Difference between revisions of "Halogen Lighting"
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Revision as of 02:59, 11 November 2008
A halogen lamp is a type of filament lamp with some improved characteristics compared to the basic GLS filament lamp. Halogen lamps were popularised in the 1970s.
How they work
In a halogen lamp, the usual evaporation of the filament is counteracted with a process which returns the evaporated material back to the filament. Halogen lamps can either be designed with higher efficacy than standard GLS filament lamps, or with longer life (or most commonly, some compromise in between). The filament is contained in a small quartz 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 and condense on the quartz envelope, combine with the halogen gas, and get redeposited on the filament. This allows the filament to be run at a slightly higher temperature for the same lifespan, or run longer at normal filament temperatures.
Halogen lighting is currently fashionable, 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 quartz glass envelope with protruding pins for 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 capsule. 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 fronted (i.e. the capsule could be touched), and others have a glass front to the lamp covering the capsule. 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 - aluminium and dichroic. Aluminium film reflectors are 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 alters its 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 double-ended halogen lamps
These are bare quartz tube lamps with a long thick filament and an electrical contact at each end. These are typically used in high power reflector lights such as security lights or flood lights.
Power consumption of linear halogens for home use is in the range 100W to 500W across a couple of common tube lengths. These lamps produce lots of heat and a dazzling light that is best used responsibly and only in limited situations.
These lamps should be operated only in fittings which will contain hot fragments from a possible exploding lamp. They also give off a small amount of UV and are best operated behind a glass shield to absorb the UV.
The filament is best positioned horizontally to ensure full rated life.
GE produces higher efficiency versions with an internal infra-red reflective coating to reflect the heat back on to the filament. These are available in 225W (to replace a standard 300W) and 375W (to replace a standard 500W), but can be hard to find.
Linear double-ended halogen lamps are also readily available in 110V versions for construction site lighting. 110v lamps have thicker filaments than the 240v versions, and are thus more rugged.
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 quartz glass must be used. One must take great care when handling this envelope not to 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 high positive pressure, unlike traditional bulbs, and 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 methylated 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
- The light will be whiter and have better colour rendition (note that some may find high power LV lamps too 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 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 increases lamp life further by reducing the thermal shock the bulb experiences at switch on from cold.
- The transformer will also typically prevent the high current surges that often occur when a bulb blows from causing the circuit fuse or MCB to also trip (a frequent problem with mains halogens)
- 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 is an 8.3% reduction in voltage. This would reduce:
- power consumption by 17.3%
- efficacy by 16.4%
- Light output by 31%
- Colour temperature by 100K
This would 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 is proportional to the current). Generally it is better to run an individual radial feed to each lamp (i.e "star wiring") rather than placing multiple lamps on a single cable run, for 2 reasons:
- it greatly reduces voltage drop for a given cable size
- it makes equalising voltage drop between the different lamps relatively easy
Obviously one can use a pragmatic combination of the two methods to ease wiring and maintain fair lighting performance.
Transmission Line Losses
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), lower the risk of causing ignition of surrounding materials, and slow smoke spread to some extent.
Compromising a Firebreak
This is not a problem specific to halogen lighting, but applies to any light fitting that needs to penetrate the ceiling (some 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.
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. 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 have a glass guard.
- Many fittings that need a glass guard have none. These are a known risk, and are banned in some countries (but not the UK).
Unlike GLS lamps and CFLs, halogen capsules produce ultra violet radiation (UVA) (as present in ordinary sunlight). Prolonged exposure to the Uv in sunlight is known to cause skin damage and can in extreme cases cause skin cancer, but halogens produce lower levels of UV output.
With modern fittings and bulbs this is not an issue since all the bulbs commonly available now have a UV absorbing coating, or are only suitable for use in fittings with glass shields which also remove any harmful UV. However if purchasing old desk lamps (or any similar fittings that could lead to you being exposed to light at close distances from the lamp) they should have glass shields in place, or have been retrofitted with bulbs that have integral explosion protection & UV filtering.
Of the filament bulb technologies commonly in use, halogen bulbs are the more efficient, with low voltage halogens more efficient than mains voltage ones. However the energy efficiencies of all filament lamps are poor compared to the other popular lighting technologies.
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 translates into energy & money waste, and hence production of more pollution. You will also get more heat introduced into the house. While this may not be a big disadvantage in the winter, it can make rooms less comfortable in the summer. If your room is air-conditioned, then you will be adding to the heat load that you will need to pay for the aircon to remove.
As with most lamp technologies, the initial purchase price of the lamp and fitting is only a small part of the total cost of ownership.
In themselves halogen lamps generally last longer than GLS bulbs (or R80 style spots), and produce more light for a given energy consumption. However this only tells half the story. The common use of spotlights in fittings typically means that more of them are required to achieve even enough illumination. Two rooms with 5x 50w halogen bulbs in each will consume 500w, which is a significant increase when compared to a single 100W GLS bulb or 25W CFL in each room.
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.
Using more bulbs also means higher purchase costs for bulbs, as many more bulbs need to be replaced per year. This means bulb failures are much more frequent, a point that has attracted complaints many times on news:uk.d-i-y. (The high relamping rate issue also applies to other filament lamp multi-bulb lighting installations, not just halogen).
Its worth paying attention to the quoted life expectancy of the bulbs you buy. The shorter life higher efficacy lamps are rated at 1500hrs, and long life lower efficacy lamps are rated at up to 4500 hours. The difference in efficacy is about 17.5%, with the shorter lived lamps giving more light per watt. For a given lighting level the shorter life lamps need less total power and thus cost less to run. The purchase cost of the bulbs is small compared to run costs. For more information on this topic see Lamp Life.
The purchase price of halogen lamps tends to be higher than GLS lamps. In fact they may cost 5 times more, which even allowing for the longer life expectancy will still add up over time. If you use 20,000 bulb hours per year, you may need 14 halogens or 20 GLS bulbs. If your halogens are 75p each and you GLS lamps 20p each, then you will be spending £6.50 more on replacement halogens per year. However this is small compared to the difference in energy use.
Note another potential cost can arise when using mains halogen bulbs since these are particularly prone to premature failure as a result of environmental influences (vibration, poor voltage regulation etc). This is another contributor to why LV lamps typically cost less over long periods, but more importantly, frequent premature failures are a real pain for some people.
Poor siting 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. On a lesser level, halogen downlights bouncing off glossy surfaces such as kitchen worktops cause a dimmed reflection of the high intensity light bulb, meaning more discomfort and glare. Reflections from stainless steel utensils can be worse. Glare impacts visibility. Its harder to see things properly when you've got a high light level shining into your eyes.
Careful planing of where fittings will be sited can dramatically reduce these problems. Using wall washer fittings close to walls or across ceilings can help give diffuse light spread without glare. Especially when the light fitting itself is concealed or otherwise not in direct view.
Halogen downlighters are spotlights, and spotlights by design give uneven illumination, halogens more so than most because they're such small (and hence directional)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. Using the widest available beam width bulbs reduces this to a limited degree.
Sparing use of halogen lighting to pick out particular architectural features in a room, or to illuminate specific paintings etc. can make for a more stimulating and interesting environment. Often less is more - a few lamps picking out interesting details will create more impact than a room full of them highlighting nothing in particular.
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 effective 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 was measured as consuming 300w.
For smaller bulbs, such as those used in a tracklight, experiments show that a lamp drawing 97W on full brightness only reduces to 41W on the minimum brightness setting.
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 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 usually gets saved many times over with a well implemented switchbank design).
- If directional halogen lighting has been used in ill chosen ways, then use of a switchbank can worsen the unevenness when set to lower output.
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, sometimes so much that when the lamp turns off again they're left temporarily blinded. A 100w GLS lamp on the PIR is more sensible for most of us.
When filament bulbs blow they can sometimes trip the MCB (or blow the circuit fuse), taking all the lights on the circuit out. Alas mains halogens are the worst culprits for doing this, due partly to their close internal conductor spacing.
This problem does not apply to 12v halogens, which are not prone to arc-over, and are current limited by transformers.
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 white light with 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.
The halogen effect without the drawbacks
If you want the style of halogen lighting but don't want all the downsides, there are 2 alternative ways of designing your lighting to avoid many of them.
Concealed Supplementary uplighting
To reduce the running costs, heat output and energy use of halogen, one can use much lower power filament downlights plus additional concealed uplighting to make up the total level of illumination.
For example 50w halogen lamps might be replaced with 10w lamps, and the rest of the light obtained from concealed uplighting. It works well if the uplighting matches the colour temperature of the halogens, at approx 3000K. Lighting with mismatched colour temperature is not convincing.
Another option to low power halogens 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 go in surface mounted fittings. You would however need to either take standard fittings and replace the lampholder or produce your own fittings, so this is not popular.
With low power non-halogen filament bulbs, the bulbs are best mounted so that the filament is a bit more visible, to give the impression of a brighter bulb. A semi-visible 10w filament bulb is more comfortable than a 50w halogen reflector lamp.
Low power drop-in replacements (LED, CFL) are available for some halogen spot light fittings. While none of these can match the particular qualities of light output from a halogen, they can be used to "make up the numbers" in multi lamp installations.
These lamps do not give a convincing imitation of halogen lighting, but are a simple, easy & energy efficient way to deal with existing halogen installations.