When using the small photoreactors described above, the LED light beam (e.g. 22 mm in diameter) is typically bigger than the side-view area of the dispersion, which is being irradiated. Therefore, the relevant measurable is the irradiance, in units such as mW cm -2 . To measure irradiance, the measurement area of the power sensor must be completed covered by the light beam, i.e. the sensor area needs to be smaller than the area of the light beam.