LED
From CPFWiki
Contents |
LED Basics
An LED (Light Emitting Diode) is a type of solid-state lighting. In keeping with the properties of a diode, electric current can only flow one direction through an LED. As the electrons cross from a material abundant in negative charge carriers to a material abundant in positive carriers active region, they lose a specific amount of energy, which results in photons of a particular wavelength. Different semiconductors used in the manufacture of LEDs will result in different wavelengths (colors) of light emitted. Currently, the brightest semiconductor materials are Aluminum Indium Gallium Phosphide (AlInGaP) for reds, oranges, ambers and yellows, and Indium Gallium Nitride (InGaN) for blues and greens. A white LED is typically a blue LED coated in a yellow emitting phosphor (this is why flashlight LED's appear yellow when they are off), or combination of phosphors, which are excited by the blue light and produce light as a result. When this yellow light mixes with the blue light from the LED, the combined light appears white to the human eye.
LED Manufacturers
Cree, Luminus, Nichia, Osram, Philips Lumileds (Luxeon), Seoul Semiconductor
CPF Post with pictures of common flashlight LED's
LED's in Flashlights
Early LED's did not give off that much light, but they were efficient. As they got brighter, they started making their way into flashlights. The classic LED's were encased in plastic with a round head. As the LED inside gave off light, the rays were shaped by the round head to go straight ahead. Many keychain flashlights use a simple LED like that. These are usually named as 3 mm or 5 mm LED's. The Fenix E01 uses a Nichia LED. To get additional brightness, some flashlights would combine multiple LED's in the head of the flashlight and maybe include some kind of reflector to shape the light.
High power LED's were developed to handle higher currents and produce brighter light. They lost the clear plastic shell and had to be mounted to a metal base to draw heat away from the LED before it could burn itself out. Lumileds Luxeon I was a 1-watt LED producing 30 to 60 lumens and was followed by the 3-watt Luxeon III (60-90 lumens, requiring more power than the Luxeon I). They also produced the K2 which could be driven at even higher currents for more output.
To compete with Luxeon, Cree started producing the Cree 7090 XR-E in 2006 with various bins (P4, Q3, Q5). The XR-E produces twice as much light as a Luxeon III at the same voltage and current. Seoul Semiconductor (SSC) produced the Seoul SSC P4. In 2007, Lumileds responded with the the small and very efficient Luxeon Rebel series of LED's.
To get even more brightness, rather than combine LED's into one flashlight, multiple LED's could be mounted to the same chip. These multi-die LED's produce 400 to 900 lumens. The Luxeon V was one of the first and produced 100-140 lumens. Seoul produces the P7 and Cree produces the MC-E, each with 4 LED's on a chip.
Bins
A particular design of LED will usually be sold in a number of different bins. Although LED production is tightly controlled, the resulting LEDs have different properties. Therefore these are sorted into bins based on flux (output) and tint. As production is refined, higher bins may become available. Thus the Cree XR-E Q5 was introduced a year or two after the earlier, less bright XR-E P4.
The Brightness Bins article summarizes manufacturer data for bins of various LED's.
Different LED manufacturers use different bin codes. Here is a CPF post with most of the different bin code color charts.
Types
Standard packages
Well-known proprietary packages
- Luxeon emitter (Lumileds)
- Luxeon star (Lumileds)
- "Jupiter" (Nichia)
- "Golden Dragon" (Osram)
- XLamp (Cree)
- "UFO"
Luxeon types explained - high dome, low dome, side emitter, etc.
Spectrum
LEDs are inherently monochromatic. There are a number of colors available.
| Color | Wavelength (nm) | LED Museum Link |
|---|---|---|
| Infrared | 1300-700 | Infrared |
| Red | 700-640 | Red |
| Orange-Red | 640-625 | Orange-Red |
| Orange | 625-615 | Orange-Red |
| Amber | 615-600 | Amber |
| Yellow | 600-585 | Yellow |
| Yellow-green | 575-555 | Yellow-green |
| Green | 540-515 | Green |
| Blue-Green | 510-490 | Blue-Green |
| Sky Blue | 485 | Sky Blue |
| Blue | 480-445 | Blue |
| Deep Blue | 440-430 | Deep Blue |
| Violet | 420-395 | Violet |
| UltraViolet | 395-275 | Ultraviolet |
Blue LED's were invented in the early 1990s by a Nichia researcher. UV LEDs are relatively new. Most are technically "Near-UV" (NUV), in that the light they emit is on the boundary between visible light and true (invisible) ultra-violet.
xenopuselectronix.com UV/Crime Scene Flashlights
White & phosphor-doped colors
White LEDs capitalize on all the advantages of LEDs and sidestep one of their major downsides - monochromatic spectrum. White LEDs use a number of tricks to do this.
White
Blue + Yellow The most common type of white LED. The underlying LED is actually Blue or Violet, but a phosphor layer shifts the light color to white. The phosphor is typically yellow or yellow-green, and is visible from the outside when the LED is off. As the LED produces light, this light excites the phosphors which give off yellow light. The two colors combine and produce a white light. Overdriving the LED often results in a bluish tint as the LED "overpowers" the phosphor, while underdriving the LED will typically result in a yellowish tint as the phosphor "overpowers" the LED.
Pink and purple are similar blends of LED light with phosphors. Pink at the LED Museum and purple at the LED Museum
UV + White Offer efficiencies similar to flourescent lighting.
ZnSe
RGB
RGB white LEDs are actually 3 (or more) LEDs in one - they use red, green, and blue elements to simulate white light. These appear in LCD computer screens and TV's.
LED Electronics (converter boards)
see Drivers.
Deterioration
LEDs deteriorate over time. Heat during operation or from improper soldering can reduce lifetime to 70% brightness (the common specification in the industry). Chip Weevils are another common source of decreasing brightness in LEDs.
LED Electronics (direct drive)
Links to CPF Threads
External Links
The LED Museum
LEDs (original page)
SMD-LED Soldering
www.LEDmonthly.com -----International Online LED Community-----
howstuffworks.com: LEDs
