Foot-candle is a unit of measurement used to indicate the intensity of light that illuminates a given area, which is also known for lighting. A foot-candle is the intensity of the light source 1-candles with a distance of one foot. Foot-candle is calculated by taking into account the source of electricity, also known as luminance, and the distance from the lit. This unit is normally used only in the U.S. and elsewhere has been replaced by the SI unit of measurement of 'lux' is equal to one lumen per square meter.

you can read what he copied and pasted and changed a few letters from here (always good to go the source you know and give credit where it is due)

http://www.ehow.com/how_6899966_calculate-foot-candle.html

This is the longer detailed formulas for calculating the number of Foot-Candles (Lumens/ft.²) coming from a bulb:

The divisor must be equal to the actual area that is being illuminated by the bulb, which is not based on your floor plans. The divisor in Formula A is calculating a spherical propagation of light, which is what the light is really illuminating.

Formula A:
[color:"000099"]Foot-Candles = Lumens ÷ (4 × Pi × d²)[/color]

Where:
d = The distance from the bulb in feet.
Pi = 3.14159

If you wanted to know how many Foot-Candles of light are striking your plant 10" away from a 400 watt HPS that puts out 50,000 lumens you would do this:

Foot-Candles = 50000 ÷ (4 × Pi × (10/12)²)
Foot-Candles = 50000 ÷ (12.57 × 0.6944)
Foot-Candles = 50000 ÷ ( 8.729)
Foot-Candles = 5,728

[color:"990000"]So, there are 5,728 Foot-Candles of light hitting your plant 10" from a 400 watt HPS with no reflector. [/color]

Now, because we are using reflectors it will actually be more. Let's calculate how many Foot-Candles hit your plant 10" away with a typical 120° reflector.

Formula B:
[color:"000099"]Foot-Candles = Lumens ÷ (2 × Pi × d² × (1 - cos(A/2)))[/color]

Where:
A = The degree of the reflector. (Most of us have 120° reflectors)
d = The distance from the bulb in feet.
Pi = 3.14159

To make this easier we already know the value of d² when the distance is 10" from the bulb. That value as calculated above is 0.6944.

Foot-Candles = 50000 ÷ ( 2 Pi × d² (1 - cos(120/2)))
Foot-Candles = 50000 ÷ ( 2 Pi × d² (1 - cos(60)))
Foot-Candles = 50000 ÷ ( 2 Pi × d² (1 - 0.5))
Foot-Candles = 50000 ÷ ( 6.283 × 0.6944 × 0.5)
Foot-Candles = 50000 ÷ (2.181)
Foot-Candles = 22,918

[color:"990000"]So, you would get 22,918 Foot-Candles (Lumens/ft.²) of light 10" away from a 400 watt HPS using a 120° reflector. [/color]

The picture below is a Blaze main cola that is 16" long. It is 10" from a 1,000 watt HPS. This HPS puts out 140,000 lumens. The illuminance in the units of Foot-Candles (Lumens/ft.²) are marked and labeled with the distance in inches from the bulb. You'll see the bottom of the cola is getting the illuminance of equatorial sun.



Disclaimer: This post was intended to simply show the wide range of illuminance that indoor plants endure when using lights. The typical rules of thumb are still the best way to calculate your lighting needs. Here are a few:

*Using 60 - 100 watts per square foot of HID lighting will provide enough light to bring out the best in the genetics that you're growing, provided that you don’t grow the plants over 3 feet tall. If you do it’s not a problem, you just won’t get bud at the bottom of the plants.

*Keep lights close to the plants to provide penetration to lower branches.

*Air cool reflectors to dissipate the heat so that the only concern you'll have on the distance of your light to the top of the plants will be the amount of light.

*Don't put 400 watt HPS lights any closer than 7 inches from the top of the plants regardless of temperature.

*Don't put 1,000 watt HPS lights any closer than 12 inches from the top of the plants regardless of temperature.

In other words, my plant above is too close to the light and you can see how it’s turning color at the very top.
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Good job! Your post is very meaningful to me and gives me some ideas. I'd better rearrange my schedule. Thanks very much.
Have a good day.

Its really helpfull to know that your lumens loose 50% strength for every 4 inches... good stuff, Cali needs to get off his damn surf board and get back on the boards!!!

Inverse square law, you should have learned that in HS>

Respect me or get bent fool, you can learn that in the school of hard knocks...lol,

JK, i see you live in my city, this isnt the mike I know from skunk fourums is it??

no its not rich this mike lives in las vegas now

Oh, I thought D-Mike was screwwing with me..lol, good to hear from you Robman.. hows the ladies comming along? What do you think about that chocolate I sent ya?

A question, I've been pondering for a while, for you to consider:

Background:

Most of the information below is from the "Marijuana Horticulture, The Indoor/Outdoor Medical Grower's Bible", 2006 edition, chapter 9 "Light, Lamps & Electricity" and I'm using "initial" lumens ratings for everything. Initial lumens are when the bulb is new and will decrease over time, not a measurement at the source as some have suggested. Lumens are what the plant "sees" and includes light that we cannot see.

Let's say that a one thousand watt HPS puts out one hundred thousand lumens (probably low, but it keeps things simple).

By definition lumen ratings are based or taken one foot from the source so (according to that inverse square law thingy), at twice that distance, two feet from the source, the lumens drop by three quarters, (one divided by two squared equals one quarter) to twenty five thousand lumens. At three times the distance, three feet, from the source, the lumens drop by eight ninths, (one divided by three squared equals one ninth) lumens for a total of eleven thousand one hundred eleven lumens, etc.

Now for my question, what happens when you move closer to, rather than further away from, the source?

Let's say I have eighteen, sixty-five watt CFL bulbs (total eleven hundred seventy watts going in, slightly more than the example above) rated at thirty six hundred lumen each, for a total of sixty four thousand eight hundred lumen.

Now, if I move these lights to six inches from the plants (one half the distance at which lumen measurements are taken), would the formula then be the same only with a distance of one half instead of doubled?

Due to the definition of a lumen, I cannot decrease the distance between the source and receiver below one foot and still call it a lumen. Or can I?

In this case the math indicates that at six inches(1/(.5 X .5)) X lumen rating goes up by four times. If calculated on a single bulb (lumen are based on single point sources), thirty six hundred lumens times four equals fourteen thousand four hundred "perceived by the plant" lumens. And with eighteen point sources that equals two hundred fifty nine thousand two hundred "perceived by the plants" lumens.

Wow, I gotta be wrong somewhere, how about it guys?

Let's see what happens (if my assumptions are correct) when the bulb is three inches (1/4D) from the plant... Wow again, sixteen times thirty six hundred lumens equals fifty seven thousand six hundred "perceived by the plant" lumens. And with eighteen point sources that equals one million thirty six thousand eight hundred "perceived by the plants" lumens.

What an I doing wrong here? Anyone?

I have six height adjustable rows of three bulbs surrounding five rows of two plants in a two foot by six foot closet. Recent temperatures are eighty two degrees in the canopy and seventy three degrees at the same distance from the floor outside the closet, using passive air intake (full louvered doors with three quarters at the top closed off) and a small fan in the ceiling (mostly to flush the odor).

From the first three plants, the largest bud/cola was nine inches around and twelve inches long. The second largest was very close to that.

I await any help on this subject as I'm growing, just not knowing. So many changes in lighting methods since my "bible" was published...

This thread is invaluable and has answered a few of my questions. Experimentation with the bulbs I have may solve the other queries. Thank you for posting this thread!