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.
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.
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: 0 ≤ α ≤ 1
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: 0 ≤ ρ ≤ 1
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: 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
For opaque surfaces, τ=0, and thus
This is an important property relation since it enables us to determine both the absorptivity and reflectivity of an opaque surface by measuring either of these properties.