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/m), while that of spectral radiosity in frequency is the watt per square metre per hertz (W·m ·Hz) and that of spectral radiosity in wavelength is the watt per square metre per metre (W·m)—commonly the watt per square metre per nanometre (W·m ·nm). The CGS unit erg per square centimeter per second (erg·cm ·s) 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.