Lumens, Lux, and Adding It all Up

Discussion in 'Indoor Growing' started by ceestyle, Jun 19, 2008.


    ceestyle Well-Known Member

    1. What is light?

    Light is electromagnetic radiation, and may be interpreted as either a wave or a particle: thus the phrase wave/particle duality. It exhibits phenomena that may be explained by its wavelike characteristics - interference, for example - while also demonstrating some that are best explained by its particle-like properties - such as the photoelectric effect.

    Treating light like a particle, a light source emits many light particles called photons. These are distinguishable by ONE unique property - their energy. When treating light like a wave, this is referred to as the wavelength, and is visible to us as a difference in color.

    The main point is this: photons of the same energy (wavelength, color) from different sources, for example MH, HPS, CFL, T5, etc. are THE SAME.

    2. What is light intensity?

    Light intensity is simply the flux of photons per unit area per unit time. More photons means increased intensity. That's it.

    3. What do lumens, lux, and foot-candles mean?


    This is the unit that defines "luminous flux", which is "radiant flux" (energy emission) adjusted for the sensitivity of the human eye. In the context of a light bulb, it is a measure of the total number of photons being emitted by the bulb (summed over all directions), adjusted for the sensitivity of the human eye to different photon energies.

    What this means is that different colors are weighted differently according to a "luminosity function" that describes the sensitivity of our eyes. You can see this function and a more detailed description here.

    *Lux, foot-candles:

    The lux is simply defined as the number of lumens per second incident upon one square meter. In other words, it is the number of photons striking a finite area per unit time, weighted by the luminosity function described above.

    The foot-candle is a derivative unit that is equal to 10.76 lux. It was defined based on the square foot, so the conversion is simply based on an area conversion.

    4. What does this mean when using multiple light sources?

    So we've established that lux are the number of photons striking a unit area per unit time, weighted by a luminosity function.

    We've also established that photons from the same light source are indistinguishable, as long as they have the same energy/wavelength/color.

    What this means is that if you put two lights the same distance from a point, and each light provides N photons per unit area at the point, with two lights you will have 2N photons per unit area at the point. Because intensity is a measure of the number of photons per unit area, the light is twice as intense, whatever unit you choose to use. Twice the lumens, twice the lux, twice the footcandles.

    An obvious practical caveat to this point comes when using multiple low-intensity light sources. Notice how I stated that the lights were at the same distance? A practical problem with CFLs, for example, is that while you can get 27000 lumens from 10 x 42W CFLs, it's difficult to get them close enough to make them useful. If you have them in a line, for example, as I've shown below, each successive light is further from the meter, and the effective increase will be reduced. They still add, but according to the 1/d^2 rule, so having a bulb 2cm farther away will yield diminishing returns. On the other hand, this can be an effective way of distributing light, whereas with HID you need to distribute the plants around your single point light source.

    5. Seriously? Prove it.

    Here is a simple experiment that demonstrates this point. Below are some 42W bulbs that are a part of a flowering cabinet. I have suspended it from some pots for the sake of this experiment. You can see there is also a 150W HPS in there; I'll do some HID vs. CFL comparisons at some point as well.

    The point is simple. Lights that are equidistant from a point contribute additively.

    Ambient light = 0


    1 x 42W = 6500 ftc ~ 65000 lux


    2 x 42W = 13100 ftc ~ 131000 lux


    Any questions?

    More examples and experiments to come.
    Last edited: Jun 21, 2008

    firebullet Well-Known Member

    Good post.

    okay, so saying the lux doubles, but what does your meter say in lumens rather then lux?
    I'mBatman likes this.

    ceestyle Well-Known Member

    Lumens aren't really meaningful in this case. To get the absolute number of lumens the meter reads, you'd just have to know the area of the photodiode in m^2 and multiply by the lux, which is lumens per m^2.

    What you can do, however, is figure out how far away the meter is from the light source, figure out the total area that the lumen output is being distributed to, and backcalculate to get lumens.

    The problem with such a calculation is that it assumes that the light is radiating equally in a spherical manner, which is certainly not the case unless the bulb is spherical.

    Let me give you an example:

    When I've got one bulb on at what is about 7", I get the reading of 65000 lumens / m^2 .

    At 7", the area the light is being distributed to is:

    4 * pi * r^2 = 4 * pi * (7")^2 = 615 in^2 ~ 3971 cm^2 = 0.397 m^2

    So for 65000 lux = 65000 lumens / m^2 , we have:

    65000 lumens / m^2 * 0.397 m^2 = 25805 lumens

    Obviously, the bulb - rated at 2700 lumens, is not putting out ten times that. It is simply that the distribution of light is not spherical. At either end of the bulb, the lux reading is much lower. You can see how important it is to properly place your bulbs.

    If instead you assume that the bulb is distributing light roughly in a cylindrical pattern the length of the bulb - approximately 3" - you get the following calculation.

    Area of distribution:

    2 * pi * r * L = 2 * pi * 7" * 3" ~ 132 in^2 ~ 850 cm^2 = 0.0850 m^2

    So for 65000 lux = 65000 lumens / m^2 , we have:

    65000 lumens / m^2 * 0.0992 m^2 = 5525 lumens

    This is still not quite right, but the order of magnitude is right. The difference can be explained by the rapid falloff in intensity as you move away from the center of the coil. In addition, the panel above the lights is covered in mylar, which reflects rather effectively.
    mrandersonnen and nedguy like this.

    chuckbane New Member

    i want to see you put 3 and 4 cfls over that meter... and then compare it to the 150 watt HPS,, that would be very interesting

    ceestyle Well-Known Member

    It will. What will be especially interesting is to compare the light distribution.

    By the numbers, each should provide about the same lumen count.

    6 x 42W = 6 x 2700 lumens ~ 16200 lumens

    1 x 150W HPS ~ 16000 lumens

    There will be errors due to my cheapass HPS and the different spectra, age, etc., but we should be able to get the point.

    What is especially interesting is that at the corners we should see better coverage from the CFLs, as they're closer, while at the center, the HPS should be better. And of course all of this will change with the distance from the canopy.

    Let experimentation begin! (but after dark, 'cause it's hot as shit in this room atm)
    Hairy Bob likes this.

    chuckbane New Member

    pics man,, PICS!

    ceestyle Well-Known Member

    I don't know what that means, but ok.
    mrandersonnen likes this.

    Seamaiden Well-Known Member

    Ask fdd. He has the powah.

    Stickied means that it's permanently "pasted" at the top of a given forum. It sticks on top no matter what. :)
    Craig1969SS likes this.

    ceestyle Well-Known Member

    we'll see how it's received. i'm getting a lot of "CFLs suck, so we don't care", but it's applicable to HID too...

    man, i should take a picture of the burn on my arm i just got moving my light meter under that HPS. *sizzle*

    chuckbane New Member

    cfls dont suck,, they just arent the proper lighting for the flowering stage
    see here
    More lumens please
    Barking Mad likes this.

    ceestyle Well-Known Member

    Yes, but considering the lumens per watt, the possibility of keeping the ballast external, and the price, I can't see using CFL for any application that uses more than 150W equivalent of HID.

    For very small grows or for maintaining a mother or two, fluorescents are fine. Anyway, that was not the point of this thread. God forbid we turn another thread into CFL vs. HID.

    Seamaiden Well-Known Member

    Damn! I hate that! Been burned by metal halides, too (working on aquaria). And I have this make-up mirror with exposed bulbs, but it lights really well, but you'd better NOT even brush against it or you WILL lost skin onto the bulb. Then you have to leave it lit so the skin dries up or you won't be able to scrape it off. :x

    I don't know or understand why this would become a debate over compact fluorescent lighting versus metal halide versus sodium vapor versus high intensity discharge lighting. None of it has anything to do with the information you've presented.

    ceestyle Well-Known Member

    Because these arguments are usually brought up in that context, so whenever people see this information, they associate it with me trying to say CFLs are better than HIDs.

    I think I just lost some hair. I'll live.

    ceestyle Well-Known Member


    BudLuvr Active Member

    Man you are a weedologist. I bet you got your degree in herbology! You know your stuff, I have to give you that.

    ceestyle Well-Known Member

    Need to find a way to make money in legit fashion in this biz. hrm. we all have our pipe dreams.
    Craig1969SS and mrandersonnen like this.

    On3Tim3OnLy Well-Known Member

    Though given, it is irrelevent ! Lumens are for Humans not plants ! Lumens measure how we see light. Our eyes prefer green light. Green light has the most Lumens, Red and Blue the least. Most fluorescent lamps are made to look bright for humans by adding huge spikes of greeny yellow light.
    Lumens only measure how BRIGHT the light appears to us.
    Lumens cannot measure light useful to plants
    Toohighmf and Craig1969SS like this.

    ceestyle Well-Known Member

    Thanks for posting the identical post multiple places. I'll respond here as well.

    If you'll notice in the text of the original post, the luminosity function spans both red and blue light, meaning red and blue both "count" toward the lumen rating.

    Therefore, you may look at the spectrum of the light, and judge how much of that light is usable by your plant. If you have primarily blue and red in your spectrum, then the rated lumens result from - you guessed it - red and blue.
    Supersweetnuggs likes this.

    Seamaiden Well-Known Member

    This is true, and for that we have two other measures that are often not well understood. They are PAR (photosynthetically active radiation) and PUR (photosynthetically usable radiation). Lumens are, for the layperson, along with color temperature ratings, the easiest way to get at what we need, which is to duplicate sunlight.
    Supersweetnuggs likes this.

    ceestyle Well-Known Member

    It's relatively straightforward for PAR:

    Take this spectrum, normalize the top peak to 1 - so that everything else is a fraction of one - and multiply it times the actual spectrum of the bulb. What you have left over is a weighted spectrum based on plant-useful light.

    A simple way to do this is to look at the PAR spectrum and look at the light's spectrum. Peaks at around 450 and 650, with a minimum at 550. You can get an idea of what light is useful with that.

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