Understanding Lumens

[lukass18]

I have been trying to get to the bottom of how lumens and light intensity work. As most of you probably know already, one lumen is one candle's worth of light per square foot of surface when that candle is held one foot away.

In a high times article, they say "while lumens are a way to measure light energy leaving the source or bulb, lux is a term generally reserved for indicating the amount of light energy actually reaching a given surface or leaf (and this is also known as illumination)"

Then there is the inverse-square law which simply says the intensity of light from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source so an object twice as far away, receives only 1⁄4 the energy (in the same time period)

With that said, if a 42 watt (true watts) cfl is rated at 2,600 lumens at one foot away, wouldn't the inverse-square law still be in effect when it gets closer? The law states that a cfl 2 feet away would emit only 650 lumens. So wouldn't a cfl one half inch away from the plant be about 10,000 lumens?

Another interesting thing from the article, "a light bulb shines 360 degrees around. If your HPS puts out 140,000 lumens, that's 140,000 lumens in all directions. If you manage to get 10 percent of that on your garden, that's only 14,000 lumens. Then consider the distance of you lamp and the Inverse Square Law, you may only be getting 2,000 of your 140,000 lumens onto your garden"

 

[Hosebomber]

Okay, there are a lot of questions in there and a few statements which may or may not be questions. First and foremost, a lumen is the measurement of brightness to the human eye. This does not directly correlate to the radiant flux or amount of overall energy put out by the source.

The article is semi-correct. Lux is the number of lumens received at point destination (the surface) in a square meter... (they left out that part).

Yes technically at ~ 1 inch you would receive ~ 10000 lumens from a 42 watt CFL.

While what they say is mathematically correct, it is not very realistic. Depending on the reflector you have, generally speaking ~30% of the photon output will be directed in the area of your grow straight from the bulb. The next 20% or so will be refracted once then directed at the grow area (losing between 8 and 25% efficiency depending on the hood), the remaining light will be reflected at least twice and may be directed back into the point source (the bulb). Of that light you will receive ~60% intensity to your grow surface. So worst case scenario you would receive over 20,000 lumens at the plant.

I cannot stress enough, people that are still using lumens to refer to the amount of light a plant needs to grow or how much light their plant is receiving is either uninformed or have very little knowledge on how plants react to photons and convert that to food, energy, growth, and reproduction.

 

[JJ Bones]

I have a question Hosebomber, not to high-jack the thread or anything.

Why is it that 1000w always beats LED? I've never known why, I understand lumens isn't the make-all, be-all however can it account for bulb intensity? The only thing I've ever come up with is that the 1000w is more intense.

I've run 1000w vs 800w of LED and the 1000w yielded about 8 oz more, could the 200w difference be that big of a deal?

 

[Hosebomber]

There are a lot of different factors. Intensity (or penetration as some call it) is one of those for sure. Another factor is most likely the wavelengths produced and in what ratio. I'm not sure what LED you was using, but I have gotten better results with LED. I most confess that I have not used a 1000 watt HPS yet as my grow space doesn't really allow me to. When I test panels it is usually 100 watts vs a 250 HPS or 200 watts vs a 400 watt HPS. Generally my panels are 50 watt panels with different ratios in each.

 

[Fuzzy Duck]

CFLs suffer far worse with the inverse square law of light effect... optimal distance is aprox 1' to 2' above plant growing tip for best effect anything greater than 6' with a CFL is rather pointless.

HPS/MH are not so bad but the bulb size is a hinderence as the larger the bulb in wattage the greater the distance of hanging height is required to prevent heat stress/burn from bulb heat...

This is why air cooled reflectors/hoods are so much better as you can get the light source closer to your plant when using HPS/MH making it more effecient & in theory a better yield.

 

[Kub’s Kush]

I have been trying to get to the bottom of how lumens and light intensity work. As most of you probably know already, one lumen is one candle's worth of light per square foot of surface when that candle is held one foot away.

In a high times article, they say "while lumens are a way to measure light energy leaving the source or bulb, lux is a term generally reserved for indicating the amount of light energy actually reaching a given surface or leaf (and this is also known as illumination)"

Then there is the inverse-square law which simply says the intensity of light from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source so an object twice as far away, receives only 1⁄4 the energy (in the same time period)

With that said, if a 42 watt (true watts) cfl is rated at 2,600 lumens at one foot away, wouldn't the inverse-square law still be in effect when it gets closer? The law states that a cfl 2 feet away would emit only 650 lumens. So wouldn't a cfl one half inch away from the plant be about 10,000 lumens?

Another interesting thing from the article, "a light bulb shines 360 degrees around. If your HPS puts out 140,000 lumens, that's 140,000 lumens in all directions. If you manage to get 10 percent of that on your garden, that's only 14,000 lumens. Then consider the distance of you lamp and the Inverse Square Law, you may only be getting 2,000 of your 140,000 lumens onto your

 

[Kub’s Kush]

I have been trying to get to the bottom of how lumens and light intensity work. As most of you probably know already, one lumen is one candle's worth of light per square foot of surface when that candle is held one foot away.

In a high times article, they say "while lumens are a way to measure light energy leaving the source or bulb, lux is a term generally reserved for indicating the amount of light energy actually reaching a given surface or leaf (and this is also known as illumination)"

Then there is the inverse-square law which simply says the intensity of light from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source so an object twice as far away, receives only 1⁄4 the energy (in the same time period)

With that said, if a 42 watt (true watts) cfl is rated at 2,600 lumens at one foot away, wouldn't the inverse-square law still be in effect when it gets closer? The law states that a cfl 2 feet away would emit only 650 lumens. So wouldn't a cfl one half inch away from the plant be about 10,000 lumens?

Another interesting thing from the article, "a light bulb shines 360 degrees around. If your HPS puts out 140,000 lumens, that's 140,000 lumens in all directions. If you manage to get 10 percent of that on your garden, that's only 14,000 lumens. Then consider the distance of you lamp and the Inverse Square Law, you may only be getting 2,000 of your 140,000 lumens onto your garden"

 

[Mayne]

BRAH