# LUX more important than WPG?



## DelawareJim (Nov 15, 2005)

I've been playing with my new LUX meter I got for Christmas and have some interesting observations.

I always thought my 110 was under lit even though it has 2-175 watt MH and 2-40 watt flourescents (3.9 watts/gal)

With the sensor resting on the center cross brace, 3" below the fixture - 6,360 LUX 
With the sensor 1" below the surface - 5,000 LUX 
With the sensor 12" below the surface - 1,200 LUX 
With the sensor 26" below the surface (resting on the gravel) - *450 *LUX.

No wonder I can't grow stems well in this tank!

My 29 gal Endler's tank with 1-65 watt, 6,700K CF (2.24 WPG) 
fixture 15" above the sensor (off the tank) - 2,300 LUX 
With sensor 15" below surface no cover - 2,100 LUX 
With sensor 15" below the surface with glass cover - 2,000 LUX

Can't explain why the CF on the 29 gal has a higher LUX reading even though it's 3" deeper, other than it's a newer bulb and the 110 gal has more surface rippling.

Natural sunlight through double pane window - 21,600 LUX.

Interesting to say the least.

Cheers. 
Jim


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## hoppycalif (Apr 7, 2005)

Lux is a measurement related to human eyes, not plant needs. The "equivalent" for plants is PAR. Measuring PAR is possibly of benefit, but measuring Lux isn't likely to be. Watts per gallon is nothing but a crude method to get into the right ballpark for an appropriate light intensity, and getting into the right ballpark is all that is really needed.


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## DelawareJim (Nov 15, 2005)

I'm not so sure here hoppy.

LUX is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. So yes, it is related to human eye perception but it is still a measurement of intensity.

PAR simply designates which wavelength of solar light from 400 to 700 nanometers is useful to terrestrial plants in the process of photosynthesis.

Since LUX measure intensity, and PAR only designates which wavelength is useful to a particiualr species, LUX is a better measure of how much light is reaching a leaf surface.

The reason that WPG doesn't work is that it essentually assumes that the same amount of light energy reaches a leaf surface at 30 inches depth as 12 inches depth.

Cheers.
Jim


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## hoppycalif (Apr 7, 2005)

DelawareJim said:


> I'm not so sure here hoppy.
> 
> LUX is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. So yes, it is related to human eye perception but it is still a measurement of intensity.
> 
> ...


PAR is a measure of light intensity in the spectral areas of use to plants, so it is very similar to Lux, which measures the intensity as perceived by human eyes. Watts per gallon actually does work very well for reasonably average shaped aquariums. Of course it is of no use at all for scientific research, but for determining the appropriate amount of light for an aquarium it works well.


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## Gomer (Feb 2, 2004)

> PAR is a measure of light intensity in the spectral areas of use to plants, so it is very similar to Lux, which measures the intensity as perceived by human eyes.


(random thought with a basis solely on my lack of memory on the mathematical definition of par )

But since lux is weighted to the human eye which is very sensitive to green, and Par is weighted to the plant equivalent..which is weighted to red and blue, but not green, doesn't that contradict lux~PAR?


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## gpodio (Feb 4, 2004)

Hey Gomer!

My memory isn't all that great either, but if I recall this correctly, PAR is generally a measure of energy between 400-700nm, so green is indeed part of the equation I believe. But different plants react differently to different wavelengths, so in reality each plant may have a different 'effective' PAR requirement. Not that any of us grow one single plant so it's not that important to us...

I agree with Hoppy, WPG is sufficient for our needs. If all bulbs carried PAR or LUX ratings we would likely be using one of these instead. Until then, as long as one gives some bonus points to thinner tubes and higher frequency ballasts, we can compare fluorescents quite fairly by their power consumption...

Giancarlo


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## DelawareJim (Nov 15, 2005)

I'm confused.

My understanding of PAR is that it simply determines that the wavelength of a photon striking a leaf surface is greater than 400 nm and less than 700 nm and photosynthesis can take place.

How is that better than LUX? That's like saying car A is better than car B just because it's red.

Also, if WPG is so good, then why do the same plants in my 20 gal tank that only has 2 WPG grow so much better than in my 110 that has 4 WPG. Literally the only difference between the 2 tanks is that the 20 gallon plants are 15" from the light and the 110 gallon plants are 26" from the light.

The WPG logic says the opposite should be true. It doesn't account of depth.

Cheers.
Jim


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## gibmaker (Jan 3, 2007)

hoppycalif said:


> PAR is a measure of light intensity in the spectral areas of use to plants, so it is very similar to Lux, which measures the intensity as perceived by human eyes. Watts per gallon actually does work very well for reasonably average shaped aquariums. Of course it is of no use at all for scientific research, but for determining the appropriate amount of light for an aquarium it works well.


I agree watts per gal does work well I have a 125 gal with only compact flourescents totalling in 440 watts 3.52 watts per gal or something like that and I can grow anything with the addition of co2


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## hoppycalif (Apr 7, 2005)

DelawareJim said:


> I'm confused.
> 
> My understanding of PAR is that it simply determines that the wavelength of a photon striking a leaf surface is greater than 400 nm and less than 700 nm and photosynthesis can take place.
> 
> ...


PAR is a measurement of light intensity in the specific parts of the spectrum that are most usable by plants. It is comparable to Lux in that both are measurements of light intensity in a specific part of the spectrum.

I don't think any of us can say that any two tanks are exactly alike except for X. There are just too many variables. If we were to set up two 20 gallon tanks at the same time, both with the same substrate, both with the same number of the same plants from the same source, both using the same light fixture and bulbs, with the same carbon source, the same fertilizing schedule, and the same water source, then the two tanks are the same at least at the start. But, once you change anything, such as tank size, date they are set up, types of plants, sources of plants, etc. , the tanks are not the same.

It has to be incredibly difficult to do real science with planted tanks. Fortunately I don't even attempt that.


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## ruki (Jul 4, 2006)

I'm different I guess.

Watts per gallon is a rough approximation which is OK and usually works OK. What don't like about it is that it's misleading because people think it's measuring efficiency like miles per gallon does. Then people start treating it like something "firm" or a "rule" which I find incredibly stupid.

Different tubes have different efficiencies, so WPG is like measuring auto efficiency by the engine cylinder count instead of the actual fuel consumed.

Lumens is alot better because it's actually measuring light coming from the tube. It also happens to be included on almost every bulb you can find. The bad thing is that it includes light frequencies not really desirable to aquariums. It also ignores reflector efficiency.

Unfortunately, PAR isn't perfect either, since it includes green light

A light meter is nice because you are actually measuring something real. For example, this should help compare reflectors against each other. It may take alot of measurements to start to make sense of things. We've been misled by aquarium fixture vendors and sellers into asuming some things that are not really true


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## hoppycalif (Apr 7, 2005)

I agree with you about using a light meter for evaluating reflectors, or even for evaluating how much light reaches the bottom of the tank vs the top of the tank. That is a usable method for doing some "science" with aquarium equipment.


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## defdac (May 10, 2004)

I've made a PUR-efficiency calculator so I know the inner workings how to take lumens/lux and convert it to PAR and then PUR and then PUR-efficiency.

Lumens/Lux is pretty much as useful as PAR since the photooptic curve (human eye sensivity) output is between 400 and 700 nm, with the most of the photons emitted in the green-yellow range.

Lumens/Lux measurements will follow PAR-measurments closely for the common bulbs we use since most bulbs have some yellow/green output. The one time it will fail is if you, for example, try to measure red and blue LED-lighting. They don't emit anything in the photooptic curve hence have no lumens/lux-measurement - but is anyway very effective in growing plants.

High lumens bulbs will have high PAR. High PAR-bulbs doesn't necessarily have high lumens. 

Problems with lux/lumens-measurements:

1) Red and blue LED:s and tri-phosphor bulbs and bulbs with high blue output will have sucky lumens/lux-readings but trigger photosynthesis very hard.
2) Water filter out blue light the least, so even if the lumens/lux meter says a very low amount of yellow/green down by the substrate - the plants might still have good amount of blue for photosynthesis.


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