The calibration and intercalibration of sea-going infrared radiometer systems using a low cost blackbody cavity

There are many infrared radiometer systems available for the measurement of in situ sea surface skin temperature (SSST). Unfortunately, the marine env ironment is extremely hostile to optical components, and to ensure the accu racy of SSST measurements, an absolute calibration of instrumentation using an independent calibration reference is required both before and after any sea deployment. During extended deployments it is prudent to have addition al regular calibration data to monitor instrument performance characteristi cs. This paper presents a design for an ambient temperature (278-325 K), wi de aperture (100 mt), reference blackbody unit that may be used to calibrat e a variety of sea-going infrared radiometer systems both in the laboratory and in the field. The blackbody consists of a spun copper cavity coated wi th well-characterized high emissivity paint (Mankiewicz Nextel Velvet Coati ng 811-21) immersed in a water bath that is continuously mixed using a stro ng water pump. The radiant temperature of the blackbody cavity is determine d from the measured water bath temperature. Results derived from validation and intercomparison experiments show this blackbody design to be an accura te and reliable reference blackbody source. However, in order to ensure tha t the best possible calibration data are obtained, extreme care must be tak en to ensure the accurate measurement of the water bath temperature, proper positioning of a radiometer in front of the cavity itself, and prevention of condensation on the cavity surface. Four blackbody units have been speci fically built for the European Union combined action for the study of the o cean thermal skin (CASOTS) program. Using these units as reference radiance sources, the authors describe the strategy adopted and present results obt ained from the CASOTS radiometer intercalibration experiment. These results highlight the need to obtain independent calibration data both before and after sea-going radiometer deployments and the need to standardize field ra diometer calibration protocols.