1. A method of measuring perfusion in tissue, which comprises the steps of: a) producing a series of optical images of the tissue under laser light by performing the following steps until a full exposure range is covered: 1) producing an optical image of the tissue with a preselected exposure time with an optical imaging device; 2) adjusting the preselected exposure time of the optical imaging device; and then 3) repeating steps 1) and 2); b) calculating a series of contrast images based on said optical images of the tissue; c) when the full exposure range is covered, determining a power spectrum of scattered light by generating a temporal autocorrelation function from the change of spatial contrast of the series of contrast images with exposure time at points of interest; d) converting the temporal autocorrelation function to its power spectrum; and then e) determining perfusion from the power spectrum.