The mole is the base unit for the amount of substance. One mole  is expressed by the Avogadro constant, which has a value of 6.022140857×1023

μmol/s/m² is a unit to express the amount of light coming from a light source per second per square meter. It is the number of photons coming from a light source that arrive every second on a given surface.

PAR is the Photosynthetic Active Radiation which designates a spectral range from 400nm to 700nm. This corresponds to the range that is visible to the human eye. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place. PAR is not a measurement or metric, it rathers defines the type of light that is relevant to plants (needed to support photosynthesis).

The amount of light per second in the PAR range can be expressed by PPF or Photosynthetic Photon Flux. It is the total number of light per second coming from a light source in the PAR range. It’s unit is μmol/s.

The amount of light per second in the PAR range per square meter is expressed by PPFD or Photosynthetic Photon Flux Density. It is the amount of PAR that actually arrives on a plant surface. It’s unit is μmol/s/m².

The Photoperiod is the period in a 24 hour time interval during which a plant is exposed to light.

Daily Light Integral (DLI) is the amount of photons (in the PAR range) received by a specific area in one day. It is expressed in mole per square meter per day. Conceptually, DLI is similar to a rain gauge. A rain gauge is not used to measure the amount of rain per second or per minute but to measure the total amount of rain received in that particular location. If you empty your rain gauge every night, you can measure the total amount of rain received during a 24-hour period. Similarly, a light meter can be used to measure how many photons of light accumulate per square meter every day.

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