I know it's a bit strange, but since a spotlight sometimes acts like it originates behind its real position due to some magnification in its optics, this is a quick way to simulate the effect of a circular porthole on a distant source in the virtual focus position of the spotlight. Since it's always preferable to apply an actual photometric distribution, this approximation is probably of limited utility. Just use a short vector if you don't know what it means, or avoid the spotlight type altogether.
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.
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.