QTR-Quadtone RIP

The AB values indicate how far and in what direction the readings deviate from neutral.

The A values are the red/green axis of the Lab color space and the B values are the yellow/blue axis. Positive A is to the red side of neutral and positive B is the yellow side of neutral.

The term “Lab” is a convenient way of referring to the 3 dimensional L*a*b* (pronounced L star, a star, b star) color space.

Since the left Y axis in the chart denotes the luminance axis in L*a*b* color space, it would be more accurate (but perhaps no less confusing) if the left Y axis was labeled L or L* and the right Y axis was labeled ab or a*b*.

So what the chart shows is a cross over in the middle values from warm gray to a cooler gray and then back to a some what warm shadow region.

Hope my answer isn’t too far into the color management weeds.

Oz