Lamp life

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For all lamp types, life expectancy is an average figure, with life varying from one bulb to another.

Life expectancies vary widely between different lamp types


Average Life Figures

GLS filament bulbs
1,000 hrs typical
long life bulbs anything upto 5,000 hours are available
(750 hour bulbs are standard in the US)
Linear fluorescent
20,000 hrs for tubes on an electronic ballast
10,000 for tubes in glowstart fittings
CFL Lamps
most are 5,000 to 8,000 hrs
Lives anywhere from 1,200 hrs to 20,000 hrs are also found
Halogen
1,500 to 2,000 typical
Long life halogens upto 4500hrs
LED
LED life expectancy varies over a very wide range, see the LED section.
Mercury
Sodium, low pressure
Sodium, high pressure
Metal Halide
Torch bulbs
anything from a few hours to over 50 hours


The Words

Lamp
'lamp' means the lightbulb itself, not the fitting into which it goes.
Efficacy
Approximately speaking this means efficiency. Technically it is a little different because of the non-linear frequency response of the eye.
Relamp
replace a lamp
GLS
General Lighting Service, traditional mains filament bulbs


Long Life Filament Bulbs

Long life filament bulbs with rated lives of anything up to 5,000 hrs can be bought for a premium.

However these are not recommended, as longer life is obtained at the expense of lower light output, meaning higher power is required for the same light level. The extra power cost far outweighs any relamping savings.

For situations where long life lamps are of real value, such as inaccessible fittings, CFLs will provide much longer life than a long life filament lamp, and are several times as energy efficient, making total cost of ownership a fraction of that of long life filament lamps.

Linear Fluorescent

Tubes achieve longer lifetimes on electronic ballasts as glow starters are not so kind to the tube filaments.

Frequency of switching does affect tube lifetimes, but in most cases not greatly. Contrary to the old myth, switching off is always cheaper than leaving on.


Miniature Tubes

Miniature tubes are more variable, but still usually achieve good life expectancy. The problem with miniaure tubes is the popularity of low quality dc powered ballasts. Many such ballasts kill tubes prematurely.


CFL Lamps

CFL life expectancy is printed on the box, and in most cases that's all there is to it.

Higher power lamps, 20w and up, have reduced lives if operated base up, as the electronics run hotter.

Short-lived CFLs generally work out more expensive per 1000 hours than better quality lamps.

CFLs suffer a higher infant mortality rate than other lamp types. This does not change the average life. To look at it another way, once the early failures are weeded out, the remaining lamps will have life expectancies longer than their declared average lifetime.


Facilities Lamps

Long lived CFL Lamps of 15,000 hours upwards are used where relamping has a financial cost, such as large business premises.

The purchase cost per 1,000 hours is a little higher than for standard CFLs. They are a good choice where bulb replacement is difficult, or when you're willing to pay a slight premium for the convenience of very long bulb life.

The highest rated life lamps are electrodeless, and use rf or capacitive excitation. The Genura is an example.

There are also specialist lamps using microwave excitation, but these are in the kilowatts power range, so not really a DIY thing.

High Relamping Cost

In some situations, unusually high costs can be paid for relamping, such as:

  • Inaccessible fittings in the homes of some elderly or disabled people, which can sometimes require a £50 callout charge to replace a bulb
  • Outdoor lamp posts
  • Roof level sign lighting
  • Underwater lights

Possible Solutions

Very Long Life Lamp

Long life facilities lamps are a simple option.

Move Fitting

In the case of elderly people paying callout charges, moving the fitting may be cheaper than repeat callout charges.

Extension Lead

One simple low cost way to solve the out of reach fittings problem for ceiling fittings in elderly people's houses is to make some short extension leads up. These have a BC bulb plug on one end and a bulbholder on the other. Instead of paying someone to replace the bulbs, they plug these extensions in, and bulbs can be self-changed in future. Using BC plugs is still legal, but selling them is not, so availability is an issue. Making plugs from CFL bases is not BS approved.

Longer Flex

Replacing the flex of pendant fittings with longer is another option that may be cheaper than paying for repeat relamping.

Rise & Fall Fittings

Rise & fall pendant fittings are also available, but are not common.

Rise & Fall Conversion

Some types of overhead light can be converted to rise & fall by fixing the bulbholder to the flex only, attaching a small ring shaped metal weight to the flex at the holder, and using a long 1 piece flex that runs all the way down to the ground, and is clipped in place there. This is especially suitable for post based fittings.

To lower the bulbholder, unclip the flex and let the holder fall. Pull the flex to raise the holder. Any bulbholder used this way must have an effective cordgrip, most don't.


World's Longest Lived Filament Bulb

The lightbulb with the longest known service life so far has been in use 24/7 nearly all the time for over a century, clocking up a service life of over 900,000 hours so far - and still going!

The bulb is currently in an American fire station, and has visitors regularly. It was made in the 1890s, and is a 4 watt carbon filament bulb.

If you need very long life lamps, they are available in the 50,000 hour region from some suppliers, but take a well stocked wallet.

If you want a 900,000 hour bulb, you'll need to borrow a time machine or have it custom manufactured, or possibly use a long life filament lamp at well below rated voltage. The light output and energy efficiency of such a lamp would be very poor.

Boosting Light Output

It is possible to boost the light output and energy efficiency of filament bulbs by raising their operating voltage slightly. A transformer can add 20v to the (mains voltage) supply for GLS filament bulbs, and thereby obtain 30% - 40% more light output with only 8% increased power consumption.

The cost of this is reduced lamp life. Mean lifetimes are reduced from 1000 hours to apx 350 hours.

Today with the availablility of CFLs this trick is a lot less useful than in the past. It can however be useful occasionally, typically where it is wished to retain an attractive light fitting that will not take CFLs, and which has too low a max power rating. A boosted bulb enables greater light output from a given power.

A few bulbs are not well suited to boosting. These are mostly some of the bulbs with the highest power to size ratio. Such bulbs may overheat and fail quickly, or in the case of 60w golfballs, solder themselves to the bulbholder in protest. Its not recommended to boost halogens or 60w golfballs.

French Bulbs

French filament bulbs are 220v. Using a French bulb in the UK is a simple way to obtain voltage boosted output for anyone that travels to the continent.

Halogen Cycle

Halogen lamps contain 0.1% - 1% iodine or bromine. Tungsten evaporating from the filament reacts with this halogen to form tungsten iodide or bromide, which does not deposit on the lamp envelope. When this gas passes the filament again (convection currents occur in these bulbs) the molecule is split and the tungsten redeposited on the filament.

This prevents darkening due to evaporated filament depositing onto the outer envelope.

The very high pressure gas fill also reduces evaporation.

Dimming of halogen bulbs does not reduce life by stopping this cycle, as bulbs normally operate well above the minimum temperatures at which this cycle works.


Dimming

Dimming greatly increases lamp life for filament lamps, but this does not make it worthwhile. Dimming heavily reduces Energy Efficiency, and the extra energy used for a given light level (in comparison to lower powered or fewer bulbs used to achieve the same light output) outweighs bulb savings many times over.


Soft Start

Soft starting is sometimes used with filament lamps to extend lamp life. This works well with halogens, but is not effective with non-halogen filament lamps.

Soft starting is of no use with fluorescent lamps, including CFLs.


Button Dimmers

These comprise a diode in a small button. It can be put in a screw-in lampholder under the bulb to reduce lamp power and reduce brightness. Power is not reduced to half, as filament resistance changes at reduced temperature.

These devices produce higher run cost compared to a lower power bulb, due to much reduced efficacy & energy efficiency.

These buttons are unsuitable for 240v lamps as they cause flicker. They are used much more in 110v countries where the thicker filaments produce much less flicker.


Rough Service Lamps

Mains filament lamps are usually fragile, and this can be a real problem when used in:

  • Droplights
  • Industrial machinery
  • High vibration locations
  • Clip-on lights

Rough service lamps are ruggedised, and more able to survive rough service. They use a greater number of filament supports.


Designing for Life

Some design points used to extend life of filament lamps include:

  • Lowered filament operating temperature
  • Thorough glass degassing & dewatering
  • A getter
  • More filament supports
  • Gas fill
  • Brass cap

These are often used as marketing points for longlife filament lamps, however filament temperature rules, with the other points having a much smaller contribution to extending life.

Filament lamp making & repair were occasionally DIYed long ago.


LED Lamp Life

LEDs have somewhat different characteristics to other lamp types. LEDs are usually very long lived devices if operated conservatively, achieving lives of 50,000 hours or more. However lifetimes are usually very much shorter in practice.

The problem is cost: producing enough LEDs to give enough light output is an expensive business, so LEDs are pushed as hard as possible to maximise output and minimise cost. The result is far shorter life and early failures.

If you need an extremely long lived very low output bulb, a modified LED lamp should fit the bill. Reducing LED current to 20mA will generally get you a lifetime of tens of thousands of hours if the lamp has not been used at higher current first.

Current can be reduced either by changing components within the bulb, or by external limiting, either with a series impedance or by running on reduced voltage from a transformer. The transformer also helps to reduce current spikes produced by the simple RC ballast in the lamp.

Single Die LEDs

Sold as ultrabright 5mm LEDs (other sizes also available) these monochrome LEDs have extremely long lives if operated within their ratings. 100,000 hour service life is unremarkable for these.

Their miniscule light output makes them of very limited use.

Pink and white LEDs lose a fair bit of light output over time, much more so than monochrome LEDs. Pink LEDs deteriorate so quickly that they are unsuitable for lighting use.

Series Bulbs

When mains filament lamps are operated 2 in series, their life expectancy increases enormously. This is occasionally done in locations where lamp failure or relamping would be problematic. The downsides are exceptionally bad energy efficiency, with far higher run cost per output than CFL, and yellowy light.

These days CFL facilities lamps are a far lower (run) cost alternative. Since series lamps are so energy inefficient, the efficiency improvement of CFLs is far greater than the usual 3x compared to series filament lamps.

Photographic Bulbs

Filament photographic bulbs are available with lifetimes of 3 and 6 hours. The 3 hour bulbs have higher output per power in.

These bulbs are usually operated on 120v by running 2 bulbs in series. When the picture is about to be taken, a switch is operated which feeds them full mains voltage. They are put back on 120v once the picture is taken.

These bulbs have long lives on 120v, it is their 240v lifetime that is so limited.

These bulbs have 2 advantages over standard filament lamps: 1. High light output per power input 2. Ability to run on reduced power most of the time

High power halogen lamps are an effective alternative to these filament bulbs, and may be operated in the same way.

Fluorescents

Large fluorescent lights (eg twin 8' fittings) with electronic ballast and Overdriven fluorescent lights (eg twin 5' fittings with replaced ballasts) with electronic ballasts can be used for omnidirectional photographic lighting.

Traditional electromagnetic choke ballasted fluorescent lights are not ideal for photography, but can be used if exposure times are kept to 1/30th second or longer.

CCT and CRI are issues with Fluorescent Lighting used for photography.

Torch bulbs

Torch bulbs vary in life expectancy, and may be anything from a few hours to 50 or more hours.

The short lifetimes are mainly due to 3 factors:

  • Battery power is relatively expensive, so there is pressure to maximise energy efficiency
  • Maximising light output for these very low power lights is desirable
  • Torch bulbs dont usually see a lot of hours of use, so short life bulbs last long enough.

When Bulbs Don't Last Long

'My bulbs keep blowing' is a fairly common complaint. It has a few possible causes:

Number of Bulbs

With a single 1000 hour mean life bulb, you'll replace the bulb once per 1000 hours on average. For many of us this was the usual lighting arrangement in childhood, and what we got used to. Modern lighting might employ 10 bulbs in a room instead of 1: what people don't always realise is this means 10 bulbs will need replacing every 1000 hours, or 1 bulb per 100 hours. And thats an average figure, half the time it will be more frequent.

Vibration

Mains filament lamps are more fragile when hot, and even slight vibration can cause premature failures. This can sometimes be an issue with ceiling mounted lights. Rough service lamps, CFLs, linear fluorescent and 12v lamps are less vibration prone types.

Holder Damage

Oxidation & pitting on the surface of the lamp holder connections can cause local heating and premature bulb failure with all types of filament lamp.

With mains filament lights the solution is to scrape the top of the pins clean. These are connected direct to the mains, so use of a fully insulated tool is required.

With halogen lights, once this condition occurs the metal heats up and loses its springiness, and the holder is not likely to be repairable.

Arcing Switch

A failing light switch can arc. This causes vibration and repeated current surges in mains filament bulbs, and premature death. It does not affect other types of lamp in this way.

Overheating

A relativly high power bulb in an enclosed fitting can overheat. This can occur with high power filament lamps, but is uncommon.

This is more common with CFLs, some of which don't like being in small enclosed spaces. Elevated temperature causes premature ballast failure, and sometimes much reduced light output.

This also occurs to some extent with higher power CFLs (20w and up) that are run base-up. These will often have shorter than rated lives in this position.

It should be borne in mind that the max power ratings found on light fittings apply to filament lamp use. CFLs are less heat tolerant, and a 25w CFL in a 25w rated fitting would quickly overheat and die.

Expectation

Experience has shown that some of these cases are down to nothing more than a difference between the realities of light bulbs and user expectation. Perhaps the change in time perception over time has something to do with it.


Equations

Some equations relate to filament lamps that can be used to calculate the expected consequences of changes in operating point.





and the following 2 with less accuracy:



Suppliers

All the lamps mentioned in this article are available from the many suppliers listed in Suppliers


See Also