Metal Halide Lighting Technologies

Considerable confusion exists in the hobby relating to metal halide (MH) lighting.    This article serves to explain some of the fundamentals of halide systems and provide the hobbyist with useful information to be used in pre-purchase research.   This article is not intended to debate the use of halides versus other technologies, but rather to explain the fundamental differences and the pros/cons, allowing the hobbyists researching lightings systems to be armed with all the facts about halide systems. 

What is Metal Halide? 

Metal halide is a type of High Intensity Discharge (HID) lighting.  HID lighting systems pass an electric current through an ionized gas in order to produce light.  In the case of MH lighting, the electric current is passed through mercury vapor that has metal salts added to it in order to produce a particular color of light.

Why is it useful in the aquarium? 

MH lighting systems are one of the most popular lighting systems for tanks, due to several advantages.  First, they are one of the more efficient lighting systems available, second only to LED.  They produce a point-source light.  Unlike fluorescent technologies, halides produce all of their light in a small quartz tube no more than several inches long.  This concentrated source of light causes  the light to produce a glimmer effect that mimics what would be seen in a natural reef, a quality that is pleasing to many hobbyists, and potentially beneficial to reef animals including corals.

Advantages of Halides

Halides have numerous advantages, particularly over older technologies such as fluorescent. 

• Efficiency - Halides are more efficient than all but the newest T5HO fluorescent technologies (provided the system and reflectors are properly engineered).  However, it should be noted that even with T5HO technologies, teh use of multiple lamps (6-8+) ends up drawing the same or more electricity in the end.   From a pure efficiency perspective, halides produce more light output per watt than any other technology besides T5HO or LED. 
• Shimmer - While this could be considered a pitfall, most hobbyists enjoy the shimmer effect created by halides that is not possible with fluorescent.  Scientifically, the shimmer lines also magnify the light output, as a lense, further concentrating PAR and there also appears to be other physiological benefits as well.
• Size - Halides pack more wattage into a smaller footprint than any other technology. 
• Slow Start - Halides take a few minutes to ignite and warm up to their full operating light output. This "slow on" feature can be more beneficial than the instant start of florescent technologies which startles and shocks fish and other animals.

Pitfalls of halides

Halides present some challenges that an informed hobbyist should be aware of when researching lighting systems. 

• Heat - Halides produce copious amounts of heat.  Often (but not always), a chiller may be necessary to mitigate the heating effect on the tank.  A great deal of hobbyists using these systems use fans, turf scrubbers, and other methods to encourage evaporative cooling in lieu of an very expensive chiller.
• Price of Entry - Halide systems (at least commercially made units) are usually very expensive compared to fluorescent systems.
• Restrike Time - Once halides have been running for more than a few moments, they require a period of time to cool before they will restart.
• Cost / Lamp and Luminary - Since the Lamp (bulb) must match the ballast, and both are fairly expensive, the trial and error process on getting the asthetically pleasing mix of color and CRI can be pricey. Moreover, lamps "season" over the initial usage, then settle into a color, so sometimes initial results seem "too white" or "too blue". Ideally, you want to see several set ups and lamps rather than engaging in trial-and-error.

Ballasts

Like all HID lighting technologies, MH lighting requires a ballast in order to properly operate the lamp.  The ballast performs two functions.  First, it provides a "kick" of high voltage to initially start the current flowing through the lamp arc tube - a process referred to as "striking the arc" or "igniting" the lamp.  Second, the ballast regulates the current flow through the lamp.  Without the ballast regulating the current, it would run out of control until the lamp catastrophically fails. 

Due to the ballast's regulating function, a ballast has to be matched with the lamp.  A ballast of a given wattage must be used with a lamp of the same wattage.  There are exceptions to this rule, as several manufacturers have created ballasts that can be used with different wattages, but unless the ballast is specifically rated as such, it must be matched to the ballast. 

Ballasts come in two main technologies, magnetic and electronic.  Magnetic ballasts are dramatically cheaper than electronic, but are less efficient and produce more heat, whereas electronic ballasts do counter these drawbacks. Additionally, quality electronic ballasts can produce more light output and longer lamp life than magnetic.  In fact, over time, even though the price of entry (purchase price) for electronic ballasts is higher, they can recover the higher initial cost through reduced electrical consumption, heat emission, and less frequent lamp changes.  It should be stated that this only applies to quality ballasts made by reputable manufacturers.  Inexpensive, bargain brand ballasts often use sub-quality components that fail long before the ballat can recover its cost.  Electronic ballasts are often potted in tar making them impossible to repair if and when they fail.  Electronic ballasts also create harmoics in the AC power line that can cause problems when operating them with GFI protected circuits.  These issues are highly variable and there is no way to tell whether a particular installation will suffer this problem. 

Lamp Form Factors

Halide lamps for the aquarium come in two distinct form factors:  single ended (SE) and double ended (DE).  Single ended lamps have an enlarged screw type base simliar (albiet larger) than a North American light bulb base: the mogul base.  Double ended lamps have an terminal on each and of the lamp, and are more long and slender.   The former is most popular in DIY systems, where the latter is more commonly found in commercially made systems. 

SE lamps are generally easier to work with, especially for the DIYer, because the reflector options are more numerous and because they do not require a UV shield.  DE lamps require a piece of glass to protect the tank inhabitants from the UV emitted by the lamp.  However, in either case, protective glass is recommended because water splashing on a hot lamp can cause it to shatter. 

DE lamps have two different socket styles.  70w and 150w DE lamps use one type of socket, while 250w and 400w take another.  SE lamps all use the same mogul base.  Regardless, it is important to make note of the system's design wattage and to replace lamps with the same wattage as what is being removed. 

Wattages

There are a great many wattages of MH lamps available, but for the aquarium, the most common wattages are as follows:

SE: 175, 250, 400, 1000 watt. 
DE: 70, 150, 250, 400 watt. 

Remember, a lamp must be operated with a matching ballast wattage.  If you, for example, operate a 250w lamp on a 400w ballast, it can cause the lamp to explode, or at least suffer a significantly reduced lifespan.  If you do the opposite, operate a 400w lamp on a 250w ballast, the lamp will probably not start reliably or will cause damage to the ballast, or suffer a reduced lifespan. 

Probe Start, Pluse Start, and HQI

This is a point of significant confusion for the hobbyist.  Halide systems come in these three flavors.  There are signficant differences between them. 

•  Probe Start:  Probe start MH systems are most common in North America in general industry.  Probe start lamps use a third electrode in the arc tube of the lamp to assist in the starting of the lamp.  This allows the lamp to be started at a lower voltage. 
•  Pulse Start:  Pulse start MH systems are more common in Europe in general industry.  Pulse start lamps use an additional component in the ballast, the igniter, to provide a very high voltage pulse (4kv or more) to start the lamp. 
•  HQI:  HQI lamps cover pretty much all DE lamps.  HQI ballasts drive the lamp harder than a comparable probe or pulse start ballast. 

The different ballast types have different levels of compatibility with each other.  HQI ballasts can start any lamp of the appropriate wattage.  Pulse start can start pulse start SE lamps or probe start SE lamps.  Probe start ballasts can only start Probe Start SE lamps.  There is no such thing as a probe start DE lamp. 

The most common mistake made regarding this information is when a hobbyist replaces a probe start lamp with a pulse start.  Doing this will not damage the system, but it can make for a very frustrating situation for the hobbyist who does not undertand the consequences of doing this.  As halide lamps age, they get harder and harder for the ballast to ignite.   If a hobbyist replaces a probe start lamp with a pulse start, the starting electrode that is needed in the lamp is missing.  A new halide lamp may work in these systems for a short time, but as the arc tube "wears in," it will get to the point where igniting becomes unreliable or stops all together, usually after only a few tens of hours are on the lamp.  The hobbyist will find himself/herself with lamps that do not work and cannot be returned. 

Vendors usually do not make a point to specify the type of lamp technology that their lamps employ.  Hobbyists must often do considerable research or visually inspect the lamp to verify that they are getting the right type.  In general, "German-Made" SE lamps are pulse start while American-made lamps are probe.  Usually, manufacturers stick to one particular technology.  Some brands that are known to produce various types of lamps are as follows: 

*Please note that sometimes manufactuers deviate.  Ushio makes mostly pulse, but some probe lamps, so research on individual lamps is necessary. 

ANSI Codes

The American National Standards Insitutite (ANSI) provides a numbering scheme to help ensure that the correct lamp technology and wattage is matched to the correct ballast.  Unfortunately, lamp manufacturers do not often make this information on their lamps readily available.  However, the hobbyist can use this information to positively identify their ballast type and wattage to facilitate lamp replacement.  Almost all ballasts have the ANSI code located on a label.  In most aquarium systems, the ballast itself is located in a metal enclosure that sits remote from the lamp fixture.  If the ANSI code is not located on a label on the outside of this enclosure, it may be necessary to open the enclosure (with the power off of course) and inspect the ballast inside to locate the code.  Once the ANSI code is known, the refer to the charts below to determine the exact lamp type that must be purchased. 

Identifying lamp technologies

Identifying SE and DE lamps is very simple.  SE lamps have the enlarged screw base (Mogul) as previously stated.  DE lamps are much skinnier and have a terminal on each side of the lamp.  70w and 150w DE lamps are often confused with the halogen lamps used in outdoor fixtures and some interior lamps, often sold at home improvement stores.  Obviously the two different lamp types are not interchangeable.  Halogen lamps cannot be used in a halide fixture or as a substitute for halide lamps, nor can 70w or 150w DE lamps be used in halogen fixtures (though some DIYers do this with extensive mechanical and electrical modifications).   250w and 400w DE lamps have a different base style that makes the distinction between the two very obvious even at first glance.

SE lamps can be distinguished between probe start and pulse start visually very easily once you know what to look for.   Probe start SE lamps have an additional component inside the lamp that is used to provide a striking arc.  Below are some examples.   SE lamps occasionally have a small silver colored disk attached to the stiff wiring structure inside the lamp.  This is called a getter, the purpose of which is beyond the scope of this article other than to mention that this is NOT the ignitor, and that the presence or absence of it is a design consideration made by the manufacturer and is not relevant to anything being discussed here.  The ignitor is a small cylindrical component, usually with a spiral pattern around the exterior.  It is about 1/2" long and about 1/8" in diameter. 

Probe Start SE halide lamp.    Note the ignitor (circled) inside the lamp.  This component is absent in pulse start lamps.	A 150w double ended halide lamp.    Note the inner arc tube, which does not exist on halogen lamps.

Lamp Lifespan

In most cases, halide lamps should be replaced every 12 months.  However, this time may be longer or shorter depending on a tremendous number of factors.  The only real way to ensure that every usable hour of lifetime is used before replacement is to use a light meter to determine when light output starts falling off.  The human eye gets used to looking at the aquarium every day, and usually does not notice the degredation in light output until a new lamp is installed and the original output is restored instantly.  Therefore, in the absence of a light meter, do not trust your eyes to tell you when it is time to replace the lamp, stick to a 12 month timeframe. 

Many hobbyists in systems with multiple lamps perfer to change each lamp at different times.  For example, a system with 3 lamps may have one lamp changed every 4 months, provided the old lamps are being replaced with the same color temperature.  This way the livestock in the tank is not "shocked" by the sudden change of light output, and the cost is spread out over the course of a year.  There is not much more than anecdotal evidence to support the necessity (or lack thereof) of doing it this way from a livestock perspective. 

Color Temperature

MH lamps come in various color temperatures across the visible spectrum.  The vast majority of halide lamps used for general industry are not suitable for reef aquarium use because their color temperature is too low, usually well below 6500K (Kelvin).  Many hobbyists are tempted to purchase these $20 halide lamps from their local home improvement store.  Unfortunately these lamps are seldom suitable.  A great deal of frustration often arises when hobbyists see the prices of aquaria-oriented halide lamps, thinking they are being gouged.  The thing to remember is that stable color temperature requires a great deal of R&D and quality control, which are a major contributing factor to the price point of the lamp. 

For most reef systems, 10,000K to 20,000K lamps are most commonly used.  The color choice of the lamp is a matter of the specifics of the hobbyist's lighting setup and his/her personal preference.  MH lamps are commonly paired with Actinic fluorescent lamps, allowing the use of the lower temperature lamps.  10,000K lamps with Actinic supplemental lights are commonly used. 

Actinics are not necessarily needed however when using a higher temperature lamp.  Many manufacturers produce lamps in the 12,000-15,000K range, which provides a medium between the very blue 20,000K lamps and the comparatively yellow 10,000K lamps.  Again however, it's a matter of personal preference. 

The lower end of the spectrum (10,000K) generally provides faster growth rates at the expense of coloration.  The upper end (20,000K) generally does just the opposite. 

Color temperature is completely independant of the halide lamp/wattage/ballast matching.  Color temperatures do not need to be paired with any specific ballast, so as long as the wattage matches the ballast, the hobbyist is free to use the color temperature of his/her choice. 

Lower color temperature lamps generally produce more raw light output than higher color temperature.  A system that is marginally suitable for a given system running 10K lamps may not have sufficient light output if those lamps are replaced with 20K lamps, so the required/desired light output should be taken into consideration when choosing lamps with a different color temperature.

It should be noted that color temperature has nothing to do with operating temperature.  All other factors being equal, a 20,000K lamp does not produce any more heat than a 10,000K lamp.  Color temperature refers to the color of the light produced, not the operating temperature of the lamp.

How Much Light?  How Many Lamps? 

There is no de-facto rule that governs how much light you should use in a tank.  How much light is a function of how many lamps you need to use and how many watts each lamp is. 

The easier of the two decisions to make is how many lamps to use.  In order to achieve even and thorough lighting, you usually want to plan for one lamp for every 4 square feet of area.  Halides, effectively placed, will illuminate a 2x2 foot area in the bottom of most tanks.  Exceptionally deep tanks (over 24") may require fewer lamps, but higher wattage, since the beam will spread further in each direction the higher they are from the bottom of the tank.   Tanks deeper than 24" may require special planning and are beyond the scope of this article since most hobbyists do not have tanks deeper than this.   To achieve proper coverage on a tank up to 24" deep, you want to place the halides approximately 34-38" above the bottom of the tank. 

Halides should never be placed above a cross brace.  Plastic cross braces are easily deformed.  Even if the halide does not melt them, the light can degrade the plastic or soften it, allowing the brace to strech as it is heated from the lamp while exposed to the pressure of the water pushing outwards on the brace.  This can cause a catastrophic tank failure.   Ergo, most hobbyists will place a halide lamp centered in the "compartments" formed by the center braces.  3 lamps on a 125 or 180g tank are common. 

The second decision, that of what wattage to use is a function of tank depth, but more importantly, the type of livestock.  SPS corals, clams, and other light-needy livestock requires more light than a soft coral tank or a fish only tank.  A hobbyist should research the type of livestock they wish to keep and make their lighting decisions accordingly.  As a rule of thumb, most 24" deep tanks use 250w halides, such as 125g and 180g tanks.  18" deep tanks, such as the 55g, might be able to get away with 175w lamps.  Remember that going overboard on wattage does nothing to help your livestock if they do not need the light, but does waste electricity and cause undue heating of the water.  Many hobbyists size their lighting systems a bit lower and place more light-needy livestock near the top of the tank.  This is perfectly fine as long as you don't mind living with that restriction and do not keep anemones.  Anemones may move to an area with less than desirable light, so you cannot count on them staying where the light is suitable. 

Reflectors can make a tremendous difference in the amount of light obtainable from any lamp and halides are no different.  Without reflectors, any lamps lumen output may be impaired by 60% or more as light is "lost" to scatter or absorption in an aquarium canopy.  Cheap or even DIY systems can function, increasing the light available in the tank significantly but the amount of light obtained from high-end "spider reflectors" such as Lumenarc reflectors can focus and direct up to 130% of a lamps rated lumens into the tank.  In some cases, this enhanced lumen yield can allow you to use the next smaller lamp with the same net output in light.  Therefore, quality reflectors should be heavily considered because their up front investment may be quickly made up in energy savings and reduced effort to cool the water.   

Bargain Lamps

The auction sites and some bargain-basement aquaria retailers will often sell halide lamps at a dramatically reduced price.  Where you can expect to pay anywhere from $70 - $100 for a quality halide lamp, you can often find halide lamps through these retail channels for as little as $20.  It may be very tempting to go this route, as conventional wisdom surrounding light bulbs tell us "a bulb is a bulb."  Not true with halides. 

Halides gain their color through an exact doping of the arc tube with halide salts.  The manufacturing process that accomplishes this is very precise.  Lamps that are not carefully designed and quality controlled will often suffer signifcant color drift, where they fail to maintain their intended color temperature.  Bargain lamps are often "factory seconds" that were rejected by mainstream manufacturers.   Although many success stories with these bargain basement lamps are out there, you should be aware before deciding to go that route of the risks and potential pitfalls, including reduced PAR values as well as "off" color (kelvin) results.