individual pulse to travel the fixed distance between individual components for the most part mirrors used to position the laser beam is easily determined also the arrival time of an individual pulse at a selected location can be accurately predicted the arrival time of an air shot i e nothing between the polygon mirror 38 and the detectors 62 therefore is also known as best shown in fig2 the time required to travel the path length in air is calculated as the arrival time when the medium is air and the arrival time when the medium is human tissue can be measured the difference between the two arrival times and the path length in human tissue can be used to calculate the relative speed of light in human tissue as shown below the determination of the speed of light in human tissue allows time gating of that portion of the avalanche photodiode response pulse desired to be measured and used for image reconstruction the first few pulses of laser energy photons that have traversed through human tissue are detected as the scout phase of the data acquisition the time difference between the expected arrival of the photons as determined by a previously run calibration and the actual arrival time of the photons is determined for example u03b4time is used to determine when the measurement of the detector response curve will commence on the.