The following practical considerations must be borne in mind while using an LDV: The signal generated at the receiver should have a high signal-to-noise ratio. Hence particles used as seeds in flow must be small, large in number and nearly of uniform size. The seeds must also be non-buoyant. As mentioned earlier, gas flows require seeding and atomized non-volatile oils such as glycerine are used for this purpose. The receiver optics must be of suitably high quality so that they can respond to optical signals of low intensity. These are usually protected by interference filters to prevent contamination of scattered light by the room illumination. When photodetectors are used with frequency counters, phase information in the signal is lost due to the squaring of the signal and the LDV setup can be used to measure mean velocity but not turbulence. Note that a change in velocity to produces a Doppler shift in frequency of identical magnitude. This problem is circumvented by the use of a Bragg cell that is described later. With reference to the LDV configuration considered in Figure 3.5, the following analysis is applicable. Writing the Doppler shift equation for each ray 1 and 2, we get