Principle of LED lamp
LED (Light Emitting Diode), a light-emitting diode, is a solid state semiconductor device that converts electric energy into visible light. It can directly transform electricity into light. The heart of LED is a semiconductor wafer. One end of the chip is attached to a bracket, one end is the negative pole, and the other end is connected to the positive pole of the power supply, so that the whole wafer is encapsulated by epoxy resin.
The semiconductor wafer consists of two parts, one is P semiconductor, the other is N semiconductor, and the other is electron. But when these two semiconductors are connected, a P-N knot is formed between them. When the current is acting on the wafer through the wire, the electrons will be pushed to the P area. In the P area, the electrons will be combined with the holes, then the energy will be emitted in the form of photons, which is the principle of the LED lamp's luminescence. And the wavelength of the light is the color of the light, which is determined by the material that forms the P-N junction.
LED can directly emit red, yellow, blue, green, green, orange, purple and white light. 
Initially, LED was used as the indicator light for instruments and meters. Later, all kinds of light and color LED were widely applied in traffic lights and large area displays, resulting in good economic and social benefits. Take the 12 inch red traffic lights as an example, in the US, the 140 watt incandescent lamp with long life and low light efficiency is used as the light source, and it produces 2000 lumens of white light. After the red filter, the loss of light is 90%, and only 200 bright red light is left. In the newly designed light, the Lumileds company uses 18 red LED light sources, including circuit losses, which consume 14 watts of electricity, which can produce the same light effect. The vehicle signal lamp is also an important field of the application of LED light source.
For general lighting, people need a white light source more. In 1998, the white light LED was developed successfully. The LED is made of GaN chips and yttrium aluminum garnet (YAG). GaN chip blue light (lambda p=465nm, Wd=30nm) and high temperature sintered YAG phosphor containing Ce3+ are emitted yellow light and peak 550nLED lamp m after stimulated by blue light. Blue chip installed in the LED based wanxingreflection in the cavity covered with a resin mixed with thin films of YAG, about 200-500nm. The blue light emitted by the LED substrate is absorbed by the phosphor, and the other part of the blue light is mixed with the yellow light emitted by the phosphor, and the white light can be obtained.
For InGaN/YAG white LED, the white light of the color temperature 3500-10000K can be obtained by changing the chemical composition of the YAG phosphor and adjusting the thickness of the phosphor layer. The method of getting white light through blue light LED is simple, low cost and high technical maturity, so it is used most.