The first serious attempts to measure optical scatter and use it to infer surface finish quality came about through the use of total integrated scatter (TIS) measurements. [1,2] In the early 1970's, one of the ASTM study groups ran a series of round robin measurements to determine the validity of comparing (TIS) measurements from one lab to another. Initial results were very discouraging. TIS values varied over orders of magnitude between labs and measurements were difficult to repeat from one round to the next at the same lab. Slowly, experience was gained in reducing operator error, limiting contamination problems and recognizing basic differences between the instruments under use. The importance of surface spatial frequency bandwidths was recognized and comparison results showed some improvement. In the mid-seventies, measurement of scatter, as a function of angle from specular, was used to obtain more information about surface finish. [2,3,4,5,6] The scatter research of the 1970's is now being combined with the computers of the 1980's to produce fast, accurate scatter inspection systems to help meet the increased demands for low scatter optics. [2,7] Although most of these systems produce the bidirectional scatter (reflectance, transmission) distribution function (BSDF, BRDF, BTDF), some are capable of producing surface statistics as well. The BRDF can be used to calculate a surface power spectral density (PSD) function, which in turn provides values of root mean square roughness and a transverse length parameter. The question of how these instruments relate to the surface statistic nproduced by surface profiling instruments is now being studied.  More basic, however, is the old question of whether BRDF (or PSD) measurements can be repeated from lab to lab. The authors decided to do a comparison between a Lawrence Livermore National Laboratory instrument, the Angle Resolved Scatterometer (ARS) , and a Complete Angle Scatter Instrument (CASI) developed at Toomay, Mathis & Associates.