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Calculating Foot-Candles


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#1 omentheduck

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Posted 14 April 2008 - 01:14 AM

*Charts below.

This is a real easy way to find the number of Lumens Per Square Foot (Foot-Candles) you are getting from your bulb at any distance. It’s the same value you’d get using the more complicated formulas below, but is real quick and simple.
  • Multiply the lumen output of the bulb by 45.84.
  • Divide by the number of inches from the bulb squared.
It’s that simple.

This would be based on the use of a typical 120° reflector. If the reflector cast a different angle, then we would need to manipulate the formula a bit as is shown below. It’s all based on spherical geometry.


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:
Foot-Candles = Lumens ÷ (4 × Pi × d²)

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

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

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:
Foot-Candles = Lumens ÷ (2 × Pi × d² × (1 - cos(A/2)))

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

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

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.

http://mycotopia.net...=1&d=1208153285

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.





I made the first graph, which was the Foot-Candles for an HPS bulb, on August 5, 2004. I could never find accurate graphs for PAR lumens, so I just made them myself. The graphs are based on pure geometry and physics.

To make the graph, you first need to calculate the number of Foot-Candles that you’d get from any distance from the bulb. This is shown above.

After calculating the number of Foot-Candles, a conversion is used to find the number of micromoles per square meter per second (umol/m²/s) of irradiance within the frequencies 400nm – 700nm (PAR).

The conversions are as follows:
The conversion is a multiplier.

High Pressure Sodium - 0.131
Metal Halide - 0.152
Sun – 0.199

After finding the PAR irradiance I just compared it to the 1,990 umol/m²/s we get from the sun by dividing by that value and showing the values as a percentage.


None of this is mine nor did I alter anything! This is just something I think that might help It has help me and i wanted to share!
all credit must go to caligrower

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#2 golly

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Posted 14 April 2008 - 09:22 AM

Thanx for posting, i like the graphs...
Which would produce more Bud, four 250s or one 1000w

A single 1000w has more lumens but from a single point ...
Four 250s can spread to cover more area...
What do u think...?

#3 spesh1

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Posted 14 April 2008 - 12:59 PM

Thanx for posting, i like the graphs...
Which would produce more Bud, four 250s or one 1000w

A single 1000w has more lumens but from a single point ...
Four 250s can spread to cover more area...
What do u think...?


To my best understanding 2 600 watters would crank out the best results with lumen per watt ratio.

#4 omentheduck

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Posted 14 April 2008 - 01:56 PM

Spesh is right ,600 watters give better lumen to watt out-put.

np on postoing it, :rasta:

#5 omentheduck

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Posted 16 January 2009 - 02:57 AM

and i hate to bump my own post but this is good info.! bump:weedpoke:

#6 growDaddy

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Posted 16 January 2009 - 07:49 AM

so according to this formula an HPS 1 foot from the plant gives less than 1/3 of listed lumens?
150w 16000 = 5093

so is this saying that a 150w HPS should cover like 3 sq ft?

#7 seven rays

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Posted 16 January 2009 - 08:47 AM

Cool stuff, omen.....

I think 4- 250's will still cover the same ground as the 1000. In other words, 1000 watts will cover 20 sq ft of area with 50 watts per sq ft of light no matter wheter it's 4-250's or the single lamp.

The real difference is shown in omen's examples above. A 1000 will penetrate the canopy and shine farther into the plant...meaning you can grow taller plants and the bottom will still recieve semi-adequate light.

With the 250's, the intensity each lamp puts out is far less...and the light will only shine as few inches into the plant before severely diminishing in intensity. 250's are great for growing SHORT plants...and a 1000 will allow you to grow larger plants.

While I love info like this...I use the 50 watts per sq ft or better general rule for ease of operation.

That's my take.... (BTW, I run mostly 600's)

good luck all

s r

#8 omentheduck

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Posted 18 January 2009 - 12:28 PM

seven seems to have it by the balls here . lol,
and np seven, happy the info could help someone,
I am running a 83 watts per sq foot and the buds seemed to pack together over night! I was only expecting about 65, tehehe,
I wish i could throw 2 400watters in there tho! damn I just thought about the heat nm, I am having trouble battling 250's lol i don't hink i co do 400's.

#9 rockawayrooms

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Posted 18 January 2009 - 12:34 PM

Awesome post bro,THX fer sharing this!!:thumbup:

#10 greenie

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Posted 18 January 2009 - 01:48 PM

excellent and helpful information, also know that all hoods are NOT created equally and you should NOT buy a cheap one. your light is very expensive, and look how much a good hood can do for your light output? A crappy one will not do that.

#11 omentheduck

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Posted 20 January 2009 - 01:12 AM

Np guys, just spreading the info!

excellent and helpful information, also know that all hoods are NOT created equally and you should NOT buy a cheap one. your light is very expensive, and look how much a good hood can do for your light output? A crappy one will not do that.


and that is very true and most of the time with cheaper hoods you have parts that are ok but when operating together the are not up to codes,
plues they put off a god awful loud hum!!!

#12 omentheduck

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Posted 26 January 2009 - 02:35 AM

Also i should add that hps do not emit enough blue spectrum so adding cfls or a metal halide will help with stretching between nodes.

#13 omentheduck

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Posted 14 January 2011 - 11:17 AM

Bump for readers

#14 omentheduck

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Posted 29 May 2012 - 09:13 AM

Bump for readers


#15 BuckarooBanzai

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Posted 01 June 2012 - 11:35 PM

Good bump. Great thread.

archive material

#16 Dipole

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Posted 02 June 2012 - 10:17 PM

Do you have the conversion to micromoles for CFLs?




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