[Radiance-general] Re: [Fwd: spotlight definition]

[Greg Ward]

The weights correspond to the conversion matrix between Radiance's RGB 
color space and CIE XYZ (the Y channel, specifically).  This is not 
very well correlated to the V-lambda curve, because the conversion 
depends so strongly on the exact choice of red, green, and blue 
primaries.  In the case of Radiance and most standard RGB systems, the 
red primary is much closer to the peak of V-lambda than is the blue 
primary.  Thus, the contribution of the blue channel to luminance is 
much reduced.

I hope this explains things for you.
-Greg

> From: M BLOJ <M.Bloj@Bradford.ac.uk>
> Date: December 16, 2003 7:22:23 AM PST
>
> Hello,
> we are trying to get our heads around the weights in the equation for
> computing luminance: L=179*(0.265R+0.67G+0.065B). We do understand
> why one needs different weights for different channels, however we are
> surprised that the red channel has a higher weight than the blue
> channel. This does not correspond to what is known about human vision
> (V-lambda, the luminous efficiency function).
> We welcome any helpful comments about this matter.
>
> Marina & Alexa