A laser range finder is developed from a pulsed infrared laser and a linear array detector.   By imaging the diffuse reflectance of the laser beam on the linear array, the distance to the target an be measured to within about 4 microns (1/25th of a typical hair diameter) at room temperature and about 35 microns (1/3rd of a typical hair diameter) in the particle-laden, reacting flow of the MFC.  The laser and detector are mounted on a precision slide and repeated measurements of probes are made to determine the rate of ash deposit formation.