Can emissivity be greter that 1? I have measured with an IR thermometer and it allows to set emissivity up to 1,5. How is this possible? I came across a paper "Cavity Emissivities Greater Than One", author Frederick O. Bartell but I don´t understand how can emissitity be greater that 1.
The article abstract can be found here:
http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1238876
I dont have the full text.
According to my knowledge NO. However, I note that this topic is beyond my interests and expertise. Petr
Actually, the answer is yes although this is counter intuitive. Specifically, the emissivity of a small particle, in relative isolation from other particles, can be greater than one. This is perhaps best understood if one considers scattering or absorption as the counterpart to emissivity. It is well established and widely known that small, isolated particles can and often do scatter or absorb more light than would geometrically fall on their cross section. Emissivity, for all the same reasons, can exceed unity under similar circumstances.
What I understood from the abstract https://www.researchgate.net/deref/http%3A%2F%2Fproceedings.spiedigitallibrary.org%2Fproceeding.aspx%3Farticleid%3D1238876
the emissivity larger than 1 applies only to a specific of a cavity. The reason is in the selected reference black body with its temperature. If the individual walls inside the cavity have different temperatures and the reference black body has a temperature lower than for example only one of the surfaces, this may happen. Then the emitted energy from the surfaces that have larger temperature is higher than from the reference black body (see Stefan Boltzman law).
Miroslav
Indeed emissivity can exceed 1. This is for particles smaller than the dominant radiation wavelength. Check this article:
V. A. Golyk, M. Krüger, and M. Kardar, "Heat radiation from long cylindrical objects," Phys. Rev. E 85(4), 046603 (2012).
Could candoluminescent materials be an example of emissivity higher than unity?
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