I
heard this on the BBC World Service Witness History programme in the
early hours of this morning. Red LEDs date back to the 60s, but became
more common in the 70s in consumer products such as calculators,
watches, instrument displays and indicators (replacing midget
incandescent bulbs). The green LED was invented in 1971 followed by a
yellow version in 1972. It would be two decades before suitable
semiconductor materials were developed to create blue LEDs in the early
90s. White LEDs can in theory be made by using red, green and blue
semiconductor chips in an LED encapsulation and mixing the emitted light
in equal proportions. However this gives poor colour rendering and
inferior "quality" white light. The breakthrough came by combining blue
LED technology with an integrated phosphor to convert the blue light
into white light. This is similar to the way a fluorescent tube has an
internal phosphor coating on the glass that converts UV to visible
light. Up until the end of the 20th century, LEDS had a quite low
luminous output and were primarily used for displays and as indicators.
However improvements in technology have increased output dramatically,
meaning they can now replace incandescent and discharge lighting in many
applications. They're also typically five times more efficient than
traditional incandescent lamps in converting electrical energy into
light and have over twenty times the lifespan (a 100 W incandescent
light bulb turns 95 W into heat and 5 W into light).
