The GU10 bulb started to appear in UK homes in the 1990s. The “10” in the name really just refers to the 10mm distance between the centres of the bayonet pins, each of which has a block on the end to lock the bulb into the socket.

GU10 bulb bayonet socket
GU10 bulb bayonet socket, showing 10mm gap

Despite the “10” referring merely to the distance between pins, GU10 bulbs quickly became synonymous with the spotlight-type bulb. They have a dome- or parabolic-shaped reflector, with a flat (or slightly convex) glass or plastic cover.

While the vast majority of GU10 bulbs still adhere to this concept, remember that GU10 is the fitting itself, not the shape of the bulb. There are some bulbs with a GU10 fitting and a more traditional light-bulb shape, but they tend to be specialist bulbs for certain custom-made appliances. That’s important because most GU10 fittings are designed to be sunk into a ceiling so they bulbs sit flush, and are thus shaped to accommodate the hemispherical reflector.

How to change a GU10 bulb

The fittings for GU10 bulbs have spring-loaded sockets, with two round holes leading to curved, narrow grooves. The blocks of the bayonet fitting are inserted into the round holes, and while applying pressure, the bulb is rotated until it stops. Once pressure is released, the springs, along with a locking step, hold the bulb in place.

To release the bulb, press it in and twist it, and it will pop out. Sometimes they are a little tricky to manipulate, in which case you can use an applying tool, which has a rubber sucker to grip the face of the bulb.

From halogen to LED

GU10s originally housed halogen lamps. These are incandescent bulbs which are brighter and more efficient than the standard tungsten bulbs that had been around since the dawn of electrification. They were also dimmable, and found their way into countless kitchens, bathrooms, living rooms and bedrooms in the 1990s and 2000s.

With the dawn of affordable LED technology, and legislation to outlaw inefficient incandescent bulbs, GU10s had to change. Rather than come up with a whole new fitting, it made sense to simply design LEDs that could fit in existing GU10 sockets, and that’s exactly what happened. Eventually, the technology was enhanced with dimmable LED bulbs, so the complete halogen experience could be achieved with a fraction of the energy consumption.

Spotlight beam angle

Since these bulbs are mounted in the ceiling and have a flat opening, they will never provide 360° illumination like a hanging bulb. They are thus technically spotlights, but they can still have pretty wide angles of illumination, by which we mean up to about 110°. That’s why it’s normal to have multiple GU10s mounted in the ceiling rather than just one or two main lamps. It’s the only way to cover the room with light.

When you’re buying GU10 bulbs, always look for the beam angle in the specifications. If you have a large room with only a few bulbs, you’ll need a wider beam to cover the area. Imagine the light from each bulb forming a cone, with the apex at the position of the bulb. If the angle is too narrow, you’ll just have several areas of intense light beneath the fittings, and everywhere else will be in relative darkness.

The only exception is if you have a particularly high ceiling, in which case the beam has more distance to spread out. In such cases, a narrower beam would actually be advantageous as the light would be more intense by the time it reaches ground level, whereas a wider beam would illuminate walls more than the floor. Narrow angle bulbs are also used for accent lighting or for illuminating objects.

Narrow spotlight beams are in the 15–25° range. Regular bulbs are in the 60–110° range, which is still pretty wide. The widest-angled bulbs can go up to more than 120°, but they’re not too common, as it’s usually better to simply install more regular bulbs.

Colour temperature

The final consideration is colour temperature. This is a measure of the “warmness” or “coolness” of the emitted light.

Warm light is white light that hints towards the red, orange or yellow part of the spectrum. It’s considered a more relaxing, homely tone that’s easier on the eye. That makes it perfect for living rooms, dining rooms and bedrooms.

Cool light has more emphasis on the cyan and blue hues, or aims for a pure, uncoloured white.  It is harsher and feels brighter for a given wattage. This tends to be used when seeing in detail is more important, such as in workshops, kitchens or makeup spaces. 

There is also the daylight colour, which imitates the natural colour temperature of the sun. It can have an uplifting brilliance, especially during the winter months. However, it can cause eye strain if you’re exposed to it for prolonged periods, especially later in the night.

If you’re interested in warm and cool light, we go into much greater depth in another article.

Come back for more bulb fitting wisdom

We’re going to cover all the main shapes and fittings here, so keep checking the help and advice pages so you don’t miss anything!