The ABCs of LEDs
Photosynthetically Active Radiation (PAR) measures the amount of light a plant needs to grow, which is light in the 400 nm to 700 nm wavelength range (the photosynthetically active wavelengths). The human eye sees the same wavelengths that drive photosynthesis in plants. This measurement is used to describe how much radiation is available or how many photons are available to make photosynthesis happen. There is no weighting factor for PAR unlike for human visual metrics. Since it’s a description, there are no units for PAR.
Photosynthetic Photon Flux (PPF) is a term used to define the measurement of PAR, which describes how many photons are being released from a light source every second. PPF does not tell you how much of the measured light actually lands on the plants, but is an important metric used to calculate how efficient a lighting system is at creating PAR. The units for this measurement are micromoles per second (µmol s-1) also written as µmol/s. This metric is obtained by using an integrating sphere or a flat-plane integration measurement.
Photosynthetic Photon Flux Density (PPFD) measures how many photons are hitting the plant -how much of the PPF is delivered to a particular target (or canopy surface area). The units for this measurement are micromoles per square meter second (µmol m-2 s-1) or sometimes written µmol/m2 s. This metric is obtained using a quantum sensor and meter. Multiple readings should be taken from different points under a fixture and averaged. PPFD describes how essential photosynthetic photons are impacting the grow area and how well those lights are working when it comes to their output.
Photon Flux Density (PFD) is the measurement of how many photons a fixture produces regardless of whether they fall into the PAR range. This measurement includes both the UV and far-red ranges. Like PPFD, the units for this measurement are micromoles per square meter second (µmol m-2 s-1) or sometimes written µmol/m2 s.
Plant Biologically Active Radiation Flux (PBAR Flux) designates the range of wavelengths of light shown to effect plant growth, from 280-800nm and is also expressed in units of micromoles per second (µmol s-1) or sometimes written as µmol/s. It’s a relatively new term that represents the total theorized portion of the electromagnetic spectrum that influences plant processes.
Yield Photon Flux (YPF) is a measure of light intensity, weighted based on the light wavelength’s usefulness to plants when it is the only color of light given to a plant. YPF weights photons in the range from 380 to 760nm based on a plant's photosynthetic response measured in micromoles per second (µmol s-1). Whereas all PPF photons are born equal, YPF considers the relative photosynthetic efficiency of different wavelengths of light, assigning less weight to assumed less useful photons.
The problem with YPF is it was developed based on short-term measurements that were made on single leaves in low light, rather than whole plants under light levels similar to those used in commercial grow facilities. It might not apply to cannabis crops or have a limited useful impact.
PAR Photon Efficacy (PPE) is the measurement of how a fixture converts power (watts) into PAR. Dividing the PPF (µmol s-1) by the actual watts used to create it (J s-1) results in PPE, which is measured in units of micromole per Joule (µmol J-1).
Photon Efficacy (PE) is a newer term that has been introduced recently into the controlled-environment agriculture (CEA) market. It’s another way to describe photon efficacy but includes photons that fall outside the PAR range such as the UV and far-red regions.