SpectralEmissivity & Emittance

| Useful Data | Practices Measuring or Avoiding | Background & Theory|

Emissivity Coefficients of Some Common Materials

We’ve been trying to preach to the inexperienced about Spectral Emissivity vs (just plain) emissivity.

The former is the subject used in Thermal Infrared Radiation Thermometry (Pyrometry, to some) and users of Thermal Infrared Imagers (Thermographic or Thermography Cameras) while the latter is the domain of radiation heat transfer considerations (except of course when spectral issues, like windows and atmospheres get in the way of the radiation transfer).

See the Emissivity Trail Pages at About Temperature Sensors if you’d like a brief rant or two.

But popular ignorance of details not withstanding, it is still a bit of a shock to see the term ‘Emissivity” a prominent feature on both instrumentation and engineering websites, Here’s another one with some sample text (no numbers here) from the Engineering Toolbox website. (Note: we corrected their misspelling of “emissivity” – as mentioned in our semi – rant pages on About Temperature Sensors, the word seems to be misspelled as often as it the term and the values are misunderstood and misused!)

The radiation heat transfer emissivity coefficient of some common materials as aluminum, brass, glass and many more

The emissivity coefficient – ? – indicates the radiation of heat from a ‘grey body’ according the Stefan-Boltzmann Law, compared with the radiation of heat from a ideal ‘black body’ with the emissivity coefficient ? = 1.

The emissivity coefficient – ? – for some common materials can be found in the table below. Note that the emissivity coefficients for some products varies with the temperature. As a guideline the emisivities below are based on temperature 300 K.
Surface Material

After Note: We have tried over the past ten years or so, with very limited success, to point out to organizations that should know better, including at least one each manufacturer of “Infrared Thermometers” and One Prominent Maker of Blackbody calibration furnaces, that they need to mend their errant ways and get with the one true religion of Spectral Emissivity.

Heck, the Church of the Flying Spaghetti Monster got a better response and Rodney Dangerfield gets more respect.

There are a few bright lights at the end of the emissivity “black hole”, the new facilities at several national Metrology Laboratories, such as the one at NIST dealing with Infrared Optical Properties of Materials and the “Modern emissivity measuring facility for industry-orientated calibrations developed at PTB“.

Hope springs eternal!

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.

Measurements of the 4.3-mu CO2 Band 2560 – 3000 K

Spectral-Emissivity Measurements of the 4.3-mu CO2 Band between 2560 degrees and 3000 degrees K
C. C. Ferriso, C. B. Ludwig, and L. Acton
JOSA, Vol. 56, Issue 2, pp. 171- (1966)

C. C. Ferriso, C. B. Ludwig, and L. Acton, “Spectral-Emissivity Measurements of the 4.3-mu CO2 Band between 2560 degrees and 3000 degrees K,” J. Opt. Soc. Am. 56, 171- (1966)