Hi Simon,
The spectral limits are set in Radiance 6.0 with the -cw run-time rendering option, which defaults to “-cw 380 780”. The actual computed bands are set by the -cs rendering option, which defaults to 3. When using “-cs 3” implicitly or explicitly, the calculation reverts to its pre-6.0 RGB approach, and any spectral primitives in the scene description are first converted to CIE-XYZ coordinates via the standard observer sensitivity curves then to RGB via the preset primaries mentioned earlier.
You still haven’t said whether you are using a previous version or 6.x, which you can determine by simply running “rtrace -version”. If you are in 6.x, then setting “-cs 24 -cw 300 2500” causes your rendered values to start at 2500 nm and include uniform-sized bands of about 92 nm down to 300 nm. Whatever spectral primitives you gave on input will be converted to this basis at run-time using an energy re-binning approach. If, for example, you specify a “spectrum” modifier that covers 380-780 nm, as might be found in the Spectral Materials Database, any modified materials will permit no energy beyond these limits. (Technically, the bounds of spectral patterns do extend slightly past the extrema, as values are centered on the end-point wavelengths and therefore spread energy half a step beyond in either direction.)
If you are restricted for some reason to using a pre-6.0 version of Radiance, then the best you can do is redefine the RGB colors to your own calculation basis, whatever that may be. The three color channels function independently for the most part during rendering with a few minor caveats, so they are open to redefinition by the user.
Is this any clearer?
Best,
-Greg
P.S. The operation of spectral pattern-type modifiers in 6.0 is such that the product of the modifier and the material color determines the final value, where all 1’s in both sets of channels will be equivalent to 100% for reflection or transmission for example.