This light sensor developed by SDEC France is designed to measure the photosynthetically active radiation (also called P.A.R. sensor). It effectively measures radiation between 400 and 700 nm.
The sensor has an output proportional to the number of photon recieved regardless of their energy levels (photon flux density : µmol.m-2.s-1).
Since the energy level of each photon depends on its wavelength, the spectral response of a P.A.R. sensor using the photovoltaic effect must be higher in the far red part of the spectrum (around 700 nm) than in the blue wavelengths (around 400 nm).
The spectral response of the gallium arsenide photodiode built in the unit is close to the theoritical curve (see dart curve below). Its sensitivity is a bit low for blue radiation. With an additionnal blue filter (glass filter, tinted in the block, “SCHOTT” type), the spectral response is well balanced.
An acrylic diffuser tightly fitted into the sensor cuts off at 400 nm. It is opaque below 400 nm and has a constant transmission (around 50%) after 420 nm.