Photosynthetic Photon Flux (PPF)
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.
Daily Light Integral
The DLI, or Daily Light Integral, is the number of photons in the PAR range that are delivered to a specific area over a 24-hour period. Where PPFD measures the number of PAR photons per square meter per second, the DLI is the sum of the per second PPFD measurements during a 24-hour period. DLI is expressed as moles of light per square meter per day, or: mol * m-2 * d-1.