Photosynthetically Active Radiation (sometimes called photosynthetically available radiation). A PAR sensor is an irradiance or scalar irradiance sensor that is equally sensitive to photons between 400 and 700 nm and insensitive to photons outside this region. In terms of collector geometry, a PAR sensor is usually equipped with either a scalar or cosine collector. At BSI, the most common units of calibration for broadband PAR are either Q^.m^-2.s^-1 (quanta) or µE^.m^-2.s^-1 (microEinstein). PAR may also be calculated by integrating the weighted visible channels from a multiwavelength instrument.