delta t sunshine sensor
source : Delta-T Devices

The PAR energy sensors measure PAR in energy units such as W m-2. They measure radiation flux through temperature fluctuation of a black body surface due to incoming radiation. The typical example of the instruments is BF5 Sunshine Sensor by Delta T devices. Upward and downward looking pyranometers can also be used to get an estimate of FAPAR, however they measure radiation over wider range of wavelengths (400 to 1100nm) and hence some corrections have to be applied to get estimates of PAR.