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Shadow-band radiometers in general, and especially the Multi-Filter Rotating Shadow-band Radiometer (MFRSR), are widely used for atmospheric optical depth measurements. The major programs running MFRSR networks in the United States include the Department of Energy Atmospheric Radiation Measurement (ARM) Program, U.S. Department of Agriculture UV-B Monitoring and Research Program, National Oceanic and Atmospheric Administration Surface Radiation (SURFRAD) Network, and NASA Solar Irradiance Research Network (SIRN). We discuss a number of technical issues specific to shadowband radiometers and their impact on the optical depth measurements. These problems include instrument tilt and misalignment, as well as some data processing artifacts. Techniques for data evaluation and automatic detection of some of these problems are described. OCIS codes: 120.4640, 120.5240, 120.5630, 120.0280, 010.1100, 010.1120.

Absolute calibration of spectral shortwave radiometers is usually performed with National Institute of Standards and Technology (NIST) or NIST-traceable incandescent lamps. We compare 18 irradiance standards from NIST and three commercial vendors using the same spectrometer to assess their agreement with our working standard. The NIST procedure is followed for the 1000-W FEL lamps from NIST, Optronics, and EG&G. A modified calibration procedure developed by Li-Cor is followed for their 200-W tungsten-halogen lamps. Results are reproducible from one day to the next to approximately 0.1% using the same spectrometer. Measurements taken four months apart using two similar but different spectrometers were reproducible to 0.5%. The comparisons suggest that even NIST standards may disagree with each other beyond their stated accuracy. Some of the 1000-W commercial lamps agreed with the NIST lamps to within their stated accuracy, but not all. Surprisingly, the lowest-cost lamps from Li-Cor agreed much better with the NIST lamps than their stated accuracy of 4%, typically within 2%. An analysis of errors leads us to conclude that we can transfer the calibration from a standard lamp to a secondary standard lamp with approximately 1% added uncertainty. A field spectrometer was calibrated with a secondary standard, producing a responsivity for the spectrometer that was within 5% of the responsivity obtained by Langley calibration using routine field measurements.

Optical depth retrieval by means of Langley regression is complicated by cloud transits and other time-varying interferences. An algorithm is described that objectively selects data points from a continuous time series and performs the required regression. The performance of this algorithm is compared by a double-blind test with an analysis done subjectively. The limits to accuracy imposed by time-averaged data are discussed, and an additional iterative postprocessing algorithm is described that improves the accuracy of optical depth inferences made from data with time-averaging periods longer than 5 min. Such routine algorithms are required to provide intercomparable retrievals of optical depths from widely varying historical data sets and to support large networks of instruments such as the multifilter rotating shadow-band radiometer.

The multifilter rotating shadow-band radiometer is a ground-based instrument that uses independent interference-filter-photodiode detectors and the automated rotating shadow-band technique to make spectrally resolved measurements at seven wavelength passbands (chosen at the time of manufacture between 350 nm and 1.7 [[micro]meter]) of direct-normal, total-horizontal, and diffuse-horizontal irradiances. This instrument achieves an accuracy in direct-normal spectral irradiance comparable with that of tracking radiometers, and it is more accurate than conventional instruments for the determination of the diffuse and total-horizontal spectral irradiances because the angular acceptance function of the instrument closely approximates the ideal cosine response, and because the measured direct-normal component can be corrected for the remaining angular acceptance error. The three irradiance components are measured with the same detector for a given wavelength. Together with the automated shadow-band technique, this guarantees that the calibration coefficients are identical for each, thus reducing errors when one compares them (as opposed to measurements made with independent instruments). One can use the direct-normal component observations for Langley analysis to obtain depths and to provide an ongoing calibration against the solar constant by extrapolation to zero air mass. Thus the long-term stability of all three measured components can be tied to the solar constant by an analysis of the routinely collected data.

In the fall of 1997 the Atmospheric Radiation Measurement program conducted a study of water-vapor-abundance-measurement at its southern Great Plains site. The large number of instruments included four solar radiometers to measure the columnar water vapor (CWV) by measuring solar transmittance in the 0.94-[micro]m water-vapor absorption band. At first, no attempt was made to standardize our procedures to the same radiative transfer model and its underlying water-vapor spectroscopy. In the second round of comparison we used the same line-by-line code (which includes recently corrected [H.sub.2]O spectroscopy) to retrieve CWV from all four solar radiometers, thus decreasing the mean CWV by 8-13%. The remaining spread of 8% is an indication of the other-than-model uncertainties involved in the retrieval. [C] 2001 Optical Society of America OCIS codes: 010.0010, 010.1110, 010.1320, 010.7340.