Delta-E (ΔE) is a measurement used to indicate how much a color deviates from an accepted standard. The higher the ΔE, the more inaccurate the color. Perfect color has a ΔE of zero. However, we need not achieve a ΔE of zero. This is because the human eye is only capable of detecting color difference at certain thresholds. The minimal detectable difference is about 1 ΔE. Calibration generally seeks to achieve a ΔE for white and all of the primary and secondary colors of no more than 2.5. However, unless a display has a fully-realized color management system (the vast majority do not), it is unlikely that calibration can achieve this level of performance for the primary and secondary colors. On the other hand, since virtually all displays have a full compliment of gray scale adjustments, calibration can usually meet this standard for white.
ΔE came into widespread use after the CIE (Commission Internationale De L'Eclairage or International Commission on Illumination) announced in 1976 two new color appearance models that were to replace the standard XYZ model for measuring color that had been in place since 1931. Why the need for a new standard? The reason has to do with the fact that researchers had known for some time that the existing standard was not perceptually uniform. Perceptual uniformity is the ability of a color appearance model to plot changes in color that accurately represent what we actually see. The standard CIE diagram shown on the Understanding Color Calibration page is NOT perceptually uniform. To see how, consider the diagram below, which is another representation of the 1931 CIE diagram.
|1931 CIEXYZ Chromaticity Diagram shown with perceptual non-uniformity indicators|
Notice that the lines inside the diagram are not of equal length. This indicates that at different points on the graph, the distance between two colors plotted on the graph is much larger than the difference people see between those colors. What the CIE wanted was a color appearance model that reduced this problem as much as possible. So, in 1976 the CIE recommended two new color appearance models that were significantly more perceptually uniform than the 1931 CIEXYZ standard represented by the chart you see above. These 2 standards were CIELUV and CIELAB, and they both improved perceptual uniformity from the 20:1 that the old system provided to about 4:1. Not perfect to be sure, but much better.
Why, you might ask did the CIE recommend TWO color appearance models to replace the single 1931 standard? Although both CIELUV and CIELAB were roughly equivalent in accuracy, CIELAB was clearly the favored model. However, it lacked something that many industries thought was essential: a chromaticity diagram that could plot the primary and secondary colors on straight lines. CIELAB offered none, but CIELUV did. Thus, the CIE offered both standards as something of a compromise. These two color appearance models also included a formula for calculating color differences. This has become known as ΔE76, which could be calculated from CIELUV or CIELAB data.
Unfortunately, color researchers began to notice that ΔE76 had it own problems with perceptual uniformity, so work continued on an even better color difference formula. In 1994 CIE approved a new formula, based exclusively on the CIELAB model. This is known as ΔE94.
CIE has continued to further refine methods for measuring color differences, including the adoption in 2000 of a new CIELAB standard (ΔE2000), which, for a variety of reasons, was never widely used. ΔE94 continues to be arguably the best and most popular of the color difference formulas. However, because CIELUV and its associated ΔE76 formula is still the only approach that offers a linear chromaticity diagram, it is still widely used in the video industry.
|1976 CIELUV Chromaticity diagram|
I continue to use the old-fashioned 1931 diagram, only because people are more familiar with it and I don't mind the fact that it tends to exaggerate certain color errors, especially in green. On the other hand, I use ΔE94 as a measurement of color difference. Because of this, you may notice that the ΔE numbers I report seem smaller than those you may have seen elsewhere. This is because many people in the video industry continue to use ΔE76 and ΔE94 numbers are almost always on a smaller scale. For example, a very large chromaticity error in green that has a ΔE76 of 50 is equivalent to a ΔE94 error of around 12. The error is the same. It is just that the method for reporting is scaled lower.