Irradiance

In radiometry, irradiance is the radiant flux (power) received by a surface per unit area. The SI unit of irradiance is the watt per square metre (W·m-2). The CGS unit erg per square centimetre per second (erg·cm−2·s−1) is often used in astronomy. Irradiance is often called intensity because it has the same physical dimensions, but this term is avoided in radiometry where such usage leads to confusion with radiant intensity. In radiometry, irradiance is the radiant flux (power) received by a surface per unit area. The SI unit of irradiance is the watt per square metre (W·m-2). The CGS unit erg per square centimetre per second (erg·cm−2·s−1) is often used in astronomy. Irradiance is often called intensity because it has the same physical dimensions, but this term is avoided in radiometry where such usage leads to confusion with radiant intensity. Spectral irradiance is the irradiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The two forms have different dimensions: spectral irradiance of a frequency spectrum is measured in watts per square metre per hertz (W·m−2·Hz−1), while spectral irradiance of a wavelength spectrum is measured in watts per square metre per metre (W·m−3), or more commonly watts per square metre per nanometre (W·m−2·nm−1). Irradiance of a surface, denoted Ee ('e' for 'energetic', to avoid confusion with photometric quantities), is defined as