Due to increasing complex structures, such as multiple metal layers, high density circuits, and wide metal buses, on a die of an integrated circuit (IC) and specially packaged devices, such as flip chips and lead on chips, the traditional front side emission microscopy is no longer effective or cannot be applied. The back side emission microscopy is the alternative. It is a very useful, necessary, and complementary method to the front side emission microscopy in failure analysis. In this paper, photon emission mechanism and spectrum from a silicon die are first discussed. It explains that the emission intensity for the most occurred two emission types (forward and reverse biased junctions) from a die back side is about the same as or comparative to that from the die front side. Next, the five factors which mostly affect the emission intensity from the die back side are discussed. Last, several examples are given.