SpectralEmissivity & Emittance

| Useful Data | Practices Measuring or Avoiding | Background & Theory|

Portable Fourier transform infrared spectroradiometer for field measurements of radiance & emissivity

By Andrew R. Korb, Peter Dybwad, Winthrop Wadsworth, and John W. Salisbury

A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the field measurement of spectral radiance from the Earth’s surface and atmosphere in the 3–5-µm and 8–14-µm atmospheric windows, with a 6-cm21 spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 µm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm21. The instrument tested here has a measured noise-equivalent delta T of 0.01 °C, and it measures surface emissivities, in the ?eld, with an accuracy of 0.02 or better in the 8–14-µm window 1depending on atmospheric conditions2, and within 0.04 in accessible regions of the 3–5-µm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made field measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3–5-µm and 8–14-µm atmospheric windows.

Key words: Fourier transform infrared spectroradiometer, portable spectrometer, infrared radiance
measurement, radiometric calibration, spectral emissivity calculation.
Reference: Korb, A.R., P. Dybwad, W. Wadsworth, and J.W. Salisbury, 1996, Portable Fourier Transform Infrared Spectrometer for Field Measurements of Radiance and Emissivity, Applied Optics, v.35, p.1679-1692. http://www.dpinstruments.com/papers/applied_optics_update.pdf

Copyright 1996 Optical Society of America

Effects of atmosphere, temperature and emittance on reflected and emitted energy

A NASA report by R. Kumar, dated Sep 1, 1977, available online and downloadable as a PDF document.


The effects of temperature and emittance on the relative magnitude of reflected energy and emitter energy from a target including atmospheric effects was studied. From the calculations of energy reflected and emitted from a target including atmospheric effects using LOWTRAN 3 programs for midlatitude summer model, the following conclusions were obtained (1) At 3.5 micrometers q is considerably less than 1 except at high temperatures and for high emittance (2) at 4 micrometers q is of the order of magnitude equal to 1 for most targets and (3) at 4.6 micrometers, q is considerably greater than 1 at high temperatures and high emittance. In addition, incident atmospheric emission reflected from the target was found to be negligible except for targets having low temperature and low emittance.