Doppler Velocity Measurements using White Light

Abstract

VELOCITY measurements, based on observations of the Doppler shift in frequency of radiation scattered by moving objects, have been made using various optical arrangements (for example, refs. 1–6), The coherency requirements of the illuminating radiation have been considered by Rudd⁵, who concluded that conventional (that is, not laser) light sources could, in principle, be used. Until recently, however, all proposed optical systems have required such critical alignment or small effective source size (to provide sufficient spatial coherence) that the use of conventional light sources was impractical. Consequently, laser light sources have had to be used exclusively. This is now not necessary and it is demonstrated here that the schlieren-interferometer arrangement proposed by Schwar and Weinberg^7,8can be used even with a white light source. To show that Doppler shifts in frequency that are very much smaller than the line width of the illuminating radiation can be detected, the velocity of a slowly moving pendulum bob was measured.