Color Accuracy is measured for each of the 24 color patches on the target. The Color Accuracy of the camera system is calculated by converting the RGB digital code values of the camera to CIELab values, then comparing them to the software CIELab aim values. The conversion of camera RGB code values to CIELab is defined by a camera’s color space setting in which the images were captured. Imaging Gauge™ performs this conversion for the four most common color spaces used in cameras: sRGB, Adobe RGB, ProPhoto and ECI
The comparison of camera CIELab values to the known aim values is accomplished using the “Delta E” calculation recommended by the International Commission on Illumination (CIE). The metric has evolved over time, with three versions available as part of Imaging Gauge™ software (1976, 1994 and 2000). The simplest of these is the earliest and a sample calculation for the dark brown color patch is shown below. For images where the calculated CIELab values are identical to the aim, the Delta E is zero. Delta E values further from zero indicate the degree to which the colors are inaccurate.
Imaging Gauge™ employs a Delta E comparison to determine the Color Accuracy of a color patch. Each color included in the Imaging Gauge™ target has a known set of CIELab values (L*, a* and b*). These values are set by the manufacturer of the patches (X-Rite, Inc.), verified by APPLIED IMAGE using a NIST traceable spectrophotometer and stored in the CIELab Values.txt file located in the Support Files folder.
The maximum and mean Delta E values are calculated for all twenty-four color patches. Theoretically, two colors that measure ΔE < 1 are indistinguishable by the human eye.
The unacceptable image was created by shifting the hue angle of the captured image. This results in changes to the chromatic patches while leaving the neutral patches relatively unchanged.
Color Accuracy has two specification limits for the Delta E metric:
Maximum is the highest allowable Delta E value for any single color patch.
Mean is the highest allowable value for the mean of all twenty-four color patches.