So far in this series on lighting systems we have looked at incandescent and fluorescent lighting and how they work. In this article we will deal with HID lighting systems and other somewhat more esoteric lighting systems that might be considered for aquaria use.

HID or High Intensity Discharge lighting systems include high and low pressure sodium lights as well as mercury vapor and metal halide (MH) bulbs. While both high pressure and low pressure sodium lights offer the greatest light efficiencies (often used as street lights) their color rendition (CRI) and color temperatures are very low leaving them unacceptable for aquarium use. Mercury Vapor lighting has somewhat better color rendition than the sodium lamps but one-third their efficiencies and greater lumen depreciation over life (see figure #1). MH bulbs offer the best combination of good color rendition, light efficiencies and bulb life making them the HID bulb of choice for aquarium use.

Table #1 shows the typical lighting properties for incandescent, fluorescent and the various HID bulbs. As can be seen MH bulbs are normally equal to or slightly more efficient than fluorescent but MH’s higher wattages and more compact size make them the lighting system of choice for high intensity lighting requirements. On the downside MH bulbs produce more heat than your typical fluorescent so normally require placement further from the tank surface (~12-18") or end up dumping significant levels of heat into the tank, which in turn may complicate its cooling requirements (i.e., expensive chillers).

MH bulbs are somewhat similar to fluorescent ones in that they require a ballast to operate (details discussed later) but differ in actual construction and operation.

Table #1: Comparison of lamp lighting properties

A Metal Halide lamp is normally composed of the following components (see figure #2 for bulb schematic):

1. A clear quartz arc tube with tungsten electrodes at either end. The arc tube is filled with gases (normally argon) along with mercury and other metals (e.g., sodium, lithium, thallium, indium, scandium, etc.) in iodine/halogen salt form, thus the name metal halide (halide refers to mercury content). These bulbs are also sometimes referred to as HQI (especially in Europe), which stands for "Mercury Quartz Iodide" (the H refers to the chemical symbol "Hg" used for mercury). The arc tube also normally has heat reflecting coatings on either end to improve its thermal characteristics during operation. This addition of metal salts serves to improve the color appearance and rendering properties as well as the luminous efficacy of the MH lamp in comparison to standard mercury lamps.
2. Starting electrode—used during lamp start-up to help provide the proper electrical potentials to initiate the lamp. Typically includes a bi-metallic contact that automatically disconnects the starting electrode upon initiation of tube electric arc due to increased lamp temperature.
3. Normally (but not necessarily) enclosed in a gas-filled hard glass outer bulb that may be clear or phosphor-coated. This outer bulb reduces temperature variations in the arc tube due to air drafts and prevents the internal interconnects from oxidizing. This outer bulb also typically acts as UV absorber, which cuts down on the level of harmful UV light emitted from the bulb. If you are using a MH bulb without this outer glass bulb or if the outer bulb is also made of quartz then you need to provide a separate UV blocking lens or shield to use it safely.
4. Bulb interconnect in the form of a base or contact pins. The two most common configurations of MH bulbs used in aquariums are the single ended mogul /medium base and the double ended pin configuration (see figure #3).

The starting process for a MH bulb is as follows:

When a MH lamp starts, the light output is about 3% to 5% of full intensity and lamp voltage is approximately 15V to 30V. As the arc tube's internal pressure and temperature increases, the output and lamp voltage increase until the stabilized operating values are reached, usually in two to six minutes. Energy from the arc tube heats the bimetal switch in the starting electrode circuit, which opens after two to four minutes. This prevents electrolysis and breakdown of the molybdenum seal.

The actual means of generating light in a MH bulb is similar to that seen with fluorescent bulbs and their low pressure mercury doped gases (i.e., excitation of electrons in the mercury and other metal ions by the electrical arc and subsequent emission of light as the excited electrons relax). The primary difference in MH bulbs is the presences of the additional metal ions (normally combined with iodide to form the metal salts) coupled with higher operating pressure/temperature in the bulb, which allows the direct generation of visible light without the need for phosphor coatings.

The presents of iodine in the metal salts helps improve the bulbs lifetime and lumen depreciation rates in similar fashion to that seen with incandescent quartz halogen/iodine lamps by preventing tungsten in the electrodes from depositing on the arc tube via facilitating its redeposition on the electrodes as it [tungsten] vaporizes during normal bulb operation.

When a MH bulb is turned off it is still hot and so the gas pressure in the arc tube is still high. This higher pressure in the bulb prevents the re-ignition of the electrical arc till the bulb cools sufficiently to drop the pressure—thus the 5-15 minutes off time needed before a MH bulb can be re-started. In some commercial applications it is possible to use a very high voltage pulse (25-50 kV) to reinitiate the electrical arc in a MH bulb without having to wait for it to cool but this requires special circuitry not normally found in those MH systems typically used in aquaria. These high voltage pulses can also be used to cold start MH bulbs specifically designed for this and eliminate the need for starting electrodes in the bulbs.

Actual failure in a MH bulb (not counting bulb breakage) is normally caused by an inability of the bulb to initiate an arc due to ever increasing voltage requirements over time from the degradation of materials in the arc tube.

Other properties of MH lamps that can effect their operation or lifetime are:

1. Operating position - MH bulbs can be designed to operate in either vertical (up or down), horizontal or universal positions. Those designed to operate specifically in vertical or horizontal positions typically produce somewhat more light output than one designed to be universal (~+10%) but performs much poorer if used in the wrong orientation as well as significantly shortening the bulbs lifetime (~15-20%). To identify what position a particular bulb is designed for look at its ID code, if you see a /BD or /BU as part of its ID number then the bulb should be mounted base down or up respectively. Bulbs marked /HOR should be mounted horizontally. If you see /U or no marking then bulb is likely a universal design and can be mounted any direction with minimal loss of performance. Most double ended bulbs are designed for horizontal mounting.
2. Duty cycle — turning MH bulbs off and on repeatedly can reduce their lifetimes. The normal operation seen in aquariums in which bulbs cycle once per day for 10 or more hours, should have minimal impact on a bulbs lifetime but shorter cycle times can reduce operational lifetimes significantly.
3. Operational voltages — variations in the applied voltages (e.g., line voltages, etc.) to MH bulbs can have significant effects on both bulb lifetimes as well as lumen depreciation and color shifting in the bulb. We will elaborate on this during the discussion on MH ballasts.
4. Initial burn-in period – most MH bulbs require a burn in period of about 100 hours before their performance stabilizes. There may be color shifts, flickering or lumen changes during this period. The reason for this burn-in period is that the metal salts in the arc tube need time to distribute themselves within the tube to provide optimal/stable operation of the bulb.

As with fluorescent lamps MH lamps require a ballast to work properly. The reasons for the ballasts are also similar to those seen with fluorescents in that it provides the proper starting voltages while limiting the current going to the lamp during its normal operation. I will not go into the details of a ballast’s operation as that was covered in part II of this series but will instead concentrate on the differences seen with MH ballasts and how they effect the properties of the MH lamps.

The most obvious difference between conventional fluorescent and MH ballasts is size. As MH bulbs typically have higher wattage ratings than fluorescent bulbs (150-1500 watts Vs. 15-215 watts for fluorescent) this should be no surprise. This means that a conventional MH ballast will also dissipate more heat than your typical fluorescent ballast and needs to be taken into account when locating the MH ballast.

As was alluded to earlier the control of the applied voltage to a MH bulb is very important to its operating characteristics including lifetime, lumen depreciation and color shift. The reasons for this sensitivity to applied voltages has to do with creating the proper electrical arc in the bulb, higher or lower than optimal voltages can cause degradation in the bulbs components especially its electrodes. This is why trying to operate a 150W bulb with a 175W ballast is not a good idea since it can shorten the bulbs lifetime and accelerate any possible color shift and lumen depreciation. More sophisticated ballasts are available for MH bulbs that try to better regulate the applied voltage (i.e. compensate for variation in line voltage as well as aging effects in the bulb itself), with the most sophisticated being of the electronic variety.

Many of the advantages claimed for fluorescent electronic ballasts also apply to MH electronic ballasts such as:

1. Lighter/smaller ballasts that dissipate less power/heat (10-15% lower).
2. Quieter operation due to higher frequency typically used (20KHz+).
3. Better bulb life, lower lumen depreciation and color shift (claims of 10-20% improved bulb life) from better voltage regulation. (more critical for MH than fluorescent).

One advantage for operating fluorescent with electronic ballasts that MH can’t claim is higher light efficiencies at higher operating frequencies. So do not expect higher initial light output using MH electronic ballasts though light loss over time should be less.

Disadvantages of electronic ballasts for MH are also similar to those for fluorescent but adds one new one as well:

1. Higher initial cost of ballast (lower operation costs due to power savings in ballast and longer bulb lifetime will offset this over time).
2. Possible electrical interference due to higher switching frequencies. The higher power ratings for MH may actually make this line noise worse compared with fluorescent electronic ballasts and require special inline noise filters for acceptable operation.
3. Higher inrush currents (explained in part II of series).
4. Possible acoustic resonances in arc tube due to higher operating frequencies leading to reduced bulb lifetimes and erratic operation.

This last issue of acoustic resonances needs further explanation. As with fluorescent bulbs MH bulbs actually turn off and on with each cycle of the AC signal applied to them (typically too fast to see with naked eye and fast enough that the electrical arc can restart without the cooling off period needed for full off conditions). The difference with MH has to do with the much higher operating pressures in the arc tube. When a MH bulb briefly switches off and on with the applied signal it sets off a pressure wave with in the arc tube. At the lower frequencies of operation with a conventional ballast (~60Hz) these pressure waves cause little harm but at the higher frequencies normally seen with electronic ballasts it is possible to excite acoustic resonances in the arc tube. These acoustic resonances can cause disruptions in the electrical arc and in the worse cases actually cause the bulb to fail.

This acoustic resonance issue coupled with the sensitivity to the applied voltage likely explains why in some cases vendors have more than one electronic ballast design for similar wattage MH bulbs from different vendors as they need to tune them for the individual characteristics of the vendor’s bulbs.

1. Higher initial cost of ballast (lower operation costs due to power savings in ballast and longer bulb lifetime will offset this over time).
2. Possible electrical interference due to higher switching frequencies. The higher power ratings for MH may actually make this line noise worse compared with fluorescent electronic ballasts and require special inline noise filters for acceptable operation.
3. Higher inrush currents (explained in part II of series).
4. Possible acoustic resonances in arc tube from higher operating frequencies leading to reduced bulb lifetimes and erratic operation.

Dimming MH lamps is very difficult and in most cases is not recommended. Due to the sensitivity to correct operating voltages it is very difficult to dim MH bulbs without significantly degrading their performance and lifetimes. In those case where special electronic ballasts do allow dimming MH lamps it normally only allows a fixed reduction in operation power and typically not any lower than about 50% power reduction. Even this lighting reduction to 50% normally requires that the bulb first come up to full intensity before it can be reduced to the lower operational level and thus not allowing a gradual increase in light intensity. The requirement to operate at full intensity before dimming is to make sure that all the metal ions are vaporized and the proper electrical arc established. If this is not done then a bulbs performance and lifetime can be significantly reduced.

If you need to vary the light intensity using MH bulbs the safest method is to use several smaller wattage units and stager their operation times or in conjunction with a second lighting system that can be dimmed (e.g., fluorescent).

Due to the high operating pressures and strong UV light generated by the arc tubes in MH bulbs never operate a MH lamp with a outer UV shield (either integrated or external). Some MH bulbs are specifically design so that if their outer envelop breaks the inner arc tube will not work and thus provides an additional safety factor.

The higher operating temperatures of MH bulbs also make their placement above a tank more of a safety risk in that splashing water on the bulbs can cause them to break or otherwise fail. If placement near the surface (<12") of the tank is desired it is recommended that some sort of optically clear shield is placed between the tank and the bulbs.

There exist other non-traditional lighting systems that might be of interest to aquarists but are either not commonly available, still too costly or still under development. This next section looks at some of these lighting system alternatives, how they work and what their strengths and weaknesses are for possible aquaria use

Xenon bulbs (the other HID): Even better color rendition than MH (used in high end video projectors) but higher operating voltages, costs ($500-$1000/bulb) and lower lifetimes (1000-2000 hrs) makes them currently a poor choice for aquarium use.

Induction bulbs: Induction lamps produce light through the use of an induction coil to create a high frequency electromagnetic field (250KHz-3 MHz) inside a gas called an electron/ion plasma. This field excites the plasma material inside the glass housing causing the mercury atoms to emit ultraviolet light. When the UV light passes through the phosphor coating on the glass housing it's converted into visible light in very much the same manner as fluorescent lamps. The induction lamp is designed to light immediately with no warm-up period or flicker. (see figure #4)

Induction lamp properties:

1. Lifetime is better than fluorescent given that they do not have any electrodes in the bulb (induction coil is outside the glass envelope of the bulb), lifetime claims are for 20K-100K hours.
2. Lumen maintenance is also better than fluorescent lamps (quoted to be 70% at 60K hours or more) again in part due to the lack of internal electrodes.
3. Lamp efficiencies are in the lower to mid range of fluorescent lamps or 60-75 lumens/watt.
4. Color temperatures run 3000-4000 K with CRI’s of about 80.
5. Wattages range from 23-150 watts/bulb.

On the down side:

1. Requires high frequency generator to operate the induction coil which may be integral with the bulb or separate unit (think of as highly specialized electronic ballast).
2. Induction lamps can not be used with dimmers.
3. Prices are as high as $200/bulb but includes the control units. Higher price should be offset some by the longer expected lifetimes.

Your not likely to find these lamps [induction] in your neighborhood hardware store as they are still fairly specialized but they might be interesting to try in an aquarium setting if higher color temperatures were available (would think color temperature could be increased as seen in many fluorescent lamps by different choice of phosphors used).

Performance claims for sulfur lamps:

1. High light efficiencies of 95-100+ lumens/watt (similar to fluorescent and MH).
2. The lamps themselves have a very long life since there are no filaments or electrodes to burn or wear out, however the magnetron microwave generators currently must be replaced every 15,000 to 20,000 hours which still yields a lifetime similar to the best MH bulb.
3. It is claimed that these bulbs can be dimmed to 20% output without change in color.
4. They are very compact, producing 135,000 lumens (equivalent to a 1500W MH lamp) from a 1425W lamp the size of a golf ball.
5. The light produced is of very high quality, similar to that of sunlight but will little UV so no shielding is required. Color temperature of current units is about 5700 K with a CRI of about 80 (but claims of higher color temperatures of 9-10K are possible).
6. Very little lumen or color degradation over time.
7. A major environmental advantage is the absence of mercury, so disposal poses no environmental hazard. Turn on time of about 25 seconds, faster than MH but slower than fluorescent.

On the down side:

1. Limited availability, with lowest wattage rating currently being about 1425W.
2. Bulb needs to be rotated by motor to reduce hot spotting and mix plasma (~1000 rpm). Bulb will fail if not rotated.
3. Current cost is $2000-4000 including light pipe (see section on light pipes for details) but producers expect it to eventually be competitive other technologies such as MH.

This technology seems to still be in the development stage but might be of interest for aquarium use in the future. The sulfur lamp has also been successfully combined with light pipe technologies and are discussed next.

Light pipes: A simple light pipe consists of a tube coated on the inside with an optical film or mirrored in some fashion which may or may not allow some of the light which is reflecting off the interior surface of the tube to pass through the tube (causing tube to glow). As noted above in conjunction with the sulfur lamp, a recently developed innovative light pipe technology was demonstrated coupled with the sulfur lamp. This was a more advanced design with two optical films, and a series of small holes in the bottom of the tube to further direct the light (see figure #6 ). In an installation in the Smithsonian National Air and Space Museum in Washington D. C., three 90-foot light pipes, each lit at one end by a single sulfur lamp have replaced 94 conventional incandescent lamps increasing light levels by a factor of three, and cutting energy use by a fourth.

Another application of light pipes is in conjunction with skylights. The light pipes allow the sunlight light collected by the skylight to be redirected to another location such as a ceiling above a tank. (see figure #7)

Advantages of skylight based light pipes include:

1. Free lighting during day, level and duration may vary (see downside issues). Some estimates for commercial units indicate equivalent light intensity to 1200W incandescent or about 250 watt MH (mileage may vary).
2. Low heat transfer.
3. Blocks harmful UV light.

On the down side:

1. Higher initial cost but could be more than offset over time due to lower electrical costs.
2. Incorporation into house structure may be difficult, especially if tank on first floor of two story building.
3. Intensity and duration of light depends on weather conditions and time of year so living location may significantly influence consistency of light availability.
4. If lighting required at night then supplemental light fixtures still required (overall operating cost may still be much lower though).
5. Unless supplemented by additional fixtures overall color temperature of light may be lower than desired (natural sunlight color temperature is about 5500 K).

I hope this series on lighting system operation has been useful, I know I had fun researching it. As indicated when I started this series no attempt was made to make claims as to what the best lighting system for aquaria is, look to other references for recommendations for this (you’ll find no small number of opinions on this to be sure). If anyone has additional comments concerning any of this information or further questions feel free to contact me at rexn@lmi.net or see me at our next general meeting.