Thermal radiation in semitransparent media

A semi-transparent medium is one that absorbs – possibly scatters – and emits thermal radiation by volume. It is also said to be a medium that participates in radiation.

This type of environment is encountered, for example, in combustion, where high-temperature gases emit and absorb radiation on a volume basis. What's more, if these same gases are laden with particles (soot, droplets), these also contribute to radiative transfer by absorbing, emitting and scattering photons. Generally speaking, radiation can be considered from either a wave (i.e., the propagation of electromagnetic waves) or a particle (i.e., the tracking of energy packets, in this case photons) point of view. Both approaches have their own advantages and disadvantages. They are used in complementary ways.

There are still other areas of great industrial interest where radiation through a "semi-transparent" medium plays a predominant role, such as high-temperature processes (glass furnaces, boilers), aircraft or rocket engines (combustion chamber thermics, infrared remote sensing), insulation design, fire prevention, and so on.

This article introduces the basic concepts of radiative transfer in media that absorb, emit and scatter photons. The various mechanisms are detailed in order to establish the basis of the physical model on which the main thermal radiation modeling methods are based.

This article describes the basic concepts of thermal radiation in semi-transparent media. It builds on the knowledge presented in the article [BE 8 210] for opaque media.