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PAR & The Light Spectrum

By Sunny Datko

Light is essential to our vision and plant growth but the ways our eyes and plants react to this light are entirely different processes. While the overall physics and science of lighting can be complex we’re going to reduce it to its bare elements here and primarily concentrate on the important plant/light interactions and how we measure that light.  In this article, we will focus on how to produce the proper spectrums for plant growth.

Photosynthesis, as we all know, is the process whereby plants use light to create sugars that the plant will use for energy and, eventually, fruit and flower production.  To better serve this process, growers need to supply their plants not only with a powerful light source but also with specific light sources that can provide the proper range of color spectrum to ensure maximum sugar production during photosynthesis.

When light hits the leaves of a plant, each leaf uses various pigments (most notably chlorophyll) to trap light energy and extract photons, which are vital in converting the light energy into chemical energy. These photons combine with water to provide the plant with the needed chemical energy to fix carbon dioxide (CO2) molecules into sugars, carbohydrates and other organic compounds the plant will use as food.

Spectrums of light are actually different sized wavelengths that give the appearance of a variety of colors to the human eye. These wavelengths are important because these pigments absorb different wavelengths, primarily in the blue and red regions, in varying amounts for photosynthetic processes.

Visible Light Color Spectrum

As photosynthesis occurs, the wavelength spectrum that is most beneficial to plant growth is found within certain areas between the 440-700 nanometer ranges of the spectrum The light that is within this region is referred to as Photosynthetically Active Radiation (PAR). The total amount of energy emitted per second in this region is called PAR watts. A PAR watt directly indicates how much light energy is available for plants to use in photosynthesis.  This is an objective measure in contrast to lumens, which is a subjective measure since it is based on the response of humans.

The human eye has peak sensitivity in the yellow-green region, around 550 nanometers. This is the “optic yellow” color used for highly visible signs and objects. Plants, on the other hand, respond more effectively to red light and to blue light. Blue light from 430 to 450 nanometers favors root growth and intense photosynthesis. Red colors at 640 to 680 nanometers stimulate rapid stem growth, intense flowering and chlorophyll production. Other colors contribute as well, but studies show that red and blue light at or near these optimal wavelengths are best for driving photosynthesis.

The graphs below show the human eye response curve and the plant response curve. Note the difference in the contours.

par & the light spectrum graphs

Having said that, plants are not sensitive to all wavelengths within the PAR region.  This is due to the specific absorption characteristics of the pigments in the leaf. Chlorophyll (green leaf pigment) is the best known and due to its relative strong reflective characteristics, the leaf uses green light the least effectively. In essence, green light is invisible to plants and simply bounces off the plant without being absorbed; therefore their photoperiod is not disturbed.  So if a green light turns on in an otherwise totally dark grow room, the plants still believe it’s dark since they can’t see that part of the spectrum.

The major benefit here is that growers can use green LED lights safely manage their flowering garden during the dark cycle. Make sure you use a green led that has a truly green spectrum from the source, not just white light passing through a green-coated bulb.  San Diego Hydroponics carries a wide variety of green lights for growers.  There are green light headlamps as well as green lights that can be worn over the ear (as shown above) or clipped to the bill of a baseball cap.  Spraying pest controls in the garden should also be done during the dark period to avoid having the plants wet when the lights are on, which can burn them.

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