The sensor may be narrowband enough that part of your measured function is just whatever slice of the blackbody radiation curve it's seeing. If you factor that out, you might be back to mostly linear.

Polynomial and special function theory remains a vibrant area of mathematical research, interweaving classical algebra with advanced analysis. At its core, the study concerns algebraic expressions ...

This function is a polynomial in two dimensions, with terms up to degree 5. It is nonlinear, and it is smooth despite being complex, which is common for computer experiment functions (Lim et al., 2002 ...