Understanding Color
Color performance is not well understood by the general public, and even magazines aimed at home theater enthusiasts often get it wrong.
The color performance of any display is measured in three ways:
Gray scale performance
This is the measurement you see most often cited in reviews. Gray scale is commonly measured as a +- color temperature deviation from 6500 degrees kelvin across a specified range of output, which is usually presented in IRE, say 20-100 IRE. This roughly corresponds to about 2%-100% of the display's available light output. Results lower than 6500K mean that the display's color temperature suffers from excessive red, while results above 6500K mean that the display's color temperature suffers from excessive blue.
This is fairly easy to understand, but it is also a little misleading. Measuring gray scale in kelvin ignores green, so a display could have an appropriate balance of red and blue and thus measure near 6500K, but suffer from a severe excess or deficiency of green. In such a case, the image would look awful. A better measurement is Delta-E (dE or ΔE), which is a mathematical measurement of the degree to which a color deviates from an established reference. Neutral gray, which is sometimes referred to as D65, has a very specific definition of 0.3127, 0.329. These are xy points on a standard graph that presents color performance in a 2-dimensional space. It is called a CIE chart and is displayed below.
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| 1931 CIExy Chromaticity Diagram |
The odd part of calibrating a display's gray scale is that you are actually trying to remove as much color as possible. It is only when the entire range of brightness is a completely neutral gray without any color contamination that the display will present ONLY the color provided by the source. Otherwise, the display introduces its own color cast to the image, which can make it look very unnatural.
Gray scale tracking is a very important part of the calibration process. However, it is not the only aspect, because there are other measurements of color that we need to take into account.
Color Decoding
Images are all recorded in RGB format and displayed in RGB format. However, they are typically stored and transmitted in YPbPr (analog) or YCbCr (digital) format. These formats compress the native RGB signal with minimal loss of signal quality. Since RGB is converted to a compressed format and that format must be uncompressed prior to its final destination on the display screen, somewhere along the chain signal processing must occur. Sometimes, when the signal is improperly uncompressed or "decoded" errors are introduced. These are called color decoding errors. They typically present themselves as errors in the brightness of primary colors and hues of secondary colors. All NTSC displays have both a Color and Tint/Hue control. The main Color control primarily affects the brightness of the primary colors and the Tint/Hue control primarily affects the hue of the secondary colors. Thus, these controls are ideal for addressing color decoding errors. Unfortunately, Color and Tint affect ALL of the colors and color decoding errors are generally not evenly distributed amongst all of the colors. For this reason, Color and Tint are of only marginal use in resolving color decoding errors.
Some displays have dedicated color decoding controls, which are essentially independent color/tint controls for red and green. These supplement the set's standard color/tint control, which will have already presumably been set correctly according to blue. However, MOST displays do not have this. In such cases, the best a calibrator can do is set the main color/tint control for one of the reference colors and hope that the other two are within specifications.
The 3 Dimensions of Color
Despite its widespread use, the correct specification for a color cannot be fully expressed by the CIE chart. The reason for this is that the CIE chart provides a 2-dimensional representation in x and y axes, but color is a 3-dimensional property: saturation, hue, and brightness. A color's brightness is that third dimension that the CIE chart does not represent.
Perhaps this would be a good place to define the concepts more clearly.
The proper brightness of a color is not a fixed property like coordinates on a graph. It is a proportional relationship. Specifically, it is relationship between the brightness of a color relative to reference white. The established HD reference for this is:
White: Red: Green: Blue:
Color Gamut
Finally, we come to perhaps the most important aspect of color performance, the gamut. This is the actual definition of the primary and secondary colors as shown by the CIE chart above. If a display is well-engineered, then when the calibrator measures red, green, blue, cyan, magenta, and yellow they will appear exactly on the CIE chart where the established reference says they should. These are xy coordinates on the CIE chart just like neutral gray.
x y Red: 0.640 0.330 Green: 0.300 0.600 Blue: 0.150 0.060 Cyan: 0.225 0.329 Magenta: 0.321 0.154 Yellow: 0.419 0.505
Because this standard provides the actual definitions for how the colors will appear with respect to saturation and hue, this is arguably the most important of all of the measurements of color. Unfortunately, it is also the one that is the most difficult to adjust. The vast majority of displays offer no way to adjust the color gamut, although some digital displays do offer full-featured color management systems that provide this ability. Most of the time, however, the consumer is stuck with whatever color scheme the manufacturer wants to provide, and it is often inaccurate.