Glossary and Definitions


The range of useful visible wavelengths for driving photosynthesis for plants. Represented from 400-700 nanometers (nm). (See figure below) PAR is not a metric for evaluating horticulture lighting systems but for describing the area of visible light useful for plant growth. 


Critical portion of spectrum including PAR, covering UV range below 400 nm and far-red region beyond 700 nm in the (280-800nm). 

Ultraviolet radiation (UVR)

A type of solar radiation with wavelengths between 280-400 nm. It ends where the colors of rainbow start.  The atmosphere absorbs all UV-C (< 280 nm), a significant part of UV-B (280 – 315 nm) but transmits most of the UV-A radiation (315 – 400nm). UVR represents only about 7-9 % of total solar radiation reaching the biosphere, but unlike other types of solar radiation, UVR is highly energetic radiation. This means that UVR can cause reactions between molecules that are hit by such radiation.  This then helps with stimulating morphogenic responses in plants.

· UVA, or near UV (315–400 nm)
· UVB, or middle UV (280–315 nm)
· UVC, or far UV (180–280 nm)


(Photosynthetic photon flux) Micromoles of PAR per second.  (μmol/s) One of the most important metrics for comparing horticultural lighting systems.  Measures the instantaneous output from the fixture at any given moment within the PAR region. 

Photon Efficacy

A useful ratio for evaluating total power draw “Wattage” with the total output of “PPF” from the lighting system giving a comparable metric for evaluating two different lighting systems.  Expressed in “μmol/J.” Where (J/s) represents the wattage. 

Photosynthetic Photon flux density (PPFD)

One of the most important metrics for evaluating quality of the intensity of light at any given point below the luminaire.  This can be used in the field on the plant canopy or from a single system to understand light spread and intensity and different mounting heights.


The duration that the plants receive light in hours over a 24-hour period versus the amount that they are in darkness. Examples, 12 hours on and 12 hours off, or 18 on and 6 hours off.

Daily Light Integral (DLI)

The total amount of photosynthetic radiation (PPFD) received over a 24-hour period.  Plants have varying lighting requirements and understanding how much light is needed over the course of the photoperiod helps with specifying the lighting intensity required.

Far Red

Are the photons with wavelengths from 700 to 800 nanometers (nm). These wavelengths are barely visible to the human eye, because it is at the edge of our eye’s visual sensitivity to light quality. 

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