LEDs-Advanced - APPLICATIONS OF ULTRABRIGHT LEDs AND DISPLAYS
The LEDs
The Innovation of SOLID STATE LAMP or LIGHT EMITTING DIODE (LED) marks a real breakthrough in the lighting industry. This device is highly versatile in its applications and is distinguished by its high reliability, long life and fast switching speed. Whereas pulsing application is the nemesis of best of filament lamps, a LED actually thrives on pulsing.
There are light generation mechanisms like incandescence, electroluminescence, chemiluminescence, triboluminescence, photoluminescence etc. LEDs employ pn-junction luminescence that occurs as a result of the application of direct current at low voltage to a suitably doped crystal containing a pn junction. It arises in two steps- excitation when electrical energy is absorbed and recombination in which energy is released as light and heat .
The light output, Iv, of an LED is proportional to the forward current, If, which again is a function of the forward voltage applied. In reverse bias beyond Vr, the LED transits from a nonconductive mode to breakdown mode.
A normal LED consists of an LED chip of around 0.3 sq. mm is attached to a 2 leaded lead frame with highly reflective cup forming contact on the p side. A fine .025mm gold /aluminum wire connects top n side pad to the next lead. An epoxy filled lens is molded on to the leads to provide protection as well as to produce the desired beam pattern.
Brief introduction to Electro-Optical Measurements of LEDs
Several areas of "light" measurement are used for defining the performance parameters of an LED. Photometric measurements relating to measurements of visible radiation as the "standard" human eye responds to it are of primary importance. The other system of measurement is radiometric, which is not limited to the eye response. Radiometric measurements are used to cover the full spectrum of radiation/photons emitted from the LED (visible plus ultraviolet and infrared).
Photometric units of measurement are important for LED product designers and those focusing on visible LED product applications (such as Big Screen Video Displays. Radiometric measurements are impor