The Science of LED Grow Lights for Your Indoor Garden

Indoor Gardening isn’t exactly a new thing, but LED’s are changing the way we light our indoor gardens.  LED lights are more efficient than traditional fluorescent and incandescent lights.  That’s because LED lights convert nearly all of their energy (95%) into light, while other lights turn a significant amount of energy into heat.  But, there’s another very important reason that LED’s are more efficient when it comes to growing plants.  With LED lights, we have the rather unique ability to customize the type of light that is emitted, and that means we’re not wasting energy to create light that doesn’t help our plants grow.  At the end of this article, you’ll understand the science behind why spyder grow light series come in many different colors, as well as why some LED grow lights cost so much more than others. 

Plants Only Use the Visible Light Spectrum for Photosynthesis

It’s important to know that plants only use visible light (the colors of light that we see every day) for photosynthesis. However, as the chart below demonstrates, the complete spectrum of light is far greater than just the visible light spectrum.  On the outer edge of the visible light spectrum is Ultraviolet (UV) light and Infrared Radiation (IR).  UV light is the invisible light emitted by the sun and other sources that will cause sunburns when we don’t wear sunblock.  IR light can only be seen with special equipment, like night-vision goggles.  Even further out from the visible light spectrum are light waves that we don’t traditionally think of as light.  These include X rays, Microwaves and even Radio Waves.

One of the most important things to understand is that scientists have demonstrated over and over again that plants only absorb visible light for photosynthesis.  Plants do react to other forms of light like UV, but that reaction is typically negative.  I’m told that marijuana growers actually use UV light to induce the production of psychoactive chemicals like THC, which seem to be produced in part as a defense mechanism against the damaging effects of UV light to the plant.

What is PAR?

PAR stands for “photosynthetically available radiation.”  PAR is made up only of visible light, because this is the only light that plants use for photosynthesis.

For decades, many indoor growers have used Lumens to measure a grow light’s efficacy, but the industry is getting smarter and turning to PAR.  Lumens are used to measure the brightness of a lamp to the human eye.  But plants and people see light differently.  Humans see yellow and green more brightly than other colors.  Therefore, yellow and green lamps may have higher Lumen values than red and blue lights that put out just as much actual light, and which plants are likely to respond better to.

PAR measures all light from the visible light spectrum equally, and does not measure light outside of the visible light spectrum, which does not help the plant photosynthesis.  So, for plants, the PAR value of a light is currently the best basic measurement of a grow light’s brightness.  Accurate PAR meters are quite expensive and generally cost $500 or more.  Inaccurate PAR meters can be purchased for much less, but there’s really no point to owning an inaccurate PAR meter. 

The best way to get PAR values for your 400W LED grow light, assuming you don’t want to purchase your own PAR meter, is to check with your reputable grow light manufacturer or provider for the PAR rating of their lights. 

How Much PAR do My Plants Need to Grow?

The amount of PAR your plants require depends on what you are growing, as well as how far away from your plants the light is.  Generally speaking, leafy greens like lettuce only need a PAR value of ~200, whereas tomatoes and other plants that flower and produce fruit require 400-500 or more PAR.  Unless you place your 600W LED grow light right on top of your produce, you will need an even higher PAR rating from your grow light, to take into account the distance between your plant and the light source.