Photosynthetically Active Radiation
PAR stands for Photosynthetically Active Radiation. PAR includes the light ranging from 400 to 700 nanometers that plants use for photosynthesis. The amount of PAR is expressed as the number of micromoles (µmoles) of photons being emitted by the light source.
PAR can be used in determining the strength and overall quality of a grow light. However, it is important to note that the amount of PAR alone is not enough. With a single PAR amount, we still don’t know how much light actually reaches the plant. It is therefore important to also know the position and distance of the light relative to the plant when determining the strength of the light fixture.
While most plants absorb and use a lot of Blue and Red light, Green gets reflected and this is the reason why most plants look green. This does not mean that plants only need Blue and Red light. Plants use all light wavelengths for different purposes, even light outside of PAR (such as Ultraviolet and Infra-Red light) are useful for plants.
Photosynthetic Photon Flux
PPF is short for Photosynthetic Photon Flux. PPF is measured in micromoles per second, or µmol/s. It expresses the number of photons or total amount of PAR emitted by a light source every second. Photons are small particles that carry the electromagnetic energy of light. The PPF value says something about the total light output that can contribute to photosynthesis.
PPF can be measured with a specialized instrument called an integrating sphere. This instrument looks like a big ball. A ray of light gets shined into the sphere, and the spherical form of the instrument will scatter the light throughout the sphere. A detector inside the sphere measures the total amount of photons emitted by the light.
PPF does not tell you how much of the light actually lands on the plants, but it says something about how efficient a light is at creating PAR.
Photosynthetic Photon Flux Density
PPFD stands for Photosynthetic Photon Flux Density and measures the amount of PAR light actually arrives at the plant. PPFD is the number of photosynthetically active photons that fall on a given surface each second, expressed in µ/m2/s. PPFD is important when it comes to grow lights, since it measures the amount of usable light that falls on a specific location of your canopy and says something about the true light intensity of the lamp. Light is generally brightest in the center of the canopy and less bright towards the edges of the coverage area, the brightness also decreases the further the light needs to travel. The closer your LED grow lights are to your plants, the more PAR light will actually reach your plants.
PPFD is a difficult metric to measure consistently, since it depends on many variables. Factors that contribute to PPFD are the distance between the LEDs and the plant and the loss of PPF from the vegetation canopy.