The authors present a retrieval technique for the inference of cloud optical depth from data obtained by a ground-based multichannel radiometer for use in climate-related studies. The basic steps of the analysis procedure are considered, including accurate calculations of the atmosphere and cloud properties. An approach is described that uses observed and model-simulated transmittances rather than irradiances and that does not depend on an absolute calibration of the instrument. This approach, which specifically deals with the transmittances, also offers substantial computational advantages. The resulting algorithm is applied (but not limited) to the measurements of incoming solar irradiance by a particular ground-based instrument, the so-called Multi-Filter Rotating Shadowband Radiometer (MFRSR). Sample results of inferred cloud optical depth obtained from MFRSR measurements in Fairbanks, Alaska, are presented.