Radiosity (radiometry)

In radiometry, radiosity is the radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area, and spectral radiosity is the radiosity of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The SI unit of radiosity is the watt per square metre (W/m²), while that of spectral radiosity in frequency is the watt per square metre per hertz (W·m⁻²·Hz⁻¹) and that of spectral radiosity in wavelength is the watt per square metre per metre (W·m⁻³)—commonly the watt per square metre per nanometre (W·m⁻²·nm⁻¹). The CGS unit erg per square centimeter per second (erg·cm⁻²·s⁻¹) is often used in astronomy. Radiosity is often called intensity in branches of physics other than radiometry, but in radiometry this usage leads to confusion with radiant intensity.

Radiosity of a surface, denoted J_e ("e" for "energetic", to avoid confusion with photometric quantities), is defined as

where

For an opaque surface, the transmitted component of radiosity J_e,tr vanishes and only two components remain:

In heat transfer, combining these two factors into one radiosity term helps in determining the net energy exchange between multiple surfaces.

Spectral radiosity in frequency of a surface, denoted J_e,ν, is defined as

where ν is the frequency.

Spectral radiosity in wavelength of a surface, denoted J_e,λ, is defined as