RADIOMETRIC AND ENGINEERING PERFORMANCE OF THE SEAWIFS QUALITY MONITOR (SQM) - A PORTABLE LIGHT-SOURCE FOR FIELD RADIOMETERS

A portable and stable source of radiant flux, the Sea-viewing Wide Fie ld-of-view Sensor (SeaWiFS) Quality Monitor (SQM), was developed as a field instrument for use in experiments away from the calibration labo ratory such as those encountered during oceanographic cruises. The SQM monitors the radiometric stability of radiance and irradiance sensors during these field experiments; a companion paper gives results acqui red during the third Atlantic Meridional Transect cruise. In conjuncti on with laboratory calibration sources, the SQM can be used to transfe r the calibration to the field experiment. Two independent lamp assemb lies generate three flux levels, and me lamps are operated at constant current using active control. The exit aperture of the SQM is large a nd homogeneous in radiance. The SQM was designed to approximate a Lamb ertian radiator. An internal heater provides operational stability and decreased warmup intervals, which minimizes lamp hours. Temperature-c ontrolled silicon photodiodes with colored-glass filters monitor the s tability of the SQM, which is better than 1%. These independent monito rs, which are integrated with the SQM, provide information on the Aux from the SQM and can be used to normalize the output from the field ra diometers during the experiment. Three reference devices, or fiducials , which are designed to mimic the optical surfaces of the held radiome ters but are not functioning detector units, are used in place of the field radiometers to produce baseline monitor signals. The front surfa ce of the fiducial is protected when not in use and kept clean during the field experiment. The monitor signals acquired using the fiducials provide additional information on the radiometric stability of the SQ M. A kinematically designed mounting ring is used on both the field ra diometers and the fiducials to ensure the devices being tested view th e same part of the exit aperture each time they are used.