Transfer Radiometers

We have two stable radiometers that we use to measure the output of various calibration sources such as an integrating sphere. These radiometers were designed to be stable, reliable, and transportable. We have used them to measure the output of various calibration spheres used to calibrate spacecraft and aircraft sensor such as MODIS, ASTER, MISR, Landsat 7 ETM+, MASTER, and MAS.

The first radiometer is based on a trap detector constructed with three large-area silicon detectors. The radiometer has seven spectral bands that nominally match seven bands of the MODIS instrument covering the range between 412 and 869 nm so we will refer to this instrument as the VNIR (Visible and Near IR) radiometer. Interference filters that are contained in a filter wheel determine the spectral bandpass. The radiometer has no powered optics so the radiometric throughput is determined by the diameter and separation of two apertures. These apertures are nominally 4 mm in diameter and are spaced 100 mm apart using invar metering rods. The detectors, amplifier, and filters are heated to about 30°C to ensure stability. The output of the radiometer is measured by a precision digital voltmeter and the data are recorded on a PC. The data acquisition program controls the gain of the amplifier and logs temperature and other ancillary data along with the amplified detector output. Calibration data are applied to the raw measurements to convert the measurements to radiance. The calibration of the instrument is traceable to NIST as we use a standard of spectral irradiance calibrated at NIST during the calibration of the transfer radiometer. The front aperture is removed for calibration so the rear aperture/filter/detector combination is calibrated for irradiance and a radiance calibration is computed using measured aperture sizes and separation.

The second radiometer is based on an Indium Antimonide detector operating at 77 K. The design of the radiometer is similar to the silicon detector based instrument in that the spectral bands are determined by filters and the radiometric throughput is determined by apertures with a fixed spacing. The 12 spectral bands are between 747 nm and about 2450 nm matching various ASTER and MODIS bands so we refer to this radiometer as the SWIR (Short Wave IR) radiometer. The apertures, detector, and detector amplifier are contained in a liquid nitrogen cooled dewar. The instrument includes a chopper running at about 20 Hz and a digital lock-in amplifier. A program running on a PC controls the lock-in amplifier and the data are logged on the PC. Again, the calibration of the SWIR instrument is traceable to NIST using the same standard of spectral irradiance. However, with the SWIR instrument the standard illuminates a Spectralon diffuser.

During use, the radiometers are typically aligned to view the center of the exit port of the sphere source along the normal to the port face. The distance to the source is set such that the radiometer field-of-view (about 5° FWHM) covers the area of the port to be used in calibration of a sensor. The source is operated until it is stable at the nominal output and then the radiometer is used to measure the output in each spectral band. The measured radiance is compared to the band-averaged spectral radiance of the source to determine if the source calibration is within expected uncertainties.