High Intensity Discharge (HID) Lights

Marijuana Horticulture

by Jorge Cervantes

Growers use HID lamps to replace natural sunlight indoors and grow outstanding cannabis. High intensity discharge lamps outperform all other lamps in their lumens-per-watt efficiency, spectral balance, and brilliance. The spectrum and brilliance of HIDs hep growers replicate growth responses induced in cannabis in natural sunlight. Compare charts on HID spectral emission with the chart on photosynthetic response, chlorophyll synthesis, and positive tropism.

The HID lamp family contains mercury vapor, metal halide, High Pressure (HP) sodium, and conversion bulbs. Metal halide, HP sodium, and conversion lamps have a spectrum similar to actually sunshine and can be used to grow marijuana. Mercury vapor lamps were the first HIDs on the market. Obsolete mercury vapor lamps are inefficient electrically and produce a poor spectrum for plant growth. Now most all mercury vapor lamps have been retrofitted with more efficient HIDs.

Researchers have created a few better bulbs with a higher PAR rating, but there is no new technology in sight. The latest glass covers of the bulb have become slightly better at letting light through, but there have been very few major technical advances in these bulbs for the last 20 years.

Incandescent bulbs are the least efficient; 600-watt HP sodium lamps are the most efficient. The brightest bulbs measured in lumens-per-watt are the metal halide and HP sodium bubs.

Originally developed in the 1970s, metal halides and HP sodium bulbs were characterized by one main technical limitation – the larger the bulb, the higher the lumen-per-watt conversion. For example, watt for watt, a 1000-watt HP sodium produces about 12 percent more light than a 400-watt HPS and about 25 percent more light than a 150-watt HPS. Scientists overcame this barrier when they developed the 600-watt HP sodium. Watt for watt, a 600-watt HPS produces seven percent more light than the 1000-watt HPS. The “pulse start” metal halides are also brighter and much more efficient than their predecessors.

High intensity discharge lamps produce light by passing electricity through vaporized gas enclosed in a clear ceramic arc tube under very high pressure. The dose, or combination of chemicals, sealed in the arc tube determines the color spectrum produced. The mix of chemicals in the arc tube allows metal halide lamps to yield the broadest and most diverse spectrum of light. The spectrum of HP sodium lamps is somewhat limited because of the narrower band of chemicals used to dose the arc tube. The arc tube is contained within a larger glass bulb. Most of the ultraviolet (UV) rays produced in the arc tube are filtered out by the outer bulb. Never look at the arc tube if the outer bulb breaks. Turn off the lamp immediately. Some bulbs have a phosphor coating inside the bulb. This coating makes them produce a little different spectrum and fewer lumens.

General Electric, Iwasaki, Lumenarc, Osram / Sylvania, Philips, and Venture (SunMaster) manufacture HID bulbs. These companies construct many bulbs with the exact same technical statistics. According to some gardeners, certain brands of bulbs are better than others because of where they are manufactured. They usually came t this conclusion because because they purchased two different brands of (1000-watt) bulbs and had better luck using one brand. What these gardeners don’t know is that many f the manufacturers buy and use the same components, often manufactured by competitors!

Pulse-start metal halides commonly use 240 volts and harbor the started in the ballast box, not in the arc tube. These systems employ physically smaller reactor ballasts that keep original line voltage within ten percent of the voltage in the arc tube.

reflective walls increase light in the growing area. Less intense light on the perimeter of gardens is wasted unless it is reflected back onto foliage. Up to 95 percent of this light can be reflected back toward plants. For example, if 500 foot-candles of light is escaping from the edge of the garden and is reflected at the rate f 50 percent, then 250 foot-candles will be available on the edge of the garden.

Reflective walls should 12 inches or less from the plants for optimum reflection. Ideally, take walls to the plants. The easiest way to install mobile walls is to hang the amps near the corner of a room. Use two corner walls to reflect light. Move the two outside walls close to plants to reflect light. Make the mobile walls from lightweight plywood, Styrofoam, or white Visqueen plastic.

Using white Visqueen plastic to “white out” a room is quick and causes no damage to the room. Visqueen plastic is inexpensive, removable, and reusable. It can be used t fabricate walls and partition rooms. Waterproof Visqueen also protects the walls and floor from water drainage. Lightweight Visqueen es easy to cut with scissors or a knife and can be stapled, nailed, or taped.

To make the white walls opaque, hang black Visqueen on the outside. The dead air space between the two layers of Visqueen also increases insulation.

Using flat white paint is one of the simplest, east expensive, most efficient ways to create optimum reflection. Artists’ titanium white paint is more expensive, but more reflective. While easy to clean, semi-gloss white is not quite as reflective as flat white. Regardless of the type of white used, a non-toxic, fungus inhibiting agent should be added when paint is mixed. A gallon of good flat white paint costs less than $25. One or two gallons should be enough to “white out” the average grow room. but do not paint the floor white – the reflection is detrimental to tender leaf undersides. Use a primer coat to prevent bleed-through of dark colors or stains or if walls are rough and unpainted. Install the vent fans before painting. Fumes are unpleasant and can cause health problems. Painting is labor-intensive and messy, but it’s worth the trouble.