The Atmospheric Optical Calibration System (AOCS) was developed at the National Renewable Energy Laboratory for making real-time, terrestrially based measurements of selected optical properties of the atmosphere. The original application of the AOCS was to provide measurements of atmospheric optical conditions pertinent to outdoor photovoltaic (PV) device performance measurements. It addresses the dependence of outdoor PV test results on the characteristics of the incident solar radiation that are dependent on these constantly changing atmospheric optical conditions. This paper describes the AOCS, its calibration and its application to PV testing. Additional applications related to the measurement of atmospheric optical properties are also mentioned.