How Big is a MPCD Unit?

[ChthonVII]

How big is a MPCD (minimum perceptible color difference) unit?

I gather (perhaps incorrectly) that MPCD is a unit of euclidean uv distance in the CIE 1960 UCS, to a given point from the nearest point on the black body curve. But I cannot for the life of me find the definition of 1 MCPD = ??? delta uv.

[KelSolaar]

Hello,

I don't think I ever seen it defined for CIE 1960 UCS, it is not typically a space where colour differences are measured and it is almost strictly relegated to colour temperature representation.

Cheers,

Thomas

[ChthonVII]

Whitepoints for CRT televisions and monitors were often specified with a color temperature plus a MCPD offset perpendicular to the black body curve. For instance, if you owned a CRT computer monitor in the 80s or 90s, there's a good possibility that the whitepoint was 9300K + 8 MCPD.

See, for instance, pp 277-78 in Poynton's Digital Video and HD: Algorithms and Interfaces, 2nd Edition:

An illuminant may be characterized by a single colour temperature number – the temperature of a blackbody radiator that exactly matches the chromaticity of the source. If the match is approximate, the term correlated colour temperature (CCT) is used.

Colour temperature is sometimes augmented by a second number giving the closest distance in the deprecated CIE 1960 [u, v] coordinates of the colour from the blackbody locus – the arcane “minimum perceptible colour difference” (MPCD) units. I consider it more sensible to specify colour temperature in kelvin for intuitive purposes, accompanied by [x, y] or [u’, v’] chromaticity coordinates.

Unfortunately, while Poynton tells what what a MPCD is conceptually, he doesn't tell us how to do the math. Hence my question: How big is a MCPD unit?

(Incidentally, by converting the commonly cited xy coordinates for 9300K + 8MPCD and 9300K + 27MPCD to 1960 UCS uv coordinates, and dividing the distance by 19, I get something around 0.0004.)