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
Journal of the Atmospheric Sciences
a. Atmospheric Science Program, Department of Physics and Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada
b. Optical Sciences Division, Naval Research Laboratory, Washington, D.C.
Chýlek, P., P. Damiano, and E.P. Shettle, 1992: Infrared Emittance of Water Clouds. J. Atmos. Sci., 49, 1459–1472.
A simple approximation has been developed for the infrared emittance of clouds composed of water spheres based on the absorption approximation for the emittance and on the polynomial approximation to the Mie absorption efficiency. The expression for the IR emittance is obtained in a simple analytical form as a function of the liquid water content and two size distribution parameters, namely, the effective radius and effective variance. The approximation is suitable for numerical weather prediction, climate modeling, and radiative transfer calculations. The accuracy, when compared to the exact Mie calculation and integration over the size distribution, is within a few percent, while the required computer time is reduced by several orders of magnitude. In the limit of small droplet sizes, the derived IR emittance reduces to a term proportional to the liquid water content.
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?
Location: Industrial Research Limited (IRL): Measurement Standards Laboratory of New Zealand
Their Client: New Zealand Refining Company
The Topic: Infrared radiation thermometry for furnace tube temperature measurement
Client benefit: High quality measurement leads to better plant control, reduced risk and increased profitability
Most of this article is from the writeup on the IRL website.
Industrial Research undertook furnace radiation modeling for the New Zealand Refining Company which operates several large furnaces in many stages of the production of gasoline (Also known around the world variously as “petrol” and “benzene”).
Thermal infrared radiation thermometry, or IR Thermometry, is the only feasible method for obtaining temperature measurements of many furnaces and the process tubes within.
The technology is not new, it was known that the measurements suffer from errors stemming from several environmental factors such as reflection of background thermal radiation and absorption and emission of the emitted & reflected radiation (from the surfaces being measured, by furnace gases.
Variations in the spectral emissivity and reflectivity of the materials comprising the surfaces being measured also influence the resulting temperature measurements.
The New Zealand Refining Company used the knowledge and background offered by Industrial Research’s Measurement Standards Laboratory in furnace radiation modelling to obtain more accurate measurements.
The service gave increased confidence that safe and efficient operating parameters were being maintained. As a result, the plant can operate more efficiently through the operators being able to better predict plant life and tube life.
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.