Well it seems apropreate to post this here so here goes
As I mentioned earlier, the most important tasks facing the indoor gardener are copying and improving upon nature. Okay, now that you are a god, you have to make a sun. Poof! Are you done yet? Probably not.
TERMINOLOGY
There are four basic terms that are used to describe light and how it affects your plants. These terms are Lumen, Foot-Candle, Watt, and Lumens per Watt.
The term "Lumen" is the basic unit of light. If light were tangible and you could somehow grab it in your hands, the amount of light that you would be holding would be described as lumens. If you took that handful of light and stuffed it into a sandwich bag, or let it loose in a dark room, the amount of lumens would be the same. Lumens do not decrease or increase as that light you were holding condenses into a smaller space, or expands to fill a room. For that particular amount of light, regardless of the space it is filling, the amount of lumens remains constant.
The term "Foot Candle" measures the amount of light intensity, or how much light you manage to shine on a given area. The foot-candle is based on how many lumens of light you distribute on a given area, which are measured in square feet. To illustrate this, if you shone one lumen of light on one square foot of space, you have achieved one-foot candle of light. You can use this formula to measure the foot candle output of any bulb. What if you were operating a 1000 watt Metal Halide bulb over a garden which measured five feet by five feet? Well, first you need to find out the amount of lumens that your bulb produces. The packaging that your bulb came in should inform you of this. A 1000 watt metal halide bulb produces 120,000 limens. The next step you need to perform is to find out the square footage of your area. The area we are using for this example is five feet by five feet, so we know that our square footage is 25' (Length (5') x width (5') = square footage (25 square feet)). Now we have to divide the amount of lumens (120,000) by the amount of square feet (25). Doing this, we come up with 4,800 foot candles. The amount of light intensity or foot candles that you produce is the most important measurement of light to a hydroponic gardener because it tells you how much light is available for your plants to "drink".
The "Watt" is probably a term that you are already familiar with. It is a common term used to measure the amount of energy a bulb requires electrically.
"Lumens per Watt" is another term, like the Watt, which is applied strictly to artificial lighting, whereas Lumen and Foot Candle can also be applied to natural light. This term refers to how many lumens of light that a bulb can produce per watt of electrical usage. The higher the ratio, the more efficient and economical your lighting system will be. Ideally, you would like to create the desired amount of lumens with the lowest wattage cost possible.
SO HOW DOES MY LIGHT COMPARE TO THE SUN?
Well, direct outside daylight in the summertime averages somewhere in the neighbourhood of 10,000 foot candles. This is equal to 10,000 lumens per square foot. If you compare this to the output of our thousand watt metal halide bulb earlier, we can see that the sun does a pretty good job.
However, the sun is subject to annoying little changes in weather that our artificial lighting is not. On an overcast day, the amount of lumens that the sun produces drops to about 1,000. If you were standing under a large, tall tree, the amount of light falling on your head is referred to as open shade. Open shade produces about 300 foot candles. In the deep shade, you would be experiencing around 50 to 100 foot candles, and under the light of a full moon you would be subjected to .02 foot candles.
So, artificial light is not subject to the interruption of inclement weather, and we can run our light for the optimum period of 12 to 18 hours per day on our plants, whereas the sun only produces six or seven hours of useable light.
WHAT LIGHT SHOULD I USE FOR MY PLANTS?
There are quite a few choices of lighting for the hydroponic gardener. However, not all lights are acceptable, even though by looking at them you might think that they are bright enough.
Following is a list of different bulbs that are suitable and unsuitable for use in your hydroponic garden.
INCANDESCENT BULBS
A regular household incandescent bulb is not very efficient. It only produced somewhere in the range of four lumens per watt. This means that if you were using a 100 watt incandescent bulb, you would only be producing 400 lumens. So, if we could reflect all of the light into a one square foot area, we would have achieved 400 foot candles. However, for this example, and all of those to follow, we will assume that we are only reflecting 75% of the light that our bulbs are producing. Even with reflective material and reflectors, some light is going to be "spilled" where it is not needed. So, our 100 watt incandescent bulb is only shining 300 foot candles on our one square foot area (400 foot candles multiplied by 75% = 300 F.C. (Foot Candles)).
Earlier we figured out that the sun produces 10,000 F.C. on a perfect day, whereas a single 100 watt incandescent bulb produces 300. If we hung that 100 watt bulb over the five feet by five feet garden we used in one of our earlier examples, we would see that that garden is receiving only 12 F.C. (300 foot candles divided by 25 square feet = 12 foot candles). If we were shooting for 1,000 foor candles for our garden, we would need to install 83-100 watt bulbs. Outside of the fact that your hydro bill will be higher than that of the Griswald's around Christmas time, you just turned your grow room into an oven. Because of this, incandescent bulbs are really unsatisfactory for hydroponics.
QUARTZ HALOGEN BULBS
Halogens are much more efficient than incandescent bulbs. They weigh in at about 20 lumens per watt. Halogens are available in a 1,000 watt bulb, and since we are trying to produce as much light as possible, we will use this for our example. A 1,000 watt halogen bulb, at 20 lumens per watt, will give us a total of 20,000 lumens of total light energy. As we did with our last example, we will multiply this number by 75% to adjust for out "spilled" light. Therefore, our 1,000 watt halogen bulb is producing 15,000 useable lumens. Our sample garden that we have used before measures five feet by five feet, giving us 25 square feet. When we shine 15,000 lumens on our 25 square foot area, we end up producing 600 F.C., or foot candles of light intensity. This is getting us closer to a useable light source, but it is still barely adequate. Also, a major drawback of the halogen bulb is that it produces a large amount of HEAT! This heat would cause foliar burn, and would be a welcome invitation for infestations. Because of this, we can conclude that halogen bulbs are also inadequate...leave them in your fog lamps where they belong.
FLUORESCENT
Fluorescent bulbs come in varying wattages, and spectrums. Ultimately, you would like to use the highest output, fullest-spectrum bulbs that you can find. When a plant is growing, it not only requires enough light, but that light should be rich in both ends of the light spectrum. There are two definitive stages in a plant's growth and they are the vegetative and flowering stages. The initial stage is vegetative. This is where the plant performs most of its vertical growth, and strengthens the main stalk. The flowering stage takes over as the plant begins to get "busy" and starts to produce its flowers. When a plant is in its vegetative state, it focuses its thirst mainly on the blue-violet end of the spectrum. When it is flowering it focuses on the red-orange end. Ultimately, you would want to incorporate a 40 watt full-spectrum tube. This would provide you with the greatest results.
These tubes produce 68 lumens per watt, for a total of 2750 lumens. Fluorescents are ideal for initial propagation because they produce almost no reciprocal heat. You can hang a four tube fixture six to eight inches above your plant canopy, and then just keep raising it as your plants grow, without fear of burning your leaves. In this example, we will scale down the size of our test garden. Earlier, we have been using a five feet by five feet garden as a reference point. For this example, we wil use a two feet by four feet garden. A two feet by four feet fixture can hold four bulbs. This will give us a total of 11,000 lumens (4 bulbs multiplied by 2750 individual lumens = 11,000 lumens). Allowing for "spilled" light, we are probably generating about 1031.25 F.C. (11,000 lumens multiplied by 75% = 8250 lumens...8250 lumens divided by eight square feet = 1031.25 Foot Candles). This is assuming that the lights are placed DIRECTLY over your plants. As you raise your lighting, your light intensity drastically drops. When you double the distance between your light and your plants, you cut the light intensity by four times.
