Reflective Hoods

Some rellective hoods reflect light more evenly than others. A refleclor that distributes light evenly-with no hot spots-can be placed closer to plants without burning them. These hoods are most efficient, because the lamp is closer and the light more intense. The farther the lamp is from the garden, the less light plants receive. For example, a 1000-wott reflector with a "hot spot" must be placed 36 inches (90 cm) above the garden, A 600-watt lamp with a reflector that distributes light evenly can be placed only 18 inches above the garden. When placed closer,

Maximum Light Requirements for Plants

Growth Stage



Hrs. of Light

















These guidelines will give plants all the light they need to form dense buds.

Less light will often cause looser, less-compact buds to form.

Place Hortilux 1000-watt fixture 4 feet (1.2 m) above garden.

You can put the Super Wide reflector very close to plants.

Hortilux Midi reflector spreads light well.

The bammered-fmish specular interior of this lamp/ballast fixUire diffuses light well.

This fixture is designed to be mounted next to a wall. It reflects light down and away from the wall.

The ballast box is attached to the reflector in this greenhouse fixture.

Place Hortilux 1000-watt fixture 4 feet (1.2 m) above garden.

The Medium reflector from Hortilux is a favorite in Europe.

Hortilux Midi reflector spreads light well.

The Wide reflector from Hortilux was one of the first European reflectors to use a deflector below the bulb.

Double fixture greenhouse fixture has the ballasts between bulbs.

This air- ventilated tube is very inefficient, leaving a "hot spot" below.

The Hydrofarm reflector is one of the best values in North America. It reflects a lot of light and "breathes" well.

You can put the Super Wide reflector very close to plants.

This fixture is designed to be mounted next to a wall. It reflects light down and away from the wall.

This Ecotechnics Diamond reflector from Spain is very efficient.

The Adjust-A-Wing is one of my favorite reflectors because it delivers the most light! The deflector under bulb alfows it to be super close to plants.

This reflector from Easy Green has holes for forced-air ventilation and can be placed close to plants.

Parabolic dome reflectors orient bulbs vertically. Although less efficient these hoods work well to grow vegetative p tants.

Gcivita invented a lamp with the re/lector inside the bulb! The reflector is the most efficient I have seen!

The bammered-fmish specular interior of this lamp/ballast fixUire diffuses light well.

covered in plastic. The bulb hangs between plants.

Hortilux sets the standard with their line of reflectors. The Deep model is to be mounted high in greenhouses.

The Butterfly reflector and deflector is one of the most interesting designs I have seen, but I have no idea about efficiency!

The "cone" reflector is one of the least efficient available. Much reflected light is wasted.

The Butterfly reflector and deflector is one of the most interesting designs I have seen, but I have no idea about efficiency!

the GOO-watt lamp shines as much light on the garden as the 1000-watt bulb!

The proper reflective hood over the lamp and reflective walls can double the growing area. Growers who use the most efficient reflective hoods harvest up to twice as much as those who don't.

Seedlings, cuttings, and plants in the vegetative growth stage need less light than flowering plants, because their growth requirements are different, For the first few weeks of life, seedlings and clones can easily survive beneath fluorescent lights. Vegetative growth requires a little more light, easily supplied by a metal halide or compact fluorescent lamp at the rate indicated in the chart below.

Reflective hoods are made from steel sheet metal, aluminum, even stainless steel. The steel is either cold-rolled or pre-galvanized before a reflective coating is applied. Pre-galvanized steel is more rust resistant than cold-rolled steel. This metal can be polished, textured or painted, with white being the most common paint color. Premium hood manufacturers apply white paint in a powder-coating process. Note: there are different shades of white, and some whites are whiter than others. Flat titanium white is the most reflective color and diffuses light most effectively. Glossy white paint is easy to clean but tends to create hot spots of light. Sheet metal hoods are less expensive than the same size aluminum hood, because of reduced materials expense.

Parabolic Drugs
Vertical parabolic dome reflective hoods distribute light evenly over a large areo and are perfect for vegetative growth.

The pebble and hammer-tone surfaces offer good light diffusion and more surlace area to reflect light. Hot spots are commonplace among highly polished, mirror-like surfaces. Mirror-polished hoods also scratch easily and create uneven lighting.

Horizontal Reflective Hoods

Horizontal reflectors are most efficient for HID systems, and are the best value for growers. 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, so 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.

Horizontal reflective hoods are available in many shapes and sizes. 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 lend to have a 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. When there is no hot spot, reflective hoods with deflectors can be placed closer to plants.

Horizontally mounted HP sodium lamps use a small reflective hood for greenhouse culture, The hood is mounted a few inches over the horizontal HP sodium bulb. All light is reflected down toward plants, and the small hood creates minimum shadow.

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.

Parabolic dome reflectors offer the best value lor vertical reflectors. They reflect light relatively evenly, though they throw less overall light than horizontal reflectors. 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 are inexpensive to manufacture and provide a good light value for the money. Four-foot parabolic hoods are usually manufactured in nine parts. The smaller size facilitates shipping and handling. The customer assembles the hood with small screws and nuts.

Four-foot cone hoods are usually manufactured in four parts, The smaller size facilitates shipping and handling. The customer assembles the pieces with small screws and nuts. Cone-shaped reflectors using a vertical bulb waste light and are very inefficient. Growers who try to save money by purchasing cone-shaped reflectors pay even more in lost efficiency.

For example, say you bought a cone reflective hood for S20 instead of the top of the line horizontal reflector for $40. First, lei's look at efficiency, The cone hood produces at 60 percent efficiency and the horizontal reflector at 100 percent, or 40 percent more. Each lamp costs $36 per month to operate 12 hours daily at SO. 10 per kilowatt-hour (kWh). If 100 percent = $0.10 per kilowatt-hour, then 60 percent efficiency = $0.06, or a loss of $0.04 for each kilowatt-hour. With this information we can deduce that $36/$0.04 = 900 hours. In 900 hours (75 12hour days) the horizontal reflector has recouped the extra $20 cost. Not only does the vertical cone yield 40 percent less light, it costs 40 percent more to operate! When this 75-day breakeven point is reached, you will be stuck with an inefficient reflective hood that costs more for

Air-cooled fixtures use blowers to direct heat generated by the bulb out ducting.

fewer lumens every second the lamp is using electricity!

Lightweight reflective hoods with open ends dissipate heat quickly. Extra air flows directly through the hood and around the bulb in open-end fixtures to cool the bulb and the fixture. Aluminum dissipates heat more quickly than steel. Train a fan on reflective hoods to speed heat loss.

Artificial light fades as it travels from its source (the bulb). The closer you put the reflector to the bulb, the more intense the light it reflects.

Enclosed hoods with a glass shield covering the bulb operate at higher temperatures. The glass shield is a barrier between plants and the hot bulb. Enclosed hoods must have enough vents; otherwise, heat build-up in the fixture causes bulbs to burn out prematurely. Many of these enclosed fixtures have a special vent fan to evacuate hot air.

Continue reading here: Air Cooled Lamp Fixtures

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