What GU10 does and does not tell you
The designation is a base type. G means a pin base, U describes the pin shape, and 10 is the spacing between pin centres in millimetres. It tells you the lamp will lock into the holder and run from the mains supply. It says nothing about output, colour, beam, colour rendering, flicker behaviour or dimming.
The specification that actually matters
| Specification | Why it changes the result |
|---|---|
| Lumens | The quantity of light. Wattage does not describe it. |
| Beam angle | How that light is distributed, and how bright the pool is. |
| Correlated colour temperature | Whether the room reads warm or cool, and whether it matches its neighbours. |
| Ra, and R9 separately | How faithfully colour renders, especially saturated reds. |
| Flicker behaviour | Whether the lamp is comfortable, particularly when dimmed. |
| Dimming compatibility | Whether it works with the dimmer already installed, and how low it goes. |
| Physical depth | Whether it clears the ceiling void and the existing holder. |
| Glare control | Whether the source is visible from a seated position. |
A list of the properties to compare. No threshold, brand or product is recommended, and no performance figure is asserted.
A practical way to choose
- Start from the room, not the lamp: what has to be lit, and how brightly.
- Fix the colour temperature first, and keep it consistent across everything in one sightline.
- Pick the beam angle from the ceiling height and the size of the area each lamp has to cover.
- Match the lumens to that decision rather than replacing like for like on wattage.
- Ask about colour rendering if timber, skin, food or fabric are involved.
- If it will be dimmed, test one lamp on the actual dimmer before ordering the rest.
Replacing an existing lamp
When a single lamp fails in a ceiling of twelve, the replacement is a matching problem rather than a specification problem. The new lamp needs the same colour temperature, a similar beam angle and a similar output as its eleven neighbours, or it will be the one everybody notices. Keeping a note of what was originally installed is worth more than any datasheet at that moment.
Common mistakes
- Buying on wattage because the base fits, and finding the room dimmer or cooler than before.
- Replacing one lamp in a ceiling of many without matching colour temperature and beam.
- Assuming any GU10 will dim on an existing dimmer.
- Overlooking physical depth, then finding the lamp sits proud of the ceiling.