Doppler velocimetry”, otherwise known as “Doppler global velocimetry (DGV)”
measures two or three components of velocity on a plane illuminated by a
laser light sheet. Like LDV, it detects the Doppler shift of light
scattered by small particles; but unlike LDV, the measurement of the
frequency shift is made spectroscopically using a high-resolution iodine
gas cell, which acts as a frequency dependent filter with very high
sensitivity. The intensity of the light from a given point in the object
plane is attenuated in proportion to its Doppler shift. The image plane
then contains gray levels proportional to the product of the local
intensity of the illuminating light sheet and the Doppler shift. To remove
the effects of variable illuminating beam intensity and other optical
elements that might affect the observed intensity, a second camera records
the unshifted image field, and the Doppler shifted image is normalized by
the unshifted image, pixel-by-pixel.
The velocity resolution of PDV is of order 1 m/s, so best application of it
involves high-speed flows. The technique is appealing because, in
principle, it involves only the measurement of pixel intensity readings.
Three-dimensional vectors can be measured using stereoscopic viewing.