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.

## 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.

## 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: 0 ≤ α ≤ 1

## 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: 0 ≤ ρ ≤ 1

## 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: 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

For opaque surfaces, τ=0, and thus

α+ρ=1

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.