Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

reflective hood is to the arc tube, the less distance light must travel before being reflected. Less distance traveled means more light reflected.

Vertical parabolic dome reflective hoods distribute light evenly over a large area and are perfect for vegetative growth.
    Horizontal reflective hoods tend 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 for 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 $20 instead of the top of the linehorizontal reflector for $40. First, let’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 $0.10 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 12-hour 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 break-even point is reached, you will be stuck with an inefficient reflective hood that costs more for 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.
Air-Cooled Lamp Fixtures
    Several air-cooled lights are available. Some use a reflective hood with a protective glass face and two squirrel cage blowers to move air through the sealed reflective hood cavity. The air is forced to travel around corners, which requires a higher velocity of airflow. Other air-cooled reflectors have no airflow turns, so the air is evacuated quickly and efficiently.

Air-cooled fixtures use blowers to direct heat generated by the