Plants use the electromagnetic spectrum for photosynthesis, referred to as Photosynthetically Active Radiation, or PAR, that encompasses the wavelengths from 40nm (nanometers) to 70nm. In addition to the spectral range of PAR, we also understand plants can detect wavelengths of light outside of this range including ultraviolet and infrared wavelengths. For plants, we use a term called Photosynthetic Photon Flux, PPF, which describes the quantity of PAR that is produced by a light source per second. This measurement is expressed in micromoles per second (µmol/s). A more important measurement used is Photosynthetic Photon Flux Density (PPFD), which describes the amount of light (intensity) used for photosynthesis (PAR) that arrives at the plant. PPFD it is given in units of micromoles of photons in the PAR range that is delivered to one square meter per second (µmol/m2/s).   This unit is important because you need to know how much light is being distributed at the crop level from the sun and/or luminaires to ensure plants are receiving the correct amount of light to hit your target DLI. PPFD should be measured at the crop canopy. Multiple readings should be taken, across the canopy, to ensure uniformity and accurate readings. To determine how much light is needed over the course of a day for your crop, we also use a measurement called the Daily Light Integral (DLI), a parameter describing the cumulative PPFD over an entire day. DLI is strongly related to photosynthesis, growth rate, and overall plant productivity. The units for DLI are similar to that of PPFD but extend over an entire day (mol/m2/day). DLI will change over the course of the year, based on your geography. Most plants need a DLI of 12-30 mol/day.  More information can be found here referring to specific DLI maps for your region.   Note: When taking light measurements in a greenhouse, DO NOT use a Foot Candle/Lux sensor (which follows the sensitivity curve of the human eye and gives inaccurate information when comparing light sources with a different spectrum). Instead, use a quality quantum sensor (PAR-sensor) which is designed to follow the sensitivity curve of plants and measures the number of photons between 400 and 700nm in µmol.