Page 126 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

120 Photonic Devices

Figure 6.11. Roughening the surface of an LED in order to improve external emission
efficiency. Glass spheres are sprinkled on the surface. The spheres are used as a mask
for sandblasting the surface. Taken from I. Schnitzer et al., Applied Physics Letters 63,
2174 (1993). Reproduced by permission from the American Institute of Physics.

would like to enhance the emission probability. The microcavity effect
was first discussed by Purcell in the context of decay of radioactive
atoms. This effect is based on the coupling between the probability of
recombination and the availability of optical modes that can carry
away the emission. Theoretical descriptions of this effect use the
framework of advanced quantum mechanics to estimate the improve-
ment in the matrix element. The effort to understand and optimize
this effect is a current research topic in the many advanced opto-elec-
tronics laboratories around the world.

Example 6.3. Traffic Lights—An Engineering Estimate
Suppose you are working as a traffic engineer for a metropolitan area
(Manhattan), and you are considering using LEDs instead of incan-
descent light bulbs for traffic lights. Could you make an initial esti-
mate to help you judge the conditions that would make this change
beneficial to taxpayers? There are three parts to this problem.
1. Properties of the lightbulbs and LEDs
Incandescent light bulbs
Power consumed = 100 W
Power efficiency = 15%

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