Identical gray patches Munsell #N5 (0.7 neutral density) are placed in the corners of the target to allow the maximum distance to be used to measure uniformity. Uniformity is measured using a relative Delta E metric between these four patches.
As shown below, the upper left patch is significantly lighter on the unacceptable image due to a specular lighting pattern. The Delta E between this patch and the other three patches are all above specification. Uniformity between the other three patches is acceptable (below 10).
Light in the Upper Left quadrant results in large DeltaE values between it and the other three quadrants.
A single value is entered for the uniformity metric. This value is used as the limit for all six combinations.