The metal halide (MH) and high pressure sodium (HPS) are the main light sources for the indoor grow industry. These systems were developed more than twenty years ago and have undergone many improvements and advances since. They are capable of producing and delivering high-intensity, full-spectrum light. They are modified streetlights, designed to grow plant material. The intensity of the light is far more potent at greater distances from the bulb than any other systems available. The MH and HPS systems come in varying wattage sizes, styles and even different light bandwidth specifications.
The most common wattage sizes are 400 and 1000 watts, although 75, 150, 250, 360, 430, 600 and 1500 wattages are also available and useable. There are also choices of horizontally or vertically mounted bulbs, and type of reflectors. These specifics are best considered after visiting your local grow store and seeing what products are available. From there, one can decide what will best suit one’s unique needs. My personal preference is for a combination of 430-watt, horizontally mounted HPS systems coupled with 400-watt, vertically mounted MH systems in small spaced flower cycles. This combination seems to utilize the best of both worlds. For larger spaces, the 1000-watt systems are virtually unbeatable.
There is a difference between MH and HPS systems in the type of light they produce. Both are relatively full-spectrum devices, however MH systems emit more in the ultraviolet range while HPS systems tend to emit more in the infrared range. Due to the fact that the late summer evening sun (the light most prevalent to budding plants outdoors) tends to emit more in the infrared range, HPS systems have been valued more for their bud cycle capabilities, whereas MH systems have been favored in the vegetative stage. I have found this to be true in regard to production. However, I have also observed that the MH systems tend to produce more colorful finished products (blues, reds and purples) when used either alone or in conjunction with HPS systems in the flower cycle. Again, experimentation will yield personal favorite results.
The difference between horizontally and vertically mounted bulbs has to do with how they disperse light. The greatest amount of light comes from the middle of the bulb, out its side. Therefore, horizontally mounted bulbs tend to direct the light down while vertically mounted bulbs tend to direct the light out to the sides. As for overall intensity of light, horizontally mounted bulbs are superior.
MH and HPS systems have two main parts: The ballast and the bulb with socket. The ballast is a large metal box that houses the transformer, capacitors, starters and other “guts” of the system. The ballast also has at least two cords attached. One cord is the electric plug, wired for either 110-volt or 220-volt usage (other wire connections are possible, but 110 and 220 are the most common in North America). The other wire leads to the socket and bulb setup that may or may not be attached to a reflector. (Generally, reflectors are always used with horizontally mounted systems and considered optional with vertically mounted bulbs.) The length of these cords is sometimes optional, prompting more planning and consideration.
The benefits and advantages of the MH and HPS systems are obvious to anyone who sees them in action. These devices radiate an intensely bright light. The disadvantages to these systems are their consumption of electricity and the amount of heat they make as a by-product. Again, careful planning maximizes the efficiency and safety of these systems. Due to their high demand for electricity, fire hazards need extra consideration when using MH or HPS systems. The electric wire supplying the MH and HPS systems should be inspected and, if need be, replaced prior to production. The wiring needs to be of adequate quality before you begin using any high amperage equipment. To avoid creating a fire hazard, make certain that the electric system supplying the grow room(s) be in perfect running order. When in doubt, have an expert check it out, preferably before you begin. There is no real solution for the consumption of electricity. It is merely considered part of the cost of production and should be accepted as such. Don’t be stupid, and try to steal power. It is a very bad idea.
There are several ways to effectively deal with the heat. The most common way of dealing with heat is adequate ventilation (see chapter 5: Air Circulation and Temperature). But there are a few other tips I would like to offer now. The first is to keep the ballast in a separate room, away from the bulb. Both the bulb and the ballast produce heat. If the cord between the ballast and the bulb is long enough, it is very simple to keep the two separated. This way, the ballast and bulb are producing heat in two locations and not in the same room as the plants.
Another tip for keeping the heat down is also a very simple one: whenever possible, run your lights at night. In most parts of the world, at most times of the year, nighttime baseline temperatures are generally cooler than daytime temperatures. This difference is usually twenty to forty degrees (except for tropical and equatorial zones). The average light cycle in the bud stage is twelve hours on and twelve off. Therefore, the most opportune time to run the lights would be between 8 PM and 8 AM —the coolest times of day.
There exist a few devices created to help lower the temperature associated with MH and HPS systems. One of these is the vented hood. This device is designed into the reflector, and can be modified to accept either a vent fan or conduit leading to a fan. It allows cool air to be drawn in to the grow room and hot air around the bulb to be vented outside of the grow room. Though these systems can be slightly expensive, they are very efficient and work well.
There is also a device that utilizes water to cool off the heat from the bulb. This system circulates water via a pump through a fan-cooled radiator and a series of tubing that runs up and around the bulb. Though this device is somewhat complicated and expensive, it works very efficiently as well.
Special caution needs to be taken when watering the plants or any time liquid has a chance of coming in contact with a hot bulb. If it gets wet while hot, the bulb may explode violently. Exercise the necessary precautions when handling liquid around the bulb.
Reflective Material
Using reflective material on the walls, ceiling and even the floors of the grow room increases the efficiency of light usage. The three most common types of reflective material are white plastic, Mylar, and flat white paint. The advantages of white plastic are that it is relatively inexpensive and easy to use. The disadvantages are that it is not as efficient at reflecting light as Mylar or flat white paint. The advantages to Mylar are that it is super reflective, relatively easy to use and it also blocks many of the rays detectable by infrared scanners and scopes (where that type of security is an issue, that is). The disadvantage is its cost. The advantages to flat white paint are that it is cheap and easy to use and also the best overall reflector of the three. The disadvantage is its permanence. These products help maximize the efficiency of any light source by directing otherwise unused light back to the plants.
Using Sunlight
I cannot let this piece end without putting in a word for good old sunlight. Let me state for the record that in my humble opinion there is no complete substitute for our beloved Sol when it comes to producing light, especially when it comes to plants. And the Sun is out there for anyone to use free of charge each and every day. There exist windows, skylights and now even “solar tubes” that are used for bringing sunlight indoors. This light would work perfectly for general plant production. The solar tube is an amazing device that is capable of collecting the sunlight and delivering it in full glory to the indoor environment. A solar tube could be built into a small private area such as a closet or pantry. The tube can also be opened or closed so that flowering manipulation could be easily maintained. It is also possible to control light cycles with windows and skylights by using a common shade and, unless some form of automatically timed device is used, a bit of daily discipline.
If a person lives on a relatively private piece of land, or has an area unseen by prying eyes, putting plants outside in buckets and bringing them in every day is another option to utilizing the Sun. This method has worked well for many people. Moving the plants in and out everyday by hand is done to force early flowering, especially in southern areas. This allows a person to grow outdoors yet harvest well before the traditional harvest time. It is a discipline. This method also allows more harvests per year than if strictly grown outdoors.
A person with a barn or garage,