The Light emitting diode (LED)

The LED is likely to be the most important advance in lighting in the early part of the twenty first century. LEDs used to be available as low power indicators in two types – one that emitted red light and the other that emitted green light.

The big advance was the development of relatively high power LEDs that emitted white light. I have a torch that contains five LEDs that gives a total output of 15000 mcd (power consumption 0.6W) using a current of 150 mA at between 3 and 3.5V.(For a definition of light intensity see: Candela)

My torch is also re-chargeable by hand.


To understand how the LED works it is important to understand the workings of the semiconductor diode – see: Semiconductor diode

It is very likely that all homes will eventually be lit by LED light sources. They consume low power, are small, fairly robust, emit virtually no heat energy and give a good clear white light.

Just like a normal semiconductor diode current flows when electrons move from the conduction band in the n type cathode" to the valance level in the p type "anode". The difference is that with the LED when the electron falls into a positive hole energy is released in the form of a photon. The wavelength of this photon depends on the energy gap of the materials of the p-n junction.

White light emitting LEDs can be made in a variety of ways. One is to coat a blue emitting LED with a material that converts some of the blue light to yellow. The yellow light stimulates both the red and green receptors in the eye. When this yellow light is combine the original blue emitted by the LED the resulting output gives the impression of white light.