the embodiment shown in fig1 and 2 is applied to determining a range of speeds in a fluid flow 10 by measuring doppler shifts of light diffused by particles 12 injected into the fluid flow . for example , and as described in the prior art documents previously cited , a flow of gas can be seeded with a mist of very fine particles of an appropriate liquid or solid which are dispersed in the flow of gas and entrained thereby . the flow 10 is illuminated by a laser sheet 14 generated by appropriate means 16 , such as beam spreading optics ( a set of cylindrical and spherical lenses ) or a rotating polygon device , from the beam 18 emitted by an appropriate laser , such as a longitudinal monomode argon laser emitting a wavelength of 514 . 5 nm or a frequency - doubled 532 nm yag laser . the laser light is diffused by the particles 12 present in the flow 10 . means for detecting the light diffused by the particles 12 include an optical system 22 for forming an image on a set of photodetectors such as a ccd video camera 24 , for example , the luminous flux transmitted by the optical system 22 passing through a beam splitter 26 that directs a portion of that flux toward another set of photodetectors , such as another ccd video camera 28 , for example , the luminous flux transmitted by the beam splitter 26 passing through spectral filter means 30 tuned to the frequency of the laser beam 18 before reaching the ccd video camera 24 . for example , the filter means 30 consist of a cell containing iodine vapor , which has an absorption line in the vicinity of the frequency of the laser beam 18 , the transmission being approximately 50 % at that frequency , for example . the ccd video cameras 24 and 28 have their output connected to data processing means 32 including data acquisition and computation means and display means 34 on which images of the observed field can be displayed , in which images the components of the speeds of the particles in a given direction are represented by different shades of grey or different colors . in fig2 { right arrow over ( v )} is the speed vector of a particle 12 illuminated by an incident laser ray 36 whose propagation direction is represented by the unit vector { right arrow over ( e )} and { right arrow over ( r )} is the unit radius of the observation direction ( the direction connecting the particle 12 to the vanishing point of the image ). the doppler shift of the light diffused by the particle 12 relative to the laser illumination is given by the equation : δ   f = fo c  v → · ( r → - e → ) in which fo is the frequency of the laser illumination and c is the speed of light in a vacuum . in fig2 the vector { right arrow over ( r )}-{ right arrow over ( e )} represents the direction in which the component of the speed v of the particle is measured by the device shown in fig1 . the light diffused by the particles 12 is picked up by the optical system 22 and some of it is transmitted to the first ccd video camera 24 after passing through spectral filter means 30 and some of it is transmitted to the other ccd video camera 28 , possibly by means of a reflecting mirror 38 ( fig3 ). the output signals from the video cameras 24 and 28 provide pixel by pixel comparison of the intensity of the diffused light received directly by the video camera 28 and that of the diffused light received by the video camera 24 after passing through the spectral filter means 30 . as can be seen on the spectral transmission curve c in fig4 the means 30 are tuned to a frequency close to the frequency fo of the laser beam 18 , the transmission being approximately 50 % at the frequency fo . as a function of the direction of movement of the particles 12 relative to the measuring device , the frequency of the light diffused by the particles may be greater than or less than the frequency fo and its transmission by the means 30 will be respectively greater than or less than what it would be for the frequency fo . for the fluctuations in the frequency fo of the laser beam 18 and drift , in particular thermal drift , of the filter means 30 to be taken into account and compensated automatically , the invention provides means 40 ( fig3 ) for generating monochromatic reference fluxes that are injected into the luminous flux transmitted by the optical system 22 and picked up by the photodetectors of the ccd video cameras 24 and 28 . the means 40 are advantageously acoustical - optical means and include a crystal in which a phase array is induced by the propagation of an acoustic wave generated by a piezo - electric crystal across which a sinusoidal voltage of frequency δf is applied . when a light wave of frequency fo ( a portion of the laser beam 18 ) propagates in the crystal , a beam at the frequency fo and a beam at the frequency fo + δf ( bragg - type operation ) are obtained at the output . these two frequencies can be transmitted by optical fibers 42 at two points of an intermediate image plane of the optical system 22 , the ends of the optical fibers 42 being oriented in that plane in the direction of the splitter cube 26 . thus the output signals from the video cameras 24 and 28 supply measurements of the intensities of the luminous fluxes at the frequencies fo and fo + δf picked up by the video camera 24 after passing through the filter means 30 and picked up directly by the video camera 28 . this provides two points on the spectral transmission curve ( fig4 ) corresponding to the frequencies fo and fo + δf , regardless of the fluctuations in the frequency fo of the laser beam 18 , the value δf being fixed and known . the measured intensities supplied by the ccd video camera 28 are used to obtain normalized signals in which are equal to the ratios ( pixel by pixel ) of the intensities measured by the video camera 24 and the intensities measured by the video camera 28 . as shown diagrammatically in fig5 the reference luminous flux generator means 40 preferably operate in the raman - nath mode to supply at the output light beams having the frequencies fo , fo + δf , fo − δf , fo + 2δf and fo − 2δf . those beams are transmitted by optical fibers and by a set of connectors 44 to three small modules 46 each associated with a detector device such as that shown in fig3 for measuring components of the speeds of the particles 12 in three different directions , in which case only one fixed direction laser source 20 is used . in each module 46 , the ends of the optical fibers are oriented toward the photoreceivers and transmit five reference luminous fluxes having the frequencies indicated above . this enables use of a larger portion of the spectral transmission curve of the means 30 , including non - linear portions , which automatically and permanently compensates fluctuations in the illumination laser frequency and drift in time of the spectral filter means 30 . the frequency difference δf previously cited can be 200 mhz , providing a working frequency range of 1 ghz around the laser frequency fo . the measured speed dynamic range depends on the geometrical aiming conditions . in the case of a standard configuration encountered in a wind tunnel , a speed dynamic range is obtained which is increased from 350 m / s ( if limited to the linear portion of the spectral transmission curve of the means 30 ) to 700 m / s , for example , in other words which is doubled . the spectral filter means 30 typically consist of an iodine vapor cell . if the temperature variation of the finger of the cell is 0 . 5 ° c ., the resulting frequency variation is approximately 5 mhz . frequency variations of the laser source 20 can be of the order of 2 mhz . they generate a measured speed uncertainty of approximately 5 m / s ( in the standard configuration ). with the invention this uncertainty tends toward zero , with automatic compensation of thermal drift of the iodine vapor cell and frequency fluctuations of the laser source . the acoustic / optical means 40 can generate frequency differences with a relative accuracy of the order of 10 − 6 , i . e . with an uncertainty of 200 hz for a frequency difference δf of 200 mhz . the measuring error is therefore related to the uncertainty of the interpolation technique used to obtain in real time the calibration curve of the spectral transmission function of the iodine vapor cell and the uncertainty related to the radiometric measurement of the video cameras 24 and 28 , which is estimated at 0 . 15 % in the case of video cameras with ten real bits .