Laser Doppler measurements of blood velocity in human retinal vessels

We have measured Doppler shift frequency spectra (DSFS) of laser light scattered from red blood cells (RBC's) flowing through individual human retinal vessels. Each spectrum exhibits a cutoff frequency which is directly related to the maximum RBC velocity at the measurement site. Meaningful DSFS can be measured in times as short as 0.1 s. DSFS obtained from suspensions of polystyrene spheres and RBC's flowing through glass capillary tubes are used to illustrate the basic principles of the technique and to aid in the interpretation of the in vivo measurements. Our measurements show that maximum RBC velocities in retinal arteries vary markedly during each cardiac cycle.