Delta Ch is a metric of gray balance error.
Delta-Ch (formerly known as Delta-F*) is an absolute (always positive) expression of the combined delta a* and delta b* values, where:
∆Ch = (∆a2 + ∆b2)0.5
Delta-Ch is the shortest line, or vector, between two points on the a*/b* graph, as shown below.
The original name for Delta-Ch was “∆F*”, where the letter F stood for the German word for color – “Farbe”. Both terms mean the same thing and may appear in various literature.
∆Ch is an excellent metric of gray balance or “nearness to neutral” because color errors are more noticeable than lightness errors in grays or near-neutrals.