Since plants and people respond differently to light, we have to use different units to describe light in relationship to plants and people. And this is an important thing to keep in mind if you are considering using LED grow lights to cultivate plants.
The unit we use to measure the amount of light visible to humans is ‘lumen’ or ‘lux’ which means the number of lumens per square meter. Lumen weighs a light spectrum along the human eye sensitivity curve, which reaches its highest pointin the green part of the spectrum.
To measure light in relation to plants, we use Photosynthetically Active Radiation (PAR), which counts all photons in the 400 to 700 nm range. The units of measure are photosynthetic photon flux (PPF) and the unit of measure is mmol.s-1 and photosynthetic photon flux density, (PPFD) and its unit is mmolm-2.s-1.