Lighting for a reef tank must meet several different, and sometimes conflicting, requirements, including initial startup costs associated with purchasing and assembling the system components and the operating expenses associated with running the system and replacing failing components. Electrical consumption of the system, the useful life of the bulbs, and the bulb costs are perhaps the most overriding of these expenses. The main types of reef tank lighting are categorized by their types of bulbs, these are: Very High Output fluorescent (VHO), power compact (PC), metal halide (MH) and mercury vapor (MV). Metal halide and mercury vapor are in a class of lamps known as high-intensity discharge (HID) lamps, which rely on the evaporation of a solid material inside an arc tube to create light. Lamp manufacturers utilize a wide array of materials to create different color temperature lamps, and the availability and stability of these materials are the main contributors that impact the life and price of a bulb. In this article, only the metal halide and the Iwasaki mercury vapor lighting options will be discussed.

Metal halide or HID lighting systems consist of several different components, which may be purchased separately, as a retrofit kit, or as a completely assembled fixture. A metal halide lamp system consists of the lamp, socket or lamp holder, ballast, reflector and power cord. Additionally, there may be many unfamiliar terms, which add to the confusion. I hope that this article will help eliminate some of the confusion about metal halide system components and help you to select lighting system components that meet your needs. Some of the more common acronyms found in the lighting industry are listed in Table #1.

In choosing lighting, it is generally accepted that increases in wattage result in increases in illumination. Consequently, the first thing to determine is what wattage lamp to use, and how many are going to be needed to provide thorough coverage for your tank. In most cases, one bulb is required per 1-2 ft². Lamps are available in 70, 150, 175, 250, 400 and 1000-watt configurations, as well as color temperatures ranging from 4200K to 50,000K (see this article for interesting comparisons of various color temperature bulb combinations). There are three types of bases that are utilized to provide the main points of electrical contact to the bulb; these bases are known as mogul, medium, and double-ended. The most popular type of metal halide lamps in the U.S. are single-ended and have an E39 screw type base, which is commonly called a mogul base. Most European lamps imported into the US have an E40 screw base and are compatible with the E39 lamp holders. Some low wattage lamps (70 and 150 watt) use a medium base, which is the same size found in typical incandescent bulbs. There are also double-ended lamps that require a pair of lamp holders, known as Rx7S for 150 watt and Fc2 for 250 watt lamps. Many lamps need to be run in a particular spatial orientation to operate correctly and produce the proper spectrum with rated lifetime. Consequently, lamps are marked, usually on the outer sleeve, by the position in which they need to be operated, such as Base Up (BU) or Base Down (BD), Horizontal (H), Vertical (V), etc. A complete listing of the various lamp position options is found grouped at the bottom of Table #1.

Mogul Base	Fc2 DE Lamp Holder	Rx7S DE Lamp Holder

175 watt Mogul base	DE150 watt HQI

Metal halide operation can be broken up into two separate phases, startup and steady-state operation, and the amount of power consumed by the system during these phases is different. Starting up the bulbs always requires more power than steady operation; up to several thousand volts may be needed. All MV and most MH lamps have an electrode located at one end of the arc tube to provide additional current to the lamp while it is starting up. These types of lamps require an Open Circuit Voltage (OCV), in which two times the lamp voltage is needed to initiate and sustain the arc. Some newer MH lamps (mainly European) have no starting electrode, so in addition to the OCV, the arc is started by a high voltage (4 kV or more) pulse that is provided by an igniter. Regardless of which type of lamp assembly is chosen, make sure to select a lamp socket that is pulse-rated high enough for the lamp that is being used (Table #2).

Unlike normal incandescent bulbs, HID lamps cannot be run directly off of 120 VAC like an incandescent lamp and take several minutes to warm up and come to full brightness. If the power is interrupted while an HID lamp is on, it will take several minutes to restrike the arc and cause the bulb to begin burning again. Restrike time is shown in Table 3 and describes the amount of time it takes for the lamp to cool down sufficiently to reinitiate the arc after power to the lamp has been lost.

There are a number of lamp manufacturers with a variety of wattage, color temperature and base styles available. Some of the more common ones are listed below.

A ballast in an HID system has generally two functions: 1) to provide voltage to the lamp so that it can initiate and sustain the arc, and 2) to provide the proper current to the lamp once the arc is started. Metal Halide lamps cannot be run directly off of 120 VAC like an incandescent lamp, because up to several thousand volts used to ignite the arc may be needed. The current must also be limited so that the lamp does not draw so much current that it destroys itself. Ballasts are typically of a Core & Coil, F-Can or electronic design. The Core & Coil is a transformer without a case along with a capacitor and igniter, if necessary. This can be a very economical choice, but it requires more wiring expertise and a metal box to house all of the components. This type of ballast will come as a kit with the coil, capacitor and igniter, if necessary. The F-Can and electronic ballast have all of these components in a pre-sealed rectangular metal box. The F-Can ballast looks just like a fluorescent lamp ballast, and it may also be called a tar ballast because of the tar-like material the can is filled with. The main problem with an F-Can ballast is that if one component such as the capacitor fails, you have to replace the entire ballast. A Core & Coil setup has a separate capacitor and igniter, so they can be replaced individually if one of them fails. The capacitor is the most likely component to fail.

Core & Coil (w/capacitor)	Core & Coil (w/capacitor & igniter)	F-Can

Current Crest Factor (CCF) is another ballast parameter that should be considered because of its effect on lamp life, and it is a measure of the maximum amount of current that is sent to the bulb. The lower the CCF is, the longer the lamp will last because there is less stress on the electrode. Table #4 below shows the CCF for several types of ballasts. Electronic ballasts often have the CCF listed in their ballast specifications.

The line voltage that the ballast is plugged into will have an effect on the lamp voltage and, therefore, it's light output. Autotransformer type ballasts tend to drop the lamp voltage when the line voltage drops. If the ballast is rated for 120 VAC and it is run on 110 VAC, the lamp will also not be running at full power and so the spectrum and intensity will vary. If a constant wattage autotransformer (CWA) type ballast has a +/- 10% change in line voltage, the lamp wattage will also vary by +/- 10%. A constant wattage (CW) type ballast has the best regulation, in which a +/- 13% line voltage change only produces a +/- 2% lamp wattage change. From these numbers it becomes obvious that the line voltage on which the ballast operates is a very important factor in maintaining the lamp wattage. If the circuit that the ballast is on also carries pumps, heaters, a chiller, etc., then the line voltage may be lower than expected, causing the lamp to discharge at a much lower intensity than expected. A separate circuit may be a good idea for the ballast in this situation, and if several high wattage ballasts are being used, then more than one circuit may be necessary for proper operation. Most ballasts come without a power cord, so one will have to be connected. Use a three-prong plug so that the frame can be grounded. If the ballast needs to be turned on and off manually, then a power switch should also be installed. Maximum wire lengths (listed in Table #5) should be adhered to for proper operation of the ballast.

Another useful resource can be found on the Advance Transformer's web site; it features a search engine that displays a list of ballast transformer model numbers that match the ANSI number of the lamp. An Acrobat PDF document containing a data sheet is available for each type of ballast that shows all of the parameters for the ballast as well as diagrams, which demonstrate how to wire the ballast to the lamp, capacitor and igniter, if required. They also have a PDF document entitled Pocket Guide to High-Intensity Discharge Lamp Ballasts that contains a lot of useful information for the potential do-it-yourselfer, including a troubleshooting guide.

There are a number of ballast manufacturers that make different styles of ballasts. Some ballasts only operate on one input voltage, while others are multi-tap and operate on several different input voltages by connecting the line voltage to the wires designated for that particular voltage. Check the ballast specifications very carefully to make sure the ballast will meet your needs and voltage range for the lamp.

A reflector will reflect more of the light downward, so that it ends up in the tank instead of on the ceiling and floor. Additionally, a good reflector can dramatically increase the spread and distribution of light while also minimizing the amount of light that is reflected back towards the bulb. Pre-made reflectors are available such as the SpiderLight™ parabolic reflector or the PFO Optimal Reeflector™. Custom-made reflectors may be obtained from Aquatic Technology, where they will cut and bend spectral aluminum to your specifications. As a fairly effective substitute for a reflector, the inside of the lighting fixture or canopy may also be painted white, or lined with a reflective material. Due to the intense heat generated by metal halide systems, fans may also be required to keep the tank from over-heating. Fans are available from electronics suppliers and most aquarium businesses that sell lighting components. However, do not blow cool air directly over the lamps, as bulb cooling will result in inefficient evaporation inside the arc tube and abnormal spectral variations of the bulb will result. It is important to remember that these bulbs are designed to operate at a pre-determined temperature.

SpiderLight™ reflector

Being armed with the knowledge of the acronyms and the manufacturer's model numbers for the ballast and socket will allow you to use an electrical supply house, or the Internet, to make a more informed purchase of metal halide components. The tables provided in this article will help match the different components. If you are not completely comfortable working with electrical wiring, please get a qualified electrician to help you, or purchase a complete lighting fixture or retrofit kit. Many of our sponsors carry both components and complete lighting fixtures.