This series of articles is for those curious souls out there that have often wondered how the lighting systems they use in their aquariums work and why we do things the way we do. I will not try to tell you which lighting system is best nor how much light you need for certain organisms – these questions have been discussed by many others and is obvious from the continued discussion that we will still be doing this for some time to come. My main objective here is to discuss the operational principles behind the various types of lighting systems used for aquariums and give some insight as to their strengths and weakness in a simple and hopefully straight forward manner.

We will primarily be discussing the three major types of lighting systems currently being used in the aquarium industry but will also speculate as to future trends and possible alternative lighting systems. The three major lighting types are Incandescent, Florescent and High Intensity Discharge (HID). This latter category of HID may not seem familiar to some but includes Metal Halides as well as Mercury and Sodium Vapor based lighting systems

One of the oldest and simplest forms of lighting used in the aquarium trade is incandescent lighting and while it has been superseded over the last decade or two by florescence lighting as the most popular for aquariums it is still found in many inexpensive systems or specialty applications.

The operation of an incandescent light bulb is simplicity itself – an electrical current is passed through a small coil of wire (called the filament) inside the bulb causing it to heat up (see diagram #1). When any object is heated up to a high enough temperature it starts to glow visibly with its color strongly related to the temperature it is heated to. As the temperature of the filament is increased by putting more and more electrical power into it, it’s color goes from a dull red to an ever whiter/bluer color. This process of a heated object giving off light (or in a more general term electromagnetic radiation) has a name and is called "Blackbody Radiation". Any object capable of being heated to a similar temperature gives off the same electromagnetic signature or spectrum. Because of this commonality of radiation behavior when an object is heated you can also tell uniquely what temperature an object is at by measuring its spectrum or Blackbody radiation signature (assuming you measure this spectrum in the dark, i.e., the only radiation from the object is due to its temperature). This relationship between an object’s temperature and its radiation spectrum results in a second term often used for descriptive purposes and is called "Color Temperature". The typical color temperature for most incandescent lighting systems is limited to about 2500-3050 degrees Kelvin. Compare this with natural sunlight which has a color temperature of about 5500 degrees Kelvin (when viewed from the Earth’s surface).

The electrical power you can apply to an incandescent light bulb is limited by the materials used in the bulb, the bulbs specific design and lifetime considerations. The material most often used in incandescent light bulbs for its filament is Tungsten because of its high melting point (~ 3270 degrees Kelvin, hence the color temperature limit). Even this high melting point would not be enough for reasonable operational lifetime for the filament in an incandescent bulb if it were not for there being very little air/oxygen inside the bulb. This lack of air or specifically oxygen in the bulb keeps the heated filament from being burned up faster. The main failure mode for an incandescent light bulb is simply the breakage of the filament in the bulb. This filament brakeage is normally caused by the gradual evaporation of Tungsten from the filament during normal operation, primarily at defect areas along the filament (no filament is perfectly formed). These defect areas in turn heat up even hotter due to becoming thinner from the evaporation process which in turn starts evaporating even faster due to the higher temperature eventually leading to brakeage of the filament and a dead bulb. Most incandescent bulbs have lifetimes in the 1000 hour range.

A variant on a conventional incandescent light bulb is the Tungsten-Halogen. This bulb incases the Tungsten filament in a small quartz capsule filled with Halogen gas (typically iodine or bromide), this configuration allows the filament to operate at somewhat higher temperatures (and thus producing somewhat whiter light) and efficiencies compared with standard incandescent bulbs. Lifetime is also increased with Tungsten-Halogen bulbs due to the Halogen gases combining with evaporating Tungsten from the filament and redepositing it back on the filament. This redeposition process results in lifetimes of about 2000 hours.

The greatest strength of an incandescent lighting system is its simplicity – you do not need additional special and costly equipment to operate it as you do with florescent and HID lighting and it is readily dimmable with minimum extra expense. On the negative side – an incandescent bulb is very inefficient at producing visible light, much of its electrical power goes into producing heat. Other down sides to incandescent bulbs are lower lifetimes than alternatives such as fluorescents and HID’s as well as limited color temperature capability resulting in inaccurate color rendering (most incandescent bulbs have a yellow cast to them due to the lower color temperatures).

Florescent lighting is probably the most ubiquitous lighting used today not only for aquariums but of lighting requirements of all types. Florescent light bulbs are more efficient than incandescent, can have better color rendition as well as longer bulb lifetimes so it is not surprising that they have displaced incandescent bulbs in many applications.

Diagram #2 shows a cross section of a typical florescent light bulb. You can see that it consists of several components – its electrodes (similar to filaments in incandescent bulbs), glass tube, low pressure gasses (typically argon or argon-krypton with a small amount of mercury added) and finally a coating on the inside of the glass tube composed of materials referred to as phosphors. We will detail the electrical operation of a florescent lighting system in part II of this series but for now it is sufficient to outline its basic operation. First, an electrical voltage of sufficient strength is applied across the florescent bulbs electrodes causing an electrical current/arc to flow between them. This current is composed of moving electrons which in turn interact with some of the low pressure gas atoms specifically those with the element mercury. This interaction of the electrical

current with the mercury atoms cause some of that atoms electrons to temporarily move to higher energy states but since these higher energy states are not stable they quickly drop back down to their original levels and in doing so cause the effected atom to emit light (think of a finger plucking a guitar string – it initially vibrates but eventually returns to its resting position all the while emitting sound in between these two states). Unfortunately this emitted light is not in the visible spectrum (i.e., we can not see it with the naked eye) but is in fact in the Ultra Violet (UV) range of the light spectrum.

To get visible light out of a florescent light bulb we have to rely on the principle of "Florescence" (thus the name florescent bulb) and is defined as the process of a material being able to absorb light of one frequency (such as UV) and then reemitting it at a different frequency (such as visible light). The component part of the florescent bulb which generates this florescence effect is the phosphor coating(s) found on the inside of the florescent bulb’s tube. There are various different types of phosphors (e.g. calcium tungstenate, zinc sulfide, zinc silicate, etc.) and each type emits visible light at different wavelengths or colors after first absorbing UV light. By selecting the right combination of phosphors and their relative amounts you can adjust the light spectrum of the florescent bulb allowing significantly more flexibility in performance than seen with incandescent bulbs. Effective color temperatures of florescent bulbs (see below for what is meant by effective color temperature) can be as high as 10,000 degrees Kelvin or above.

It should be noted that most light spectrums from florescent bulbs are discrete, consisting of many individual spectrum spikes rather than the more continuous light spectrum seen with incandescent bulbs and natural sunlight (see diagrams #3 & 4). Because of this difference in light spectrum between florescent and incandescent bulbs you should take any "Color Temperatures" quoted for florescent bulbs with a grain of salt. Florescent bulbs are not "Blackbody" radiators and therefore trying to apply color temperatures is at best an approximation. In general a higher color temperature florescent bulb will normally be whiter or more blue in color but two florescent bulbs having the same quoted color temperature may still look different due to their light spectrums not being quite the same.

Another advantage of this light spectrum tailoring possible with florescent lights is that they normally produce much less heat than an incandescent bulb to produce the same amount of visible light making them much more electrically efficient (i.e., takes less energy to produce the same amount of visible light, typically 3-4X more efficient).

The main failure mode for florescent bulbs (assuming you don’t brake them) is from depletion of the electron emission coating found on the electrode/filaments. The filaments used in fluorescent bulbs are normally tungsten coils coated with a special material (typically barium oxide) that emits lots of electrons when heated (~ 1300K). When this coating on the filament is gone the bulb can no longer maintain the electric arc in the tube. On top of this electrode failure mode it is also possible to lose bulb light intensity over time or have the effective color temperature shift. These reductions in a bulb’s light performance over time can be caused by a several factors including degradation of the bulb’s phosphors, darkening of the tube’s glass (both due prolonged exposure to UV light and the elements generating it) and evaporation of the coatings use on the electrode’s filaments (which in turn can react with mercury and cause darkening of the bulb near its ends). Phosphor degradation rate will depend on several things including the specific phosphors used as well as the bulb wattage relative to its surface area. This means that the light properties of tubes using higher wattages for a similar sized tube such as High Output (HO) and Very High Output (VHO) bulbs will normally degrade faster than standard output bulbs. Most standard output florescent bulbs claim 10,000-20,000+ hour operating lifetimes but can fall to 80% or less of their original light intensity after 10,000 hours operation. Depending on your requirements for light intensity and color fidelity you may need to replace bulbs more often than indicated by full failure lifetimes.

One additional comment concerning florescent light bulbs – while the mercury vapor inside the bulb produces UV light when excited, little UV radiation is emitted outside of conventional bulbs due to both its absorption by the phosphors on the inside and also the fact that glasses typically used for tubes block most UV light. One special purpose aquarium florescent bulb where this is not true is the bulb used for disease control found in so called UV sterilizer units, this bulb normally has no phosphors on the tubes inside and the tube itself is made of more expensive quartz rather than glass because it [quartz] does not block UV light. The light emitted in these UV sterilizer bulbs is referred to as UV-C and consists of the shorter wavelength (higher frequency) UV spectrum. This UV-C radiation is dangerous to living organisms allowing these bulbs to be used to kill free floating forms of disease and parasites when used properly in a sterilizer unit.

There are some definite down sides to using florescent lighting systems and most of these are related to the more complex electrical requirements needed to operate them, in particularly the need for electrical "Ballasts". The next article in this series on lighting system operation will discuss in more detail the various electrical requirements for florescent lighting systems, the pros and cons of conventional and electronic ballasts and also how they work. A third article will then follow that covers HID systems and other more exotic or possible future lighting options.