PAR stands for photosynthetically active radiation and is otherwise known as the range on the UV spectrum that is conducive to plant growth. PAR is to plants as lumens are to humans. Lumens focus mainly on light with wavelengths in the 500’s—often called green light—while PAR encompasses a range of wavelengths between 400-700, which includes green, red, and blue light. Therefore, lumens are human-centric and do not reflect a true measurement of PAR and plants’ receptiveness to those aforementioned wavelengths.
The total measure of PAR is properly quantified by photosynthetic photon flux (PPF), and the amount of PAR that is absorbed by the plants is correctly represented as photosynthetic photon flux density (PPFD). To have a true indication of PAR at a particular height, there must be several PPFD measurements taken and averaged; typically this is done with a quantum meter. To verify the validity of any given measurement, three variables should be present: the distance of the quantum meter from the light source, the number of measurements taken, and the minimum and maximum ratio.
In addition to PPFD, it is also important to consider how efficient the lighting system is at turning input energy into photons of PAR, which is referred to as photon efficacy. In order to properly determine the efficacy, the PPF and input wattage must be applied to the formula; the higher the number, the more efficient the system is at converting its input energy into PAR. The values are essential in predicting the overall quality of LED lighting systems, which determines the future of many crops. Make an educated purchase: seek out manufacturers with the proper spread of information listed, and be sure to adequately review the derivatives of the listed values.