The PAR sensor measures global solar radiation from 400 to 700 nm, which approximates the spectral band active in photosynthesis. The response of the instrument falls sharply to zero on either side of this band, and between 400 and 670 nm it increases monotonically from about 50% to 100% (see Spectral Response for a plot). There is no protective glass dome over the receiving surface. Irradiance from the sun passes through a small white visible bandpass filter/diffuser in the shape of a horizontal disk at the top of the instrument. Within the instrument radiation is directed through a series of colored glass filters and onto a silicon photodiode detector. The factory-supplied calibration converts the signal from the detector to a flux of photons in micro-mole (of photons) s-1 m-2. (One micro-mole equals 6.022 E17 photons.) For the solar spectrum, these units may be converted to watts per square meter by dividing by 4.6.
