Under most circumstances a 1000-watt lamp with no shield should be kept no less than two feet from the top of the canopy. A 400-watt lamp can be placed as close as 15 inches. A fluorescent tube can be placed about six inches from the top of the plant.
The distance is not to protect the plants from the lamp's brightness, but from the heat that it generates. At its peak, the Sun is brighter than even 1000-watt lights. The amount of light measured 24 inches directly under a 1000-watt horizontal HPS lamp with high efficiency reflector measured about half of what a plant would receive at noon on a cloudless day in early July at the 38th parallel. It would equal the light intensity of a September day at noon, when the flowers are ripening. There was fall-off as the meter was moved from the center.
Heat from the lights causes the buds to "run." Instead of blocky buds on compact stems, the branches stretch and new growth appears, with thin groups of flowers. There are several ways to deal with the heat. Swift circulation/ventilation systems can be set up to move heated air away from the canopy and out of the area. This works best when there is a constant source of cool air. For instance, in Holland, which has cool air most of the time, heated air is replaced with a constant flow of filtered street air. In large spaces the air draft results in a low roar of white noise.
Another method of dissipating the heat is to use a light mover. The heat is radiated to any point for only a few moments. The mover disperses both light and heat. With a reasonable circulation system, the heat is variable, but not intense.
By far the most efficient method of eliminating the heat, enabling the lights to be placed closer to the plants, is by using air-cooled lights. The lights are enclosed by the reflector, which holds a glass plate that captures most of the lamp's heat. Using 4-inch duct tubing powered by an inline fan, the lights draw in cool air from outside the space to cool the light. Heated air is pushed out through the exhaust duct on the other side of the reflector. This air has never been in contact with the garden. It has no odor, just heat. It can be used to heat a space or just vented to the outdoors.
With air cooling, a 1000-watt lamp can be placed at 15-18 inches from the top of the canopy and a 400-watt lamp can be placed about 12 inches above it.
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