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Plant Lighting Fundamentals

Since light plays such a critical role in a plants successful growth it’s important to have the proper quality and quantity of light available to the plant as it needs it. Insufficient light levels will reduce a plants overall weight and develop symptoms of stress, decreased nodule density and smaller leaves. While too much light can damage the plant from excessive IR heat radiation or extreme UV radiation.

While a plant benefits to some degree from the light wavelengths or spectra that the eye see’s, plants respond best to spectral regions at the outer edges of peak human vision. If the artificial light spectrum is narrowly emitted or missing altogether, then the plants will not develop to its fullest leafy vegetative or bulky flowering stages that natural sunlight would have intended.

Standard visual measurements used for classifying visible light levels and color temperatures, such as Kelvin, CRI, Lux, Foot Candles or Lumens, are not entirely applicable when selecting or measuring plant lighting.  For example; Kelvin temperatures are used to describe the color of light within our visible spectrum.   Growers who may be overly influenced by a lamps  Kelvin rating and visible color may believe that their plants PAR needs have been entirely met. However with the two charts shown below we demonstrate how Kelvin temperatures and visible light have little contributive influence on the plants maximum PAR regions of UV and IR light. Kelvin values were first developed to allow artists, photographers window dressers, etc. as a way of communicating how something will look under different  temperature lamps.  Kelvin ratings have no  absolute spectral values in determining if that rating will benefit a plants PAR requirements. To a grower, a Kelvin rating should be used as a ballpark indicator of how much red or blue  light is being emitted within the visible range and not for an indication of PAR Value.

The chart below depicts the electromagnetic spectrum of light wavelengths from the low level ultraviolet (UV) on into the infrared (IR) wavelengths.  Measured in nanometers, the wavelengths consist of both visible and invisible light.  Of particular importance to plants would be wavelengths within spectrums known as the PAR regions which are mostly outside of our visible light regions.  While plants do benefit to a small degree from the wavelengths within our  visible spectrums,  plants respond best to PAR wavelengths within the UV and IR regions outside of our visible light ranges.

As it relates to proper light selection we’ll introduce you to the importance of two biological reactions that occur within a plant; Photosynthesis and Photomorphogenesis.  Plants absorb light by a green pigment within the plant known as chlorophyll. When chlorophyll absorbs light and turns it into energy it is through a chemical process within the plant called Photosynthesis.  As Photosynthesis occurs, the wavelength spectrum that is most beneficial to plant growth is found within certain areas between the 380-720 nanometer range of the spectrum. The light that is within this region is referred to as Photosynthetically Active Radiation (PAR).

A plants spectral lighting needs will change as it grows. Since spectrum plays an important part in the success of the plants growth developmental bioligists refer presence of these light mediated changes that the plant absorbs through a variety of receptors Photomorphogenesis.

As shown within the chart below, you can see the average PAR ranges for most plants that should be available for maximum chlorophyll absorption. Within these ranges plants will respond very well to the emitted light wavelengths.

Chlorophyll Absorption Chart

When measuring light QUANTITY for a plant we look to measure how many PHOTONS, (the minimum unit of energy involving light) that are emitted from the lampare falling each second within a square meter. Photons are such a small unit of measurement that they are referred to as MICROMOLES OF PHOTONS or more often just MICROMOLES to describe a measurement of how many photons are arriving at a plants surface from the emitted light source. For reference 2000 micromoles would be a sunlight level measurement of light.

Of most value to the grower and his plant would be the number of photons being measured at the plant, per second, per square meter, within the PAR ranges of 380-720 nanometers. This value is then known as the PHOTOSYNTHETIC PHOTON FLUX DENSITY (PPFD) level that can be measured in the field for lamp intensity.

Meters that measure these (PPFD) values are often referred to as QUANTUM METERS since a quantum is the amount of energy carried by a photon. These meters will provide entire spectrum measurements of the total number of photons per second values as well as measure the YIELD PHOTON FLUX (YPF) of the lamp which is as we’ve seen by the plants photomorphogenis requirements will assist the grower in identifying that the lamp has the proper PAR spectrum for maximum photosynthetic repsonse at that stage of plant growth.

Another way growers like to measure light for plants is by PAR WATTS. What this refers to is how much light energy is available between the 400-700 nanometer ranges that the plant requires for Photosynthesis. What is extremely important to know the efficiency of the lamp being considered. Growers should be careful when considering these values and not to correlate higher PAR WATT values with more successful yields since with energy efficient lighting such as induction the PAR Watts per Square foot may measure 70% less than an HID and while still delivering micromoles in excess of the HID within the plants PPF and YPF requirements. As such we publish our lamp output values in Watts/Region values which allows the consumer to see how much energy the lamp emits in the three regions of greatest importance to known photosynthetic response.

To learn more about Measuring Plant Lighting click here

Blending the Art and Science of Indoor Gardening

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