"How Light Works" Tackling the question...

Read this first!
It seems to me to be the case that many new growers are getting badly mislead when trying to select the best lighting for their grow setups. I have decided to write this thread to try to explain some of what's actually happening. It is meant to be a “sister” thread to my lighting comparison thread located here: http://forum.grasscity.com/general-...out-using-150w-hps-dont-bother-heres-why.html I am not a professor by any means, and I don't market or sell anything. I consider myself to be an average guy with a decent IQ, and the ability to ask the right questions in the right places. Also: I use a lot of simple comparisons to get my point across. Some may not be directly relevant. Some may illustrate a point using different properties of physics/etc. I'm not doing this because I'm stupid. I'm not doing it because I 'm trying to publish bad information. I'm not doing it because I think it's cute to make stuff up and hear myself talk. I'm doing it because I want this to be readable on as close to the high school level as possible. Please try to keep that in mind while you read this, and I hope you gain something from my efforts

 

Before I try to talk about the basics of how different types of light “work,” I think it is important to discuss light itself as we know it. So, I'll begin by posing the question again: what the hell is light anyway?

If you want a straightforward cut and dry answer, then it would sound something like this: “Light in the traditional household sense, is electromagnetic radiation with a wavelength, or combination of wavelengths in the 400-700 Nm range which is visible to the human eye.”

That wasn't very helpful now was it? So I decided to dig deeper and digest as much valid information as I could. Before I get all technical I'd like to share my thoughts on the matter first.

-->OPINION: First of all, I think that many people overlook how relatively recent the proper scientific understanding of light is. I don't wish to discredit many great scientists and philosophers throughout history, BUT- most of the understanding we have about light comes from the 1900's. See Einstein and also Max Planck. I think that lots of physical science type concepts, such as gravity, inertia, momentum, even chemistry were more deeply understood prior to our current understanding of light. So keep that in mind too. Years ago in high school physics class my teacher told us that scientists were trying to prove that electricity, gravity, and magnetism were all displays of the same phenomena. What does this mean? Why is this relevant? I'm trying to say bear with us... Don't just bear with me in this thread, but bear with the lighting industry in general. There's a lot they are still working on, even if the industry still lies to us to sell inferior product- trust that they are working on it...
-->OFF RANT NOW

Ok so you're back. Anyway- Like I said earlier. Light is electromagnetic radiation. So how exactly does that concept work again? What do the nanometers mean? Wavelengths? Wavelengths of what? Again here's how I understand it:

All the visible light we are able to see when we open our eyes is made up of millions of “photons.” Photons are basically waves of “energy” which pass through space. The light and colors of objects we see are the end result of photons (light) leaving a light source and either travelling straight to our eyes, or bouncing off other objects before being reflected to our eyes. The “color” you see when you look at an object is simply the photons (of light) which are not absorbed by the surface and are thus reflected back to your eyes.

Gosh- it sounds like I just wrote a paragraph about light, and then replaced the word “light” with the word “photon” everywhere huh? Well, it's kinda like that, and it's kinda not like that so much at all. And again, please remember- I'm not a physicist...

 

I found some useful information for answering that question here: http://home.howstuffworks.com/fluorescent-lamp1.htm it’s a site explaining the operation of a fluorescent light, but it gets into photons a little bit... In order to explain what I think to be true of photons, I'll use a diagram showing a few different atoms.





As you can see, and hopefully remember; the atom is composed of 3 basic parts: protons, neutrons, and electrons. The protons and neutrons stay together at the center of the atom, and this core (called the nucleus) is orbited by one or more electrons.

Now for our purposes, we are mostly concerned with the electrons. If you ever read or learned anything about atoms/chemistry, then you'll probably remember about “electron shells.” It's hard to explain, but follow along: Different atoms have different amounts of electrons. The electrons revolve/rotate around the nucleus much like a satellite orbits the earth. In order to distribute the electrons “evenly” or “properly” or whatever, the electrons belong to different “shells.” It's complicated, but (to oversimplify it a bit) basically the first (inner shell) holds 2 electrons, and once that shell gets filled, we move to the next one which holds 8 electrons, and the list keeps going (with different maximum numbers for future shells) until there are no more electrons left. By definition, outer shells cannot be filled with electrons until all lower shells are filled, and also, shells cannot accommodate more electrons than their defined maximum number.
So why again does this matter and how does it relate to light? See the quote in bold below (the non bold parts are things I added). According to the article I read:

“When an atom gains or loses energy, the change is expressed by the movement of electrons. When something passes energy on to an atom -- heat, for example -- an electron may be temporarily boosted to a higher orbital (they use the term orbital, I called it a shell: same thing) (farther away from the nucleus). The electron only holds this position for a tiny fraction of a second; almost immediately, it is drawn back toward the nucleus, to its original orbital. As it returns to its original orbital, the electron releases the extra energy in the form of a photon, in some cases a light photon.

The wavelength of the emitted light depends on how much energy is released, which depends on the particular position of the electron. Consequently, different sorts of atoms will release different sorts of light photons. In other words, the color of the light is determined by what kind of atom is excited.

This is the basic mechanism at work in nearly all light sources. The main difference between these sources is the process of exciting the atoms.”

 

Again, just my humble understanding, but here's how I think the pieces fit together somewhat: First off- Photons are a form of ENERGY, not matter. They don't have mass or weight. They have energy and momentum. I don't expect to get the whole picture right, hell even Einstein had trouble with that, but I think I have a good Idea of what's going on... Basically, photons, are the “re-radiation” of some sort of input energy (either heat or otherwise) which are emitted as a result of the atom taking on, or losing some energy. In short, they are waves of energy period. Think of those drag racer cars that get up and over 100mph in like 2 seconds. You know the ones with parachutes? Well, I guess the car is like the electron leaving orbit, and the photon given off is like the parachute that slows the car back down before the stop line. Different energy levels result in different wavelengths (different colors), which is kind of like saying that if that same car goes 200mph and pulls the parachute at the same spot, then it'll need to be a way bigger parachute to still stop the car before the same stop line..

First off: note 2 things. 1) I said the photons are emitted, not created etc, and 2) I said the energy is re-radiated

Why would I be careful to put it this way? Because according to the laws of physics, energy cannot be created or lost. It exists somewhere, perhaps in a stored form, it is acquired or leveraged, distributed etc, and when all is said and done at the end of any task you should be able to account for where all or most all of the energy went as the net total of work performed and energy lost to heat etc.

So, back to the electrons. What happens is they are in their orbits etc, and something happens to disturb their normal position. This requires energy input. It is usually heat, electricity, or both. The result of this input of energy is that some of the electrons in the source material become “boosted” from their normal positions. That is, they sort of “go rogue” and fly off from their orbit to higher positions than usual. They do not stay this way for long. They snap back to position and resume their “usual” paths almost immediately. Basically that is why a photon is emitted.

Still doesn't make sense? Okay. Most things that are disturbed from a position of rest experience a natural “settling” period. Think of when you flick a car antenna with your finger- it flops around a bunch of times before resting again. Same thing with a springy brass doorstop inside your house. Another good example would be a still glass of water: if you tap the side, it sloshes around for a bit before settling back down. Anyway, what the article is telling us is that this is not the case with the disturbance of an electron. When the electron gets disturbed it does not experience any sort of settling effect- it returns to its normal state with little deviation. It basically just snaps back to where it was supposed to be almost immediately. Since the electron doesn't “twang” around like a guitar string before it goes back to where it should be, there's some extra energy going unaccounted for there. The extra energy is what is then emitted as a photon. In a sense I guess you could say photons are the “equal and opposite reaction” to the initial action of an electron leaving its shell briefly. This, again, is why I said “emitted” and not created- the frequency/wavelength of the photon represents the balance of the leftover energy which was originally added.

As a side note: It's not hard to understand and accept the idea of things “re-radiating” energy. For example- ever go tanning in a booth and get a nasty burn? Notice how the burn “felt warm” for days afterword? It didn't feel warm because of your body heat- the heat the burned tissue seems to be emitting is actually, in part at least, energy that your skin absorbed in the tanning booth. The energy is being “re-radiated” by the affected tissue. Not all the energy is re-radiated of course because some went towards all that tissue damage you see, but some is re-radiated nonetheless.

 

Ok. So if you've followed me this far, then you know that photons are the energy (sometimes light energy if the wavelengths of the photons released happens to be between 400-700nm) that is released when an atom experiences a net change in energy (gains or loses energy) and that change “boosts” one or more electrons out of their “orbit” and into “higher altitudes” temporarily.

So, here we have Exhibit A. It's a video.


You can watch, and I'll narrate what's happening. In the video you can see a Bunsen burner (propane lab torch) burning. What the technician does next is dips a small piece of platinum wire into water (to get it sticky) and then into a pile of lithium chloride (a chemical) powder. Next, they place the wire in the flame, and what you see is a beautiful reddish-pink flare coming off of the wire. Eventually the lithium burns up in the flame and the cool show ends. They do it again in the same video with sodium chloride (table salt) and get an orange color flare for a few seconds and the video ends. I'm just going to focus on the lithium for now.

Here's the video: [ame=http://www.youtube.com/watch?v=QNojS6ZZ4og]YouTube - Alkali Flame Test[/ame]

So who can tell me what they just saw? Did the lithium “burn red?” The answer may surprise you. The answer is no. The lithium did not burn red. What you actually just witnessed was a prime example of photon release. When the lithium powder on a stick was placed into the flame, its atoms became “excited” by the heat energy it was suddenly surrounded with. This caused its atoms' outer-most “valence electrons” (the electrons in the outer-most shells) to temporarily jump out of their orbits and return briefly. This is what caused the release of photons.

So.... While that wire was sitting in the flame, thousands (millions or billions maybe) of electrons became excited from the heat, left and returned to their orbits', and released the balance of the energy absorbed as photons in the process. These photons are waves as we know. In this case, the wavelength of the waves created was in the red range, so if you consult a chart, you could very well argue that the visible photons emitted by this experiment must lie mostly somewhere in the 570-700Nm Range.

Next- On to the sodium chloride (table salt) they test next in the same video. You'll notice two things. 1) We learned earlier in the article I shamelessly reposted here that changing the material use will change the color, and so you have it- lithium “burns” red, and sodium chloride “burns” yellow. 2) Sodium “burns” yellow? Wait a minute! HPS lights are yellow too! You know, High Pressure Sodium? There you go. Proof that the material used matters, and a practical example of why HPS light is so yellow. Also, the lab technician in the movie comments on this as well.

As a last note- remember that I'm specifically trying to avoid saying that these items “burn” a certain color. It's tough to back up my point, because yes the material in the video is burning. That's why the experiment ends: eventually the stuff on the end of the wire burns all to sh*t and won't work anymore. BUT- the lithium is not "burning red." It is taking on heat energy and re-radiating that energy as light photons in the red range of the spectrum while it is burning. It's kind of one of those things that works two ways: these materials will give off cool colors while they burn because of the heat, but under the right circumstances you can get those colorful displays without burning the material. This is the concept around which artificial lighting is based- otherwise every artificial light source we use would somehow need to involve fire.

 

If any of you recognize me- you'll know that one of my biggest pet peeves is the Kelvin Color Temperature system. If you haven't heard my rant before, I'll pitch it to you one last time...
This:


Is a chart of Kelvin color temperatures. On the left are temperatures in degrees Kelvin. You can see that they correspond to a color somewhere in between red and blue. Also, the chart lists a few types of common lights on the right, and the relative color of light they produce.
Now don't get me wrong- this chart is great for some purposes, but not so great for getting useful growing information. It actually raises more questions than it answers- so I'm gonna go there We're gonna get the can opener and bust this can-of-worms wide open...

 

Alright- first off most household lights are sold with the “color temperature” listed on the box. In fact, I believe it's becoming the law that it must be that way here in the USA soon. Anyway, when you look at it, it's just a temperature. “2700K” (twenty seven hundred degrees Kelvin), 3500K etc. But isn't Kelvin a measure of temperature like on a thermometer? Isn't it a measure of heat? Temperature of light? You may be asking yourself questions like: 2700 degrees of heat where? 2700 degrees Kelvin = 4400F, so does that mean the bulb operates at that high a temperature in my house? That can't be right?! OK. So they didn't explain that part on the box lol.

The answer is a little confusing: the Kelvin color temperature system is not a direct measure of heat, but instead a reference to the “temperature of an ideal black body radiator.” WHAT? Ok- don't panic. It's actually pretty easy.

In physics, an “ideal black body” is “an ideal black substance that absorbs all and reflects none of the radiant energy falling on it.” Basically what they are saying is that this is an imaginary material which is capable of absorbing all forms of radiation (including photons at the wavelengths that make up the visible light spectrum) that get shined upon it, zapped through it, or otherwise transmitted its way. It is important to understand that this is a concept, not a reality. In other words: there are many substances found on earth which behave very close to an “ideal black body,” but no such true perfect black body material exists. Also, remember that we said this magical substance absorbs ALL forms of energy, including the entire visible light spectrum? That means that there's no light to get reflected back to our eyes (because that bastard substance absorbed it all lol) so this mysterious material would appear to be perfectly black. As you might imagine, the substances we use here on earth as substitutes are also pretty damn black. Examples include: carbon, graphite, and tungsten although there are many others.

Anyway, back to the point. Let’s say you take an ideal black body radiator (we'll say graphite was substitute, it really doesn't matter) and heat it up. You heat it up bad-ASS. I'm talking like you put it over some sort of industrial heater in a steel foundry. Then you watch... Let's say you've got a really cool infra-red laser thermometer so you can take the temperature of the material accurately while you do this also... Here's what you're going to see happen- At room temperature the black body will appear- you guessed it black. As it heats up it will begin to glow and as the temperature increases, the visible color given off will change. The visible color given off will start in the red range, and change to yellow, white, and lastly blue as the temperature gets hotter and hotter. This is because once the black body reaches a certain temperature, its atoms become “excited,” which will allow its electrons to jump orbit and release the photons you will see as a glow of visible light. And just like we said when we talked about photons earlier: as you increase the temperature (put more energy in), the color changes (the wavelength of the photons being given off by the black body changes). So it all “jives” with what we discussed earlier...

So, by this model; at 2000 degrees Kelvin, an “ideal black body radiator” will glow a reddish orange color. At a temperature of 5000 degrees Kelvin that same black body radiator will glow a baby blue sort of color. So what does this mean? How does that have anything to do with light bulbs? Here's the pitch- when you buy a light bulb that says 2700K on the box, they're saying that the light bulb will glow the same visible color as a block of graphite heated to the same temperature. That's it.

So now you can see that Kelvin color temperature is a VISUAL reference.

Side-Note: Think of hardened lava. It's black. By a very basic model it fits the profile of a “black body radiator.” For this reason, scientists are able to accurately estimate the temperature of hot flowing lava based on its color. In other words, if an object is really black at room temperature, and it's the kind of material you can heat up to high temperatures without setting it on fire, then you can probably estimate the temperature of the material based on the color and making a comparison to the Kelvin chart above.

Don't mistake me when I say this. You might think I am trying to say that only black objects will glow at high temperatures. That is not what I am trying to say at all. What I mean is that only black bodied objects will exhibit visible colors consistent with the temperatures chart above. Other materials may or may not give off photons at high temperatures providing they don’t burn first, but if they do not have the characteristics of a black body, their colors and temperatures will not match the chart above. Example: hot steel is “lemon” colored at about 1725 degrees Fahrenheit. Converted to degrees Kelvin that’s 1214K. That doesn't even make the scale in the chart above. If it were going to match the Kelvin chart, I'd guess the temperature would have to be at least 3000K, which it's not, it's 1214K.

 

Below you will find a series of images. They are screenshot captures from a java applet I found here: Blackbody Radiation















What you are seeing is a simulated image that is supposed to look like a spectral output curve from an ideal blackbody radiator... You are basically pretending that you have a huge block of graphite that you are using as a light bulb by heating it up, and the screen shows you what the spectral distribution graph for that light would look like.

We'll start with the screenshot from a simulated 2500K blackbody radiator. As you would expect from what we discussed earlier- most of the output is in the red range. As we look through the sample images I have provided: you will see that the spectral output curve for a perfect blackbody radiator shifts to the left as the temperature increases.

This leads me to mention something not previously mentioned: the reason for this shift in the curve. Light at different wavelengths carries different amounts of energy. The lower the wavelength in nanometers- the more energy being carried. I am guessing (open to correction) that the inverse consequence of this is that light at lower wavelengths takes more energy to produce- even if the light is being produced using a technique other than a blackbody radiator. For example- take (2) 26watt GE brand CFL's, one 2700K and the other 6500K. The 2700K bulb is rated at 1700 peak lumens, and the 6500K bulb is rated at 1600 lumens, even though the wattage is the same. I may be wrong here but the most obvious explanation is: The bluer wavelengths take more energy to produce (because they carry more energy per photon) to you will get fewer of them for the same wattage because more energy input (watts) is needed to emit photons at those wavelengths. I'm guessing that the types of phosphors used are somehow related to that function also. Another cool little fact that still supports my guess would be LED's- red LED's usually have a voltage requirement which starts at 2 volts, while blue LED's usually have voltage requirements starting at 4 volts...

 

So now that I hope you have a decent understanding of how the Kelvin color temperature scale works; I'll present the first problem I have with this approach. In case you hadn't noticed already, the Kelvin color temperature system is based around human eyesight. Think about it- they're giving you a light bulb of some sort, and they're giving it a number that basically says this light gives off a color that looks (to your human eyes) the same color as hot coals at the same temperature look (to your human eyes). Now that you know what the numbers really mean, doesn't that sound like complete bullsh*t or what? Like how the hell is that useful for plant growth purposes? It just plain isn't.

→ On Rant: It really pisses me off when I hear people preach Kelvin color temperatures, lumens, and LUX like they're the gospel end all answer all figures to know in terms of getting a decent setup going. YES these are great figures to have, and they absolutely have their purpose, but there are other figures to consider and some of these figures are more important than others. Also, I think the myth of blindly relying on these figures is a "revenue generating mechanism" that has been propagated by the greedy and seedy grow lighting industry. I personally believe it is of great importance to understand the “why” behind how artificial indoor grow lighting works in order to make the best informed decisions. In other words: these figures can still work for or against you in some cases.
→ Off Rant

So, If Kelvin color temperature has to do with how we see light, then how do plants “see” light? Why in nanometers of course!

 

If you have ever bought a decent replacement HID bulb (HPS or MH) you've probably seen the spectral output graph on the side of the box. Here's a random sample for a 250Watt HPS I found:



When you look at the charts, there are features about them that are the same and some that aren't. First off, the bottom will always be wavelength in nanometers. If it isn't, then you're looking at some other kind of chart you probably don't want. The left side however will sometimes differ from chart to chart. This one says “relative spectral power.” and it spans what appears to be a percentage scale. Some use a different figure or unit of power etc for the vertical axis. It doesn't make one chart any better or more relevant than the other... It just means that the two charts cannot necessarily be compared directly to each other. Now why would they do that? So you have to take their word on it with all the sales hype because you can't really compare statistics on the bulbs directly to one another unless you're comparing products within their line. Doesn't that sound a little scandalous? It's kind of like shopping for a new mattress. Try that recently? You can't compare sh*t because it's named differently everywhere you go. The names of the same friggin' mattress will be different at every single chain furniture store, department store, and online store. The manufacturers do this on purpose so you can't price compare. Once you find something you think you like you're stuck paying their price. Sleazy Huh? Anyway, back to the light- it wouldn't really matter what unit of measure they used on the left side of the chart as long as they were all the same so you could compare apples to apples. They don't do this, probably for sales reasons and also for the lack of an industry standard for the units of measure on the graphs.

So regardless of what units are used for the vertical axis, the charts are basically trying to tell us the same thing: Which wavelengths of light the bulb emits the most.

Why does any of this matter? Because plants don't “see” light the same way we as humans do. As humans, we see light with our eyes. The “color” we “see” is the product of rods and cones in our eyes responding to the light (photons) that strike their surface, get absorbed, and become signals to our brain, which then renders an image somehow. Plants don't play that way at all. Plants “see” light, in terms of wavelength in nanometers that stimulate chemicals in the leaves etc to go to work... To be more precise, vigorous plant growth revolves (basically) around stimulating 2 substances: chlorophyll-A and chlorophyll-B with light. The problem? Plants respond differently to different wavelengths of light. There is an excellent article about this here: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/biobookps.html
And I'll try to get into some of it later myself.

 

So anyways if you've been reading this thread, or some of my scarce postings, you know I just can't stand being forced to rely on a Kelvin color temperature alone when examining a potential light source. So I'm going to beat Kelvin Color Temperatures up a little bit more, and then I'll try to lay off it a bit; although I can't promise anything

Here's the deal- Light as we understand it today is the product of centuries of sometimes conflicting research. The way I understand it, when Einstein won the 1921 Nobel Prize in physics for correctly explaining the photoelectric effect. It is more or less at that point that light was, for the most part, finally “understood.” That's great, but know this: The conflicting research, theories, and scientific studies in the few decades up to that point were a “pissing contest” of heroic proportions.

Yes it's true- The scientific community at the time just “couldn't get their sh*t straight” so to speak. The biggest problem was that early theories about light stated that light was moving particles of some sort. Today we understand that is not true. Light is basically discrete moving packets of energy. Starting around 1860, a handful of scientists turned their attention to that black body radiation stuff I was discussing far earlier. You see, they didn't at all understand it the way I tried to explain it, so there was quite some debate.

Basically scientists were trying to predict the emissions of a black body when the black body itself was radiated with frequencies instead of heat. Some could explain and somewhat accurately predict emissions from the black body when using low input frequencies but not at higher frequencies of input. Other researchers were able to do just the opposite. If you go on Wikipedia and look up terms like “max Planck,” and “ultraviolet catastrophe” you'll get the idea.

What I'm getting at is that there were a bunch of experiments before all this came about. Some of them were failures, and others lead to accepted scientific truths. Kelvin Color Temperature was one of the ones that beat out the scrutiny so to speak. That's great, but the problem is that The concept of Kelvin Color Temperature itself, near as I can tell, was developed before light had even been measured in nanometers yet. That's fine, because for the final time: Kelvin Color Temperature is a reference based around human eyesight. It's not useless, but it has its limitations.

Still don't believe me? Check this link out:
http://www.uky.edu/~jholl2/technology_pdfs/KelvinColorTemperature.pdf
It describes how lights are used in broadcast television, but it also reinforces my point: Kelvin color temperature is a visual reference. This doesn't make it useless, but it does make it limited.

 

OK. So I've spent a bunch of time assaulting the Kelvin Color Temperature system, so I'm going to try to spend some time on the statistics I feel matter much more.

Here it is in a nutshell: Plants can use quite a bit of light in the visible light spectrum for plant growth, but some wavelengths of light are utilized by the plant far more efficiently than others.

So which wavelengths are used most efficiently?

 

This image below is a pair of charts I like to loosely refer to as the “photosynthetic response curve.” I don't believe it applies specifically to one plant vs another. I think this is a direct graphical representation of how the two chlorophyll molecules (is that right?) react to the different spectrums of light. I have also seen graphs with specific species of other plants (in Latin) written on them. They do not always match this curve. I think that is because there's a little wiggle room in there caused by a species-to-species difference in plant's response which might not have to do with the chlorophyll at all. Don't ask me why. I don't know, but I feel the curve shown below is best.

 

Ok, so I'll admit it right here- I've got a HUGE problem with the concept of lumens and lux... I'm going to start talking about why it isn't important here. I'll start first with illustrating why the concept of using lumens for any sort of plant growth purposes is a preposterous notion in the first place.

 

So everybody seems to think that the term “lumen” is a cut-and-dry way of measuring light output. I'm going to try to re-work your thinking about that a little bit for your benefit.

What are lumens? Look it up on wikipedia. First sentence? “The lumen (symbol: lm) is the SI unit of luminous flux, a measure of the power of light perceived by the human eye”. There's your first clue- they basically state lumens are a unit used to describe how much light that is useable to the human eye a light source produces. Don't believe me? Here's another supporting argument: how many lumens does a 1watt infrared LED produce? Answer- 0 lumens. These diodes are rated in “W/sr” instead.
That's right. Because you can't see the light the diode produces, it can't be expressed in lumens! More on that later.

So How are lumens calculated? Wikipedia “luminosity function” and hang on...

First you’ll see this chart:



Don’t get intimidated, it’s simple. The chart is trying to tell you that our eyes respond differently to different wavelengths/colors of light. The human eye is most sensitive to light at the middle of the visible light spectrum (yellow light) and less sensitive to light in the outer reaches (blue and red) of the visible spectrum. The chart above is showing The averaged response (of the rods and cones etc) of how our eyes react to different colored light. Why does this matter? Because Lumens are a measure of how much light a source produces that is useable to the human eyes. As such, the curve depicted in the image is applied to the calculation. More on that in a minute…

Scroll down and look at the equation for lumens:



Now I’m no physicist, but I do understand SOME of what’s going on here. “F” is electromagnetic flux (in lumens)… “J” is the spectral power distribution of the radiation (power per unit wavelength)… The lowercase “y” with a bar on top of is the symbol for the “standard luminosity function.”

Does anyone else see what’s happening here? Maybe My hacked up diagram will help make some sense of it…



Now does it make a little more sense? In order to assign a “lumen” rating to a lamp, part of the calculation involves multiplying the spectral distribution of the lamp by the human eye response curve. Why is this such a big deal?

As you remember earlier when we examined the averaged photosynthetic response curve, peak absorption for chlorophyll occurs at about 450nm wavelength for blue light, and 660nm or so for red light. The problem is simple: 1) lumens are meant to be a measure of light that is useful to the human eye 2) Plants just so happen to favor light at wavelengths that the human eye is least sensitive to. 3) because of this; quantifiable the amount of light a fixture produces which is useful to plant growth becomes almost completely obscured inside the data. See chart below:



Basically what I’m trying to say is that when you calculate how many lumens a fixture produces in a lab: the equation used doesn’t give equal consideration to all the different wavelengths. Unfortunately for us, the equation uses a curve shaped the opposite of the photosynthetic response curve, so the data gets skewed to the greatest extent possible

And so the process of the lighting industry taking advantage of growers (and their wallets) begins…

 

So, now that you know where the concept of a “lumen” comes from, and how little it matters, I’m going to “keep firing” so to speak… OK. So I’m sure you’ve all seen this chart fly up on the forums whenever the topic of LUX floats in…



Illuminance Example
10−5 lux Light from Sirius, the brightest star in the night sky[2]
10−4 lux Total starlight, overcast sky[2]
0.002 lux Moonless clear night sky with airglow[2]
0.01 lux Quarter moon
0.27 lux Full moon on a clear night[2][3]
1 lux Full moon overhead at tropical latitudes[4]
3.4 lux Dark limit of civil twilight under a clear sky[5]
50 lux Family living room[6]
80 lux Hallway/toilet[7]
100 lux Very dark overcast day[2]
320–500 lux Office lighting[8][9][10]
400 lux Sunrise or sunset on a clear day.
1,000 lux Overcast day[2]; typical TV studio lighting
10,000–25,000 lux Full daylight (not direct sun)[2]
32,000–130,000 lux Direct sunlight



è ON RANT First off, copying and pasting a chart from wikipedia doesn’t make you an expert in anything… Thanks.
è OFF Rant

OK. So what does this chart mean? Basically, lux= lumens per square meter. That’s it. These are just a list of some common scenarios you can use as a visual reference. Notice I said visual reference? You know, like that human eye deal we were talking about earlier?

I hate it when I see someone simply post this chart up on a forum, and then tell people to simply get the best HPS they can, and get it as close to 32,000–130,000 lux as they can. That’s not quite right. Why?

1) because LUX is an extension of Lumens as a form of measurement; you have stripped the importance of the correct wavelengths of light directly out of your measure of the light being used.
2) The chart above gives no consideration to the spectral output of the lamp source. It is simply trying, yet again as always, to give you a visual reference of what a certain amount of LUX looks like, based on something you’ve seen before…

So why is this a problem? Because it infers that you can match the output of the sun using lux alone as a guideline. This is just not true.

Take for example a LOW pressure sodium lamp. LPS is one of the most efficient sources of light on the planet, in terms of lumens/watt. It’s about 200 lumens/watt. The catch? It produces light on only two dominant spectral lines very close together at 589.0 and 589.6 nm. The averaged spectral output is almost completely on the 589.3 nm wavelength, and looks like this:



So here’s a great example of why LUX SUX! Lets say I buy a low pressure sodium lamp to grow plants with. It’s a great idea right? 200 lumens per watt is king shit of the HID luminous output world so who could go wrong there right? Let’s say My growing area happens to be one square meter, and I somehow get my hands on a 600watt Low Pressure Sodium light. (I’m not even sure if they exist). 600watts * 200 lumens/watt = 120,000 lumens/ one square meter! That means that bad boy will be illuminating the grow area with 120,000 lux of light! According to our precious lux chart- that equals nice intense direct sunlight, so we’re in great shape right? Wrong. Look at that puny spike at 589nm and look at the photosynthetic response curve I posted earlier... ALL of the light produced by the LPS light is almost completely useless. So I guess 120,000 lux doesn’t instantly mean the same as direct sunlight huh?

 

That’s a good question. I’m not sure myself. I don’t think it’s worth the paper it’s written on in terms of plant growth purposes. So what about the 32,000–130,000 lux benchmark? Here’s my thoughts on the matter. When someone used one of those lux meter thingies (looks like a multi-meter and probably costs about 150 bucks) to come up with the 32,000-130,000 figure- they doubtlessly checked a variety of different locations. One other thing is certain- each time they did this they were measuring the same light source- the sun. Granted the spectral composition of the light you receive from the sun differs a bit depending on your global location, but it’s way closer to being “all the same” than when you compare different fluorescent bulbs etc. To me this means that 32,000-130,000 lux as a benchmark only counts for anything if you are going to be growing in the sun! So unless you have a grow light with the exact same visible spectral distribution as the sun (like the luxim inductive light), or you're growing outdoors, what does lux mean to you? What should lux mean to you? NOTHING…

 

Ok- I realize that incandescent bulbs are not suitable for plant growth, but they are a great start for explaining how artificial light so I'm going to start there. (check out this article: http://home.howstuffworks.com/light-bulb2.htm )

As you know the basic (not a 3-way) light bulb has a few basic parts. The “base” is the metal part at the bottom that's threaded like a screw. There's the glass bulb. And, there's a “filament” inside that’s supported by a couple of wires. Either side of the filament made from tungsten metal is wired down to the two electrical contact points on the base. The inside of the glass has had all the air sucked out so that it is a near vacuum. Sometimes they suck all the air out and fill the bulb with an “inert gas” (a gas that doesn't react chemically with anything else very well). They do this for reasons I won't get into.

Remember the black body radiator experiment? That's what's going on here. Basically they (Thomas Edison) picked tungsten for a couple reasons- 1) it has properties that make it a near ideal black body radiator and 2) it can reach temperatures high enough to give off light without melting into a liquid.

So: when you flip the switch, electricity flows through the tungsten metal filament, which causes it to heat up. It heats up A LOT. Once the thin metal ribbon in there gets hot, its electrons become excited, get “boosted” and release photons of light. Here's a cool fact: when you buy an incandescent light bulb, the color temperature is basically the same temperature that the filament is designed to heat up to during operation. That's right: the simplest form of a blackbody radiation experiment you can do is just turning on an old style light bulb! For example; if an incandescent bulb says 2700K, then the filament inside is probably getting really close to 2700 Degrees Kelvin when the light is on! Of course the glass on the outside doesn't get that hot, but that's for other reasons which don't really matter much.

Note: This little fact about the color temperature being the same as the temperature of the filament only holds true of incandescent lights. With fluorescent and HID lighting- the primary light source is not a tungsten filament. These other types of lights usually have “CCT” written somewhere before or after the Kelvin temperature of the bulb. “CCT” stands for “correlated (sometimes the word 'corrected' is used instead) color temperature. In other words- because the source of light with these lighting types is not a black body type material the bulb must be assigned a temperature which accurately describes the color of the lamp.

 

OK so the answer to this question is simple: Incandescent light bulbs are unsuitable for plant growth purposes because they don't produce enough light period. Although they produce most of their light in the red range which is good for flowering, they produce light using heat, so they're doomed to be less efficient. This quote from wikipedia says it best “A typical 100 watt tungsten filament incandescent lamp may convert only 2% of its power input to visible white light, whereas typical fluorescent lamps convert about 22% of the power input to visible white light.”

And that's pretty much just about it... Even if you find a way to vent all the heat away from the bulbs, they simply don't produce enough light. Even if heat wasn't a concern, I don't think you could stuff enough of them into an enclosure to produce any results because they are also pretty big compared to how much light they put out too, so you'd probably run out of room for more bulbs before you reached your desired light intensity... Plus incandescent bulbs suck for VEG, and will make your plants stretch unreasonably...