I forget the name it has, but that is a technique used to examine silicon — all the PN junctions in the logic will emit light when they pass current. CMOS logic

I forget the name it has, but that is a technique used to examine silicon — all the PN junctions in the logic will emit light when they pass current. CMOS logic doesn't have very many (forward-biased) junctions though. IDK if the technique is still useful there.