Measuring the intensity of light using a PAR meter
Posted: Sun Jan 28, 2018 11:22 pm
If you are going to use artificial light, it is a good idea to buy a light meter that is capable of measuring photosynthetically active radiation or "PAR". The standard units for counting PAR is called "photosynthetic photon flux density" or "PPFD", and it is essentially the number of photons that hit a square meter surface area per second... only photons with a wavelength in a range that is used by plants counts. So an artificial light that looks bright to the human eye, may have a very low PPFD. In addition, how far away the artificial light is will also dramatically impact the PPFD count. For these reasons, it is important to use a PAR meter to measure whether or not you are hitting your plants with enough light.
Here is a picture of the PAR meter reading 363.1 PPFD, just above the topmost point of my venus fly trap. This is 6 inches below 2x T5HO Sunblaster fluorescent lights and 1x Sunblaster LED light. This is incredibly bright as far as artificial lights go, largely due to the single Sunblaster LED light, which outputs 2.3 times as much PAR as a single T5HO fluorescent light. The foil is there because it helps reflect PAR back to the plant. I measured the same point above my flytrap with and without the foil, and the foil increases the PAR by about 50 PPFD.
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Here is picture to give an appreciation for how much PAR can vary based off of distance from the artificial light. Just moving a couple of inches closer to the 3 light setup increases the PAR from 363.1 PPFD to 420.7 PPFD.
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Here is a picture showing how very little PAR comes out of a typical fluorescent light used in residential homes. I measured a random fluorescent light that is used in my kitchen. I have the PAR meter's sensor 6 inches below the light and yet the reading is pitifully low, just 28.94 PPFD. While the light is bright from the perspective of a human's eye, a venus fly trap will not be able to survive under so little PAR. Typical fluorescent lights used in homes have lots of yellow light in them, which is a wavelength of light that the human eye perceives as being the "brightest". If the human eye saw a blue light and a yellow light, both emitting the same amount of energy, the human eye would perceive the blue light as being "dim" and the yellow light as being "bright". Plants love blue light, but yellow light is not useful to them.
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A very informative measurement for you all to see is the PAR of direct sunlight beaming through a window. The meter reads 803.7 PPFD. Remember, my 3 light setup measured half that amount of PAR a few inches below the lights! Direct sunlight is intensely rich with PAR.
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Here is a picture of the PAR meter reading 363.1 PPFD, just above the topmost point of my venus fly trap. This is 6 inches below 2x T5HO Sunblaster fluorescent lights and 1x Sunblaster LED light. This is incredibly bright as far as artificial lights go, largely due to the single Sunblaster LED light, which outputs 2.3 times as much PAR as a single T5HO fluorescent light. The foil is there because it helps reflect PAR back to the plant. I measured the same point above my flytrap with and without the foil, and the foil increases the PAR by about 50 PPFD.