# Watts vs Lumens



## IllusionalFate (May 9, 2008)

Why is the effectiveness of grow lights measured in lumens (luminous flux)? Lumens is the measure of how strong light appears to the human eye, and watts measures the total amount of light emitted (radiant flux). The absence of human eyes on cannabis plants would lead me to believe that watts are a more accurate unit of determining a light's effectiveness for growing. Could anyone with more knowledge shed some more light (no pun intended) on this subject?


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## nickname (May 9, 2008)

watts is just a rating of power, lumens are relative food candles as they appear to the eye.


the way to measure for plants is called par lumens or par watts


Since plants use energy between 400 and 700 nanometers and light in this region is called Photosynthetically Active Radiation or PAR, we could measure the total amount of energy emitted per second in this region and call it PAR watts. This is an objective measure in contrast to lumens which is a subjective measure since it is based on the response of the subjects (humans). PAR watts directly indicates how much light energy is available for plants to use in photosynthesis.


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## lyfr (May 9, 2008)

nickname said:
			
		

> watts is just a rating of power, lumens are relative food candles as they appear to the eye.
> 
> 
> the way to measure for plants is called par lumens or par watts
> ...


 now thats an explanation!


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## IllusionalFate (May 9, 2008)

nickname said:
			
		

> watts is just a rating of power, lumens are relative food candles as they appear to the eye.
> 
> 
> the way to measure for plants is called par lumens or par watts
> ...


 Seeing how lights don't give you the PAR watt rating and only the entire spectrum of light energy emitted, or total watts, the only way we can judge how much light a light puts out is the total lumens or total watts. Although both innaccurate for growing plants as they tell you the entire spectrum of light energy emitted instead of PAR, wouldn't watts be the less innaccurate unit of the two? Lumens seems completely irrelevant as it deals with the human eye's sensitivity to light, at least watts tells you the amount of light energy there is.


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## nickname (May 9, 2008)

what you want to look at in terms of bulb is the lumens per watt efficiency, and also to look at the cri, the color rendering index, hps bulbs only have a cri of 20-30 which actually makes them not the best choice for growing.  A marijuana plant is highly efficient at processing light in the red spectrum, the plant needs more of the spectrum though to reach its true potential.

An enhanced halide bulb has a cri of around 80+ there is a new bulb coming out soon called a high pressure metal halide that is supposed to have a cri of around 90+,

I'll have a look for an article for you then i will come back here with a link


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## nickname (May 9, 2008)

or you can check here where rbh has kindly retyped the whole thing

http://www.marijuanapassion.com/forum/showthread.php?t=25637


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## Puffin Afatty (May 9, 2008)

IllusionalFate said:
			
		

> Why is the effectiveness of grow lights measured in lumens (luminous flux)? Lumens is the measure of how strong light appears to the human eye, and watts measures the total amount of light emitted (radiant flux). The absence of human eyes on cannabis plants would lead me to believe that watts are a more accurate unit of determining a light's effectiveness for growing. Could anyone with more knowledge shed some more light (no pun intended) on this subject?


 
 _Lux is apparently the measure of the penetration of the light.  this is by far a more useful measure to growers.  also, the specific spectrum of light emitted, matched to the specific needs of marijuana.  Also, check into UV B, often neglected in growing indoors._:hubba:


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## liermam (May 9, 2008)

Luminous flux is the measure of light emitted from the bulb, measured in the units lumen (lm)

Illuminance is the measure of light falling upon a surface, taking into account the distance between surface and light. This is measured in Lux (lx), and can be derived simply by using the following formula:

E = P / (4 * Pi * d^2)


Where P is lumen output, and E is illuminance, and D is distance. 

With this, certain lights might be more effecient for certain setups, assuming you can put the lower wattage light closer.

There is also the measurement of candela, which is a bit harder to explain. Its the measurement of intensity of light. I'm not really sure I can explain it well enough.

But what I can say is Watts are not a measurement of light at all. Lumens are undeniably better for judging total light output. Maybe not the best, but definetely better than watts.


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## IllusionalFate (May 9, 2008)

So wikipedia is incorrect on this then?

From: http://en.wikipedia.org/wiki/Luminous_flux


> In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.



This suggests that lumens only refer to the sensitivity of light to the human eye, and does not measure the amount of actual light emitted. It says radiant flux is "the measure of the total power of light emitted" and that the SI measurement for radiant flux is watts.

From: http://en.wikipedia.org/wiki/Radiant_flux


> The SI unit of radiant flux is the watt (W), which has dimensions of energy/time or, in SI units joules/second.


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## nickname (May 9, 2008)

who really cares, just get a bulb mang


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