Radiometric and Engineering Performance of the SeaWiFS Quality Monitor (SQM): A Portable Light Source for Field Radiometers

Abstract

A portable and stable source of radiant flux, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Quality Monitor (SQM), was developed as a field instrument for use in experiments away from the calibration laboratory 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 acquired during the third Atlantic Meridional Transect cruise. In conjunction with laboratory calibration sources, the SQM can be used to transfer the calibration to the field experiment. Two independent lamp assemblies generate three flux levels, and the lamps are operated at constant current using active control. The exit aperture of the SQM is large and homogeneous in radiance. The SQM was designed to approximate a Lambertian radiator. An internal heater provides operational stability and decreased warmup intervals, which minimizes lamp hours. Temperature-controlled silicon photodiodes with colored-glass filters monitor the stability of the SQM, which is better than 1%. These independent monitors, which are integrated with the SQM, provide information on the flux from the SQM and can be used to normalize the output from the field radiometers during the experiment. Three reference devices, or fiducials, which are designed to mimic the optical surfaces of the field radiometers but are not functioning detector units, are used in place of the field radiometers to produce baseline monitor signals. The front surface 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 SQM. A kinematically designed mounting ring is used on both the field radiometers and the fiducials to ensure the devices being tested view the same part of the exit aperture each time they are used.