Radiosité (physique)

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 (), 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 (). The CGS unit erg per square centimeter per second () is often used in astronomy. Radiosity is often called in branches of physics other than radiometry, but in radiometry this usage leads to confusion with radiant intensity. Radiosity of a surface, denoted Je ("e" for "energetic", to avoid confusion with photometric quantities), is defined as where ∂ is the partial derivative symbol is the radiant flux leaving (emitted, reflected and transmitted) is the area is the emitted component of the radiosity of the surface, that is to say its exitance is the reflected component of the radiosity of the surface is the transmitted component of the radiosity of the surface For an opaque surface, the transmitted component of radiosity Je,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 Je,ν, is defined as where ν is the frequency. Spectral radiosity in wavelength of a surface, denoted Je,λ, is defined as where λ is the wavelength. The radiosity of an opaque, gray and diffuse surface is given by where ε is the emissivity of that surface; σ is the Stefan–Boltzmann constant; T is the temperature of that surface; Ee is the irradiance of that surface. Normally, Ee is the unknown variable and will depend on the surrounding surfaces.