AN2042

Light emitting diode and colour vision

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Light emitting diode and colour vision

Light-emitting diodes (LEDs) used for illumination are solid-state devices that produce light

by passing electric current across layers of semiconductor chips that are housed in a

reflector, which in turn is encased in an epoxy lens. The semiconductor material determines

the wavelength and subsequent color of the light. The lens converts the LED into a

multidirectional or unidirectional light source based on specification.

The first generation of LED was based on Gallium Arsenide (GaAs), Gallium Arsenide

Phosphide (GaAsP), Gallium Phosphide (GaP) technology, but thanks to the growth of solid

state technology, new structures have been introduced based on Aluminum Indium Gallium

Phosphide (AlInGaP), Indium Gallium Nitride (InGaN) or Gallium Aluminum Arsenide

(AlGaAs), mainly for the high brightness LEDs branch.

In Figure 1 the basic LED structure and the energy bands are shown.

Figure 1.

Light emitting diode structure

The junction in an LED is forward biased and when electrons cross the junction from the n to

the p type material, the electron-hole recombination results in a process called

electroluminescence: when the applied voltage drives the electrons and holes into the active

region between the n-type and p-type material, the energy can be converted into infrared or

visible photons. This implies that the electron-hole pair drops into a stabler bound state,

releasing energy on the order of electron volts by emission of a photon of energy, according

to (Equation 1).

Equation 1

The human eye is excited in response to electromagnetic radiations with wavelengths in a

tight range of the electromagnetic spectrum, as shown in Figure 2, from 400 nm to 700 nm

which corresponds to extreme red and violet respectively.

E

g

h

c

υ

h

c

λ

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