Spectral Emissivity & Emittance

NASA Portable Infrared Reflectometer Designed and Manufactured

The optical properties of materials play a key role in spacecraft thermal control. In space, radiant heat transfer is the only mode of heat transfer that can reject heat from a spacecraft.

One of the key properties for defining radiant heat transfer is emittance, a measure of how efficiently a surface can reject heat in comparison to a perfect black body emitter.

Heat rejection occurs in the infrared region of the spectrum, nominally in the range of 2 to 25 micrometer.

To calculate emittance, one obtains the reflectance over this spectral range, calculates spectral absorptance by difference, and then uses Kirchhoff’s Law and the Stefan-Boltzmann equation to calculate emittance.

Portable infrared reflectometer for evaluating emittance. Photo from NASA

A portable infrared reflectometer, the SOC–400t, was designed and manufactured to evaluate the emittance of surfaces and coatings in the laboratory or in the field.

It was developed by Surface Optics Corporation under a contract with the NASA Glenn Research Center at Lewis Field to replace the Center’s aging Gier-Dunkle DB–100 infrared reflectometer.

The specifications for the new instrument include a wavelength range of 2 to 25 micrometer; reflectance repeatability of ±1 percent; self-calibrating, near-normal spectral reflectance measurements; a full scan measurement time of 3.5 min, a sample size of 1.27 cm (0.5 in.); a spectral resolution selectable from 4, 8, 16, or 32 cm^–1; and optical property characterization utilizing an automatic integration to calculate total emittance in a selectable temperature range.

The computer specified to drive the software is a laptop with a menu-driven operating system for setup and operation, a full data base manager, and a full data analysis capability through MIDAC Grams/32 software (MIDAC Corporation, Irvine, California).

Spectral scanning is achieved through the use of a Fourier Transform Infrared (FTIR) Michelson interferometer. In addition, the reflectometer’s size and weight make it conducive to portable operation.

Although most of the planned uses for the instrument are expected to be in the laboratory, some field operations are anticipated. The only requirement for field operation is a source of power (115 V alternating current).

NASA Glenn took delivery of this world-unique, portable infrared reflectometer in January 1999. It is a resounding success, and an evaluation of thermal control materials for NASA and aerospace customers is currently underway.

Find out more about this research.

Glenn contact: Dr. Donald A. Jaworske, (216) 433–2312, Donald.A.Jaworske@grc.nasa.gov

Author: Dr. Donald A. Jaworske

Headquarters program office: OSS (ATMS)

Programs/Projects: Space Power, ISS, Aerospace Industry

Measuring the spectral emissivity of rocks and the minerals that form them, By Miroslav Danov, Dimitar Stoyanov, and Vitchko Tsanev.

It is an online paper at the SPIE news room website. The tagline for the paper reads:

“A new ground-based technique measures minerals in their natural conditions, a prerequisite for satellite data processing”.

The paper discusses a new measurement technique that uses both a scanning FTIR spectrometer and a gold-plated hemispherical mirror and provides data from tests using limestone as the test subject material.

Several references are cited, as follows:

Jingmin Dai, Xinbei Wang, Guibin Yuan, Fourier transform spectrometer for spectral emissivity measurement in the temperature range between 60 and 1500°C, J. Phy. 13, pp. 63-66, 2005.

S. Fonti, Spectral emissivity as a tool for the interpretation of Martian data: A laboratory approach, 32nd Annual Lunar and Planetary Science Conference, no. 1279, pp. 12-16, 2001.

A. M. Baldridge, P. R. Christensen, A laboratory technique for thermal infrared measurement of hydrated samples, 38th Lunar and Planetary Science Conference, pp. 2407, 2007. Lunar and Planetary Science XXXVIII, held March 12-16, 2007 in League City, Texas. LPI Contribution No. 1338

Z. Wan, D. Ng, J. Dozier, Spectral emissivity measurements of land-surface materials and related radiative transfer simulations, Adv. Space Reg. 14, no. 3, pp. 91-94, 1994.

T. W. Stuhlinger, E. L. Dereniak, F. O. Bartell, Bidirectional reflectance distribution function of gold-plated sandpaper, Appl. Optics 20, no. 15/1, 1981.

The spectral library is hosted by the Mars Space Flight Facility at Arizona State University (ASU) consists of thermal infrared emission spectra (typically 2000 - 220 cm-1) of a variety of geologic materials.

It is open and free, but one needs to register with a valid email address and take the time to learn how to access the data and obtain plots. It is not a trivial task.

Each spectrum comes with descriptive information, sample quality, and a comments field that describes any appropriate, related information.

To quote from the introduction to the library about the sources of data:

“Emission spectra were acquired using a Nicolet Nexus 670 interferometric spectrometer equipped with a CsI beamsplitter and an uncooled deuterated triglycine sulfate (DTGS) detector; the spectral range of the instrument is from 2000 — 220 cm-1 (5 — ~45 microns). Both the spectrometer and the sample chamber/glovebox were continuously purged with nitrogen gas during sample analysis to minimize atmospheric H2O and CO2 which also have absorption features in the 2000-220 cm-1 region of the spectrum. The particulate samples were heated in an oven to 80°C to improve the signal to noise ratio during spectral analysis (this temperature is maintained during analysis by placement of the sample cup on a heater element). The samples were raised into a water-cooled sample chamber that closely approximates a blackbody cavity [Ruff et al., 1997]. A total of 270 scans at 2-cm-1 sampling were taken over ~7 minutes and averaged together by the spectrometer. In the case of a hand sample, active heating during measurement is not possible. Hand samples were taken directly from the oven and placed into the sample chamber and 180 scans were taken over a period of ~5 minutes to minimize the effects of sample cooling. The spectral calibration method is a variation of method 1 of Christensen and Harrison [1993] as described in detail by Ruff et al., [1997].”

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References cited above:

“Christensen, P.R., and S.T. Harrison, Thermal infrared emission spectroscopy of natural surfaces: Application to desert varnish coatings on rocks, J. Geophys. Res., 98 (B11), 19,819-19,834, 1993.“Christensen, P.R., J.L. Bandfield, V.E. Hamilton, D.A. Howard, M.D. Lane, J.L. Piatek, S.W. Ruff, and W.L. Stefanov, A thermal emission spectral library of rock-forming minerals, J. Geophys. Res., 105,9735-9739, 2000. {ED NOTE: PDF DOWNLOAD}

“Feely, K.C. and P.R. Christensen, Quantitative compositional analysis using thermal emission spectroscopy: Application to igneous and metamorphic rocks, J. Geophys. Res., 104, 24195-24210, 1999.

“Lane, M.D. and P.R. Christensen, Thermal infrared emission spectroscopy of salt minerals predicted for Mars, Icarus, 135, 528-536, 1998.”"Lane, M.D., Midinfrared emission spectroscopy of sulfate and sulfate-bearing minerals, American Mineralogist, in press, 2006.

“Ruff, S.W., P.R. Christensen, P.W. Barbera, and D.L. Anderson, Quantitative thermal emission spectroscopy of minerals: A laboratory technique for measurement and calibration, J. Geophys. Res., 102, 14,899-14,913, 1997.”

Further reference publications related to the work at ASU may be viewed on the ASU website.

This linked website discusses the background and flight of the instrument and also provides some interesting spectral emissivity curves for Quartz (SiO₂), Feldspar* and Hornblende** and an equal mixture of the two,

The TES instrument first flew aboard the Mars Observer spacecraft that was lost. The TES instrument was rebuilt and launched along with instruments aboard the new Mars Global Surveyor spacecraft.

The purpose of the TES device is to measure the spectral distribution of thermal infrared radiation emitted from Martian surfaces. The TES technique, can tell us much about the geology and atmosphere of Mars.

One can learn much about this method and the device by visiting the Arizona State University website pages that provide much more detail and background and reading through the TES News Archives.

[NOTE: The above curves actually exist on the Arizona State University website on their webpage address: http://tes.asu.edu/MARS_SURVEYOR/MGSTES/mixed_spec.gif]

The Thermal Emission Spectrometer is a scientific instrument and also Thermal Emission Spectroscopy is a measurement technique.

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* K-feldspar end member KAlSi₃O₈, Albite end member NaAlSi₃O₈ or Anorthite end member CaAl₂Si₂O according to the Wikipedia article on Feldspar.
** The general formula (for Hornblende) can be given as (Ca,Na)_2-3(Mg,Fe,Al)₅(Al,Si)₈O₂₂(OH,F)₂ , according to the Wikipedia article on Hornblende..