To understand what a horticulture LED lighting system is delivering three points are key to evaluate:
Amount of PAR (Photosynthetic Photon Flux) is coming from the lamp?
How much of this light (Photosynthetic Photon Flux Density = PPFD) is available at plant level?
What is the efficiency of the lamp to create PAR light from electrical power (Photon Efficiency)?
PPF is photosynthetic photon flux. PPF measures the total amount of PAR that is produced by a lighting system each second. To measure the complete amount of light the lamps are place inside a sphere that is called an Ulbricht Sphere. Especially for the larger lamps specialized external companies are required to conduct these measurements. The unit for PPF is expressed in micromoles per second (μmol/s). To fully understand the lamps this is very helpful metric and required for a comparable efficiency calculation.
PPFD describes the PAR light that arrives at a given surface, here usually the plant, and describes how much light is available at and for the plants. For a complete description of a lamp many spots need to be measured under the lamps, often in an area of 1x1 m². The values are measured in micromoles per square meter per second (μmol/m²/s).
By sampling an area with multiple measurements under a lamp, the light distribution can best understood, to archive a homogeneous PPFD on the plant level. Directly under the lamp the values are highest and a single measurement will not provide full information. For a comprehensive overview complete information will define: measurement distance (ideally based on real life installations), the measured area and at least nine measured points. Average and min/max values can already be mention in specs or can be calculated from the given values.
Plants use PAR
Plants primarily use wavelengths of light within the visible range of 400 to 700 nanometers (nm) to drive photosynthesis, which is why this range is also called Photosynthetically Active Radiation (PAR). PAR is a much used (and often misunderstood) term related to horticulture lighting. PAR is not a well-defined metric like meter or kilo, but a practical definition to measure light relevant for plants. The values used in horticulture lighting build on this spectral definition for their numbers so the different manufactures can be better compared with a strong tie to plant growth.
Measurements in PAR do NOT reflect or can include the latest trends to extend spectrum to UV and IR in growth lamps.