Light bulbs
This page clarifies the confusing array of different types of light bulb that are now availble.
Summary
In years gone by there was only one kind of light bulb - the tungsten filament incandescent bulb. And it came in a fairly limited range of powers and shapes, and with just a couple of different kinds of base. In the persuit of greater efficiency, these were superseeded first by the compact florescent and then LED types, and each brought new sizes and formats. A confusing array of types, shapes and sizes now graces the retail shelves, and it's common to see a perplexed customer staring at them helplessly. This page should make everything clear.
Safety
- Always turn off the switch before changing a light bulb as it may be easy to touch live contacts with the bulb removed.
- If you have to stand on a chair to change a light bulb, make sure it's steady and that there's no risk of you falling off.
- It's very important not to touch the quartz envelope of a halogen bulb where it's not protected by an outer glass envelope (such as the capsule bulbs used in some desk lamps). Finger grease will etch into the quartz when it gets hot and cause premature failure, possibly releasing shards of very hot quartz.
Types of bulb
Filament, florescent or LED
There are three kinds of bulb in domestic use:
- Filament or incandescent bulbs. These contain a thin wire which glows white hot when a current passes through it. They are very inefficient and have a relatively short life. Standard "tungsten" types can no longer be legally sold.
- Halogen bulbs are a newer type of filament bulb, somewhat more efficient and with a longer life. They contain a halogen gas which helps preserve the life of the filament (still made of tungsten) and so can be run hotter and hence more efficiently. The envelope is made of quartz instead of regular glass so as to withstand the higher temperature, and this may be mounted inside a standard glass bulb.
- "Vintage" or "antique" bulbs are available for decorative use, often rated at 25W, 40W or 60W. These generally have a large glass envelope and a filament in a large coil or strung between a number of supports. They give a warm amber glow but are extremely energy inefficient. If you get close you will be able to feel the heat on your face. (Some LED filament bulbs have a very similar appearance but much lower wattage - don't be fooled.)
- Compact florescent bulbs are basically just a standard florescent tube bent into a compact shape. Both their life and their efficiency are several times greater than a filament bulb. A florescent tube cannot be connected directly to the mains but needs a ballast and a starter, and a compact florescent, instead, has an electronic circuit in its base. They tend to be bulkier than their filament equivalents and so don't always fit an existing lamp. They can take a minute or so to reach full brightness.
- LED bulbs are the newest, most expensive, but also much the most efficient type. In theory, they could be made to convert nearly all the electrical energy into light. They can be made smaller than compact florescent, so are often made in shapes similar to traditional filament bulbs. Consequently, they are more likely to fit an existing lamp or light fitting.
- Some are now made with the LEDs arranged in "filaments" to resemble a traditional bulb. The more spread-out light source is more decorative and may be less likely to give you spots before your eyes if you should look directly at them.
- "Vintage" or "Virtual filament" LED bulbs are designed to mimic "vintage" bulbs. Some are actually LED filament bulbs (as above) sometimes with a flexible LED filament wound into a spiral. In others, the light is directed into a piece of transparent plastic and escapes through its edges and through etchings on its surface, giving a similar decorative effect.
Commercially, metal halide lamps are also used. These are often seen in shop displays or for illuminating a wide area in a superstore as they give a very intense white light. However, they take a minute or two to start and run extremely hot and so are not used domestically except as the light source in a data projector. Like a florescent tube, they work by creating an electrical discharge through a gas or vapour, but contain a mixture of gases to produce white light.
Dimmable?
All filament bulbs are dimmable though they become even more inefficient as they are dimmed. If you want to use a compact florescent or a LED bulb on a dimmer switch you must choose one which is marked as "dimmable", and ensure that your dimmer switch is a modern one designed for low energy bulbs.
What shape and kind of base?
When replacing a bulb, be sure to check which sort of base it has. Mains bulbs have either a bayonet cap (BC) or Edison Screw (ES). There are different sizes of ES and less often different sizes of BC. It's a good idea to replace an old filament bulb with a much more efficient compact florescent or LED equivalent, which will also run much cooler. However, compact florescents don't always fit an existing bulb holder or shade. If a compact florescent won't fit and you find the LED equivalent too expensive, at least go for a halogen.
Candle-style bulbs, often in a group of three or more, are used in decorative light fittings. Compact florescents couldn't be made sufficiently compact for this format so they have transitioned straight from halogen to LED.
Reflector bulbs are available in LED equivalents of the older types for use in suitable light fittings. These allow particular areas of a room to be highlighted, such as a kitchen work surface.
Linear bulbs have been used for many years, for example over a shaving mirror, in a cupboard, or over a work surface. These were either filament or florescent. LED equivalents are now available for the linear filament bulbs and also for standard florescent tubes.
Choosing the brightness
Old fashioned tungsten filament bulbs were sold in several different powers or wattages, most commonly 40W, 60W and 100W. Compact florescents were initially sold as (for example) 60W equivalent. However, quite often a so-called 60W equivalent compact florescent didn't actually give as much light as a traditional 60W filament bulb, causing mistrust of the ratings.
Nowadays, whilst bulbs still usually come in powers equivalent to the familiar 40W, 60W and 100W classifications, they are rated in lumens, which is a measure of the actual light they produce. This allows a direct comparison between LED bulbs, compact florescent and halogen. The precise lumen ratings vary slightly but typical values are given in the table below. The applications are for a small to medium house. Larger rooms will often need several light fittings.
| Tungsten equivalent | Lumens | Typical applications |
|---|---|---|
| 25W | 300 lumens | Cupboard light, or candle-style bulbs used several together. |
| 40W | 470 lumens | Bedside lights, cloakrooms. |
| 60W | 810 lumens | Reading light or table lamp, bedrooms, bathrooms, hall, landing. |
| 100W | 1520 lumens | Living room, dining room. |
Warm white or cool white?
LED bulbs are sold either as "warm white" or as "daylight" or "cool white". This is a matter of personal preference but warm white is closer to what we are used to from old fashioned filament bulbs. Cool white may seem harsh by comparison and is best avoided for bedrooms or rooms used in the evening as the extra blue in the light is not conducive to sleep.
How long will it last?
Halogen bulbs are generally designed to last 2,000 hours, roughly double the lifetime of an older tungsten type. A compact florescent might last 10 - 20,000 hours but will grow dimmer as it ages. A LED bulb should last 50,000 hours. Its life is limited not so much by the life of the LEDs as by the electronics in its base.
Premature failures are far from unknown in compact florescent and LED bulbs, often due to poor quality electronic components or poor thermal management in cheap Far-Eastern examples. You may wish to keep the receipt if you decide to buy a relatively expensive bulb so you can claim in the case of an early life failure.
Rather than failing outright, a LED bulb sometimes starts flickering due to the intermittent failure of one of the many individual LED chips it contains. If you're lucky it may settle down as the intermittent fault becomes solid, but the chances are it'll annoy you into changing it before that happens. Perhaps you could retire it to the cupboard under the stairs where its flickering would be less of a problem.
Why do they fail?
Although LED lightbulbs are much more efficient than the old incandescent types, the LED chips are run at quite a high current in order to get as much light as possible. This causes them to get quite hot, and often to heat up the control electronics, usually in close proximity. Heat always accellerates failures.
Each individual LED chip is bonded to a substrate which provides one connection, and a thin wire is bonded to the top of the chip to make the other connection. Heat is likely to stress a weak bond and increase the chance of failure.
The control electronics often includes one or more electrolytic capacitors. These contain a small amount of liquid, and although they are meant to be completely sealed the liquid can dry out, causing them to fail.
All electronics these days uses lead-free solder except in medical and other safety-critical applications. Lead-free solder is more brittle than traditional lead-tin solder, occasionally causing solder joints to fail.
Can I fix a dead LED lightbulb?
Whilst most electronics are repairable in principle at least, with lightbulbs the main obstacle is often the non-destructive disassembly. But even if you have to smash a glass bulb, you might still be able to reuse some of the components, and diagnosing the fault will certainly be instructive.
A series of YouTube videos demonstrates the teardown and diagnosis of a range of different LED light bulbs. No expensive test equipment is required.
Some LED lamps are built around standard easily obtainable surface mount multi-chip LED packages. A common type is 5mm square with 6 connections, and contains 3 individual LED chips. Often, one or two of the LED chips in a package will fail, and if you have the capability to do surface mount rework you should be able to replace failing packages.
Faulty electrolytic capacitors should be easily replaceable. If possible, choose a replacement with a higher temerature rating as the failed component may well have been under-rated. Such a replacement may be slightly bigger than the original, which could be a problem if space is tight.