The heart of the CatEye Stadium is its metal halide bulb. This is a completely different technology for turning electricity into light than that used by all other bicycle lights. Do not confuse "metal halide bulb" with "halogen bulb". The two may sound alike, but they denote utterly different technologies in lighting.
The world of lights we commonly use to illuminate other things with can mostly be divided into two main categories: Incandescent lights, which use a filament, and Arc Lights, which use an electrical arc running thru a gas plasma. The world of Arc Lights can be further divided into the world of high pressure arc lights (in which light is produced in a physically small bulb of high pressure gasses) and low pressure arc lights, where a physically big tube of low pressure gas plasma is employed.
Examples of incandescent lights include the 60 and 100 Watt light bulbs most of us use in our homes, most car headlamps, all flashlights, and all current bicycle headlamps. Incandescent lights come in a number of variants, which go under the names of "vacuum", "krypton", "xenon", "halogen", and "quartz-halogen".
Examples of high pressure arc lamps include mercury vapor lamps, high pressure sodium arc lamps, and metal halide arc lamps. All of these are found in street lamps, and industrial and stadium lighting.
Examples of low pressure arc lamps include flourescent lights, of the sort we often use in homes and offices, germicidal UV lamps, and neon sign lamps.
All current popular bicycle lighting systems use incandescant bulbs. Most of these (including NiteRider, VistaLite, Night Sun, BLT, Turbo Cat, and previous CatEye products) employ a particular variant of incandescent bulb called a "halogen" bulb. All incandescant bulbs work in fundamentally the same way: Electricity is forced thru a filament that poses a resistance to the electricity. This causes the filament to be heated to extremely high temperature. It's a basic law of physics that bodies radiate electromagnetic waves with a frequency spectrum determined by their temperature. At the extremely high temperatures of the filament of incandescent lamps, the waves emitted are in part in the visible light spectrum. The "halogen" varient has some gas in it that combines with atoms boiled off the filament and allows these filament atoms to be re-deposited on the filament later, providing for longer bulb life with hotter filaments. But they make light from electricity in the same way as other incandescent bulbs.
The problem with halogen and all other incandescant bulbs is that they are very inefficient. The first incandescant bulbs made by Edison, which used a carbon filament, were around 1% efficient in turning electricity into light. As incandescant light technology improved over the years, tungsten replaced carbon as the material for the filament. As noted above, tricks such as putting halogen element gas into the bulb were employed to allow filaments to last longer, even when run hotter. In the somewhat more than a century since the incandescant bulb was first invented, efficiency of them has been improved from 1% to about 10%.
The remaining 90% of the electrical energy poured into a modern icandescant bulb is turned largely into invisible infra-red radiation, which we know as heat. Thus, the "20 watt" halogen bulb used in a Night Rider bright beam is actually putting out 2 watts of visible light. The remainder of the battery power poured into that lamp is put out primarily as heat.
There are other ways of making light from electricity. Over a hundred years ago, it was found that if you touched two carbon rods hooked to a powerful electrical source together, some of the carbon would vaporize. If you then moved the rods apart just a little, current would flow thru this carbon vapor as a spark "arced" thru a gap between the rods filled with that vapor. This is the "Carbon Arc" lamp. Though crude, open to air, and requiring constant attention to keep the gap between the rods proper as more and more carbon boils off the rods, the carbon arc lamp produces a very intense, and very pure white light. It's also more efficient in turning electricity into light than an incandescant. Carbon arc lamps were used for years in movie projectors. In certain senses, the carbon arc lamp is the grand-daddy of today's mercury vapor, sodium vapor, flourescent, and metal halide lamps.
In 1901, only a couple of decades after Edison introduced the incandescent bulb, a now-forgotten inventor named Peter Cooper Hewitt invented an arc lamp that used mercury vapor. The vapor was enclosed in a glass bulb. This was the first enclosed arc-type lamp using metal vapor. In 1934, a high pressure varient of this was developed, which could handle a lot more power in a smaller space. Mercury vapor arc lamps are roughly 20% efficient in turning electricity into light, Unfortunately, their light is particularly cold, harsh, blue-green in color. Still, because of their efficiency, they were used for a long time in stret lamps.
The low pressure mercury arc lamp of Peter Cooper Hewitt is the very direct parent of today's modern flourescent lights. It was found that these low pressure arc lamps would put out large amounts of ultra-violet light. Folks then figured that if they coated the inside of the light bulb with a flourescent chemical (one that absorbed UV light and re-radiated that energy as visible light) they could make an efficient light source. Early flourescent lamps used beryllium in their flourescent material, creating a horrible toxic waste situation (beryllium is hideiously toxic to the lungs when inhaled). But later this was replaced with safter flourescent chemicals. Combined with modern electronic technology, the latest flourescent lamps are as much as 40% efficient in turning electrical energy into light. However, they can't be made to produce a bright point source of light that can be focused into a nice beam for bicycling, so this technology is not of interest for bicycle lighting.
Further experimentation with metal vapor arc lamps resulted in the high pressure sodium lamp. This produced a distinctive yellow color of light. High pressure sodium metal arc lamps have been designed to be very efficient. Some can turn over 50% of the electrical power put into them into visible light. Their yellowish light is much more psychologically warm and pleasant. They replaced mercury vapor in streetlight applications.
A different type of sodium arc lamp, the low pressure sodium arc lamp, is the most efficient arc lamp known today in turning electricity into visible light. These can be made to have a staggering 80% efficient in turning light into electricity. However, these are physically big bulbs, and cannot be used to produce a bright point source of light that can be turned into a useful beam for bicycle and automobile applications. Worse, their light is a pure monochromatic yellow, which literally cannot render colors AT ALL. This has greatly limited the number of applications of this highly efficient lamp.
About 40 years ago General Electric began to experiment with more complicated mixtures of stuff inside of mercury arc lamps, in order to get better efficiency and a more pleasing, white color. Starting with mercury metal vapor, they added iodine salts of other metals (indium, scandium, sodium, and thallium). Iodine is one of the halogen elements, and thus a compound of a metal and iodine is called a "metal halide" salt. By 1962 they had applied for patents on this varient arc lamp, which they called a "Multi Vapor Metal Halide" lamp. The "Fountain of the Planets" at the 1964 New York World's Fair was illuminated by GE's new metal halide lamps. Metal Halide lamps are more efficient than their parent, the mercury vapor arc lamp. They can be as much as 50% efficient in turning electricity into light. They tend to not be quite as efficient as the high pressure sodium vapor lamp, but they put out a much more white light. This white light does tend to be somewhat biased toward appearing a bit bluish. These lamps have in places replaced sodium lamps for use in street lights.
The problem with using any of the arc type lamps is that you need very high voltage to start up the lamp... up to 6000 or more volts. Then you need moderately high voltage to run the lamp, in the range of 100 or so volts. Early mercury, sodium, and metal halide arc lamps also took a while to get to full brightness... as much as 5 minutes. When turned off and turned on again, some would take even longer to reach full brightness... up to fifteen minutes. They also were made only in relatively high wattage (200 or more) versions. For all these reasons, they were not available for use on bicycles or cars, where only a 6 or 12 volt DC source of power was avalable, and where there's a need for very rapid turning on and off of the lamp.
In the 1970's and 1980's new developments in electronics (integrated circuits, microprocessors, MOSFET power transistors, and designs for ultra high efficiency "switching" power converters) set the stage for development, in the 1990's, of mass produced, inexpensive electronic chips and circuits dedicated to turning 12 volts DC into the from 100 to 6000 volts to drive a metal halide arc lamp. Such chips were devised to sense the condition of the lamp, and to allow safely forcing it to full brightness far faster than the older arrangements for arc type lamps. Such power conversion is not 100% efficient, of course, but it is staggeringly close to 100% (around 90% or so). The small loses in the power converter circuit are more than made up for by the greatly increased efficiency of the metal halide light arc lamps.
With this technology available, refined metal halide light bulbs were produced in the 35 watt range. These bulbs put out light equivalent to 140 watt halogen incandescant light bulbs. The combination of these smaller metal halide arc lamps and the new electronics for running them off 12 volts has been installed in top of the line Porche and BMW automobiles for the last year. It's possible that, as the cost of this technology decreases, all automobiles will eventually use metal halide arc lamps as headlamps.
CatEye is the first maker of bicycling equipment to attempt to utilize this dramatically far superior technology for a bicycle lamp. I have no formal connection with CatEye, but I must salute them for their courage and vision in attempting to bring this technology to us cyclists. CatEye obtained a 21 watt metal halide arc bulb, and used the same electronics as are being employed in $120,000 BMW and Porche car headlamps. They've packaged this in a fashion that physically looks like an ordinary bicycle lamp. Their lamp is, however, about four times as efficient in turning light into electricity than all other bicycle lamps (which are all incandescent). The light it puts out is equivalent to the light from a 70 or 80 watt halogen bulb. CatEye has combined this lamp with a very carefully-thought-out reflector (CatEye's specialty is reflector design).
With vastly more light available, night bicycling is qualitatively far safer. The road can be lit both further ahead and, even more important, far more brightly to the sides of the bicycle. It's my impression that CatEye's Stadium lamp represents the technology of bicycle light that will eventually replace all of the existing incandescent bicycle lighting systems in serious off road and touring bicycling applications. This because it's not just a little better. The difference is like the difference between night and day.
Though I presented the world of devices to convert electricity to light as divided only into the two categories of incandescent and arc lights, there are devices that turn electricty in to light in wholly different ways.
Light Emitting Diodes (LED's) are among the most efficient devices for turning electricity into light, rivalling the efficiency of low vapor sodium lamps. Like incandescent lights they have the advantage of running directly off low voltage batteries without the need for fancy, expensive electronics to convert voltages. They are totally "solid state" devices, and last many times longer than an incandescent bulb. Unfortunately, they can't be made at this time in sizes that can accommodate more than about a tenth of a watt of electrical input. Thus, you need a great number of them to put out much light, and this raises problems with cost, and (from the point of view of bicyclists) problems with finding a way to combine all those separate light sources into a single useable beam. Also, one can't make a white LED. One has to use separate red, green, and blue LEDs. And high output blue LEDs, a relatively recent invention, are still extremely expensive. For these reasons, they are not a practical approach to high output bicycle lighting, tho I have been told that at least one designer tried to use a large bunch of LEDs in a bicycle light. It never made it to the mass market.
Solid state and gas filled lasers are yet another sort of device that turns electricity into light. Unfortunately, these are hideously inefficient, and produce monochromatic light, utterly useless for rendering colors. Thus, they have no place in the world of bicycle lighting at this time.