# Lux per Square Inch?????



## swylie (May 10, 2007)

Of course you're right that those units make no sense. You may be interested in this discussion of light per area in a planted tank. It's an analysis of how much light Takashi Amano uses in the tanks published in his books. You'd think that lighting intensity would pretty much be proportional to the illuminated surface area. It's not, though. In Amano's tanks, p=kv^0.5 where p is power and v is volume. 'Course there's a lot of variability in his tanks, and it all depends on the efficiency of your illumination and the plants you want to grow etc. etc.


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## jbolinger (Oct 13, 2007)

swylie said:


> ....It's not, though. In Amano's tanks, p=kv^0.5 where p is power and v is volume. 'Course there's a lot of variability in his tanks, and it all depends on the efficiency of your illumination and the plants you want to grow etc. etc.


Yes, I think I have seen this before. Interesting, but you still need to know how much light reaches the the plants and not how much coal the power company is burning (at least in this area):wink: !

Note that for the 'typical' tank sizes that most of us have it is the 2-3 watts per gallon

Jim


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## AndrewH (Dec 24, 2007)

(I'm also an engineer )

I think I see where the quoted went wrong...

1 Lux (lx) = 1 lumen in 1 square meter

1000 lx = 1000 lumens in 1 square meter

1 footcandle = 1 lumen in 1 square foot

The example within the quote I think takes lux and turns it into footcandles, but not correctly. It doesn't seem to be converting from metric to imperial. 1 fc = 10.764 lx.

He needs to stay within the same measuring system. Either use lux or footcandles.

55 gallon at 110 watts = ??? footcandles or lux (at the surface). Of course we all know the wattage rating means nothing when it comes to lumens.

To calculate the lux or footcandles you would have to have the lumen output of the light source regardless of what wattage it is.

Plus, the lux or footcandle reading at the surface of the water isn't 100% of the lumen output of the light source. You have reflector light loss, fixture design light loss, etc.

The lux or footcandle reading 1" below the surface of the water will be considerably less than the surface. The surface will reflect some of the light, and the suspended partials within the water will reflect or absorb some of the light, which will only get worse the deeper you go.


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## AndrewH (Dec 24, 2007)

It also seems the quoted is comparing illuminance (lumens/meter) and irradiance (watts/meter) which is very hard to compare.


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## Left C (Nov 15, 2003)

I believe that Tom Barr recommends measuring PAR instead of LUX.

PAR is based on the parts of the spectrum that the plants can use.

LUX is based on what the human eye sees, I believe.


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## AndrewH (Dec 24, 2007)

I've read about the PAR vs. LUX a while back and it makes good since, and I agree.

But most light is measured in lumens by the manufacturers and they do not provide the PAR information unless you specifically request the data sheets. And even if you get the data sheets, they're sometimes pretty hard to read (understand).

Generally speaking you're looking for the red and blue wavelengths and not the yellow and greens. Now there is not a conversion from K to wavelengths (nanometers), so unless you know the nanometer rating of the lamps you'd have to go by your best guess and others' experiences with the different K ratings.

And if you did take all the time to gain the nanometer ratings of your lamps, calculate the lux to maximize the plants growth, you'd still have to take into consideration the effects of the water on the manufacturer's data. Light through air and light through water are two different things.


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## jbolinger (Oct 13, 2007)

AndrewH said:


> It also seems the quoted is comparing illuminance (lumens/meter) and irradiance (watts/meter) which is very hard to compare.


In determining RF (radio) signal strength, Watts/(Meter^2) is used. Also Volts/Meter. These are what I am familiar with. They are independent of 'color'.

How do these compare to the illumination units?

Jim


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## AndrewH (Dec 24, 2007)

jbolinger said:


> In determining RF (radio) signal strength, Watts/(Meter^2) is used. Also Volts/Meter. These are what I am familiar with. They are independent of 'color'.
> 
> How do these compare to the illumination units?
> 
> Jim


Well, it goes into the wavelengths of each color vs the wattage to produce that color... green is the most efficiant wavelength so it has the highest lumen per watt rating (somewhere around 680 lumens per watt).
Since most of the light is a mixture of all of the colors in varining amounts to determine the exact watts/meter^2 rating of the light you'd have to know all of the colors and their efficiancies. Example: 3% green, 45% blue, 34% red, and 18% yellow make up the light source you're measuring (this is just a simplifed example). Green is 680 lumens per watt, blue is 550 lumens per watt, red is 500 lumens per watt and yellow is 425 lumens per watt. Take the total wattage and the percentages of each color, then you can calculate the watts/meter^2 as compared to lumens/meter^2, but it'll change with each light source (and even from one light bulb to another).


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## jbolinger (Oct 13, 2007)

AndrewH said:


> Well, it goes into the wavelengths of each color vs the wattage to produce that color... green is the most efficiant wavelength so it has the highest lumen per watt rating (somewhere around 680 lumens per watt).


No, No, No!

The amount of electricity you use from the electric company has _nothing_ to do with the 'color' or 'wavelength' of the radiation!!

I was talking about *electromagnetic field strength*. For radio waves, field strength is measured in either Watts/Meter^2 or Volts/Meter and has nothing to do with the frequency or wavelength of the radio signal.

What you are talking about is the efficiency of the 'transmitter'---that is, how much power you get out, in the electromagnetic field, in relation to how much you are using from the electric company. Lumens is a unit of 'intensity' or 'electromagnetic field strength'.

My question was simply: What is the relationship between the *units of measurement* for radio waves (watts/meter^2) and for light waves (Lumens)? Both are electromagnetic waves, so there _has_ to be a relationship between them.

Jim


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## swylie (May 10, 2007)

Lumen is a measure of intensity that's weighted according to the sensitivity of the human eye, so x green photons per time will have a higher lumen rating than x red photons per time. Watts is of course a function of the wavelength. PAR is unweighted, while PUR is the plant equivalent of the lumen, it's weighted according to the absorption spectra of plant photopigments. Chlorophyll does the same electrochemical work per photon regardless of the wavelength of the photon. Therefore the PUR (photosynthetically useable radiation?) system weights reds and blues about equally even though blue photons have more energy than red photons.

For our purposes, any measurement (such as lumen) that is based on the human visual spectrum is of limited use. You can say that 2x of a particular bulb will put out twice the lumens and twice the PUR compared to 1x, but you can't say the same when comparing different bulbs. PUR is the number you really want, but an unweighted number such as PAR will suffice.

Dunno if that contributes anything to the discussion...


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## jbolinger (Oct 13, 2007)

swylie said:


> Watts is of course a function of the wavelength.


Watts is _not_ a function of wavelength! Since you mentioned photons below, watts is a function of the _number_ of photons, not their wavelength. And keep in mind here I am talking about the _strength or intensity_ of the radiation, not the power used to generate it. Photons with shorter wavelength will have higher energy regardless of their number.



swylie said:


> Chlorophyll does the same electrochemical work per photon regardless of the wavelength of the photon. Therefore the PUR (photosynthetically useable radiation?) system weights reds and blues about equally even though blue photons have more energy than red photons.


If this is so, then why all the bother with lights of a certain color temperature, that is wavelength? Any ole' light bulb would do!

If blue photons are weighted tHe same as red, this would mean that fewer of them are needed? That is, the required Watts/Meter^2 is less

Granted, some lights (LEDs for example) may be more _efficient_ at different wavelengths (photon energies), but that does not mean that the wavelength and power are directly related.

Jim


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## swylie (May 10, 2007)

jbolinger said:


> Watts is _not_ a function of wavelength! Since you mentioned photons below, watts is a function of the _number_ of photons, not their wavelength. And keep in mind here I am talking about the _strength or intensity_ of the radiation, not the power used to generate it. Photons with shorter wavelength will have higher energy regardless of their number.
> 
> 
> 
> ...


Wha? I'm pretty sure I'm correct. I wasn't talking about watts of power consumption either, I was talking about watts of incident radiation. Since a blue photon is shorter wavelength than a red photon it contains more energy. It would take fewer blue photons to illuminate with a flux of 1 W/m^2 than it would red photons. Is this not correct? If we're thinking about monochromatic light, wouldn't radiant flux be a function of quanta _and_
wavelength?

As for photosynthesis, I'm again pretty sure that I'm correct. Look up the "action spectrum" for photosynthesis and compare it to the absorption spectrum. There's an action spectrum for _anacharis_ farther down on this page. Red and blue produce roughly equal growth rates. I'm almost positive these charts are in terms of equivalent quantum flux, not in terms of equivalent power.

Photosynthesis is a quantum process. A photon strips an electron from a photopigment. I suppose if it's a blue photon then there's some energy wasted that's dumped into the plant as heat.

Anyway, the real expert on this topic is Tom Barr. I'm a dilettante.


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## jbolinger (Oct 13, 2007)

swylie said:


> It would take fewer blue photons to illuminate with a flux of 1 W/m^2 than it would red photons. Is this not correct? If we're thinking about monochromatic light, wouldn't radiant flux be a function of quanta _and _wavelength?


Hmmmm. A bell just went off in my head. Yes, you are right. The same thing happens with radio waves, of which I am more familiar. If you have a radio signal at 10 MHz, for example, and collect the energy over an area of 1 Meters^2 you will get X watts. If you then go to 100 MHz, for example, _with the same transmitter power_ you will only need 0.1 Meter^2 to get the same X watts! I've always thought of it in terms of wavelength, but photon energy also makes sense. Thanks, Swylie, you taught me something today!

Still, a previous post in this thread implied that the power controlled the 'color', or the 'color' controlled the power somehow. My point is that you can have a 100 Watt Red light and a 10 Watt Red light---they both produce the same color/wavelength. The difference is that the 100 Watt light is kicking out more photons each with the same energy as the 10 Watt light.



> As for photosynthesis, I'm again pretty sure that I'm correct. Look up the "action spectrum" for photosynthesis and compare it to the absorption spectrum. There's an action spectrum for _anacharis_ farther down on this page. Red and blue produce roughly equal growth rates. I'm almost positive these charts are in terms of equivalent quantum flux, not in terms of equivalent power.


I'll have to look into this. My botany education dates back 40+ years ago!



> Photosynthesis is a quantum process. A photon strips an electron from a photopigment. I suppose if it's a blue photon then there's some energy wasted that's dumped into the plant as heat.


...Or maybe it comes out as Green?

Jim


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## swylie (May 10, 2007)

I think you might appreciate this reference. It's a fairly technical discussion of lighting for plant growth with a focus on comparing different lighting sources against each other. It has data for a lot of different fluorescent tubes. It discusses various units of illumination such as lumen and PAR.

More importantly, the author seems pretty authoritative.


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