Understanding PAR in Lighting

Dec 26, 2014 No Comments by

Every industry that utilizes light, has their own way of measuring the intensity, perceived color and overall output of particular lighting apparatus. In photography, foot-candles describe the intensity of light on the subject, while kelvin describes color and Lumen is a measurement of light output.eye plant PAR graph

For horticulturalists using artificial light, these terms are insufficient, plant and fruit growers are more interested in the spectrum of the light used most: Photosynthetic Active Radiation or PAR for short.


Before LEDs with specific spectrum, it was much more difficult to target PAR and lights would require inefficient filters in order for horticulturalists to study the effects of different spectrum on plants. Plant Biology is well understood and the effects of spectrum on plant growth are well documented.


Spectrum absorbed by plants

So we see Green, Yellow and Orange are used by plants in only small amounts with large spikes between 400 and 500 nm And again between 640 and 660 nm, this is not to say other spectrum are not used at all as we can see small bumps between 550 and 660 nanometers. If these small percentages are ignored, adverse plant growth may be detected and other deficiency symptoms may arise.


So now we understand PAR shows the intensity in the nanometers absorbed by plants.



Lets look at a standard HPS PAR Graph

hps spectrum

Now compare HPS PAR against the chart ‘Spectrum Absorbed by Plants’ notice the critical 400 to 520 nm range where most plant biology functions, it’s output in these ranges are quite low,with a good percentage of its light output in the 540 -640 nm range where almost no photosynthesis or plant biology is occurring. What happens to this light? You can see exactly what happens to this light, the photons bounce off the leaves into your eye, making the plants appear to be bright orange, these are the unabsorbed photons of light in the wavelengths that are unused by plants they simply bounce off unused. This even explains why most trees and plants appear green to us, the 550 nm is bouncing of the leaves and into our eyes.

PAR Graph from GrowthStar Spider 4x

PAR Graph from GrowthStar Spider 4x


Daylight has perfect PAR Levels

So when choosing horticultural lighting, Keep in mind PAR and its effects on plant biology.


About the author

15 years horticulture experience to draw from
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