The LI-190R Quantum Sensor measures Photosynthetically Active Radiation (PAR, in µmol of photons m^-2 s^-1). It provides accurate measurements—in the open, in greenhouses, under plant canopies, or in growth chambers—for most broad-spectrum light sources, including natural sunlight, artificial, or mixed sources.

Plants use light in the wavelength range from about 400 to 700 nm to drive photosynthesis. The efficiency with which plants use light varies somewhat across this range, but McCree¹ showed that measuring PAR provided a consistent way to predict plant photosynthetic response regardless of the spectrum of the light source.