The LI190R Quantum Sensor, manufactured by LI-COR, accurately measures photosynthetic photon flux density (PPFD), which is the number of photons in the 400 to 700 nm waveband incident per unit time on a unit surface. It uses a silicon photovoltaic detector mounted in a cosine-corrected head. A shunt resistor in the sensor’s cable converts the signal from microamps to millivolts, allowing these sensors to be measured directly by a Campbell Scientific data logger..