A halogen lamp is a type of filament lamp with higher efficacy than standard GLS filament lamps. Compared to a normal filament bulb it will produce a whiter and brighter light for the same amount of energy consumption. It achieves this by running a tungsten filament at a much higher temperature than that of a normal bulb. The filament is contained in a small quarts glass envelope which is filled with one of the halogen group gases. This design allows the "halogen cycle" to take place. Here tungsten atoms that evaporate from the filament as tungsten vapour, combine with the halogen gas, and get redeposited on the filament. This allows the filament to be run at the higher temperature without also reducing the life of the filament.
Halogen lighting is currently very "trendy", however there are cons and well as pros, and this article attempts to address some of these.
Types of halogen bulb
Halogen bulbs are typically found as capsule bulbs, spot bulbs, or short linear strip bulbs.
The capsule bulbs typically have a very small physical size and simply consist of a quarts glass envelope with protruding wires to allow electrical connection. They produce light omni-directionally and rely on the light fitting for any masking or directional reflection. Capsule bulbs are also available in very low power (from about 2W) forms for panel illumination and other tasks requiring embedded but bright light sources.
These typically contain a capsule bulb built into a small enclosure that forms a reflector behind the bulb. They project a quite well defined beam of light forward with relatively little spillage from the rear of the bulb. Hence they do not usually need additional reflectors to be built into the fitting.
Some lamps are open ended (i.e. the capsule could be touched), and others have a glass front to the lamp covering the open end of the reflector. Due to the explosion risk associated with halogen lamps it is important that the open type are only ever used in fittings which have their own front glass shield. They should never be used in open fittings.
Reflector lamps are often available in two forms - normal, and dichroic. The normal ones have an aluminium filmed reflector that is basically completely opaque. This projects all the light and heat forwards. The Dichroic bulb has a rear reflective coating that is not reflective to all wavelengths of light. They are designed so that some of the wavelengths near the infra red end of the spectra will be able to pass through the reflector rather than being reflected out. This reduces the heat projected forward by the lamp, and also improves it colour rendition slightly. These lamps also have a characteristic pinkish colour spill from the back of the lamp which makes them more attractive when used in some display lighting situations. Dichroic lamps are usually only found in low voltage halogens rather than mains ones.
Spot lamps are available with different beam widths. These start with very narrow beam widths of 12 degrees, ideal for feature lighting with little overspill, but hopeless for area lighting. Other beam widths such as 24, 36, 38 are also available. The wider beam widths being more suited to task lighting or general lighting in small areas.
Linear strip bulbs
These are bulbs with a long heavy filament contained in a glass tube with a terminal each end. These are typically used in very high power lights such as security lights or flood lights. Power consumption starting at 150W, with 300W and 500W also being common. These bulbs produce lots of heat and a dazzling light that needs to be used responsibly and only in limited situations.
Lamp handling safety
Halogen lamps require the close fitting glass envelope to allow the halogen cycle to operate (it only starts working at several hundred degrees celsius, and it is not uncommon for the glass envelope to reach in excess of 700 degrees during normal operation). The very high temperature of operation would melt a normal glass envelope, hence a special quarts glass must be used. One must take great care when handling this envelope to not allow any contaminants to come into contact with the glass, and that includes finger prints. If this is allowed to happen it can start a process that will weaken the glass and cause premature failure of the lamp. Note also that the lamps are designed to run at a higher positive pressure than normal bulbs, and hence will often explode should the quartz envelope fail.
It is safest to install halogen lamps using clean gloves. If you do accidentally touch the glass, then clean any trace of grease from the bulb surface using mentholated spirit or isopropyl alcohol (IPA) cleaner.
Beware handling halogen lamps or fittings that have been running recently, not only will they stay hot for a long time, they are also more at risk of damage when hot (the filament is weaker and could break, and the glass envelope will be softer)
Low Voltage or Mains Halogen
Mains halogen lamps are designed to run directly from a 240V mains supply like most normal lamps. Low voltage ones are designed to run from a 12V AC supply provided by a transformer or small switched mode power supply ("electronic" transformer). While the mains ones are marginally easier to fit, and have a lower installation cost that is about the limit of their advantages. The LV lamps will produce more light for a given power, and the light will be whiter and have better colour rendition (note that some may find high power LV lamps to clinical in some circumstances). The filament in the LV lamps is also thicker (since it runs at a higher current) and hence more robust. This gives the bulbs longer life and also makes the bulbs a little less vulnerable to vibration (which will quickly cause a short life for any bulb, but particularly mains halogens). The transformers and control gear used for LV lamps also typically includes a "soft start" capability that increased lamp life further by reducing the thermal shock the bulb experiences at switch on from cold. LV lamps are also available in dichroic versions which are rarely found in mains voltage halogens.
One transformer can power several lamps if required. It is important to match the lamp load to the transformer rating reasonably closely since many will have a minimum load below which they will not work correctly. You can also get transformers designed for powering just a single lamp. These are easy to retrofit to mains halogen installations since the transformer may be pushed through the mounting hole of many of the inset light fittings.
Transformers can fail, so it is important to think about how one would be replaced should it do so. Building it irretrievably into the fabric of the building is not usually a good idea.
One feature of LV lamps is that they run at lower voltages and much higher currents than their mains equivalents. This has a number of implications:
One must pay more attention to voltage drop. A loss of 1V due to cable resistance on a 240V bulb is unlikely to be visible. However a 1V drop on a 12V bulb will represent nearly a 10% reduction in voltage. This will be very noticeable in the light output. The current consumption of a 50W bulb will be over 4A. So a run of half a dozen will draw in excess of 25A. If wiring a number of lamps from a single transformer, a very substantial gauge of wire will be required to not only carry the current, but also minimise the voltage drop (which will be proportional to the square of the current). Generally it is better to run an individual radial feeds to each lamp (i.e "star wiring") rather than placing multiple lamps on a single radial. Obviously one can use a pragmatic combination of the two methods to ease wiring and maintain lighting performance.
One (relatively rare) problem that can also occur, is transmission line losses. Most electronic transformers will produce their LV output at a frequency much greater than 50Hz, however some take this to an extreme and have very high output frequencies. Normally this is not a problem, but if the selected frequency is too high, you can start to see transmission line effects causing unexpected voltage drops at the end of cable runs to the lamps. Keeping the distance between lamp and transformer to a minimum, and using heavy gauge wire will help to moderate this. (purchasing good quality transformers from a respected manufacturer however will usually eliminate the problem altogether!)
Halogen capsules run extremely hot. Contact with paper or plastic quickly causes ignition.
- This is not a problem with downlighters using reflector lamps, as these have lower surface temps and gravity normally keeps flammables away.
- However it is a serious shortcoming of halogen uplighters.
Upturned clay flowerpots are also occasionally found used as hoods, these will not work as effectively as a real intumescent hood (see below), but they will space insulating materials away from the light fitting which will allow it to run cooler (hence longer bulb life), and lower the risk of causing ignition of surrounding materials.
Compromising a Firebreak
This is not a problem specific to halogen lighting, but applies to any light fitting that needs to penetrate the ceiling (typically spots, downlights, wall washers etc). When you pierce the ceiling you also bridge the fire & smoke barrier between the floors of the house, permitting quicker spread of fire and lethal smoke in house fires.
One solution to reduce this risk is to use intumescent fire hoods. These are floppy hoods that fit over the light fitting, and in a fire they will swell up and block the hole, helping to preserve the fire break.
Of all the filament bulb technologies commonly in use, halogen bulbs are among the most efficient. With low voltage halogens being better than mains voltage ones. However it has to be remembered that the energy efficiency of any filament lamp will be poor compared to alternative technologies if all you are interested in is the most light per watt of power consumed.
Careful choice of light fittings will make a big difference to the overall practical and energy efficiency of the lighting. For example, attempting to cover large floor areas using nothing but downlighters can not only prove to be an expensive to run solution, but will also tend to give uneven lighting. The use of "wall washer" fittings will tend to give a much more effective and aesthetically pleasing general lighting coverage than downlighters. Using concealed linear fluorescent lighting to provide general background illumination, and then using halogen spotlights for feature lighting or task lighting will often give far more effective and attractive lighting in a room.
Note that poor choice of lamp fitting also translates into
- energy & money waste
- higher indoor temperatures
- increased aircon running costs when aircon is used.
Don't worry about the cost of the light fittings, you'll pay 4 figures extra to run them.
2 rooms with 5x 50w halogen bulbs each is 500w of halogen lighting, versus 200w of filament or 50w of CFL.
If these are on 6 hours a day, annual energy run cost = 6*365*0.5*10 = £110 at 10p per unit.
Compare this with GLS at £44 a year, or CFL at £11 a year.
So the extra electricity cost over CFL is £99 a year for just 2 rooms. Per 25 year product life that's £2,475. For 2 rooms only. Light your whole house in halogen and it'll be a fair whack more.
And the extra electricity cost over GLS is £66 a year for just 2 rooms. Per 25 year product life that's £1,650 extra for 2 rooms only. Again, light your whole house in halogen and its more.
Then there's the cost of the bulbs. For 2 rooms with 10 bulbs total, with 1500hr low voltage bulbs @ 75p each thats 6*365*10 = 21,900 hours of bulb operation per year, or 14.6 bulbs. @75p each that's £11. Not much. Over 25 years its £275.
Compare with GLS 1000hr bulbs @ 20p each, 4380 bulb hours = 4.4 bulbs = 88p
Compare with CFL, 4380 bulb hours is apx 1 bulb per year @ apx £2.
Total extra cost of halogens for only 2 rooms over 25 year product life is:
- compared to GLS: £1650 + £274 = an extra £1,900 per 2 rooms.
- compared to CFL: £2475 + £273 = an extra £2,750 per 2 rooms.
Poor sighting of halogen downlighters can make it impossible to lean back comfortably in your own home. Lean back, face upwards, and you're looking into a very intense light source that contains UV. This means pain.
On a lesser level, halogen downlights bouncing off glossy surfaces such as kitcen worktops cause a dimmed reflection of the high intensity light bulb, meaning more discomfort and glare. Bouncing off stainless steel utensiles its worse. Much worse.
Glare impacts visibility. Its harder to see things properly when you've got a high light level shining into your eyes.
Halogen downlighters are spotlights, and spotlights by design give uneven illumination, halogens more so than most because they're such small light sources. Uneven illumination means dark underlit areas. To make up for this one has to use many lights scattered round the room, and raise the overall lighting level so that the dimmer areas are adequately lit.
The result is uneven illumination, which does not aid clear vision, and over intense illumination, which is wasteful as well as uncomfortable.
Relamping (replacing light bulbs) is a normal inconvenience of any lighting. However the tendency to use multiple lights in a single room will increase the number of lamps that need replacing.
Mains halogens typically require replacement more often than LV ones. Note also that lamps are available in different qualities and hence life expectancies. The lowest cost mains halogens are often only rated at 1500 hours, which is not much better than a conventional GLS bulb. Their fragility can also result in premature replacement. Better quality LV lamps may have a design life of 4500 hours, so often it is worth paying extra for lamps in value terms. If you have a set of 5x 1500hr (mean life) bulbs in a room that means 5 are replaced per 1500 hrs of operation, which is one per 300 hours on average. 3 rooms with 5 in each means a bulb for each 100 hours of use.
Compare this with a single GLS lamp at one per 1000 hours, or CFL at one per (typically) 5,000-8,000 hrs.
Note the premature failure problem applies to mains halogen lamps, but not to low voltage halogens, nor to high power mains halogens (300w,500w). The high relamping rate problem applies to all multi-bulb lighting installations.
Halogen lamps run at high pressure when hot. Due to high pressure and high temperature, lamp envelopes do occasionally physically fail. Thankfully this isn't too common, but when it occurs you've got lots of pieces of quartz lamp envelope flying out at speed, all hot enough to set fire to things. If you're in the vicinity, you really have a problem.
The solution to this is a toughened glass guard. This prevents fragments of shattered bulb flying out of the fitting.
- Most reflector bulbs have this built in, as the outer glass of the bulb is a separate item pre-fitted to the inner halogen capsule. So these are not an explosion risk.
- When using small capsule bulbs, all fittings should have a glass guard fitted.
- For high power linear halogens, often used outdoors, the fitting should always have a glass guard.
- Many fittings that need a glass guard have none. These are a known risk.
Unlike GLS lamps and CFLs, halogen capsules produce ultra violet. UVA, present in ordinary sunlight, is a known carcinogen.
Glass covers filter this out, and halogens require a glass cover for reasons in the section above. However not all fittings have glass covers. Those that don't will produce UV.
The level of UV produced is much lower than that in sunlight, so the risk will be very much lower too. On the other hand sunlight does cause a lot of cases of skin cancer.
When filament bulbs blow they sometimes trip the MCB, taking all the lights out. Mains halogens are worse at this for 3 reasons:
- Halogen lighting uses many more bulbs, which increases the odds of a trip by several times.
- Halogens have higher switch on surge than GLS filament bulbs, which adds to any arc current to give higher odds of the MCB tripping.
- Halogen capsules are small compared to GLS filaments, much increasing the odds of arc-over.
This problem does not apply to 12v halogens, which are normally soft started and current limited by electronic 12v transformers.
One of the advantages of halogen lighting is that it is dimmable and hence highly controllable. It is important to remember however that dimming is an aesthetic measure designed to control illumination levels, and not is not an energy saving measure. (Dimming filament bulbs reduces can reduce their light output significantly, but this does not translate into an equal reduction in energy consumption. To take an extreme example, A 500w halogen spotlight dimmed such that it produces the same light output as a 40w GLS bulb, can still be consuming 300w).
An alternative way to effect dimming is using a switchbank (i.e. arranging multiple lamps into separately switchable groups such that groups of lamps may be switched on or off to change lighting level). To work well, the lighting layout and the choice of fitting needs to be carefully considered:
- For greatest control with LV halogen lighting it is more useful to use one transformer per bulb (or one for two) rather than running multiple lamps from a single large transformer. (The extra cost of additional transformers will get saved many times over with a well implemented switchbank design).
- If directional halogen lighting has been used in inappropriate ways, then use of a switchbank may compound the uneven lighting problems.
Halogen is a good choice for PIR use in principle, if not in practice. The problem is simply the popularity of excessively high power fittings. There is simply no reason to use 500w on most properties, doing so just wastes energy and dazzles people, so when the lamp turns off again they're left totally blind for a while. A 100w GLS lamp on the PIR is more sensible for most of us.
Advantages of halogen lighting
For all the limitations of halogen lighting (or any filament technology for that matter) there are many situations where this type of lighting has specific advantages. Careful lighting design can make for very effective "mood" lighting. Wall washers can provide a subtle indirect and diffuse light with more "interest" than the flat blanket illumination provided by big a central light fittings. Combined with dimmer switches, a very restful environment can be created with no direct light sources to catch your eye, and no strong shadows.
The narrow beam width of some halogen spotlights can make for very effective task lighting, directing light just where it is needed without too much overspill. Small halogen reading lights in a bedroom for example, can provide discrete illumination of a book for one person, without disturbing another trying to sleep.
Halogen spots are also an effective safety measure for task lights in workshops and machine shops since they can direct bright light where needed to avoid the strobing problems associated with a standard fluorescent room lighting (where the 50Hz flicker produced can result in stop motion effects that may obscure movement of rotating or moving machinery).
LV halogens in particular produce a very white light, that gives good colour rendition. The small source of the light also produces more specular reflections. Hence halogen lighting can be very effective for display lighting, since it will add "sparkle" to ornaments, glassware, and other objet d'art. The use of dichroic lamps and fittings can enhance this effect, while at the same time reducing the heat radiated forward from the bulb, which can be valuable in confined display cases.
If you really want the style of halogen lighting but don't want all the problems, there is one way to do that. That is to use very much lower power downlights, eg 5w instead of 50w, using non-halogen uplighting to provide the bulk of the required lighting. The small downlighting bulbs can be CFL or filament.
Some 3w, 5w, 7w and 9w CFLs will fit into R80 spotlight fittings.
If you really want the filament look, another options is car lightbulbs. These provide various low wattage bulbs that will all run off a standard 12v halogen transformer, and some (eg double ended interior light bulbs) are small enough to be go in surface mounted fittings. You will however need to either take standard fittings and replace the bulbholder or produce your own fittings.
With non-halogen filament bulbs the bulbs are best mounted so the filament is more visible to give the impression of a brighter bulb. A semi-visible 5w filament bulb is a lot more comfortable than a 50w halogen reflector lamp!