Why are LEDs different?

LED lighting is very different from earlier lighting sources in that it is produced using semiconductors. A light-emitting diode (LED) is a semiconductor that emits light when current flows through it. Because it is such a new technology, there has been no long-term research into LED safety and its health and environmental impacts. In particular the light propagation and distribution characteristics of LED lighting means it cannot be regarded as a like for like replacement of other lighting technologies.

How LEDs work (the detail)

A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Each LED emits light of only one particular colour. In street lights, this is usually blue. To make white street lights, a powerful blue LED is shone on to compounds called phosphors that absorb blue light and emit yellow light. This yellow light combines with the blue light and appears white to the eye. 

Most white-light sources emit a range of wavelengths, which, when combined, produce the colour of light perceived by the human eye. The resulting shade of white depends on the blend of phosphors and is measured on the colour-temperature scale. 

Colour temperature

Colour temperature is expressed in Kelvins, using the symbol K, a unit of measure for absolute temperature. It is measured on a numbered scale, where the higher the number, the ‘cooler,’ or bluer the light. 

Examples of the colour temperatures of different forms of lighting include:

• 1700K – Low pressure sodium lamps
• 2400K – Standard incandescent lamps
• 2550K – Soft white incandescent lamps
• 2700K – Soft white LED street lights
• 3000K – Warm white LED street lights
• 4000K – Neutral white LED street lights (the most commonly used type)
• 5000K – Cool white LED street lights, Tubular fluorescent lamps
• 6000K – Sunlight.

Early ‘white’ LEDs were very blue and harsh on the eye. Adding more phosphors to a ‘white’ LED makes its light look warmer and less harsh, but at the expense of reduced efficiency, as energy is lost in converting high-energy blue photons to lower-energy photons. This means that ‘warmer’ LEDs are slightly more expensive to run and achieve marginally lower electricity savings. White LEDs also affect circadian rhythms and cause sleep disturbance when they shine into people’s homes.

Direction and distribution

Unlike other forms of lighting, LEDs are highly directional with light emitted in an arc of around 60 degrees, rather than 360 degrees common in other lighting. LED light is usually emitted from a small, flat, surface. Because of this, the vast majority of LED luminaire designs suffer from the drawback of attempting to illuminate wide areas, a distance away from a small, flat light source.

Also, LED light is not distributed evenly across its beam but concentrated in the centre of the beam (on its axis). This causes problems of glare, for example, car headlights can be blinding to pedestrians and oncoming traffic when cars go over a speed bump or over the brow of a hill and shine directly in people’s eyes. 

A recent RAC survey found that “the headlights of some newer cars are so bright they are causing a road safety hazard for drivers with as many as two-thirds of motorists saying they regularly get dazzled by oncoming headlights even though they are dipped”.  There are similar issues with LED street lighting where areas directly under the light are brightly illuminated, but with poor illumination between lights (the zebra effect).