Color Rendering Index (CIE 1995 Ra)
The Color Rendering Index is a standard for evaluating the spectral
fidelity of an illuminant. It is computed by comparing the color
rendition of a set of colors to the reference light source, a black
body radiator or daylight source, which has a CRI of 100.
A spectral instrument is needed to compute the CRI, and for the
value to be completely accurate and the illuminant to be a good
light source, the illuminant must be within a certain tolerance of
the black body or Daylight color locus, and the CRI Readout will
annotate the value with (Caution) if this is not the case.
If the color is extremely unlike an illuminant, then the CRI may
return Failed. The tolerance dictated by the CIE 1995
standard is 0.0054 Duv from the the Black Body locus for
temperatures below 5000ºK, and the Daylight locus above 5000ºK. You
may decide to use the CRI values even if they are annotated with
(Caution), if you judge the illuminant to be close enough to the
illuminant locus to be useful.
By default the R9 value will also be shown, which can be useful in
judging how lighting renders saturated Reds, as poor Red rendering
isn't always evident in the Ra value. An R9 value above 0 is
generally considered a minimum requirement for rendering of reds,
with a value above 50 being considered good.
You can also turn this extra information off, or choose to display
the worst R value from the first 8 test patch colors (the ones used
to compute the overall Ra value), or the worst from all 14 CIE 1995
test patch values, or all 14 R values. (You will have to scroll the
display to see all of them.)