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To: mauser96 who wrote (1981)	2/10/2000 3:00:00 PM
From: Guy Gordon	Read Replies (2) \| Respond to of 10713

Lucius: Is this the section you refer to? About 30 percent of the light generated inside the chip makes it way out of the brightest LEDs. Semiconductor materials have very high indices of refraction and so can trap a great deal of light when configured in a square chip. An epoxy encapsulant around the LED chip reduces the refractive index mismatch and allows more light to be emitted. For some LEDs, the light escaping the chip (extraction efficiency) can be 4 percent or lower. Transparent substrates and thick semiconductor layers increase the extraction efficiency. Making LED chips more spherical, which is now not practical for mass production, could also significantly increase extraction efficiency. I'll look into it further. I'm quite open to being proven wrong. This is the first reference I've ever seen to an advantage in the transparency of the substrate. RE "I'm not interested in playing games, just trying to find out the facts..." I assumed so.

To: mauser96 who wrote (1981)	2/10/2000 5:36:00 PM
From: John Walliker	Read Replies (3) \| Respond to of 10713

Lucius, Sorry Guy, I believe you may be incorrect. Not all the light goes out laterally in an LED, or perhaps there is some scattering effect. In an LED the photons of light are emitted in random directions. The optical properties of the semiconductors and the substrate affect which directions deliver useful light to the outside world. In a laser there is stimulated emission of radiation. The light intensity, initially from LED action, builds up between a pair of parallel mirrors (made from the cleaved or etched sides of the die). As photons pass atoms (or molecules) which have been given a higher energy level by the electric field in the laser diode they stimulate the coherent emission of additional photons. Coherent emission means that the wavelength, direction and phase (relative position of peaks and troughs in the electric and magnetic fields of the light) are the same for the stimulating and stimulated photons. Hence the light from a laser does have an inherent dominant direction while that from an LED does not. John