Metal halide lamps are excellent lamps for reptiles because …
However, they do have some disadvantages:
These disadvantages are only minor in the context of terrarium lighting, and once you are used to metal halide lamps, the thing about the electrics does not seem that complicated any more.
Side note: UV-metal halide lamps, like the brands solar raptor, lucky reptile bright sun, … have a different spectrum. Their visible and UVA light is not as nice, the spectrum is spiky and the amount of UVA is quite huge.
There is a huge variety in metal halide lamps for different applications. Metal halide lamps range from 20W to several thousand watts and come in different shapes and sizes.
These lamps seem easiest at the beginning, because they look like normal incandescent bulbs and also some reptile metal halide lamps from the pet store have this form. But often they are more expensive and you need to do all the electric wiring yourself.
Although these lamps have a normal E27 base, they must not be used with normal (plastic) E27 sockets. Metal halide lamps are ignited with 5000 V and a normal E27 would not withstand this strain. You must at least use a ceramic socket, best is a socket that is specified for 5kV ignition voltage.
This high ignition voltage and a stable current during operation is generated by the ballast. The ballast is switched between the wall plug and the lamp.
Two kinds of ballasts exist: Electronic ballasts and conventional ballasts. Conventional ballasts are heavier, sometimes make a buzzing noise, waste more energy, but are more robust against voltage fluctuations. Sometimes conventional ballasts are divided in three parts: A coil, sometimes called ballast, a condensator and an ignitor (pictures below). An electronic ballasts is always only one device, they are much lighter, consume less electric energy, sometimes they produce a high frequency noise. In most cases I would recommend electronic ballasts. The wiring is printed directly on the electronic ballast. Attaching the cables is technically very easy, but depending on the legal regulations in your country, your insurance and your willingness to take risks, you should ask an electrician to check your installation.
Always use a ballast that is specified for the same power as the lamp! Never use a 150W ballast or a 50W ballast for a 70W lamp! You risk an explosion of the lamp, and the lamp is filled with toxic mercury. Even if the lamp does not explode, the lifetime is reduced because the power does not fit to the lamp and the spectrum of the lamp is altered. Metal halide lamps do not function like normal incandescent household bulbs, so do not try to transfer your experience with these lamps to metal halide lamps!
The cable from the ballast to the wall plug is like any cable from any other household device. The cable from the ballast to the lamp socket needs to be a special cable for high voltage. It should at least be a cable that is thermally more stable than standard cables. Best is a cable that is specified for 5kV ignition voltage.
If UV-metal halide lamps are available in your country, you can use the installation sets that are sold for the metal halide uv lamps.
Metal halide lamps with a housing are more widespread and used in many stores (clothing, food, cars …) with a rail fixture system. The housing contains the socket for the lamp, a reflector and the (often conventional) ballast. The adapter for the rail can easily be removed using some force and replaced by a normal cable with wall plug. In some countries, these lamps can be bought very cheap, when shops close and the lamps are not needed any more. In Germany the best place to buy them is ebay. If you know where to buy these lamps in your country, please leave a comment! These lamps are often fitted with a conventional ballast. You can continue to use this ballast, but if the ballast becomes noisy or is to heavy for your setup, in most cases, there is enough space in the housing to replace the conventional ballast with an electronic one.
There are hundreds of different housings and different reflectors. Some make very good lamps for basking zones because the collect all the light and heat in a small area. Others diffuse the light and can be used similar to fluorescent tubes for general lighting. And some reflectors are just badly designed and do not give a nice distribution of the light.
Different lamps exist. First, there are quartz and ceramic lamps. Their spectra do not differ much (see above), but ceramic lamps have a slightly higher colour rendering index (>90) than quartz lamps (CRI ~80), and ceramic lamps are more efficient (brighter when using the same power) and live longer. When you by a used lamp that contains a quartz illuminant, you can continue to use this. But when you buy a new lamp, I would recommend a ceramic one.
There are dozens of different sockets for metal halide lamps. You need to buy the one, which fits to the housing. Widespread in Germany are TS and T sockets. TS is a two sided lamp. It is more widely used with reflectors that spread the light. T is a one sided lamp that is very good in a round reflector to create a basking zone.
The housing usually has a clear glass front in front of the lamp. Never remove this glass front! It protects you and your pets from glass splinters in the rare event, that a lamp explodes. It also protects the lamp from water drops, which helps to prevent an explosion. An - also very important - it blocks UVC radiation from the lamp, in case the lamp itself does not have a proper UV-stop.
For used housings for metal halide lamps, search “70 W metal halide” on ebay. In Germany you will find houndreds of used lamps that were used in retail lighting before. In other countries this might be different. If you see metal halide lamps in retail stores (clothing, fruit and vegetable section of a supermarket, cars, …) you could ask the owner, where they buy their lamps.
For metal halide lamps with an E27 base, search “Philips CDM R” in google.
Once you have find the first lamp, you will surely find more and search for the best price.