Making a Granger Rainbow for Debugging Color Management

The Granger Rainbow is an image which presents all saturated rgb or lab colors, possible or impossible, in a smooth palette.  When printed such an image allows one to assess both the gamut of a device and the smoothness of a rendering, from one image.

To create a Granger Rainbow in Photoshop, in Lab space, we will cross two gradients. The gradient tool can be selected by typing "g". Starting from a filled white canvas, we first deploy a  "Spectrum" gradient preset that cycles through all hues from left to right.  

Then we apply a "black and white" gradient to the L channel only, from bottom to top, to get the light tones in the sky where we expect them. Tip: Hold the shift key down while drawing the gradients in order to force a square orientation.

Edmund Ronald