Lots of people have been asking about their light setups so I thought I'd post how to find out exactly how much light your plants are getting. This only bothers with amounts of light, not the color temperature and index, so remember to get the right ones! This doesn't factor in everything so it is just a way to get a very close approximation
a. The power of your bulbs. This is represented as P and is measure in Lumens. Lumens measure the intensity of light.
It should say on the box your lights came in how many Lumens your light emits. This measurement is taken from the surface of the bulb.
b. The distance between the light and the plants. Your plant lights are most likely going to emit light sphericaly so this distance is actually the radius of a circle. Because this circle only touches one point on the entire surface area of the plants, and you most likely have more than one light you have to deal with this by averaging the distances your lights are apart, as if the all the light is coming from one source.
c. The surface area you are lighting. A=πr²
2. The Math
Now for some math and physics. We want to find out the amount of lumens per square meter that the plants are getting. To factor in the amount of light lost from traveling to your plants you need to calculate its new intensity. For this we use the equation for intensity of electromagnetic waves, which is E=P/(4πr²). E is the intensity you want to find out. P is your total amount of Lumens added from each bulb and r is the distance (in meters) between your average 'light' and straight down to the center of the surface area of your plants. Your Intensity, E, will be in Lumens per square meter. This unit is known as luminous flux or lux for short.
Here is a chart of lux values from This website for you to compare your own to:
Outdoor Illuminance (lux)
Bright sun 50K - 100K
Hazy day 25K - 50K
Cloudy bright 10K - 25K
Cloudy dull 2K - 10K
Very dull 100 - 2K
Sunset 1 - 100
Full moon 0.01 - 0.1
Starlight 0.001 - 0.001
3. Light Loss
unnecessary, but interesting to know. To find the amount of the light lost traveling to the surface area of your plants, or change in P, you take your initial sum of Lumens and subtract the final sum of Lumens. To find your final sum of Lumens take the area of the plants you are lighting and multiply it by the intensity.
USE MYLAR AND A REFLECTOR! This will greatly increase your light amount! For each reflective surface find the distance between the light source to the center of the the reflective surface and to average center spot on your plants. Mylar reflects somewhere near 90% of light so calculate the light intensity reflected by each surface onto your plants and multiply them by .9. Now take your orignal Intensity and add each of the reflective surface's intensity's to it for a new E.
5. Light Gain
After finding your new intensity because of reflective surfaces, use it to find your new change in P. To do so take your new Intensity and multiply it by the area you are lighting then subtract this new number from your P initial. Negative means you gained light and positive means your still losing light but probaly a lot less than before!
This is probaly still a little hard to understand or do so I'll take you through it by calculating the light in my new closet growshelf thats in the other thread. Hope this helps:
a. I have 4 Philips fluorescent tubes that are 2250 Lumens each. Adding them all together gives me a total of 9000 Lumens.
b. Distance from the center of these lights straight down to the surface of my growing area is about 8". 8" is about .2 meters, so my r is .2 meters.
c. The Surface area the light is going to at an 8" radius is A = π(.2m)² = .13m²
E = (9000 lm)/[4π(.2m)²] = (9000 lm)/(.5m²) = 18,000 lm/m² = 18,000 lux
This is equivilant to a very bright cloudy day.
3. LIGHT LOSS
P1 - P2 = (9000 lm) - [(18,000 lm/m²)(.13m²)] = (9000 lm) - (2,340 lm) = 6760 lm
I don't have mylar yet but I plan on it. With 3 reflective surfaces I can measure the distance the light goes by starting at the light source and measuring the distance to each reflective surface then the distance to the spot below the light where the plants are. Surfaces 1 and 2, the left and right sides, are both 8" high. the distance from these walls to the center spot is 19.5". Drawing my ray of light gives me two symetrical triangles that I need to find the length of hypotenuse's for. Using Pythagoreans theory of A² + B² = C² and solving for C (the hypotnuse) and then doubling it will give me a distance, r. 2C = 2[(A² + B²)^(1/2)] = r. Converting my inches to meters and plugging them in I get [(.5m)² + (.1)²]^(1/2) = (.25m² + .01m²)^(1/2) = .26m²^(1/2) = .5m. My third surface, the back, is 8" high and 9" away from the center. Repeating the triangle process I get .3m
5. LIGHT GAIN
Starting with the two surfaces where the light bounces at the plants from a distance of r = .5m I plug this into the intensity equation. E1&2 = (9000 lm)/[4π(.5m)²] = (9000 lm)/(3.14m²) = 2866.24 lux. Then I do the last surface E3 = (9000 lm)/[4π(.3m)²] = (9000 lm)/(1.13m²) = 7964 lux. Adding all of the light intensitys gives us the final total. E = Eo + E1 + E2 + E3 = (18,000 lux) + (2866.24 lux) + (2866.24 lux) + (7964 lux) = 31968 lux. My cloudy bright day just went to an slightly hazy sunny day.
To see how much light was lost with the reflective surfaces I subtract the new P final from P initial. P = P1 - P2 = (9000 lm) - [(31968 lux)(.13m²)] = (9,000 lm) - (4120.5 lm) = 3879.5 lm
as you can see the light loss for reflective surfaces is half of what it would be with no reflective surfaces. When you add a reflector right next to the bulb you can do some serious damage
Hope this helps, please correct any errors if you can because I know there are much more accurate and complex ways to do it! What are your guys' light conditions?