The CMS I'm building is for creative imaging—I want to model some specific subtractive processes from the physical world digitally. In the process, I had to come up with a way to convert RGB to spectral power, and was surprised to find that while there's a Spectral -> XYZ function (CMF), there's no industry standard inverse (XYZ -> Spectral). So I came up with a function which allows you to specify three RGB peaks, along with their widths, and it solves for the correct peak levels to reproduce an XYZ equivalent to the input color. This is my idea of fun! haha