The comparison of BRDF (Bi-directional Reflectance Distribution Function) measurements made on the same system but at different times or days is a method of determining potential sample or system change. If the measurements lie within the acceptable error region of the system, confidence is gained in the results after repeated testing. However, when a system is replaced by a new, improved facility, both the time delay associated with the change and the different character of the systems may preclude easy, direct comparison. When the test pieces are carefully stored to minimize contamination and when both systems use a common standard, similar results can be anticipated in a comparison test. Such comparisons are needed and can be made. Two mirrors were tested, using an in-house designed and fabricated BRDF measuring system. The mirrors were immediately replaced in their protective boxes and stored. A comparison was made between the mirror test results and tests from similar mirrors. No abnormal results were found. The initial system was subsequently replaced by a new system, the LMSC BOSM (Bi-directional Optical Scattering Measurement) Facility. After installation and evaluation, the two reserved mirrors were retested on the new system and comparison between old and new data is shown.© (1987) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.