Delta E
Delta E turns up in 2 fields: powder coating, paper optics. Here is what it means to the people in each one.
Delta E in powder coating
A single number for how far a color is from its standard, read by a spectrophotometer.
A spectrophotometer reads lightness, red-green and yellow-blue values of a coated part and the standard panel, and delta E combines the differences into one figure. A delta E around 1 is about the smallest difference most people notice side by side, and architectural specs often cap batch-to-batch variation near that. Readings shift with gloss, film build and cure, so panels are read in the same conditions.
"Delta E is 2.3 against the standard, mostly in the yellow, I think the oven's overbaking."
Delta E in paper optics
Single number for the total color difference between a sample and its target, usually in CIELAB units.
The basic form, Delta E*ab, is the straight-line distance between two points in CIELAB space. A difference under about 1 is hard for most people to see side by side, while 2 to 3 is obvious to a trained eye on a white or pastel. Formulas such as dE00 weight lightness, chroma and hue differently to track perception better, so a spec must name the formula as well as the limit.
"Delta E to the master is 1.4, inside the 2.0 limit, but the customer grades by eye under their own lights."