 The photon histogram records an increase in photon emissions during times that the transistor switches on or off.By detecting the combined photon emissions of pairs p- and n-channel transistors contained in logic gates, it is possible to use the resulting histogram to determine the locations in time of the rising and falling edges of the signal at that node.The waveform produced is not representative of a true voltage waveform, but more accurately represents the derivative of the waveform, with photon spikes being seen only at rising or falling edges.