I’ve made this Node Group which Converts Wavelengths from 390nm - 790nm to Color, exactly like in a Rainbow.
Input is either direct Nanometer from 390-790 (actually more 720 due to 720+ = Infrared)
or Manual 0-1
IMPORTANT: If you use Manual 0-1 set NM to 390 (or lower), since the Manual Value is simply added to (mapped) Value from Wavelentgh nm Input!!!
It is not entirely correct, but close enough.
Possible Usecases: everywhere where you need Rainbow colors (ring a bell?). Just plug-in a Gradient in Manual 0-1. Soap Bubbles…
Just another example why i love Cycles