displays.

     
All color video display devices share one critical calibration: color balance. This is true whether the device displays standard NTSC signals, standard computer-format signals, proprietary high resolution signals or HDTV signals. To display the most accurate colors, every video display’s color balance must be adjusted to match the color balance of the signal source. This exact balance of red, green and blue to make exactly the right color of white and neutral grays in the picture (often called White Balance) must be correct in both low brightness and high brightness parts of the picture. To understand how we see, specify and measure this color balance, let’s review some basics of light and color.

Human Sight Characteristics
     Light is electromagnetic energy within a narrow range of frequencies that are higher frequency than microwaves, but lower frequency than x-rays. If seen by itself, each different frequency, or wavelength, of light energy is perceived by the human eye/brain as a different, fully saturated, color. In a certain respect, the eye/brain acts like a tuned, high-frequency radio receiver (see Figure 1).

Three characteristics define the way the human eye/brain sees light that is either radiated from or reflected off an object.

  • Brightness: The eye sees the total amount of light energy radiated or reflected by an object, in comparison to other light sources or lighted objects in the field of view, as the brightness of the object.
  • Saturation: A single frequency of light, at any energy level, is seen as a fully saturated, vivid color. The saturation of that color decreases as any other light frequency, or any combination of light frequencies, is added to the original light. If equal energy of all visible light frequencies is added together, the result is zero saturation, pure white light.
  • Hue: For any combination of light energy other than pure white, the dominant perceived frequency of the combined light energy is known as the hue of the light.

Light Measurement
     To specify different amounts and combinations of light energy, we would like to measure light in a way that corresponds closely to the way we see light. In the video-display industry, two types of measurement units are used to measure light and relate it to the human sight characteristics: luminance and chromaticity.
• Luminance is a type of light measurement closely related to the human sight characteristic of brightness. The US measurement 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. (A footcandle, on the other hand, is a measurement unit of illuminance, the light falling on an object.)
• Chromaticity is a type of light measurement related to the human sight characteristics of hue and saturation. This combined method of chromaticity measurement was developed by the CIE (International Commission on Illumination) in 1931 and is used by the video industry for all display color measurements. The CIE developed a chromaticity diagram that graphically depicts the relationship between the hue and saturation of light sources.
     The CIE Chromaticity Diagram (Figure 2) plots the pure spectral colors (hues) around the curved border (380-780nm). The results of mixing any of these fully saturated spectral colors are shown at the base and interior of the diagram. Any visible color can be specified by the x coordinate and the y coordinate position of that color on the diagram.

White Reference
     The CIE coordinate pair of x = 0.333, y = 0.333 (E) specifies the white light produced by mixing equal light energy of all wavelengths (zero saturation). The color of any point immediately surrounding the equal energy white point would also appear white, if seen by itself with no other color reference.
     It would seem logical that equal-energy white (E) would be used as the standard color of white for video display systems. However, because a more bluish white appears brighter, the CIE coordinate pair of x = 0.313, y = 0.329 (CIE standard illuminant D65) is the white “color” that was chosen as the standard white reference for all video and computer display systems. This allows displays to appear brighter without producing additional light energy output, yet doesn’t shift the color of white enough to be detrimental to color accuracy.

White From Red, Green, Blue
     On the CIE chromaticity diagram, if any three color points are chosen, the area included by the connecting triangle represents the range of colors that can be produced by mixing the three chosen colors (Figure 3). The three points are known as the primary colors. The connecting triangle encloses the full range of colors (gamut) that can be created 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 give us color vision. The rod receptors give us black-and-white vision, especially in small detail and low light.
     Each of the red, green and blue cone receptors has a different response to different colors (frequencies) of light. The average response of the human eye receptors to light across the visible spectrum is shown by the Standard Observer Response graph, developed by the International Commission on Illumination (CIE) (Figure 5).
     We call this tristimulus vision because there are three types of receptors that individually send information to our brain and allow us to perceive different colors for the different mixtures of light energy within the visible spectrum.

Tristimulus Measuring Devices
     Tristimulus color measurement devices are called colorimeters (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 each of the three types of cones in the eye, only in electronic-signal form. This information allows us to compute a different measurement result for the different mixtures of light energy within the visible spectrum, in a way that duplicates the response of the human eye/brain combination. To accurately predict the response of the human eye to a combination of light energy at different frequencies, a tristimulus color measurement device must “see” light exactly the same way the human eye sees light, as documented by the CIE Standard Observer Response graph (Figure 5).

Display Technology Challenges
     Each of today’s new display types—plasma, LCD, DLP, LCOS, D-ILA, etc.—produces light energy with a spectral power distribution (SPD) that is usually quite different from the average spectrum produced by CRTs. This is a change from working only with direct-view CRTs because all CRT phosphor sets produced strong peaks of light and low levels of light at pretty much the same color frequencies. Some of the new display types produce strong peaks of light at color frequencies where CRTs produce very little light. Each of the new display types can still produce a standard color of white by adjusting the relative balance of colors in the red, green and blue portions of the spectrum (Figure 7).
     Because the new technology displays may produce strong peaks of light at just about any color frequency, it is 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 response as the three color sensors of our eye (Figure 8).
     If a colorimeter uses optical filters that are accurate to the human-eye response at all frequencies of light, not just high output CRT frequencies, the instrument will accurately measure all displays of the past, present and future, no matter what their SPD.
     Displays can be made to have accurate colors. Also, adjacent displays can be made to produce the same accurate colors, whether they are multiple projection displays in the same facility, or the individual cubes of a video wall. Accurately measuring and calibrating the color of each display is the key to producing beautiful end results.

Thomas Schulte, an application engineer at Sencore, Inc., for the past 16 years, has taught and worked in the video industry for many years.

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