LED
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 LEDs 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), Samsung, Seoul Semiconductor
LED Gallery with pictures of common flashlight LEDs
Driving the LED
As the available voltage to a LED increases, the current (in amps or milliamps) drawn by the LED increases. LEDs are usually more efficient at lower voltages and currents, but get brighter as more power (power being volts times amps and measured in watts) is applied. LEDs are evaluated at different currents and a forward voltage (or Vf) is measured as the voltage drop across the LED at a particular current level, 350mA or 700mA being pretty common benchmarks. In flashlights, an electronic driver is there to regulate the amount of power delivered to the LED either by controlling the voltage and/or current to the LED.
As more power is applied, more waste heat is generated and must be carried off. At some point a LED will be overdriven which will shorten its life from the tens of thousands of hours a properly driven LED should last. As the LED is overdriven the yellow phosphor on the LED starts to burn and "angry blue" light is emitted. If the light is turned off quickly, the LED may avoid permanent damage, but otherwise the LED will literally burn with brown spots on the LED. Generally when that happens, the maximum output will now be significantly lower.
LEDs in Flashlights
Early LEDs did not give off that much light, but they were efficient and they would last many thousands of hours before burning out. As they got brighter, they started making their way into flashlights. The classic LEDs were 5mm in diameter encased in clear epoxy resin 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 LEDs. The Fenix E01 uses a 5mm Nichia LED. To get additional brightness, some flashlights would combine multiple LEDs in the head of the flashlight and maybe include some kind of reflector to shape the light.
High power LEDs 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 using Cree's LED die. In 2007, Lumileds responded with the the small and very efficient Luxeon Rebel series of LEDs.
To get even more brightness, rather than combine LEDs into one flashlight, multiple LEDs could be mounted to the same chip. These multi-die LEDs 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 LEDs on a chip.
Luminus developed the SST-50 and SST-90, larger LEDs requiring currents of 5 amps and more, but giving off a lot of light. Cree responded with the XM-L which has a larger (than the XR-E/XP-E series of LEDs) single die and can be driven up to 3 amps.
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 slightly varying properties. Therefore LEDs 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 light output data (lumens) for bins of various LEDs.
LEDs are also binned by the resulting tint of the LED. See pages for each LED manufacturer for a chromaticity chart showing tint bins. Some companies base their tint bins on ANSI White standards.
Some LEDs are also binned by Vf, or forward voltage.
Datasheets
Manufacturers usually have a datasheet available for their different models of LED. These have a great deal of valuable information, including the binning and labeling information (sometimes in a separate document) which tells you how to read product number codes and codes for the different bins for output, tint, forward voltage, and CRI. Different tint bins are described with coordinates and a chart is usually included showing the tint bins graphically along the black body curve. The datasheet also includes physical dimensions of the LED and characteristics like viewing angle. Graphs tell how the output varies with temperature and current, as well as how the forward voltage increases for given currents.
Here's an example: A manufacturer makes a LED available in bin X that gives 120 lumens at 350mA, but you want to know the output at 1500mA. In the datasheet find the relative output (or relative luminous flux) graph. At 350mA the line should indicate a relative output of 100%. To find the output at 1500mA, find 1500mA on the horizontal axis, go up to the line and then over to the relative output reading, which might be 320%. So just multiply 120 lumens times 3.2 to get an output of 384 lumens at 1500mA.
Since some LEDs are now binned at 85 degrees C instead of 25 degrees, the datasheet allows you to correct the output for temperature and make fairer comparisons between LEDs.
The datasheets usually do not tell you the bins that are actually available. Just because there are 80 different tint bins does not mean you can get the LED in all of those tints. And while a datasheet might show a maximum output bin, realistically that may not be available or sometimes even higher bins will be available. Manufacturers usually update the datasheets a few times during a product's lifetime.