Filament lamp

From DIYWiki
(Redirected from Filament)
Jump to navigation Jump to search
Gluehlampe 2.jpg


Lightbulb (the bulb only)
Fitting, luminaire
The whole lighting appliance, into which the lamp goes
Light output per power input. However this does not take into account that perceived brightness per output varies considerably depending on the spectrum of the light, so it doesn't tell us the brightness per power in figure.
Light output brightness per watt input. The best measure of lighting energy efficiency, and the one usually used.
The base of the bulb, the bit that connects to the bulbholder.

More: Lighting Glossary


The first metal filament incandescent lamps from the early 1800s used a platinum filament in open air. They were very short lived and prohibitively expensive, and never considered practical.

The practical carbon filament lamp was invented in the late 1800s, and provided lighting at around half a percent efficiency for several decades. Carbon lamps are seldom seen outside of museums today.

Osmium filaments improved efficacy and light quality, and were soon replaced by tungsten, which was introduced in 1910. Today's filament lamps have a tungsten filament. Gas fill was introduced in 1913, preventing blackening and improving light output. Practical frosting was invented in 1924. The development of the coiled coil filament in 1936 increased efficiencies further, and this is the type of GLS filament lamp we use today. Efficiencies vary but are in the region of 2%.

Halogen lamps, popularised in the 1970s, feature some improvements which raise the efficiency of filment lamps a little. However they come with their own issues.

The latest chapter in the filament lamp story is HIR lamps with IR reflecting envelopes. By reflecting IR back to the filament, less electrical power is needed. These are still low efficiency lamps though.


4 Bulb bases 5359-3.jpg
  • BC or B22d: Bayonet cap, Britain's most popular lamp base.
  • ES: Edison screw, our 27mm version of America's most popular lamp base
  • SBC: Small bayonet cap
  • SES: Small edison screw, 14mm
  • M17: 17mm screw cap, used for some appliance lamps
  • MES: Miniature edison screw, used for torch bulbs
  • Tubular: 221mm and 284mm filament striplights with a connector at each end.
  • Linear halogen: short linear lamps

Miniature lamp bases

BC and ES bulbholders can be installed on 5A, 6A or 10A lighting circuits. Most domestic lighting circuits are 5A or 6A.

Note that prior to the 17th edition of the wiring regs, there was a restriction on using SES and SBC bulbholders on lighting circuits protected at 10A. This restriction was lifted in the 17th edition.

SES and SBC lamps are often more expensive, and add yet more lamp types to stock. They're useful where the smaller cap size is really needed, eg chandeliers, and existing fittings such as fridges. Otherwise ideally they're better avoided.

Popular domestic filament lamp types

Tubular filament lamp
R65, R80 & R50

PAR reflector lamps (parabolic aluminium reflector) produce a narrower beam and greater light concentration than non-PAR reflector lamps.

  • GLS A series - pictured at top of article, 15w to 200w
  • Golfball - small round, upto 60w
  • Globe - large round, apx 4" dia, typ 60-100w
  • Candle - decorative for chandeliers etc
  • R39 - 39mm reflector, 30w
  • R50 - 50mm reflector, typ 40w
  • R63 - 63mm reflector, typ 60w
  • R80 - 80mm reflector, typ 60w
  • R95 - 95mm reflector, typ 100w
  • PAR38 - apx 5" reflector, hardened glass, rainproof. Typ 100-150w
  • Crown silvered
  • Tubular filament - striplight, 221mm 30w & 284mm 60w
  • 12v halogen reflector, 20w - 150w
  • 12v halogen capsule (G4) 5w - 150w
  • 240v halogen (GU10, G9)
  • 240v halogen GLS replacement
  • Linear halogen - 150w, 500w, 1kW & more

Filament lamp specs

These figures apply to standard filament lamps. Long life lamps will have lower output and energy efficiency figures.

  • Power consumption is in watts (w)
  • Light ouptut is in lumens (l)
  • Energy efficiency is in lumens/watt (l/w)
  • Average life is in hours (hrs)

For GLS, Golfball, Candle, Globe, R50, R80, R95, PAR38: 1000hrs, typical figures only:

  • 15w
  • 25w 200-250l 8-10 l/w
  • 40w 400l 10 l/w
  • 60w 660-700l 11 l/w
  • 75w 900l 12 l/w
  • 100w 1300l 13 l/w
  • 150w 2100l 14 l/w
  • 200w 3000l 15 l/w

Tubular filament:

  • 30w 190l 6.3 l/w 1000hrs
  • 60w


  • 12v halogen reflector
  • 12v halogen capsule
  • 240v halogen reflector (GU10)
  • 240v halogen GLS replacement
  • Linear halogen
    • 150w
    • 300w
    • 500w
    • 1kw

Appliance Lamps

Cookers, fridges and microwaves commonly use small filament lamps for interior lighting. Some of these have:

  • hardened glass to reduce risk of breakage & contaminating food with broken glass
  • higher operating temp spec (for oven lamps)
  • M17 bases (17mm)

These lamps are usually designed to run at below 2700K to achieve longer life, at the expense of reduced light output and slightly yellowy light colour.

The differences of appliance lamps are optimisations rather than essential features, and in many cases standard SES lamps can be fitted. These have shorter lifetimes and are easier to break, but are widely available and lower cost.

Appliance lamps not rated for oven use may often be fitted in ovens. These then run outside the manufacturer's specs, but seem to be ok in practice.

Replacement of hardened glass lamps with unhardened is only advisable where the lamp is behind a physical shield to prevent accidental breakage. Lamps housed in open wire cages should preferably be the correct appliance lamps.

For appliances using hard to find lamp types, replacing the holder with an SES is an option. Many Sharp microwaves use 120v lamps, requiring a minor change in wiring if this is done.

Lamp life

Standard British GLS filament lamps last an average 1,000 hours. This is an average figure, individual lamp life varies.

Filament lamps are batch tested, and lamps not meeting the manufacturer's claims would be in breach of the Trades Descriptions Act. In practice its rare to find a batch of filament lamps not meeting their claimed lifetime. Some in the batch will last less time, some more.

See main article Lamp Life

Premature failure

Premature filament lamp failure may be caused by

  • overvoltage
  • vibration, even slight
  • pitted bulbholder contacts or switch contacts

However most complaints of high failure rate seem to be down to unrealistic expectations. Bear in mind that a lighting scheme with 10 times as many bulbs will experience 10x the rate of failures, and a lot of people grew up with one bulb per room.

Overvoltage can be monitored with a multimeter. If frequent or sustained, the electricity supplier will normally adjust the supply voltage.

Vibration from foot traffic on the floor above occasionally kills pendant lamps quickly. Perhaps this could be solved by suspending the flex inside the rose with a light spring or a little soft foam.

CFLs are much less susceptible to all the above issues.

See main article Lamp Life for a full discussion.

Increasing lamp life

Filament lamps with up to 5000 hr life are available. The energy efficiency of these is much worse than standard 1000 hour lamps, and the total run cost for a given light output significantly higher.

Increasing filament lamp life always reduces energy efficiency and increases total cost per lumen, so is only a good idea where relamping causes a specific problem, and for some reason other longer lived lamp types (such as CFL) are rejected.

Ways to increase lamp life

Filament lamps can be wired in series to achieve very long life, but this produces exceptionally bad energy efficiency and excessive run cost per lumen.

A transformer may be used to reduce the supply to filament lamp by 20v to triple its life. The lamp then behaves as a long life lamp, with the same poor energy efficiency.

American lamps are sometimes run with a diode in series to achieve long life. This works with US 100w bulbs, but not British ones due to the voltage difference. On 240v bulbs it causes severe flicker. (It works here with 500w lamps, but at such powers its a very wasteful way to get long lamp life.)

Bulb Boosting

Light output and energy efficiency of filament lamps may be boosted by running them on slighly increased voltage. This can be of use for light fittings with max power ratings below what's wanted. It does much reduce lamp life, so easy relamping access and low costs lamps are best for this approach.

Halogen lamps are not suitable for boosting. Nor are 60w golfball bulbs, these already run very close to temperature limits, and may solder themselves to the bulbholder in protest.

There are 3 ways to achieve the boost:

  1. Transformer boost 240v to 258v
  2. French 220v bulbs on our 240v.
  3. Rectifier and capacitor booster

Transformer boost

An 18v transformer is wired to change 240v to 258v. See Droppers for details of transformer boost wiring.

  • A 60w bulb requires a 6w or more 18v transformer
  • A 100w bulb requires a 9w or more 18v transformer
  • A 150w bulb requires a 13w or more 18v transformer

French 220v bulbs

When used on our 240v, french 220v bulbs run at above design voltage. With filament bulbs the result is 30% more brightness and about 300hr life expectancy. More info on 220,230 & 240v

Rectifier & capacitor

Mains is rectified with a bridge rectifier, then a small capacitor placed across the rectified ac. This increases rms voltage without increasing peak voltage, leaving the power still compatible with CFL lights as well as filament.

Capacitor value is adjusted to give the wanted rms voltage boost, which is typically 20v. An rms reading voltmeter is used to set the right amount of boost. Without an rms meter, the resulting operating point would only be a rough guess, and lamp life could vary a fair bit from expectation.

A bleeder resistor is added across the capacitor to discharge it when power is off.

Required part specs:

  • Bridge rectifier: rated 1kV, 15x lamp run current
    • 4A for 60w, 6A for 100w
  • Capacitor: Class X 250v ac, or 400v dc (not class X)
    • non class X caps marked 250v dc are definitely not suitable
  • Bleeder resistor: 270k or higher, 400v rated
    • minimum 0.5w. If using 0.3w or 0.25w Rs, use 2x 220k in series to achieve enough voltage rating


A 20v rms boost results in:

  • 16% efficacy boost
  • 14% power consumption increase
  • 31% light output boost
  • lifetime reduced from 1000 hours to 280 hours
  • Colour temperature increased from 2700K to 2800K

			With 20v boost
Property		60w	100w
--------		---	----
power consumption	68w	114w
light output equiv	80w	130w


                  CFL     LED     GLS of other powers
                  ---     ---     ---
Standard 240v     yes     yes     yes
220v bulbs        yes     yes     yes
Transformer 18v   barely  barely  lower powers yes, higher powers often
BR&C boost        yes     no      higher powers yes, low powers no

Including a back to 240v switch is a good idea with voltage boosters, giving full compatibility.

Bulb Stocks

The less different types of bulb are used, the easier it is to keep spares stocked.

The more common the bulb types, the more convenient buying them will be, as popular lamps are available from more retailers.

The proposed government ban of most filament bulbs on political grounds failed to happen, so your fittings are safe from the politicians for now.


These large reflector lamps have a thick hardened glass envelope and may be used outdoors with no protection from rain (in suitable fittings). They are expensive filament lamps, and consume money on purchase costs if used indoors. Because of their high price, PAR38s are often 1500hr lamps, producing additional electricity costs too.

Indoor PAR38 fittings can be fitted with GLS 100w bulbs where the bulb is not directly visible. These give a much wider beam than PAR spotlights, in most cases this is fine. Occasionally the fitting interior is not white or silver, in which case light output would be reduced with GLS lamps. Painting the interior white solves it.



Dimmer 4476-3.jpg

Dimming greatly reduces energy efficiency of all filament lamps. As an example, a 500w halogen lamp dimmed to put out the equivalent of a 40w bulb was tested & found to consume 300w. Dimmers also preclude use of CFLs and LEDs (although dimmable CFLs are now available, at a price).


Switchbank new 1083-6.jpg

Switchbanks switch more than one bulb separately to control light level. They don't affect efficiency, and are compatible with all lighting types. But they're less flexible than dimmers, both in installation and use. Retrofitting usually requires running a new piece of cable.

Switchable Boost

Another dimming option is to use bulb boosting with a switchable boost setting. This increases energy efficiency and reduces run cost, but it reduces lamp life significantly, and only gives 2 brightness settings. Real lamp life will depend on the relative amount of time at each setting, and will vary from 280 hrs to 1000 hrs.

3 light levels can be had by switching the 18v boost polarity, causing an 18v reduction and consequent drop in light output, thus giving a choice of 222v, 240v, 258v. The result is better energy efficiency than a lamp on a dimmer covering the same lumen output range.

Coloured lamps

Novelty lamps

Filament lamps are available in an array of colours. Since these colours are achieved by filtering out the unwanted colours in white light, all coloured lamps have much lower efficacy than white lights. Blue filament lamps have especially poor efficacy, and when colours are mixed its common to use higher power bulbs for the blues.

There is more than one method of coating the lamps. Some coatings produce an even light with good transmission, and some produce an uneven thick coat with patchy light transmission. The latter are easily spotted, they look like a layer of applied paint.

Kids sometimes like to paint white lamps to achieve additional colours. Coating evenness is an issue, and the lamp needs illuminating to get a sufficiently even coat. Applying wet paint to a powered bulb tends to cause an explosion, so a separate light source must be used.

Soft tinted lamps

Lightly tinted lamps can create a warm soft atmosphere. Efficacy is below standard filament lamps, but much better than that of strongly coloured novelty types. These are mostly available with peach and pink tints.

Daylight lamps

Daylight lamps have a blue coating to produce a higher colour temperature (colder) light. These are used when it is necessary to view colours as they would appear in outdoor light. The resulting spectrum is not perfect, but close enough for most artistic tasks (they lack the uv of daylight).

Again these have reduced efficacy. 100w bulbs rated at 500l were spotted, versus 1300l for white GLS.

For regular users of daylight lamps, its cheaper to fit a CCT changing gel over a standard GLS lamp, and this avoids the need to buy expensive lamps, and stock another lamp type.

Halogen lamps with their 3000K operating CCT are a bit closer to daylight spectrum, and a bit more efficient. Halogen lighting plus the right filter gets a more energy efficient daylight setup.

Painted lamps

Painted lamps have limited life. if you wish to keep your work you may prefer to paint a glass globe and fit this over a bulb. Dead painted GLS lamps may sometimes be salvageable by cutting the base off and inserting a small lamp, such as a halogen capsule. Globe lamps give a bigger painting canvas.

Coating evenness is an issue, and the lamp needs illuminating to get a sufficiently even coat. Applying wet paint to a powered bulb tends to cause an explosion, so a separate light source must be used.

Violet lamps

Used in cheap electric flykillers. Not very good at attracting flies. For cut price flykillers, a halogen capsule lamp is more effective.


Halogen gu10 5355-2.jpg

Halogen lamps are a more modern form of filament lamp. Halogen lamps contain high pressure fill gas to reduce filament evaporation, plus iodine or bromine that returns evaporated metal back to the filament via the halogen cycle. These enable the filament to be operated at higher temperature, achieving some improvement in efficiency.

In principle halogens are slightly more efficient than GLS, but in practice the types of lighting halogens are usually used in result in much worse energy efficiency than GLS.

Halogen lamps necessarily operate at very high temperatures and high pressure. This brings risk of fire in some cases, and to a lesser extent explosion. To operate safely the lamps should be guarded with toughened glass. Halogen reflector lamps usually have this guard built in, but outdoor PIR halogen lights often lack guarding. Reflector lamps recovered from small projectors should not be used, they have no glass guard.

Linear halogen

Linear halogen lamps are available in wattages of:

  • 150w
  • 300w
  • 500w
  • 1kW

All but 150w have more light output than is useful in most domestic houses. The majority of outdoor yards and gardens also only need 150w. 500w fittings have their uses for lighting large areas, but are much overused. The result is unpleasant lighting and waste of energy and money.

Outdoor 500w lamps need to be mounted high up to illuminate a wide area effectively. 1kW lamps require street lamp posts or similar to make them effective at achieving large area coverage. Without sufficiently high mounting, high power lamps effectively cover an area much smaller than their capability, and cause severe glare. 500w lamps can be mounted by an upstairs window to make relamping easy, but ensure curtains can't land on them.

Due to the high power consumption of 500w & 1kW lamps, when continuous lighting is wanted at these power levels its usually better to look at some form of Discharge Lighting instead. Discharge lighting is available in yellow, golden and white. Discharge lamps can hugely reduce energy consumption and run cost.

Indoor sun

Linear halogen lamps are occasionally used to provide artifical sunlight where natural light is insufficient. This may be achieved by mounting a high power lamp near the ceiling by the front door, allowing it to shine through an interior window to produce a pattern that replicates that of the sun shining in through an exterior window. Where this arrangement is not suitable, its possible to mimic window shadows in other ways to make the appearance semi natural.

HIR lamps

Linear halogen lamps are now available with IR reflecting envelopes. By reflecting heat back onto the filament, less electrical energy is used. They're called HIR or IRC lamps.

  • 375w HIR replaces 500w halogen
  • ? HIR replaces 150w halogen

Halogen A-line

Halogen GLS 5357-4.jpg

GLS-style lamps with halogen capsules inside give an efficiency improvement and a mean life expectancy increase, though the improvements are nothing great. The downside is much worse glare if the bulb is viewable directly.

TCO for 60w GLS lamp: 25p purchase cost + 1000hrs x 0.06kW x 12p/unit = £7.45 / 1000hrs

TCO for 42w 1500hr halogen A-line lamp: £2 purchase cost + 1500hrs x 0.042kW x 12p/unit = £9.56 / 1500hrs = £6.37 / 1000hrs

Saving = £1.08 per 1000 hrs per bulb.

Dimmed PIR lights

PIR lights that run the bulb dimmed suffer a sizeable defect, and often waste thousands of pounds over their lifetime. For details see Dimmed PIR Lights

Mixed lighting

Reduced energy use can be achieved by mixing reduced level background fluorescent lighting with filament lights. This works well if the fluorescent source is 2700K and high CRI. Trying this with another tube CCT doesn't work as well. Most retailers need to order in 2700K tubes. Where the plan is to retain the appearance of filament lighting, low CRI tubes should be avoided. 2' & 4' tubes are a useful size for this.

Colour temperature (CCT)

  • GLS lighting is 2700K
  • Boosted GLS is 2800K
  • Halogen lighting is around 3000K
  • Long life lamps are below 2700K
  • Tubular filament is below 2700K

Other filament lamps

Sunbeam lamp, 1890 (Forty Years of Electrical Progress).jpg
Soft white
About 11% less lumen output
Carbon filament
About a fifth the efficacy of modern lamps, yellowy white light, very occasionally used for background heating
Photo lamps
High power GLS lamps with 3 and 6 hr life expectancy. Run 2 in series most of the time, just switch them to full power while taking a shot, then back to reduced power. This way the 3 or 6 hr life applies to the time spent in full power mode, low power running doesn't use up the short lifetime to any noticeable extent.

Other lamp types

See Also