SpectralEmissivity & Emittance

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ET10 Reflectometer Measures Emissivity

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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The RET Theory

Ircon, Inc., a leading producer of industrial radiation thermometers, line scanners and quantitative thermal imagers, in its training programs for many years used to teach something they called the RAT Theory.

Reflectance, Absorbtance and Transmittance, or the coefficients of them, abbreviated as R, A &T must sum to 100%, or R + A + T=1.

An easy way for newcomers to Infrared radiation thermometry to remember a very important concept.

The associated concept is that Absorbtance=Emittance, or A=E. Or the RAT theory could be written as R+E+T=1 and renamed the RET Theory.

So, while not as easily recalled, the RET Theory name just didn’t catch on as easily as the RAT Theory.

(BTW, whenever I tried to teach some basics of Radiation Thermometry, I used to call it the TAR Theory because I thought it might “stick” better- it didn’t – RAT wins by a landslide every time.)

All this is a lead in to the wonderful resources by the folks at  LabSphere for those who want to know or learn how to measure emittance or absorbtance through the roundabout way of measuring reflectance and transmittance first and then doing a bit of math.

They have a readily downloadable 26 page PDF document entitled “A Guide to Integrating Sphere Radiometry and Photometry”.

It explains far more than the RAT or RET or TAR theories about optical radiation metrology.

I think it and many of their online aids are well worth a read.

The InfraRed Sea Surface Emissivity (IRSSE) model

From Paul van Delst’s Work Page at The Cooperative Institute for Meteorological Satellite Studies of the University of Wisconsin – Madison Space Science and Engineering Center.
The InfraRed Sea Surface Emissivity (IRSSE) model was developed for use in the Global Data Assimilation System (GDAS) at NCEP/EMC. Previously, the GDAS used an IRSSE model based on Masuda et al (1988). The Masuda model doesn’t account for the effect of enhanced emission due to reflection from the sea surface (only an issue for larger view angles) and the implementation was based on coarse spectral resolution emissivity data making its application to high resolution instruments, such as AIRS, problematic.

The old IRSSE model has been upgraded to use sea surface emissivities derived via the Wu and Smith (1997) methodology as described in van Delst and Wu (2000). The emissivity spectra are computed assuming the infrared sensors are not polarised and using the data of Hale and Querry (1973) for the refractive index of water, Segelstein (1981) for the extinction coefficient, and Friedman (1969) for the salinitiy/chlorinity corrections.

Infrared Sea Surface Emissivity

“The InfraRed Sea Surface Emissivity (IRSSE) model was developed for use in the Global Data Assimilation System (GDAS) at NCEP/EMC. Previously, the GDAS used an IRSSE model based on Masuda et al (1988).

“The Masuda model doesn’t account for the effect of enhanced emission due to reflection from the sea surface (only an issue for larger view angles) and the implementation was based on coarse spectral resolution emissivity data making its application to high resolution instruments, such as AIRS, problematic.

“The old IRSSE model has been upgraded to use sea surface emissivities derived via the Wu and Smith (1997) methodology as described in van Delst and Wu (2000).

“The emissivity spectra are computed assuming the infrared sensors are not polarised and using the data of Hale for the refractive index of water, Segelstein (1981) for the extinction coefficient, and Friedman (1969) for the salinitiy/chlorinity corrections.

“Instrument spectral response functions (SRFs) are used to reduce the emissivity spectra to instrument resolution. These are the quantities predicted by the IRSSE model.”