A brief explanation of the topic of emissivity, that gets into the concept of spectral emissivity… a little. It deals with the total emissivity used in radiant heat transfer, but the basic concepts apply at any wavelength.
Visit http://www.fuji-piezo.com/emissivi.htm online to read their explanation.
New device hides, on cue, from infrared cameras
November 26, 2012
Tunable material developed at Harvard boasts nearly 100% absorption on demand
Cambridge, Mass. – November 26, 2012 – Now you see it, now you don’t.
A new device invented at the Harvard School of Engineering and Applied Sciences (SEAS) can absorb 99.75% of infrared light that shines on it. When activated, it appears black to infrared cameras.
Composed of just a 180-nanometer-thick layer of vanadium dioxide (VO2) on top of a sheet of sapphire, the device reacts to temperature changes by reflecting dramatically more or less infrared light.
Announced today in the journal Applied Physics Letters, and featured on its cover, this perfect absorber is ultrathin, tunable, and exceptionally well suited for use in a range of infrared optical devices.
Perfect absorbers have been created many times before, but not with such versatile properties. In a Fabry-Pérot cavity, for instance, two mirrors sandwich an absorbing material, and light simply reflects light back and forth until it’s mostly all gone. Other devices incorporate surfaces with nanoscale metallic patterns that trap and eventually absorb the light.
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In: Remote Sensing of Environment, Volume 45, Issue 2, August 1993, Pages 225-231.
by John W. Salisbury a, Dana M. D’Aria a and Floyd F. Sabins Jr.b
aDepartment of Earth and Planetary Sciences, Johns Hopkins University, Baltimore U.S.A.
bChevron Oil Field Research Company, La Habra, California U.S.A.
With all the interest on the Gulf Oil spill and recent accounts of the use by British Petroleum and others of Infrared Thermal Imaging to search for surface oil slicks, it seemed very timely to be sure we had included some links and summaries of articles dealing with the thermal Infrared optical properties of crude oil on seawater.
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Novel approach to assess the emissivity of the human skin
J. Biomed. Opt., Vol. 14, 024006 (2009); DOI:10.1117/1.3086612 Published 6 March 2009
by: Francisco J. Sanchez-Marin, Sergio Calixto-Carrera, and Carlos Villaseñor-Mora
Centro de investigaciones en optica, Loma del Bosque 115, Lomas del Campestre, Leon, Guanajuato 37150, Mexico
To study the radiation emitted by the human skin, the emissivity of its surface must be known. We present a new approach to measure the emissivity of the human skin in vivo. Our method is based on the calculation of the difference of two infrared images: one acquired before projecting a CO2 laser beam on the surface of the skin and the other after such projection. The difference image contains the radiation reflected by the skin, which is used to calculate the emissivity, making use of Kirchhoff’s law and the Helmholtz reciprocity relation. With our method, noncontact measurements are achieved, and the determination of the skin temperature is not needed, which has been an inconvenience for other methods. We show that it is possible to make determinations of the emissivity at specific wavelengths. Last, our results confirm that the human skin obeys Lambert’s law of diffuse reflection and that it behaves almost like a blackbody at a wavelength of 10.6 µm.
Editor’s Note: Back in the 1960s there were several serious projects mounted by the US Army Medical Research Laboratory’s BioPhysics Division on determining injury thresholds of laser radiation on human skin analogs. The article “THRESHOLD LESIONS INDUCED IN PORCINE SKIN BY CO2 LASER RADIATION” by Brownell, Arnold S. ; Parr, Wordie H. ; Hysell, David K. ; Dedrick, Robert, USAMRL Report No. 7327, June 1967, is available as a pdf download at: http://handle.dtic.mil/100.2/AD659347.
Although not fully described in the article, the measured results compared favorably with a semi-infinite solid model of heat conduction for a surface that was essentially black (10.6 micron spectral absorptivity or emissivity very close to 1.0) or fully absorbing at 10.6 microns. This editor was a member of the USAMRL BioPhysics Division staff at that time and helped with the dosimetry of the experiments described.
San Diego CA, USA –Surface Optics’ ET10 measures emissivity values in two most commonly used spectral regions, 3 to 5 and 8 to 12 microns.
Its main application is to produce emissivity values for the infrared cameras.
Advanced IR cameras require the input of an emissivity value for accurate temperature calculations. The emissivity values obtained from tables can be far from real leading to large temperature uncertainties.
The ET10 can be used in the lab or in the field and on small or large objects. With the ET10 one can measure emissivity of any surface in just a few seconds.
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