I'm sorry I'm bringing this topic up again, but I need to expound some of the topics.
With reference to this question, I was wondering if someone can help me out in expanding some of the details.
In the answer to that question we have:
However, since $L_i(l_k)$ is usually not a constant, this integral is not separable. Yet, in some cases (like when dealing with diffuse lighting or, more specifically, diffuse ambient lighting), the information that $f(l_k,v)$ carries is low-frequency enough that it can reasonably be approximated as a constant with respect to the distribution of $L_i(l_k)$. Experimentally, it can be shown that the final result is not too different from what you would expect.
I'm not sure I understand the specific bit "the information that $f(l_k,v)$ carries is low-frequency enough" and also the bit "with repsect to the distribution of $L_i(l_k)$".
I've been reading something about radiometry lately. The only clue I have is that since $f(l_k,v)$ is also function of the wavelength, which is inverse w.r.t. the frequency. Therefore low frequency means high wavelength, which in turn translates some specific assumption about the colour of both incoming and reflected light I guess. Experimental results are also mentioned and I'd like to find out those if they've been published.
Is my reasoning correct? If yes, I still have the feeling I'm missing something. If not what am I missing yet? Experimental results are also mentioned, can you point out any paper so I can read about those?