Light theory

Grow Lights – Theory of Light

In this chapter you will learn about the different aspects of light in gardening. It clears up all the different keywords and hopefully all technical terms that occur when talking about light.

Definition of Light

“Light” is actually defined as a thin part of the electromagnetic spectrum. Exactly the one we can see with our human eyes. 

Light can be divided into parts called photons, which act in theory as light particles. According to physics you can treat light as a particle and a wave simultaneously. We will need both ways in this chapter!

Short wavelenghts means high frequency. Higher frequency correlates to higher energy per light particle. Long radio waves, which can expand to several meters in wavelength have a very low energy per photon.

Electromagnetic Spectrum with the visible part in the middle. From: Wikipedia

Defining a light source

If you want to learn about lightsources such as Light Emitting Diodes (LEDs), High-Pressure Sodium lamps (HPS), Compact Fluorescent lamp (CFL) or Ceramic Discharge Metal Halide (CMH) you will not only have to look at the specs each of these lamps have, you also have to look at the theory on which the light is created. Every single light source used for growing has its advantages and disadvantages.

All light sources used for growing work with a the same rough principle:

Turn electrical energy (input) into light (output)

The devil is in the details. As always. What I will describe now is awful lot to learn and difficult to have alook through. But its worth the reading, and after this you will be more able to decide wether a light is “good” or “bad” for your cause.

What light does a plant need?

Plants have two main sources of energy: The root environment (soil, water, aeroponics, nutrition…) and the light that shines on its leafs. The process in which the leafs turn light energy to “food” is called photosynthesis.

Not every light that a plant is exposed to, is usable to do photosynthesis. A human eye can basically see every color between 390 nm and 700 nm, but is blind for colors that have lower wavelengths (ultra-violet) or higher ones (infrared). Humans are most sensitive for the color green. As opposed to plants. Keep that in mind when you get to the unit section

Plants have a similar spectrum, but see actualy green light the least. Thats obvious, since leafs are commonly green, so they reflect the green parts of the spectrum. The three ingredients in a leaf that are responsible for the reaction are Chlorophyll-a and Chlorophyll-b as well as Carotenoids.

Bottom: Spectrum of photosynthesis rate –  There is a big dip in the middle.

Top: If you build the sum of the three top spectra, and take into account, that the different parts that are responsible for the photosynthesis are available in unequally you get the lower spectrum. Chlorophyll-a is obviously the main driver for the process.
Source: 
https://en.wikipedia.org/wiki/Photosynthetically_active_radiation

If you look at the lower spectrum in the picture above you can get a sense of what kind light a plant likes to receive: two broad bands of light between 400 nm and 500 nm and between 650 nm and 700 nm. One is in a blueish region, whilst the other is rather red. 

The Relative Quantum Efficiency

The three molecules (molecule groups) Chlorophyll-a ,Chlorophyll-b and Carotenoids are not the only driving factor of photosynthesis. In of the most cited articels from K.J. McCree (1972) he measured how the plants react to light of different wavelengths. He found, that the area around the green light (550 nm) is actually well used by the plants. Not as good as blue or red light, but up to significant margin! So green light is not inherently bad for plant growths.

Mc Cree curve showing the photosynthetic efficiency of green leaf material. The experiment is from 1972, so it might be time to reevaluate. In this graphic we see the impact of  red and blue lights in growing, but you can also imagine that green light is also important.

Measuring light – A jungle of units

Now that we have the favourite colors of a plants light dinner, we need to figure out, how much they need! This is determined by the amount of light a source can emit. This amount can be described with various units of measurement. Some of them make sense for growing (PAR, PPFD), others don’t (lm, lux), and the rest is not normalized and/or very subjective in a way that it is depending on the experimental setup one uses to measure it.(also PAR/PPFD).

The following part will enlighten you about the different units of measurements scientist and light distributors characterize light.
Note: I tried to sort the units in order if importance, starting with the most important one.

PPF and PPFD – Photosynthetic Flux
Photosynthetic Photon Flux Density

Lets dive right into things. So photosynthesis is a quantum process. Just think about it this way: A leaf is plastered with billions of little molecule-machines trying to catch light particles in the exact right energy range. The more machines are hit per timeframe the better. So PPFD counts the amount of photons (µmol) that hit a certain area (m²) in our timeframe (s). This results in the unit of measurement:  [PPFD] = µmol/m²/s

A PPFD value of 900-1000 µmol/m²/s is needed to saturate a canopy area of plants if you use a warm white spectrum!  How to get an even canopy using LST!

PPF on the other hand only counts the Photons emitted by a light source, but does not take into account any absorption effects. So if a photon is emitted through a source and runs right into a the case of the light, its counted towards PPF but not PPFD. [PPF] = µmol/s

The counted Photons also have to be from a certain spectral area between 400 nm and 700 nm. Everything else is not counted!

Two different lights: Blue has a smaller wavelength and can transport more energy per photon than the longer red. Source: http://www.shorstmeyer.com/wxfaqs/Blue_Sky/bluesky4.html

Example 1: Lets say we have two lights emitting 100 photons a second on a normalized area. One is blue, one is red. The blue photons contain more energy. The PAR value would be higher for the blue light. The PPFD value would be actually dead even because both lights emit the same amount of photons in the active range.
So which light is “better”?
Answer: The red one for flowering, the blue one for vegetative growth. This is also the reason “common grow lights” give you both parts of the spectrum which results in a pink look of the lights. (blurple if you want…).

Cannabis plant under a grow light with a blue/purple spectrum
Cannabis plant under a grow light with a blue/purple spectrum. Source: homegrow-pro.com

Example 2: We add a green light to the show with an output wavelength of 555 nm monochromatic light. Its PAR values would be better than the red light, but worse than the blue ones. PPFD would be dead even again. Do we take the green one now?
Answer: No! A green light is not effective to grow cannabis. The plants will not be able to do photosynthesis with green light, as effective as with the use of fullspectrum systems, because they simply reflect alot of it.

Grower inspects his plants with a green headlight.. Source

Conclusion: When reading about PPFD values in the description of your light, always take a look at the spectrum as well. If the light “wastes” alot of intensity on green wavelength the number is not as useful anymore! If you have a high PPFD value it can mean less if it has mostly a green spectrum.
Its also interesting to look at the light distribution a light creates at certain distances to the source and wether the source created an even output! If there is a disproportion between PPF and PPFD its often times a lack of optical diffractors that distribute the light uneven to the plants. Take a look at a HPS lamp that creates light in a very narrow area inside its tube. This creates a very uneven distribution on the surface area: Buds on the edge of the light cone will develop more poorly.

PAR – Photosynthetically active radiation

This is easy to figure out: You imagine a perfect black surface and shine a light on it. The hypothetical surface (m²) absorbs the energy at a 100% effectivity which you can measure. Within a certain timeframe we can now measure the power (W) emitted by the light in [PAR] = W/m². The counted photons are again in the 400 nm to 700 nm range.

Here is a subtle difference to the PPFD value we learned about in the last section:

Example : Imagine two light sources. One monochromatic red and one monochromatic blue one. The red one has an output of 200 photons a second with 700 nm in wavelenghts, while the blue one puts out 100 photons with 400 nm. PAR does not care if the plant can use this light, it just takes the values because they are in the range where we observe PAR. So regarding PPFD we learned that the red one should be twice as good as the blue one. But on PAR level both are about even, because the blue light can carry more energy than the red one and PAR only cares about energy over time on a certain surface.

Conclusion: PAR can be at a high value, because it gets pushed through high intesity of blue parts in the spectrum (more blue wavelengths). Always check PAR, PPFD and the spectrum.

Light distribution under a metal-halide light: perpenticular to the source direction the light is most intensive. PAR values might look good, but whats measured at a surface level is also important! 
PPFD takes uneven distribution into account

W – Real Power Draw (Watt)

The Wattage of a lamp is a good ballpark number to start searching for a light source. Its actually the first value I look at, and from there you start to compare lights on the same level!

The power draw from the wall can directly calculated into money that appears on your power bill. So this is often the first thing you want to check. After this, you find out which light converts this money to the best possible PAR/PPFD/spectrum combination.

Many light suppliers tend to market their lamps with this value to its customers. 

WARNING!
Blue/purple lights from various distributors will name their lamps after “artificial HPS equivalent Watts” to trick the customer. You might think that a 600 W LED with the an awesome blue/purple spectrum is the deal of the day. Be aware that these companies hide the true power draw in the description at amazon or eBay. And they are good at it. Don’t get fooled.
If you think you can have 600 W of high quality LED plug-and-play light for 100$ you are WRONG.

Always check for the real power draw.

  • If you’re going for HPS/CMH/Lowball LED light: Calculate with at least 50 W/ft² for decent results.
  • If you are using High-Quality LED Lights (COBs or Quantumboards): Calculate with at least 35 W/ft² for decent results.
This Phlizon is marketed as a 600W LED Grow-light for mind-boggling $89.99. Its actually a 108W shit light, thats not even suitable to light up a 2×2 tent. ONLY BUY THESE LIGHTS IF YOU ARE ON A BUDGET! This scam is actually the “Amazons choice”…….

lm, lm/W – Lumen, Lumen per Watt

Now we’re really deep down the cave. But still: Companies try to catch you with values like this! 

Lumen is one of the least scientific units around, but still has some meaning for interior architects and astronomers. The unit is an abbreviation from the units candela (which literally refers to the output of a burning candle) and the so called luminosity function.

Quote:  “[…]
luminous flux is 
weighted according to a model (a “luminosity function“) of the human eye‘s sensitivity to various wavelengths. ” (Source)

So if you are following you might wanna ask yourself something. 
First remember: The human eye can best distinguish green wavelengths, because it is most sensitive around the 555 nm value. A plant is literally blind in this region.
WHY would you buy a light only because it has awesome lumen values?!

As always: check the spectrum! If the light is mostly green it will have awesome lm values. And perhaps, if its a modern LED, also great lm/W values. But it might not really suitable for growing.

NEVER BUY A GROW LIGHT ONLY BECAUSE OF THE LUMEN OR LUMEN/WATT VALUES. THE UNIT IS MADE FOR HUMANS, NOT FOR PLANTS!

How The Pro chooses a light

When it comes to the decision for the grow lights people always hassle. The market is flooded!

  1. Determine the size of your grow area and find out how many of the sweet Watts you want in there. The unit might not mean much, but you gotta start somewhere! Example: 3×3 tents have 9 ft² of space, so we need about 315 Watts of High Quality Growers Masterrrace LED Awesome Lights, or 450 Watts of Lowball LED / Good-old-fashioned HPS or CMH lights. Make sure your light has a PPFD value of 700-1000 at canopy level, has a full spectrum (warm white) and an efficiency of 120-200 lm/W (remember that you can use this number only for lights with similar spectra).
  2. How is your budget? Great LEDs are expensive, but worth the money. Pre build ones come for $1-$4/W in various forms. So youre looking potentially at $315 – $1260 just for the light. If you want to save money you can choose low-quality LEDs for like $250 that will reach 300 Watt if you buy 2-3 fixtures. Or you just go for a HPS setup for about $150 as a bundle with reflectors and balast. (I wont go into the details here)
  3. Lower your expectations! Great, you just want to take a look into growing and chose the entry level LEDs. Thats totally fine! You can grow awesome weed with these lamps! But if you ever feel you are missing out on something, come back and read about the real LEDs.
  4. Did you really get the HPS?! Thats also fine. They are working for decades and give awesome results. Come back if you think about buying an AC unit and have nightmares about your power bill…
  5. You cant decide between two or more lights? Compare PPFD/PAR/spectrum values at the same Wattage and find your favourite! A light should at least bring the PPFD up to 700 – 1000 in the sweet spot.
Spectrum of a Mars Hydro 300 W light normalized by maximum intensity. The spectrum is good if you just consider the PAR spectrum, but misses out heavily between the red and blue lines if you take the McCree curve into account as well.

LEDs recommended by The Pro

I’m not getting paid for it (yet) but here are some lights i will approve. I saw the results from growers and if you want high yield, dense buds and fast growing weed with low internodial distances buy from them. They also tend to run much cooler than HPS lights…

Timber Grow Lights (US)
Horticulture Lighting Group (US)
Cob Shop (US)
Greenception (EU)
San Light LED (EU)
Rapid LED (US)

Build your own!

Don’t try to buy a good grow light on amazon.com

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6 thoughts on “Grow Lights – Theory of Light”

  1. Yo, brah!! That’s sick!
    But I ain’t understand a wrd, know what im sain
    I have to read that shit again tomorrow morning.
    Anyways dude, you rock!

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