Solar radiation instruments are used to measure the spectral range of radiation from the sun. Product categories include radiometers, and quantum sensors. Pyranometers measure broadband solar irradiance incident on a planar surface and calibrate temperature response in units of radiation flux. Radiometers measure the net radiation level by subtracting the reflected solar radiation from the sum of the incoming direct beam and the diffuse solar radiation. Quantum sensors measure the photosynthetic light spectrum or photosynthetic photon flux density (PPFD) as the number of photons incident per unit type on a unit surface. These solar radiation instruments use either thermopile arrays or photovoltaic detectors. Thermopile arrays provide an electrical output proportional to the absorbed radiation. Thermocouples may be connected in series for a higher voltage output, or in parallel for a higher current output. Photovoltaic (PV) detectors consist of photovoltaic cells which measure radiation by generating a potential at a junction between two materials.