### There are two sorts of Duv (Delta u v and Delta u' v') in lighting - what is the difference?

Unfortunately there are two sorts of Duv, and they are both to do with the color of light sources, which makes it really confusing. Here they are:

Duv on a CIE 1960 u-v chromaticity diagram: This measures how far away the color is away from the Plankian locus of black body radiation. It's assumed that this locus is natural light in at various times of day. So the closer the u-v coordinates of the light is to that locus the more "natural" and "high quality" the light is.

D u'v' spatial chromatic nonuniformity: The other "Duv" is the spatial chromatic nonuniformity of a given light source. In English this means that Delta u' v' is a measure of how far the color of a light source varies at different angles. For example a LED light might be more blue in the horizontal gamma=90° direction than it is the the vertical gamma=0° direction. Here's an invented example:

The colors have been exaggerated but you can see that at gamma=0° there is more purple/red than at gamma = 90°.

So what is an acceptable nonuniformity in the color of a single light source? Well IESNA LM 79 08 says that if Delta u'v' is less or equal to 0.001 of the average u'v' of the source, then the source can be assumed to have the same spectrum in all directions.