As promised the September meeting was a techie dream come true! We were able to check out the photosynthetically active radiation (or PAR - the component of light that plants can actually use for photosynthesis) as well as the spectra produced on not one but four different LED grow-light fixtures! Don’t worry if you are not as techie though. Not all of our meeting are this technical. Mostly we talk about growing carnivorous plants, so if this level of detail is not your thing, don’t let it stop you from attending out next meeting in November. Details to come. Any questions, as always please feel free to contact us at our yahoo.com e-mail address, username: umcps

On to the details! For those unfamiliar with what most of these LED fixtures look like, here is a picture:

You may, very reasonably, be asking yourself why you would want to use a light fixture with that sort of color to it. It certainly doesn’t make the plants look very nice. These bulbs are not lights for admiring your plants under. Nope. They are meant to make them grow while using as little energy as possible. They are “tuned” to your plants not your eyes.

Let’s explain what that means. Our eyes make use of a very different part of the light spectrum than plants leaves do for photosynthesis. While we are very tuned into yellow and green (baring types of color blindness), plants don’t use green very well - which is why they look green to us. Plants use reds and blues best. Below is a spectrum image of the wavelengths of light plants use, which we have borrowed from here (which is a really great site talking about photosynthesis in general!):

You can see that light in the green bands, around 550 on the bottom axis are used pretty poorly. Whereas, the blues at about 420, and the reds, about 650 are what plants absorb best. So if you wanted to target the lights to use as little energy as possible while getting plants to absorb as much as possible, you would want to use red and blue lights. See where that magenta color is coming from now? That’s the idea behind the production of LED grow lights. They are tuned to me as useful to plants as possible.

Now we know what light plants use and the type of absorbency they are trying to produce. So how did the four LED fixtures do? Here is the spectra from the first one. It is a generation 1 (or Gen 1) fixture, and early type that only used 3 bulb types. Blue, red, and orange.

You can see that it really just has peaks on the red and blue with a little orange near 625. So that’s pretty good right? Well it turns out plants get information and collect light from the other parts of the spectrum as well, and so it helps to have a little light in those regions as well. The next spectrum is from a Gen 2 fixture, that includes a white LED as well. This fills in some more of the spectra as you can see. Oh and don’t worry about the intensity on the vertical axis - we had to adjust them at different distances to get the highest peak to fit on the graph, so these graphs are of relative intensity between fixtures.

On the Gen 2 graph above you can see that the addition of the white LED bulbs fills in that saddle between the red and blue parts of the spectra better. But it still doesn’t replicate the curve of the light that plants absorb quite correctly. And that why people developed Gen 3 fixtures, of which we tested two. This first spectrum is from the first one we tested, which we’ll arbitrarily call “type 1”. This one includes 9 types of LED bulbs covering a range of the spectrum, including one infrared (IR) bulb type.

You can see the IR bulb’s influence as the small bump to the right of the red peak. The shape is a rather impressive approximation of the absorbency curve for photosynthesis. The last spectra is from the other Gen 3 fixture, here called “type 2” because we tested it second. It also has 9 types of LED bulbs. This is the curve from that fixture.

Interestingly this one is a little peakier, if that can be a word. And the general consensus was that it didn’t match the optimal as well as the type 1 fixture did. Despite it supposedly having IR bulbs too, we didn’t really see a signal for those. Unfortunately because the fixtures are individually made and purchased on e-Bay there is not a lot of assurance about which exact spectrum you might get. Once these sorts of fixtures become more common and produced on a larger scale, there might be more standardization.

The last thing, and if you’ve read this far you can consider yourself very techie, is the PAR readings.

in (µmol m-2 s-1) @12 in @24 in

Gen 1 354 92

Gen 2 250 74

Gen 3, Type 1 260 65

Gen 3, Type 2 178 51

Given how bight these bulbs were to the eye the PAR readings seemed at first very low, with numbers no too different from T5 or T8 fluorescent fixtures. But those fluorescent bulbs throw off a lot of light plant’s aren’t using. Those of you paying close attention may object to that last sentence. PAR is supposed to measure the photosynthetically available light! So how can the same PAR mean different efficiency coming from different fixtures?

It’s a good question and the answer is in the details of how PAR is measured. The short answer is that PAR weights all wavelengths of light equally between 400 and 700 nanometers. But as we can see from the photosynthesis absorbency graph earlier that’s not how plants work. So why was PAR designed that way? PAR was meant to measure light from black body radiation, things like sunlight and light from incandescent bulbs. For those things it is very good metric. For LED bulbs that produce a very narrow band of light, it becomes more problematic. This means that with the same PAR reading a well tuned LED might allow a plant to photosynthesize at a higher rate than a fluorescent bulb with the same PAR reading. So, it becomes difficult to compare directly. Someone is going to have to come up with a new metric if we all move to LED fixtures.

That’s it! Hope it was interesting for those who read this far. Please feel free to leave comments if you have any thoughts on this topic.