Equivalent Filter-Radiometer Brightness Temperatures from a Fourier Transform Spectrometer

Abstract

The NOAA Environmental Technology Laboratory has operated radiometers in a variety of field experiments. The infrared instruments include a Fourier Transform InfraRed spectro-radiometer (FTIR) that measures spectral radiance from 500 to 2000 cm-1 at 1-cm-1 spectral resolution, and several filter radiometers (Barnes Precision Radiation Thermometer model 5, PRT5) that measure brightness temperature (Tb) in a 1.48-µm-wide (130 cm-1) band centered at 10.7 µm wavelength. The FTIR calibration uses 77-K and 300-K blackbody sources to cover the entire expected range of atmospheric brightness temperatures, while the PRT5 calibration uses a blackbody source whose temperature is varied between 300 K and about 205 K with a bath of alcohol and dry ice. The PRT5 instrument is smaller and easier to operate than the FTIR, but the FTIR has the advantage of a larger calibration range, a broader bandwidth, and a finer spectral resolution. In this paper we show that the FTIR spectral radiances can be converted to PRT5-equivalent brightness temperatures and used to compare the two instruments, correct the uncalibrated cold PRT5 data, and provide PRT5-like data in the absence of a PRT5.