Abstract: A method of determination of blood flow including projecting a beam (B) of laser generated light (L) to move (SM) over a surface (T) beneath which blood flow in a vessel or vascular bed is to be determined, collecting (RD)light returning scattered from the beam by the surface and by blood flowing beneath, measuring a spectrum of frequencies in said collected light, determining (DA),(F) from differences in said frequencies blood flow (VS) in a vessel or vascular bed beneath said surface. A reference for the detected frequencies is provided by light returned from the surface. A map of blood flow in a false-colour flow scale can be displayed(DU).