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         A Little About Metal Halide Lighting


The advent of metal halide lighting, during the eighties, within the reefing community, played a large part in making successful reef aquariums possible. Up until that time, it just wasn't possible to deliver light with the intensity and spectrum required to maintain SPS and LPS corals for any length of time in reef aquariums. Today, of the various types of light sources generally employed in reef tanks, metal halide are generally considered to provide the highest output lighting available - although this may be changing with the recent advent of LED lighting. Metal halides produce about 90-100 lumens for each watt of power.

Concentrated light

Metal halide lights are able to concentrate their output into a very small space -  other light sources, fluorescent lights for example, produce an even, almost diffuse, broad area of illumination. The concentrated output of metal halides is often referred to as point source lighting. It is this concentrated light that produces the visible rippling effect that so many reefkeepers consider to be one of the advantages of metal halides. In addition, the concentration of light increases the light intensity. This combined with well designed reflectors, allows the illumination from metal halide lamps to reach relatively deep into the water column.

Color temperature and wattage

Metal halides are available in many color temperatures, from 6500 Kelvin up to 20,000 Kelvin. The temperatures most often available are 6,500 K, 10,000 Kelvin, 12,000 Kelvin, 14,000 Kelvin, and 20,000 Kelvin. Common output values in watts are 70W (usually only available in double ended bulbs), 150W, 175W, 250W, 400W, and 1000W. The three most often employed bulbs within the hobby are 175W, 250W, and 400W.

A quick glossary

When shopping for metal halide lighting, you'll come into contact with quite a few technical sounding terms. Halides, metal halide, HQI, etc.

Let's take a look at a few:

  • Halides - a group of minerals containing one of the halogen elements (chlorine, fluorine, bromine, and iodine) as a building block, a salt of any halogen acid .

  • Metal Halide - a lamp which generates light by passing an electrical current from one electrode to another through a metal vapor, to form an arc between the electrodes. Like most high-intensity discharge lamps, metal halide lamps operate under high pressure and temperature. Because of this, special fixtures are necessary for safe operation.

  • HQI - Hydrargyrum Quartz Iodide is a particular kind of high-intensity discharge (or HID) lighting. Light is produced by an electrical arc being passed through a gas. Hydrargyrum is the Latin name for the element mercury.

Bulb configurations

 Metal halide bulbs are available in two basic types:

  • single ended (SE) bulbs - These are the bulbs most encountered in the United States.

    One of the most important advantages of the SE bulbs is that the arc tube (the light producing assembly) is enclosed in a protective glass envelope. This envelope not only protects the inner components and arc tube from oxidation, it is also very effective in preventing harmful UV light from escaping and possibly harming delicate corals, other invertebrates, and even fish - because of this, the vast majority of SE bulbs need no additional UV protection.

    It should be noted that although some manufacturers of SE bulbs recommend that the bulbs be positioned so that the "nipple", located on the arc tube, is pointing away from the direction the light is needed. Using my Apogee PAR meter, I have never been able to verify this to be accurate.

    They are designed to be used with a ceramic mogul socket (E39).

  • double ended (DE) bulbs -  Most DE bulbs do not have a protective protective glass envelope and must, therefore, be shielded with  a piece UV proof glass placed below the bulb.

    DE bulbs are designed to be used with special two piece sockets (one piece is shown in the pic, below).

I thought I'd mention that although there are many claims that DE bulbs are better than SE bulbs in somer espects and SE bulbs are better than DE bulbs in other respects, I have never been able to ascertain any appreciable differences between the two.


All metal halide lamps require ballasts to regulate the current flowing through the arc tube. Ballasts also ensure that the proper voltage is delivered to the arc. It is important that the proper type of ballast is used with a particular bulb. Some metal halide bulbs contain a third electrode to start the arc when the lamp is first lit. Others, such as pulse-start lamps don't contain a starting electrode, so they require an igniter to generate a high-voltage pulse to start the arc.

Electronic ballasts combine igniter and ballast into a single package. These ballasts use high-frequency to drive the lamps. They have less loss than a line-frequency (60Herz) ballasts, they are more energy efficient. Some electronic ballasts, like the ones I use in the Glass Reef are "dimmable", with the output being adjustable from approx.  70% to 100%.


No matter what kind of MH bulb is used. No matter what ballast is used to drive them. A reflector is needed to optimize and control the light output. A well designed reflector, properly matched to the tank setup with which it will be used, ensures that a maximum of light that is distributed in as even a manner as possible will be achieved. In my opinion, the choice of reflector is just as important, and sometimes more so, than the choice of bulbs to be used. Be diligent in your research. Do not skimp when purchasing the reflectors for your reef tank. Buy the best you can afford.

Reflectors like the one in the next pic were once considered top notch. Longer!

Today's modern reflectors (like the LumunBright shown below) do an excellent job of concentrating and evenly distributing light. The model shown below is intended for use in installations where the reflector cannot be seen by people observing the reef tank. It is a "bare bones" model and there has been no attempt to make it "attractive". It is meant to be incorporated into a canopy that will hide it from view.

This is the same reflector, as above, but has been enclosed in an attractive housing. In this form, it can be employed as a pendant, fully visible to the viewer.

There are downsides

There is a downside to metal halide lighting. They produce much more heat than, for instance, a T5 fluorescent installation. That heat has to be dealt with - cooling fans, a chiller, or possibly both must be considered.  The initial cost can be a shock! Metal halide fixtures are more expensive than fluorescent systems at first glance - although in large T5 systems where as many as 20 and more T5 bulbs are in play, the cost of T5 bulbs can far exceed that of MH bulbs in that same sized system.

You can't use an MH bulb forever. After about eight months (just an average) a slow shift in spectrum occurs as does a steady lessening of the amount of light produced. This happens so slowing that we don't notice it, but our corals do. I make sure to use my MH bulbs for no longer than nine months, or so.

Which bulbs should I use?

Ask  20 reef enthusiasts about which MH bulbs deliver the "best" spectrum, PAR value, and look, and you'll get 20 different answers. It's probably true that the current "hot favorite" in the hobby is the Radium 20,000 K.  I tried them but found them to be too blue. I switched back to my 12,000 K ReefLux's. My honest opinion is, if you stick with one of the quality bulbs available on the market these days, and all other conditions in your tank are conducive to coral health, you'll get great growth. No particular bulb is the magic bullet.



Jorge says...
I had a question....I currently have a 250w mh and its ballast...... would i be able to replace that bulb with a smaller watt bulb or must I always keep the 250w?


GlassReef: It depends on your ballast. There some ballasts that provide the ability to to switch between two or more wattage values. Without a switching capability you will have stick with 250W bulbs. Trying to do otherwise will almost certainly damage your bulbs.
26th January 2012 1:30pm

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