The term PAR (Photosynthetically Active Radiation) refers to the wavelengths of light which can actually be used by the plants to photosynthesize. The more of these wavelengths of light that your marijuana plants receive the better. In general, the PAR values are measured in two ways, those are, PPF and PPFD.
PPF or Photosynthetic Photon Flux calculates the amount of usable lights that are being emitted by the grow light. The Photosynthetic Photon Flux is measured in micromoles per second (μmol/s).
The term PPFD stands for Photosynthetic Flux Density. The Photosynthetic Flux Density is an even more important measure because it not only specifies the amount of PAR that is emitted by the LED grow light but it also tells how much of that light is actually falling on your cannabis plant. The Photosynthetic Flux Density is measured in micromoles per square meter per second (μmol/m²/s).
There are some manufacturers that will only provide you with the PPF values and some will only provide you with measures that only indicate from right in the center of the light’s footprint. Both of these tactics allow sub-standard manufacturers in order to make inflated claims about their LED grow lights by only stating the PAR values of the grow light very selectively. So, you need to make sure that you are getting what you need, in order to do that, you should always check the diagrams of the PAR values across the whole footprints and even at different heights as well.