Pretty sure the 3rd edition already contained that, no? At that time (not sure if that's still the case now) the choice was a compile time option, iirc.
In the 4th edition, there's no support for RGB rendering--it's always and only spectral.
And admittedly the spectral rendering option in earlier editions wasn't great. We didn't always correctly distinguish between illuminants and reflectances, used a fixed binning of wavelength ranges (vs stochastically sampling wavelengths), and had a fine-but-not-state-of-the-art RGB -> Spectrum conversion algorithm. All of that is much better / state of the art in the 4th edition.
Oh, thanks a lot for the clarification! And thanks for the work on the book in general. :)
I need to look into the changes then. The spectrum implementation of the 3rd version served as inspiration for an X-ray raytracer I've been working on.
