emittance vs emissivity: what's the difference?

[Todd C. Secoy]

around here, folks use the words EMITTANCE and EMISSIVITY interchangeably.

i have begun to suspect, however, that the meanings may be slightly
different.

also, how valid is the assumption: absorptance = emittance ? (gray body?)
does this have to be true?

maybe someone can shed some light :> on this issue.

-todd
sec...@ssvax1.ssd.loral.com
palo alto, california

[JBCooper3]

In article <1994Sep2.2...@wdl.loral.com>,

SEC...@SSVAX1.SSD.LORAL.COM (Todd C. Secoy) writes:

>around here, folks use the words EMITTANCE and EMISSIVITY
interchangeably.
>i have begun to suspect, however, that the meanings may be slightly
different.
>also, how valid is the assumption: absorptance = emittance ? (gray
body?)
>does this have to be true?

I don't have my handy radiometry book around (as if anyone would keep this
kind of thing in their heads), but I'll give it a go. A material's
*emissivity* relates to, in essence, how close it comes to a blackbody. It
is usually spectrally variant. A perfect blackbody has an emissivity of
unity (1). Emmisivity is usually denoted by epsilon, and can be thought of
in the same way as reflectivity (ie, reflectivity + transmissivity = 1).

*Emittance* is the radiometric flux from a source per unit area of the
source. Various symbols are used for it; I try to stick with Hudson and
use W. Emittance can also be spectrally variant.

Radiometry isn't trivial and doesn't stick in your brain if you don't use
it all the time. Some good references (IMHO) are:
William Wolfe, ed., IR Handbook (2nd ed),
Smith, Modern Optical Engineering, and
Hudson, Infrared System Engineering.
Fred Nicodemus (sic) has published a few good basic papers covering
terminology and theory, but I don't have the references handy. I'm sure
others out there in NetLand can clarify all these points. Hope this helps.

John
jbco...@aol.com

[Ken Ellis]

Absorptivity = emissivity as long as the surface is in thermal
equilibrium (Kirchhoff's Law). If not, deviations from this equality
are generally second-order effects. Jack Salisbury at JHU has
published a paper (I think in JGR) how Kirchhoff's law breaks down for
soil and snow surfaces which have vertical structure. In most cases
Kirchhoff's Law can be assumed to be valid.
--
_____
\ | Opinions expressed here are solely
\ ERIM Ken Ellis | those of the author and do not
/ el...@erim.org | necessarily represent those of
/____| | his employer.

[David Tilbrook]

As far as I am concerned, the basic radiative flux of a surface is
described by its spectral exitance or emittance as it is otherwise
referred to. The RATIO of the emittance of a surface to that of a
blackbody at the same temperature is designated the emissivity of that
surface.

This is definition that my project group uses in my project.

Any correspondance relating to the emissivity of surfaces would be
welcomed e-mail me on tilb...@chem.surrey.ac.uk

Regards

Dave Tilbrook
**********************************************************************
Polymer Group, Chemistry Dept.
University of Surrey, Guildford,
Surrey, UK

Tel 44 (0) 483 25 92 91
Fax 44 (0) 483 30 08 03 (Mark Chemistry PG)
**********************************************************************