Q: How far should my grow light be from my plants?
A: Artificial light fades fast.
The fact that artificial light fades fast needs to be balanced by the fact that grow lights generate heat. Another consideration is that the light corona (area receiving full light) expands as the light is raised. In other words, the higher the light, the more plants can be covered.
Q: You may cover more plants, but will the light be as bright?
A: Artificial light fades fast.
So, as you raise the light further from the plants, the intensity of the rays shining on the plants will diminish.
Q: So how fast will the light fade?
A: Let's take a 400 Watt Metal Halide with a Super Metal Halide Bulb. A good average figure would be 32,000 lumens measured right next to the bulb. The 32,000 lumens figure is for a "burned-in" bulb, which is warmed up--it's a good average; remember it's a Super Metal Halide not a regular one-- a regular Metal Halide would average out at about 28,000 lumens.
As the light rays travel, they will lose about one quarter of their energy for every foot travelled.
So, at
1 foot away there will be about 24,000 lumens,
2 feet away there will be about 18,000 lumens,
3 feet away there will be about 13,500 lumens,
4 feet away there will be about 10,125 lumens,
5 feet away there will be about 7,595 lumens,
6 feet away there will be about 6,000 lumens,
7 feet away there will be about 4,275 lumens,
8 feet away there will be about 3,200 lumens.
So, you can see artificial light fades fast.
Q: Do you have to calculate each foot by foot? A: No, there are other fancy ways, but just taking the number at each one foot mark and multiplying by three quarters or .75 each time works and is the simplest. But you can't take this formula and make it work by say trying to get the reading at 3 feet by multiplying three quarters by three or dividing the lumens number by 3. That's just not the way to make this easy formula work.
Q: How big an area can I light with a 400 Watt Metal Halide? A: The rule of thumb is a four foot by four foot area. But this is primary light--like the strength of light you would use for a tomato or pepper plant. Low light plants can take advantage of the outer corona. Of course, there are limitations.
Here's a list of the usual recommendations:
a 1000 Watt HID Light has a primary light corona of about 8 feet by 8 feet
a 400 Watt HID Light has a primary light corona of about 4 feet by 4 feet
a 250 Watt HID Light has a primary light corona of about 2 1/2 feet by 2 1/2 feet
a 100 Watt HID Light has a primary light corona of about 1 foot by 1 foot.
These numbers should just be used as a guide. Lots of people have grown healthy strong plants somewhat further away in the secondary corona.
Q: What happens if you give a plant too much light? A: The truth is we don't have the technology to give a plant more light than it can handle. Any problems the plant gets are from heat and not from light.
Some researchers have taken four (yes, 4) 1000 Watt HID lights and lighted a four foot by four foot area. Needless to say the plants grow very quickly.
Q: You're kidding me, right? A: No, they made it work.
Q: How the heck can that work? A: First, separate the light from the heat. They used water cooled lights.
Q: Isn't that dangerous? A: If it's done wrong--yeah. If you have your tubes and engineering done right, though, it kicks butt. But the margin of safety is not all that large.
Q: Then what? A: Then you have to use hydroponics to give the plants nutrients because the plants can't pull the nutrients out of soil fast enough. It's somewhat like a plant growing in soil under sunlight is a pedestrian walking along. Regular hydroponics and grow light is like driving a car at 45 MPH. This is like cruising the freeway at 120 MPH.
This brings up a real good point: when you are growing faster, you have less time to react to problems. The faster you are growing, the less time you have.
