Table of Contents
Everything around us constantly emits radiation and emission characteristics of a body is represented by emissivity. This means that every body, including our own, is constantly bombarded by radiation coming from all directions over a range of wavelengths.
Coming directly to our topic let us discuss different terms related to radiation.
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Irradiation
Definition of irradiation.
The radiation flux incident on a surface is called irradiation and is denoted by G.
When radiation strikes a surface, part of it is absorbed, part of it is reflected, and the remaining part, if any, is transmitted, as illustrated above Figure.
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Absorptivity
Definition of absorptivity.
The fraction of irradiation absorbed by the surface is called the absorptivity (α). It is the ratio of absorbed radiation (G abs) to incident radiation (G).
Its value range: 0 ≤ α ≤ 1
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Reflectivity
Definition of reflectivity.
The fraction of radiation reflected by the surface is called the reflectivity (ρ). It is the ratio of reflected radiation (G ref) to incident radiation (G).
Its value range: 0 ≤ ρ ≤ 1
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Transmissivity
Definition of transmissivity.
The fraction of radiation transmitted is called the transmissivity (τ). It is the ratio of transmitted radiation (G tr) to incident radiation (G).
Its value range: 0 ≤ τ ≤ 1
The first law of thermodynamics requires that the sum of the absorbed, reflected, and transmitted radiation energy be equal to the incident radiation. That is,
G abs + G ref + G tr = G
Dividing each term of this relation by G yields
α+ρ+τ=1
An opaque surface is a surface which does not allow transmission of radiation through it. Wood, metal, concreate are some examples of opaque materials.
For opaque surfaces, τ=0, and thus above equation reduces to
α+ρ=1
It is an important property relation since, it enables us to determine both the absorptivity and reflectivity of an opaque surface by measuring either one of these properties.
We hope this article has helped you in understanding various terminologies related to radiation. In case of any doubt kindly feel free to use the comment section.