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Grow Light Reflectors
Excerpt from Gardening Indoors by George Van Patten
Reflective hoods increase available light by more than 30 percent. The proper reflective hood over the lamp and reflective walls can double or triple the growing area. Gardeners who use the most efficient reflective hoods can harvest up to twice as much produce as those who don't.
Reflective hoods come in all shapes and sizes; some are more efficient than others for specific applications. The trick is to buy the proper reflective hood for the proper application.
Horizontal Reflectors
Horizontal reflectors are the most efficient and the best value for frugal gardeners. A horizontal lamp yields up to 40 percent more light than a lamp burning in a vertical position. Light is emitted from the arc tube. When horizontal, half of this light is directed downward to the plants. Only half of the light needs to be reflected. Horizontal reflectors are inherently more efficient than vertical lamps/reflectors because half of the light is direct and only half of the light must be reflected.
The most efficient horizontal reflective hoods produce up to 40 percent more light than most vertical reflectors.
Horizontal reflective hoods can have various reflective shapes that direct the light downward. The closer the reflective hood is to the arc tube, the less distance light must travel before being reflected. Less distance traveled means more light reflected.
Horizontal reflective hoods tend to have hot spot directly under the bulb. To dissipate this hot spot of light and lower the heat it creates, some manufacturers install a light deflector below the bulb. The deflector diffuses the light and heat directly under the bulb. Reflective hoods with deflectors can be placed closer to plants because there is no hot spot.
PL reflective hoods are computer designed for even light distribution.
HP sodium lamps mounted horizontally use a small reflective hood for greenhouse culture. The hood is mounted just a few inches over the long horizontal HP sodium so that all the light is reflected down toward the plant beds but the small hood creates a minimum shadow. Several manufacturers have a protective glass covering to protect the lamp from water spray when irrigating.
Vertical Reflective Hoods
Reflectors with vertical lamps are less efficient than horizontal ones. Like horizontal bulbs, vertically mounted bulbs emit light from the sides of the arc tube. This light must strike the side of the hood before it is reflected downward to plants. Reflected light is always less intense than original light. Light travels farther before being reflected in parabolic or cone reflective hoods. Direct light is more intense and more efficient.
Horizontal reflectors are the most effective and best all around value.
Parabolic dome reflectors offer the best value for vertical reflectors. They reflect light relatively evenly, even though they throw less overall light than horizontal reflectors. Cone-shaped and other reflectors using a vertical bulb waste much light and are very inefficient. Gardeners that try to save money buying cone-shaped reflectors pay even more in lost efficiency.
Large parabolic dome hoods distribute light evenly and reflect enough light to sustain vegetative growth. The light spreads out under the hood and is reflected downward to plants. Popular parabolic hoods inexpensive to manufacture and provide a good light value for the money.
Polished parabolic reflectors are dome shaped. They tend to concentrate the light directly under the source. They work best with light movers than concentrate intense light directly over plants.
Parabolic dome reflective hoods distribute light quite evenly. They are inefficient in their use of light, but when growing clones and vegetative plants that require lower light levels, parabolic domes are a safe choice.
Lightweight reflective hoods with open ends or plenty of vents dissipate heat the best. Aluminum dissipates heat quicker than steel. Train a fan on reflective hoods that heat up and dissipate heat slowly. Extra air flows directly through the hood and around the bulb in open-end fixtures to cool the bulb and the fixture.
Because artificial light fades as it travels from its source (the bulb), the closer you put the reflector to the bulb, the more intensely light will be reflected.
Enclosed hoods with a glass shield that covers the bulb operate at higher temperatures but keeps water from causing damage to the lamp. The glass shield also serves as a barrier between plants and the hot bulb. Enclosed hoods must have enough vents; otherwise, heat builds up in the fixture causing premature bulb burnout. Many of these enclosed fixtures engage a special vent fan to evacuate hot air.
No Reflective Hood
One option is to remove the reflective hood if the garden is too tall. With no hood, the lamp burns cooler. You can paint the ceiling white to provide reflection, but somewhat less than a reflective hood. If the lamp is too close (less than 24 inches) to the ceiling, install a non-flammable heat shield to protect the ceiling.
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