Quote Originally Posted by 420NLGuy
Yes, light dissipates the farther you are from the source. Now we agree on that. Therefore the opposite is also true: light increases in intensity the closer you get to the source. Do we agree on that? If not, please tell me why you disagree.

There are many links to show this. Here are a few:

Inverse-Square law for light
Inverse Square Law for Light

Intensity: The Inverse-Square Law
Intensity: the Inverse Square Law

Overview of Inverse Square Law of Lighting
Overview of Inverse Square Law of Lighting - AFB Senior Site - American Foundation for the Blind

CFL: Calculations and Room Design
CFL: Calculations and Room Design - Marijuana Growing

Can you tell me how many lumens/sq. ft. that a plant needs in flowering? I've read in many forums that it should be at least 30,000 lumens/sq. METER. That converts to 2800 lumens/sq. FOOT. That is the number I used to calculate the distance.

I also made a mistake in the distance. I said it was 2.6", but it really is 9". If you hold a 1560 lumen bulb 12" away, you get 1560 lumens. If you hold it 9" away (3" closer), you get 2800 lumens.

Here is the corrected calculations:
[attachment=o226702]

Notice that I have been putting my bulbs 2" away. That close results in a huge increase in lumens: 56,000 lumens at 2" away.

Don't believe it? Here is a screenshot of a little spreadsheet that calculates the lumens at various distances.

[attachment=o226703]

Note that the lumens at 12" is whatever the rating of the bulb is. That's because the lumen rating of all bulbs are measured at 12". So moving it closer gets you more lumens. Much closer gets you much more lumens. It's the inverse-square law in action.

The formula to calculate the lumens from a certain distance is:
L = (Lumen rating)/[Distance/12]^2

So if your lumen rating is 1560, then at 2" away, the lumens at the plant is: 1560/[2/12]^2 = 1560/[0.167]^2 = 1560/0.028 = 55,936 lumens.

At 12" away, it should equal the bulb's rating. So:
1560/[12/12]^2 = 1560/[1]^2 = 1560/1 = 1560.

PS: I'm not smoking anything when I write this. :rasta:

we must all be morons and you are a freaking genius!

"a foot-candle is how bright the light is one foot away from the source. A lumen is a way of measuring how much light gets to what you want to light! A LUMEN is equal to one foot-candle falling on one square foot of area.

"RADIANCE is another way of saying how much energy is released from that light source. Again, you measure it at the source."

"Illuminance is the intensity or degree to which something is illuminated and is therefore not the amount of light produced by the light source. This is measured in foot-candles again! And when people talk about LUX, it's illuminance measured in metric units rather than English units of measure. To reinforce that, LUX is the measurement of actual light available at a given distance. A lux equals one lumen incident per square meter of illuminated surface area. They're measuring the same thing, just using different measurement units.

"Nowadays we use the term CANDELA instead of candlepower. Candlepower, or CANDELA is a measure of how much light the bulb produces, measured at the bulb, rather than how much falls upon the thing you want to light up."

"But here's where it gets tricky. The further away you move the light from what you want to illuminate, the less bright the light seems! If you measure it at the light, it's just as bright. But when you measure at the object you want illuminated, there is less light! A Physics teacher is going to tell you that light measured on an object is INVERSELY PROPORTIONAL to the distance the object is from the light source. That's a very scientific and math rich way of saying, the closer you are to the light bulb, the brighter that bulb is. Or, think of it this way. You can't change how much light comes out of your light bulb. So, to make more light on an object, you have to either move the light closer, or add more lights"


you can find out more here.


"Wattage is a measure of electricity consumption, that's all. Technically wattage has nothing to do with light (how much light does a 1000 watt hair dryer give off?), however if you are talking about wattage of bulbs, and the ratio of the amount of light given off by a certain type of bulb to the wattage it consumes is somewhat predictable, then sometimes we talk about wattage as if we were talking about light intensity.

Lumens is a measure of light intensity at a fixed distance of one foot on a fixed area of one square foot. Furthermore, lumens measures only certain (visible to humans) frequencies of light. But again, the relationship is somewhat linear so we often use the term lumens when talking about light intenstity. Lumens is getting closer to the concept, but in reality lumens is a standardized measure for comparing bulbs one to the other, not for measuring how much light is being given off.

For example, take a bulb rated at 10,000 lumens and move it half the distance closer to the plant, and what happens to the lumens? Nothing -- it's still a bulb rated at 10,000 lumens. But as we know that light intensity is inversely related to the square of the distance, moving the light to half the distance increases the light intensity four-fold."


-shake