SpectralEmissivity & Emittance

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Handbook of OSML Libraries: Emittance

CRMHT – CNRS Centre de Recherche sur les Matériaux à Haute Température, Orléans, France
Mesure indirecte de l’émittance (Includes sample data for Silicon Dioxide)

Mesure de la réflectivité et de la transmissivité normales spectrales (10 à 40 000 cm-1 soit 1 000 à 0,25 µm).

L’émissivité normale spectrale se déduit indirectement par calcul de ces deux grandeurs par application des lois de Kirchhoff ,

i.e. at each wavelength, Emissivity =1 – Reflectivity – Transmissity

Emittance-WN Handbook of OSML Libraries
E – dielectric function
N – complex refractive index
RT – reflectivity, layer transmissivity
WN – wave number
OSML Source : [Emitttance-WN]
Function Group : [Optical Functions]
Emittance-E
Emittance-E (AE) represents the fraction of the incident radiation that is absorbed (Kirchhoff law) by a sample with a plate shape. Its expression take into account for multiple reflections (no interference effects) and depends on the dielectric functions of the incident medium Ei, those of the material Eo and the thickness d of the sample.
Function signature : Emittance-E(x,Ei,Eo,Thickness)Units


The spectral dependence must be expressed in wave numbers (cm-1) and the thickness in (cm).
Emittance-N
Emittance-N (AN) represents the fraction of the incident radiation that is absorbed (Kirchhoff law) by a sample with a plate shape. Its expression take into account for multiple reflections (no interference effects) and depends on the complex refractive indexes of the incident medium Ni, those of the material No and the thickness d of the sample.
Function signature : Emittance-N(x,Ni,No,Thickness)Units


The spectral dependence must be expressed in wave numbers (cm-1) and the thickness in (cm).
Emittance-RT
Emittance-RT (ART) represents the fraction of the incident radiation that is absorbed (Kirchhoff law) by a sample with a plate shape. Its expression take into account for multiple reflections (no interference effects and depends on the reflectivity R and the layer transmissivity T of the sample.
Function signature : Emittance-RT(R,T)
Planck-WN
Planck-WN (PWN) is the wave number version of the Planck function. Its expression depends on the temperature T.
Function signature : Planck-WN(x,T)
Constants : C1=1.1910 10-6 (W.m2) C2=1.4388 (cm.K)

See Also : [Optical Functions] [Reflectance-WN] [Transmittance-WN]

Handbook of OSML Libraries

ASTM E423 – 71(2008): Standard Test Method for Normal Spectral Emittance

Standard Test Method for Normal Spectral Emittance at Elevated Temperatures of Nonconducting Specimens ASTM E423 – 71(2008) – www.ASTM.org

1. Scope

1.1 This test method describes an accurate technique for measuring the normal spectral emittance of electrically nonconducting materials in the temperature range from 1000 to 1800 K, and at wavelengths from 1 to 35 ?m. It is particularly suitable for measuring the normal spectral emittance of materials such as ceramic oxides, which have relatively low thermal conductivity and are translucent to appreciable depths (several millimetres) below the surface, but which become essentially opaque at thicknesses of 10 mm or less.

1.2 This test method requires expensive equipment and rather elaborate precautions, but produces data that are accurate to within a few percent. It is particularly suitable for research laboratories, where the highest precision and accuracy are desired, and is not recommended for routine production or acceptance testing. Because of its high accuracy, this test method may be used as a reference method to be applied to production and acceptance testing in case of dispute.

1.3 This test method requires the use of a specific specimen size and configuration, and a specific heating and viewing technique. The design details of the critical specimen furnace are presented in Ref (1), and the use of a furnace of this design is necessary to comply with this test method. The transfer optics and spectrophotometer are discussed in general terms.

1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

2. Referenced Documents

E349 Terminology Relating to Space Simulation – www.ASTM.org

Full document current and on sale at the ASTM web store.

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.