How White LEDs work / Colour Produced

Like all LEDs, the white LED consists of a semiconductor PN junction, which emits photons when excited by forward current. The photons are emitted at a single frequency, which is determined by the type of semiconductor material that the junction is made from. In the case of the white LED, the wavelength of the photons is approx 460nm, which is blue. Yellow phosphors are placed in front of the PN junction, which convert some of the blue light into a broad spectrum light of a yellow colour. The resulting mix is perceived by the eye as white light.

There has been much debate over the colour of the light produced, many cavers preferring the yellowish glow of a halogen bulb. If we compared the spectrum of a halogen bulb to that of a typical white LED we would see the halogen bulb produced far more light of a green and red colour. This helps explain why these colours appear less vivid when lit by LED light. In most LEDs the phosphors have been optimised to produce light of a band of wavelengths where the human eye is most sensitive.

Typical Cool White LED Response

The hue of the light is known as its colour temperature. Halogen bulbs produce more energy at the red end of the spectrum and can be described as producing ‘warm white’ light. White LEDs are available as both ‘warm white’ and ‘cool white’, however, as the ‘cool white’ LEDs produce a hue nearer to daylight, and are more efficient, they are normally preferred and are therefore, much more common.

White light can also be produced by mixing Red, Green and Blue light in equal quantities. Although not as efficient, this can lead to better colour rendition. When using multiple 5mm white LEDs, a small number of cyan and orange LED’s can be added to help improve the colour rendition by filling in the missing parts of the colour spectrum.