Spectral rendering, part 2: Real-time rendering

Based on the insights from part 1, we have the means to define illuminant and reflectance spectra for our scene data.
Then the color of a pixel arises from an integral over a product of spectra.
The main goal of this blog post is to find efficient ways to evaluate such integrals using Monte Carlo integration and importance sampling.
Additionally, we discuss how to define a BRDF based on known reflectance spectra, how to implement that and look at results.
I have extended my educational path tracer to support both spectral and RGB rendering.
You can take a look at this code to see specifics of how all of this is implemented (see the links at the bottom).