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.
It's a great text, and remains one of my go-to examples when someone asks for examples of well executed technical book.
I was only ever peripherally involved in this area as a but found myself revisiting the origin just because it was put together so cleanly.
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.
Is it something similar to Wenzel and Jo's one, or Alex Wilkie's Sigmoid variation?
Edit: ah: RGBSigmoidPolynomial...
You can use LuxCoreRenderer with Blender, though!