Ultra Violet (UV) Spectrum

As promised, I have researched the UV spectrum issue; and as it turns out 380nm (nano meters) is a crucial spectrum most green leafy plants need (like Bell Peppers) in their vegetative state even. Neither Blue nor Royal Blue LEDs supply this spectrum.

Before getting into the science of plants' need for UV light, lets check the Data Sheet for CREE Inc.'s Blue and Royal Blue LEDs. (CREE is the top producer in America of high quality LEDs which last longer -guaranteed 50,000 hrs- and keep more true to their color after 50,000 hrs (80%) than the next leading manufacturer's -Phillip's Lumileads @70% after 50,000 hrs.
Here is the data sheet.

Notice on pg 2 of CREE's data sheet there is a chart for Dominant Wavelength (nm). Looking at Royal Blue the CREE LEDs (Phillip's Lumideads are almost exactly the same) the minimum dominant is at 455nm and dominant max at 465nm. Keeping this in mind scroll down to pg 3; here we can see distribution curves for each color LED CREE produces. The royal blue LED distribution curve is most left curve and peaks right past 450nm and dramatically falls to ~6% radiant power at 425nm.

What this means is the bulk, ~85% or so, of light the LED produces is rang bound from 425nm to 480nm. Which is excellent except we are missing everything to 380nm. Now how do I know 380 is intregal?
I did some searching online since I was curious about UV and ran into LEDgrowlights.info and there Light Wavelengths are posted for plants and whats necessary and not necessary in terms of specific spectrum. Now UV can be dangerous to not only your skin but eyes! So even though LEDgrowlights.info states "315 - 380 nm Range of UVA ultraviolet light which is neither harmful nor beneficial to most plants" still treat this are of the spectrum with respect. Now, wavelengths greater than 380nm are safe, and is just about the spectrum area of Black Lights. Black Lights still can not only cause eye fatigue but over exposure will lead to skin aging, so again be careful and if dealing or exposed with these wavelengths do what you would when in direct sunlight: where Sunglasses and Sunscreen (no kidding).

OK, back to the point, 380nm to 400nm, according to LEDgrowlights.info, is where the visible light spectrum begins and the "process of chlorophyll absorption begins." Still our High Power Royal Blue LEDs from either CREE or Lumileads dont even touch the outer bound of this spectrum range. We are not out of luck though!

I have found wholesale suppliers that provide both High Power and High Brightness LEDs. (High Power is the more useful -more light out put despite the misnomer of High Brightness LEDs, which are the smaller traditional 'looking' LEDs except clear) What I havent worked out just yet is what proportion of these UV LEDs or even spectrum one needs relative to the visible blues and reds (and Infrared or Near Red) LEDs.
I'll touch on IR and red later but for now, we know we need not just blue or even just Royal Blue but UV LEDs to fill in the very important near Blue/Violet side of the scale.

So Now Why is This Important?
Ok several reasons: Pigments, Energy & Protein- there are basically two types of reactions that go on inside a plant- light dependant and light independent. As horticulturalists, we appreciate the science of plants but if your like me we want things to not just start growing but really take off, so we'll possibly touch on light independent later.

Light Dependant reactions are the first stage of photosynthesis. (see wikipedia.org) If you remember 9th grade bio, this is the mechanism plants use to make food -very important for all growing stages of a plant's life. Now these reactions "take place on the thylakoid membrane inside the chloroplast" and is called the lumen (dont confuse this with a measure of light irradiance). The lumen side of the chloroplast is where the light dependant reactions take place because of Photosystem I and II where they "absorb light energy through proteins containing pigments, such as chlorophyll." "When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains a higher energy level." In otherwords the photon that gets absorbed by the chlorophyll pigment is exciting a domino effect eventually leading to food production for the leaf cell and plant all together.

Now I dont want to bore anyone, my basic point is that pigments are really what alows for specific light colors to feed the plant (as well as other things). And these plants have evolved, if you will, to utilize a high energy form of light, to get food on the table.
Going back to LEDgrolights.info, we see at the bottom of the page a list of wavelengths to each pigment. Though this is a generalization it helps a lot with knowing what to get in a Grow Light or in Building Your Own which I plan to do.

The table is as follows:

Beta-carotene 450nm 480-485nm dual peak
chlorophyll a 430nm 662nm dual peak
chlorophyll b 453nm 642nm dual peak
phycoerythrin 590nm single peak
phycocyanin 625nm single peak

To get an idea of what this looks like would look like, and I stress idea, Im borrowing a graph from Instructables.com.


Still notice this graph and table appear to show no great need for 380nm to 430nm and a simple explanation for this is that this graph and table is for a typical plant vegging. You may even notice that different plants have stronger shade of green on there leaves or even differ in color. Which leads me to reference another source: Jorge Cervantes The Indoor/Outdoor Grower's Bible (ISBN: 978-1-878823-23-6). Which is an excellent source for any horticulturalist! Looking at pg. 160, Cervantes breaks down the exact Photosynthetic response. Unfortunately, he provides the data graphically and I do not have a photo I can link to. You may purchase his book at Barnes & Noble or at Amazon.com.
Still, I can tell you according to Cervante's graph the Phototropic response absorption begins at 350nm with a high ~75% (out of 100%) absorption and peaks at 380nm with ~82% absorption! (High absorption at a specific wavelength implies a neccisary wavelength) The Phototropic response then troughs slowly down to ~470nm @59%, which is still high, and then rises to a peak of 490nm and then to 510nm at 97% and 95% absorbance.

Why those spectrum range's? Circadian Rhythm!
Everything, and that includes plants all the way down to life forms you cant see but still need sun light, work on a biochemical rhythm just like you and I; and as you know, you'll feel well with only the right amount of sleep (darkness) and too much or too little will make you feel, as well as act, differently -this is the same and perhaps more profound with plants.

From wikipedia.org, "Light resets the biological clock in accordance with the phase response curve (PRC). Depending on the timing, light can advance or delay the circadian rhythm. Both the PRC and the required illuminance vary from species to species..." In plants Cryptochromes are responsible for the PRC- "Cryptochromes possess two chromophores: pterin (in the form of 5,10-methenyl-6,7,8-tri-hydrofolic acid (MHF)) and flavin (in the form of flavin adenine dinucleotide (FAD)). Both may absorb a photon; in the plant Arabidopsis thaliana the pterin appears to absorb at a wave length of 380 nm and flavin at 450 nm."

So some light is used by plants to make food, other wavelengths cause the plant to wake up, (absence there of to sleep), vegetate, flower and fruit. I'll end the topic on Blue light with noting the morning, just before and just after sunrise, the sky appears in all shades of blue. Not just that but the sunlight in general leading into and transitioning out of Spring is very Blue as opposed to those Indian Summers with very dramatic Red and Orange sunsets. We see these dominant colors primarily due to the rotation of our Earth about her axis and rotation about the Sun -the Circadian Rhythm. And thats why you need more than just visible Blue light!

Tomorrow, I'll discuss the other side of the spectrum and why IR light is necessary.

-Power Bower!

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