Figure 1 shows the interfering high fequency light and the resulting beat at low frequency. This is what the photo diode of the HRF camera records and what is saved in the SER-files: 256*64 recordings, each of ~2 seconds duration. You can see the time measurement either if you load a SER file by the HRF software or load it into SDLF.
Figure 1: Two waves of slightly different wave length (incoming and reflected light) interfering; frequency shift is caused by the moving blood cell
Figure 2: Same scenario, but the cell moving faster resulting in a higher frequency shift
The algorithms in HRF or SLDF analysis software again take the time signals of each pixel recording and determine the different frequency shifts. The distribution and magnitude of the frequency shifts are equivalent to distibution of blood cells moving at different velocities.