Histogram Readout Method and Circuit for Determining the Time of Flight of a Photon

A histogramming readout circuit is described. The readout circuit comprises a time to digital converter (TDC) configured to continually report time-stamps defining an arrival time of a laser clock and a signal output from a photosensor. Memory is provided for 10 storing TDC events. A programmable processor is configured to implement a state machine. The state machine being operable to save a time-stamp when a TDC event is detected; determine the time of flight of each of the photons detected by the photosensor; use each calculated time of flight to address a memory location; build up a histogram of the TDC data values using the memory locations as time-bins; and maintain a pointer to a maximum memory location where the highest number of TDC event resides. A calculator is operable to read the value of the maximum memory location and one or more adjacent time-bins either side for processing.