Spectral Emissivity & Emittance

For your reference, there are several that deal with emittance that may be of interest to you.

For your convenience, they are on the IRINFO.org web pages at the following links:

www.irinfo.org/tip_of_week_2004.html#t02092004

www.irinfo.org/tip_of_week_2004.html#t04052004

www.irinfo.org/tip_of_week_2003.html#t09292003

www.irinfo.org/tip_of_week_2004.html#t08022004

www.irinfo.org/tip_of_week_2005.html#t09122005

www.irinfo.org/tip_of_week_2005.html#t09192005

www.irinfo.org/tip_of_week_2005.html#t09262005

www.irinfo.org/tip_of_week_2007.html#t05282007

Enjoy!

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

Emissivity & other infrared-optical properties FAQs at the evitherm website,
evitherm is the European Virtual Institute for Thermal Metrology

Click on the number below for an answer on the evitherm website…

C1. What is the emissivity of a surface?
C2. Why is emissivity important?
C3. How is emissivity used?
C4. Is it easy to measure emissivity?
C5. Is it possible to predict or calculate emissivity?
C6. What type of emissivity should I use for my application: total emissivity or spectral emissivity?
C7. What is the emissivity of painted metal surfaces and how does it depend on layer thickness?
C8. Which surfaces behave like a grey body?
C9. What is the emissivity of a layer of gas?
C10. Where can I find information on the emissivity of a given surface?
C11.How can I measure the emissivity of a surface using an IR-thermometer?
C12. What is the difference between emissivity and emittance?
C13. What is a radiant barrier?
C14. What is a low-e coating?
C15. What is low-e glass?
C16. What is a selective absorber?
C17. Is a knowledge of emissivity important for contactless temperature measurements?
C18. What is infrared thermography?

Effects of Preoxidation Treatments on Spectral Normal and Total Normal Emittance of Inconel, Inconel-X and Type 347 Stainless Steel

Authors: Wayne S. Slemp; NATIONAL AERONAUTICS AND SPACE ADMINISTRATION HAMPTON VA LANGLEY RESEARCH CENTER

Abstract:

The spectral/normal-emittance values of several oxidized surfaces prepared by varying the preoxidation treatments or oxidation time for inconel, Inconel-X, and type 347 stainless steel were determined at temperatures of 900, 1,200, l,500, and 1,800 F over a wavelength range of 1 to l5 microns. Polishing, grit blasting, etching, or combinations of these preparations were used as preoxidation treatments. These values were compared for 900 and 1,800 F to determine the effects of these treatments on the spectral-normal-emittance values. Significant effects of preoxidation treatments and oxidation times on the spectral normal emittances of oxidized inconel, Inconel-X, and type 3k7 stainless steel are presented. In general, if a grit-blasted surface is etched before being oxidized, the final oxidized surface will have a lower emittance but will be more adherent and uniform. Of the two types of grit used in this study, the coarser grit provided the higher emittance. Polishing provided the lowest emittance of all specimens tested. In the one set of tests in which oxidation time was varied (on the inconel specimens), increasing oxidation time increased the emittance; however, increasing the time beyond 2 hours produced no further effect.

The Optical Properties measurements laboratory at The USA National Institute of Standards & Technology (NIST), a part of the Optical Technology Division of the PHYSICS Laboratory has been developing a full spectral emissivity (emittance) measurement capability.

The laboratory has established high accuracy infrared reflectance and transmittance capabilities for wavelengths between 1 µm and 18 µm. Near normal absolute spectral reflectance and transmittance of both specular and diffuse samples can be measured from near ambient to 200 °C using a custom integrating sphere and Fourier transform (FT) spectrometer. Additional capabilities for specular samples include transmittance down to 10 K using an optical cryostat, as well as variable angle transmittance and reflectance using a custom goniometer and polarizers.

_{Layout of the NIST Setup for Direct and Indirect Infrared Spectral Emittance Measurements}

Spectral directional emittance can be determined indirectly form reflectance and transmittance measurements described on the NIST Page: Infrared Spectrophotometry. These capabilities have limits of temperature, measurement geometry and sample type.

To expand the spectral emittance capabilities, a separate facility has been developed for its measurement at NIST using the direct method of radiance comparison of the sample with a blackbody(BB) reference source.

The facility consists of a set of reference blackbody sources mounted on a motorized stage for selection; interchangeable sample heater/mounts on motorized translation and rotation stages; a removable visible/near-infrared integrating sphere for measuring the sample temperature above 500 K; and low scatter interface optics to image the 3 mm to 5 mm central region of the sample or BB source onto a water cooled field stop.

Each BB contains calibrated platinum resistance thermometer (PRT) or thermocouple (TC) temperature sensors.

The spectral emissivities of the BBs have been calculated using a Monte Carlo ray tracing algorithm with input of the measured spectral reflectance of the cavity wall materials or coatings.

_{Integrating sphere for non-contact temperature measurement with sample heater in place}

_{Heaters for Transparent (Left) and Opaque (Center) Samples; Heater (up to 600 °C) and Set of Samples for Emittance Measurements (Right)}

The sample emittance is determined through a series of measurement steps.

The first step is a measurement of the sample’s hemispherical-directional reflectance at the measurement temperature and at a single wavelength matched to the filter radiometer.

The second step is a relative radiance measurement of the sample to a BB at the same wavelength.

The third step is to compare the sample spectral radiance to that of the reference blackbody source as a ratio with the FTIR .

_{Three Steps of Spectral Directional Emittance Scale Realization}

Finally, here’s a few results:

_{Spectral emittance of SiC and Pt10Rh samples}.

References

• Infrared spectral emissivity characterization facility at NIST,
  L.M. Hanssen, S.N. Mekhontsev, and V.B. Khromchenko,Proc. SPIE 5405, 112 (2004).
• Temperature- and angle-resolved infrared spectral directional emissivity of SiC, Alumina, and Pt for temperatures up to 1000 °C, C.P. Cagran, L.M. Hanssen, M. Noorma, and S.N. Mekhontsev,Intl. J. Thermophysical Prop. (submitted 2006).
• Use of a high temperature reflectometer for surface temperature measurements,
  L.M. Hanssen, M. Noorma, A.V. Prokhorov, S.N. Mekhontsev, and C.P. Cagran, Intl. J. Thermophysical Prop. (submitted 2006).