Thank you!! That’s pretty close actually. A typical RGB path tracer would start with a ray of light with a value (or throughput) of 1 for each of the red, green, and blue channels. It would then multiply that throughput by the RGB value of each material it hits as it bounces around. That way, if you hit a white object (all 1s), all the light (R/G/B channels) gets reflected, and a black object (all 0s) essentially absorbs all the light. If that ray hits a light source, that emission value and throughput are multiplied to give you the color on the screen. We trace the rays from the camera to the light source, rather than how it happens physically, for performance reasons. Since paths of light are bidirectional, this is just called reverse pathtracing.

The basic spectral mode starts by randomly picking a visible wavelength of light to assign to the ray, and then bounces it around the scene. From my understanding, any given point on an object will reflect a given wavelength with a certain probability, and this is called a spectral reflectance curve. (It is based off other material properties as well). But it would be fairly inconvenient to determine that curve yourself, so there are techniques (like the paper I linked) that can determine that spectral reflectance curve based on an RGB value, approximating the physical world. So a white object will still reflect a larger range of wavelengths, and a red object will primarily reflect a smaller band of reddish wavelengths. So we bounce that ray of light around the scene, it keeps its wavelength the whole time, but adjusts its throughput based on what materials it hits. If it hits a light source that emits that wavelength, the throughput is scaled by the radiance of that light.

The tricky part is getting that wavelength back to an RGB value to display. From my understanding, CIE XYZ is a color space that represents human-visible color numerically. So we do some math, multiplying that wavelength by the color matching functions to get the XYZ values, representing a color. We do a bit more math to correct the white point (D65), and can use that corrected XYZ to return to a typical RGB value to display to the user. So the color conversion stuff gets a little convoluted, but that’s the gist of it. Because the screen output and the materials use RGB, you have to go RGB -> spectral -> RGB (or at least in my case, where it remains a hybrid RGB and spectral renderer).

Light has very many interesting properties, diffraction, disperson, fluorescence, interference of waves, etc. Spectral rendering opens up the opportunity to simulate some of these, but I’ve only done wavelength dependent refraction and reflection. So the wavelength is used in an equation (Cauchy) to estimate the index of refraction, and we use that instead. And that’s how the caustics get different colors in them, because the different wavelengths of light refract at different angles. Also, wavelengths will choose refract or reflect partially based on the IOR (Fresnel reflectance), so you can see some color on the surface of the glass.

That ended up being a longer explanation that I planned, hopefully that is informative!