About radiometry,our particular interests are the wavelengt of electromagnetic radiation between approximately 380 nm and 780 nm, which account for light visible to humans.The lower is blue,the middle is the green,the upper is the red.
The SPDs of real-world objects can be quite complicated,it looks like as follows.
A renderer doing computations with SPDs needs a compact, efficient, and accurate way to represent functions.
A general framework for it is to find a basic function to represent SPDs,and many different basis functions have been investigated for spectral representation in computer raphics.
The Spectrum Type
By default,pbrt uses the more efficient but less accurate RGB representation.
Both of the representations are based on storing a fixed number of samples of the SPD.