Irradiance detector is a sensor that measures the radiative power per surface area. For example, the irradiance of bright day light is about 1000 W/m², meaning that 1000 W of power are absorbed by one square meter of horizontal surface. Spectral irradiance is abbreviated E(λ) where λ is the center wavelength of the detector. Calibration units common for our irradiance sensors are µW^.cm^-2.nm^-1 for narrow bandwidth detectors, or either Q^.m^-2.s^-1 (quanta) or µE^.m^-2.s^-1 (microEinstein) for broadband sensors such as PAR (400–700 nm). The SI unit for spectral irradiance is W^.m^-2.nm^-1.