Screw Cap or Bayonet: Which Lightbulb Should I Buy?

Choosing the right light bulb is easy when you know what you need. If you’re not sure which light bulb is suitable for your lamp or light fitting have no fear, we’ve put together this light bulb buying guide here to help you. 

To choose the right light bulb, you will need to know:

1. 1. 1. Which type of light bulb cap or base your light fitting needs
   1. 1. Which shape of light bulb you need
   1. 1. Which type of lighting technology you need (e.g. LED)
   1. 1. What level of light bulb brightness you need
   1. 1. What colour temperature do you need?
   1. 1. Whether you need a dimmable or non-dimmable bulb
   1. A Quick Guide To Bulb Base Codes

Step 1. Light Bulb Caps and Fittings

Light bulb bases (also known as caps) are the part that connects the bulb to the light fitting, and they come in several shapes and sizes that can often be confusing, to begin with. The type of bulb you need to choose will depend on the types of fittings you have in your home, and buying a bulb with an incompatible cap or base for your fittings will render the bulb useless. 

Choosing the right light bulb cap fitting is the first essential step to light bulb buying, as the wrong cap will not work with your light fitting.

The main types of light bulb caps are as follows:

• • Bayonet Cap

• • Small Bayonet Cap

• • Edison Screw Cap 

• • Small Edison Screw Cap

• GU10 Cap

Screw Caps (Edison Bulbs)

The Edison screw cap bulb is named after the inventor of the light bulb, Thomas Edison himself, and is one of the most commonly seen types of light bulb caps on the market. Largely unchanged since its invention, the screw cap bulb has a threaded metal base that screws tightly and securely into place within the light fitting.

The most commonly used screw cap bulb is the E27, which fits most standard light fittings in the UK. There are also smaller versions (E14 and miniature E10s), which have a thinner screw base and are most commonly used for decorative lighting, i.e. candle-shaped bulbs for chandeliers.

Bayonet Caps (Twist and Lock)

If your light fittings do not take screw cap bulbs, odds are they require bayonet cap bulbs; the commonly used bulbs in the UK. Also known as the ‘twist and lock’ bulb, a bayonet cap bulb has two pins which retract and then expand into slots inside the fitting when twisted. Standard bayonet cap bulbs are 22mm in diameter (B22d), but there are smaller (SBC) sizes available.

GU10 Lamps

Although GU10 caps look very different from traditional bayonet cap bulbs, their ‘twist and lock’ mechanism means they are technically part of the bayonet cap family. The letter ‘G’ indicates that the bulb has a two-pin (or bi-pin) base, and the number 10 indicates that the pins are 10mm apart.

GU10 lamps were traditionally the base cap for halogen bulbs, but manufacturers are now making LED retrofit versions which are much more energy-efficient.

LED GU10 bulbs are most commonly used as kitchen and bathroom downlights and another domestic recessed lighting, as well as retail showroom lighting. Another common GU base is the GU5.3 (also known as the MR16), which are used for very similar purposes but use a simple push-in mechanism instead of a twist and lock. GU5.3 bases also used to be designed for halogen bulbs before LED retrofits were created.

Each type has a code, which is always displayed on the packaging and will help you match up your old bulb with the correct new one.

Less common bulb types include MR16, MR11, G4 and G9, which are traditionally halogen bulbs. 

Halogen bulbs have been phased out since September 2018, but if you have these light fitting types, you should easily be able to find retrofit LED versions to replace old ones.

Here’s a little more information about common the most common light bulb types:

How do I know what type of bulb fittings I have?

There is nothing more frustrating than buying a new bulb only to get it home and realise it has the wrong type of cap for your fittings. To avoid this, it is best to be as prepared as possible before you hit the shops, which means having a good look at your fittings.

If Your Fittings Still Have Bulbs In:

The fastest and easiest way to find out which type of fitting you have (and therefore which kind of bulb you need to buy) is to simply remove the bulb that is currently in the fitting and check what type of bulb it is. Most bulbs will have the base code and wattage/lumen levels written on them.

If Your Fittings Are Empty:

If there is currently no bulb in your fitting, then you will need to look closely at the type of fitting and do a quick diameter measurement. 

A quick inspection will also tell you if you require a screw cap (the inside of the fitting will have spiralling ridges (threads), or a bayonet cap (the inside of the fitting will be smooth with two L-shaped slots either side).

For empty spotlight fittings, you can measure the distance between the pin slots to determine whether you need a GU10, GU5.3 or another sized bi-pin lamp.

Step 2. Light Bulb Shapes

Now you know which light bulb cap you need, you need to decide which bulb shape you want to get. 

Light bulbs come in all shapes and sizes, and multiple shapes can be used for each fitting type. For example, a classic-shaped light bulb and a candle-shaped bulb can both be found with standard bayonet cap fittings, so this stage is in part down to preference and also in part down to the style, shape and size of your light fitting.

Please also note, the names for light bulb shapes are not universal in the same way as the cap fittings are, so there may be some variation here. 

Not sure which light bulb shape you need? Here are the most common light bulb shapes you can purchase; if you still have your old bulb simply match it up to the closest shape on our chart below.

If you don’t have your old bulb for reference, you can pick which bulb you think would look best in the fitting in question. 

Downward-facing pendant ceiling lights usually suit traditional and globe-shaped bulbs, while upward-facing chandelier fittings often look great with candle-shaped bulbs, and table lamps can use practically any bulb shape providing it fits safely within the lampshade.

Step 3. Light Bulb Technology

Do you know which type of light bulb technology you should be using for your light fitting? 

The four most common types of light bulb technology that most people are familiar with include:

• • Incandescent

• • Halogen

• • • LED
  • Energy-Saver / CFL (Compact Fluorescent Lamp)

Incandescent bulbs and halogen bulbs have now been phased out and are no longer being manufactured, so you won’t be able to buy these any longer. LED light bulbs are your best course of action as they are the most energy-efficient and last the longest. Some halogen bulbs are still available to buy, though these will not be around for long and only have an average lifespan of approximately two years.

Making the switch to LED bulbs is easier than you might think, as most types of halogen and incandescent bulb light cap fittings are available to buy as retrofit LED lights.

Step 4. Light Bulb Brightness

If this is the first time in a long while that you’re shopping for a new light bulb, you may notice that things are a little different now. Most of us have spent years getting used to shopping for light bulbs by wattage, but technology has moved on. With the phase-out of incandescent and halogen bulbs, watts are no longer a relevant measurement of light bulb brightness.

Light bulb brightness is now measured in Lumens, which is purely a measurement for brightness rather than energy consumption. You will find that the wattage indicator on LED light bulbs looks very low, but this is because LED lights require less energy to produce a brighter light.

Not sure which light bulb will match the brightness of your traditional 40 or 100-watt bulbs? Use our handy Lumens to Watts conversion chart to find the right bulb to suit your light fitting and your lighting requirements. Or, if you already have an LED bulb, check the bulb to find the lumens rating.

Step 5. What Colour Temperature Bulb Do You Need?

As bulbs have come up to date with technology, you can now buy a range of colour temperatures. 

As well as the brightness, the colour temperature of your lightbulb can alter the way a room feels. From creating an ambient atmosphere to providing white-clarity, lighting can be amended to suit the room’s purpose, and it’s essential to keep in mind when you purchase your lightbulbs. 

Daylight Temperature

The brightest white, daylight temperature bulbs will range from 4,600-6,500k. Perfect for security lighting, these light bulbs will brighten up space with clarity. 

Cool White

The cool white temperature will be the ideal fit for bathrooms or kitchens, adding a functional tone of white to the room. These bulbs will be 3,100-4,500k.

Warm White

The ideal choice for an ambient glow in bedrooms, living rooms and dining rooms, warm white light bulbs will be perfect for creating a relaxing environment throughout your home. Warm white bulbs range from 2,000k to 3,000k. 

Step 6. Dimmable or Non-Dimmable?

Next, you need to consider whether or not you want your light fitting to be dimmable. 

Dimmable light bulbs are great for spaces that are used for more than one purpose, i.e. reading, working and watching TV, as you can set the lights to the exact level of brightness you need at any one time.

However, not all bulbs are compatible for use with dimmer switches, so you need to take care when choosing your new bulb. 

The good news is, LED light bulbs are available to buy in both dimmable and non-dimmable versions, so you will always be able to find the right bulb to suit your needs. The packaging will always state when a bulb is dimmable, so always check closely before buying.

CFL ‘energy saver’ bulbs are not dimmable, while the remaining halogen bulbs on the market can usually be used for both dimmable and non-dimmable purposes.

Step 7. A Quick Guide To Base Codes:

The first letter(s) indicate the type of base, for example:

• • E= Edison (screw)

• • • B = Bayonet
  • G = Multi-pin / Bi-pin

The number(s) indicates the diameter of the base or distance between the pins in millimetres, for example:

• E27 = Edison 27mm
• B22 = Bayonet 22mm
• E14 = Edison 14mm
• GU10 = Bi-pin 10mm between pins

There is also sometimes a final letter at the end, which simply indicates the number of pins present, for example:

• S = single pin
• D = double pin / bi-pin
• T = triple pin

So, for example, a standard bayonet cap would have a code like this: B22d

Hopefully, we’ve helped you to gain an insight into which light bulb is best for you and stopped you from being overwhelmed when you go shopping but, if you’re still in doubt, simply take your bulb along to the shop and ask for assistance in finding a matching new bulb.

If you’re still unsure about which of the different light bulb bases you need for your fittings, simply give the team at Moonlight Design a call on 020 8925 8639 or email us at info@moonlightdesign.co.uk and we’d be more than happy to help.