Most professional cultivators are aware that cannabis requires one bandwidth of light during the vegetative stage and another during the flowering stage. As a result, many growers use metal halide (MH) bulbs for vegetative plants and high pressure sodium (HPS) bulbs for flowering plants. However, because they commonly provide plants with 18 hours of light during vegetative growth, MH bulbs are widely considered to be workhorses of the industry.
In fact, MH bulbs are one of the most efficient sources of artificial white light. They are available in 175, 250, 400, 600, 1,000, 1,100 and 1,500-watt types, and come either with or without phosphor coating. Although frosted bulbs are easier on the eye and produce less ultraviolet light, they also produce fewer lumens than clear bulbs and have a slightly different color spectrum.
Inside MH bulbs
MH bulbs are available in different sizes and shapes to fit different reflectors, and are rated by the manufacturer according to both their wattage and the size of their outer envelope. The MH system is a complex one that requires electricity to be passed through vaporized argon, mercury, thorium iodide, sodium iodide and scandium iodide gases within the arc tube. As a result, it takes about five minutes for an MH bulb to “warm up” and start producing its characteristic white light. If the power fails or the light is turned off for any reason, it takes about five to 15 minutes for the gases contained inside the arc tube to cool before the light can be restarted.
But starting an MH bulb requires an incredible amount of voltage, and turning it on and off more than once a day puts stress on the bulb, shortening its life span. New bulbs require about 100 hours of operation before all of their components stabilize, so that they produce a consistent bandwidth of white light.
Standard MH bulbs operate most efficiently when hung vertically. When operated horizontally, the arc bends due to the excessive heat, which in turn results in uneven heating of the arc tube wall. This causes the bulb to produce fewer lumens and have a shorter life span. Fortunately, special MH bulbs are designed to be operated in horizontally and are stamped “HOR” on the crown or base. In addition, there are universal bulbs that can be operated in any position, but they commonly produce up to 10 percent less light and have a shorter life span than standard vertical bulbs.
The average lifespan of an MH bulb is about 12,000 hours, which equals nearly two years of 18 hours per day. But even though it may continue to operate, the bulb will gradually produce less and less light near the end of its life. As a general rule, MH bulbs should be replaced every 12 months or 5,000 hours of operation. It’s recommended to record the year, month and day that you first use each bulb so you will know when it’s time to change, rather than waiting for it to diminish or burn out.
In addition, an MH bulb may produce a slight stroboscopic effect that will cause the bulb to appear to be bright at first, then dim a bit, then brighten again in repetition due to the arc being extinguished and re-illuminated 120 times per second. But this is entirely normal and doesn’t indicate a malfunction in the light bulb, fixture or ballast.
Techniques for effective MH bulb use
Although there are other sources of light available (such as compact florescent lights, T8 florescent tubes and mercury vapor HID lamps), nothing else on the market—other than high pressure sodium bulbs—is more efficient than MH bulbs for cultivating cannabis. But to maintain peak efficiency in your garden:
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