Why does my energy-saving light bulb sporadically shine after it's been switched off? © Getty Images

How do colour-changing light bulbs work?

A tiny computer can create different colour combinations using coloured LEDs which correspond with the colour-sensitive cells in the eye.

Asked by: David Connolly

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Colour-changing light bulbs use red, green and blue LEDs to create their range of colours, switching them on in different combinations using a tiny computer (‘microcontroller’). Red, green and blue LEDs are used because our eyes have colour-sensitive cells called cones that are roughly attuned to these three wavelengths of light, and we see colours as different combinations of these wavelengths.

If you want yellow, the bulb turns on red and green. For cyan, it turns on green and blue. For white, it turns them all on. To fine-tune the shades of colour, the bulb also pulses the LEDs very quickly – if it pulses blue so that it’s on only 50 per cent of the time, for example, we see a darker blue. With these tricks, colour-changing bulbs can create millions of subtly different colours.

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