the measurement volume 24 the optical transmitter 22 may be a galilean or keplerian telescope when the object 26 in the illustrated example constituted by aerosols 26 in the measurement volume 24 are illuminated by the measurement beam 20 the aerosols back scatter a small amount of light forming a signal beam 28 towards an optical receiver 30 that images the measurement volume 24 onto one or more detector arrangements 33 in the detector system 32 the operation of which is further explained below the angle 34 between the reference beam 18 and signal beam 28 incident at the detector system 32 comprising one or more detector arrangements 33 leads to formation of a fringe pattern 36 of intensity variations overlaying a speckle pattern 38 that is formed by illumination of the object 26 in the measurement volume 24 by the measurement beam 20 the combined fringe pattern 36 and speckle pattern 38 is illustrated to the right in fig2 showing the intensity pattern as it could be detected at the surface of a detector arrangement 33 in the detector system 32 the fringe distance is determined by the angle 34 the longitudinal direction of the measurement beam is equal to the common direction of propagation of the measurement beam and the signal beam when the object moves in the direction of propagation of the measurement beam 20 the fringe pattern 36 shown in fig2 moves in the speckle pattern 38 to the left or right i e perpendicular to the direction of the individual fringes as determined by the direction of movement of the object in the direction of propagation of the measurement beam 20 thus both.