Hence

As discussed above, the output of the photodetector oscillates at a frequency . Hence the measurement of the frequency shift enables one to measure velocity. Note that the Doppler shift is independent of , the receiver position. Instead the sensitivity of LDV is adjusted by changing and hence . The angle is accurately measured from the fringe pattern that forms at the measurement volume owing to interference of the beams 1 and 2 (Figure 3.6). The fringe spacing can be shown to be given by from which can be determined.

Figure 3.6: Fringe Pattern at Measurement Location.