LED Types:
5mm/Nichia: This is the ultra-common low-power LED that gives off a wide spot of directed light. They come in many colors, and the "white" version is usually bluish, to some degree. They cost about $0.15 each. The Vf of these LEDs varies with color, with red 5mm LEDs having a Vf of a little over 2V and white 5mm LEDs needing about 3.5V. An ordinary white 5mm LED will produce from 5-10 lumens at a safe drive level of about 20mA. Because of this very low current requirement, these LEDs are ideally suited to keychain flashlights that run on small button or coin cells, as these cells can't handle much current anyway.
Luxeon I/III: These have been the standard range of high-power LEDs for years. They cost between $8 and $15, depending on bin. Prices dropped along with demand upon the release of the new high-efficiency power LEDs (explained below). The only difference between Luxeon I and Luxeon III is their speced drive currents. Lux Is are rated for a particular light output level at 350mA, and Lux IIIs are rated for a particular light output level at 700mA. This has led to the common marketing practice of calling Lux Is "1-watt" LEDs and Lux IIIs "3-watt" LEDs. This is misleading, because these wattages only describe the speced capabilities of these emitters (Vf of 3.4V * 350mA = 1.2W, and Vf of 3.7V * 700mA = 2.6W) and not the actual power consumed by the LED in any particular flashlight. Thus, a "3W LED flashlight" could be running at any actual drive level.*
A common Luxeon I driven at a decent 1W will provide between 30 and 60 lumens, depending on bin. A common Luxeon III driven at a decent 3W will provide between 60 and 90 lumens, depending on bin. They will emit those lumens in some sort of pattern: Lambertians emit more of their light straight forward, creating a nice 180-degree flood. Batwings are similar, but emit slightly less light in the center. Side-emitters emit most of their light to the sides.
* Some misleading sellers advertise LED flashlights as "10W," "15W," and so on. Even if these figures were accurate, they would only relate to power delivered by the batteries, as most of those dozen watts would have to be burned up by a resistor if such a flashlight were to last more than a few minutes. The LEDs used in these flashlights could never actually take 10-15W of power.
Luxeon V: Starting at around $10 each, these are LEDs with four "dice," or light-emitting chips. They still fit into a small package, but the larger emitting surface makes for focus and throw challenges. It's all too easy to get a "donut hole," which is a beam that has a darker spot in the center (like a donut; get it?
), and it's much more difficult to throw a small, tight spot with these than with the Luxeon I and III explained above. The benefit of these LEDs is that they have more output, with most samples reaching 100-140 lumens at proper drive currents (around 700-900mA), depending on bin. The downside with this package is the higher Vf of 6-8V, which necessitates specialized driver circuits and consumes more battery power. Luxeon V LEDs are available in Lambertian and Side-Emitting packages (explained above).
Luxeon K2: These came out a couple years ago, and they bore the hopes of flashaholics everywhere. Functionally, they were like Lux IIIs speced for drive currents in the neighborhood of 1.5A, with increased lumen output and heat tolerance. However, multiple delays, limited availability, and competition with the new high-efficiency power LEDs (explained below) made it a disappointment. It wasn't a leap forward so much as a shuffle in a general direction. For these reasons, they never met with much success.
Cree 7090 XR-E: These came out in the fall of '06, generating a flashaholic frenzy. They could be driven at voltages and currents simliar to Luxeon IIIs, but they were twice as efficient, meaning that in two flashlights identical except for the emitter, the XR-E flashlight would have twice the output! LED flashlights producing 150 lumens became a reality. Better bins released over the coming months provided even more efficiency and output. Unfortunately, they are not direct replacements for Luxeon lights due to a unique package (explained below). They cost between $6 and $15, depending on bin and seller.
SSC P4: Released a few months after the XR-E, these use the XR-E's EZ1000 die, giving them the same high efficiency and output as the XR-E. However, they have a significantly different package, bearing more resemblance to the old Luxeons. They are approximately $7 to $15, depending on bin and seller.
Edison Opto KLC8: These are high-efficiency power LEDs also using the EZ1000 chip in a package similar to the Lux III, but with some differences (explained below). They are around $4-$6.
Luxeon Rebel: These came out in mid '07. They have the same efficiency characteristics as the XR-E and Seoul, but the package (and intended application) is very different from the other high-power LEDs. They are ideally suited to surface-mount applications assembled with reflow soldering.
Practical considerations: Lux Is and IIIs have a hard acrylic dome, smooth 180-degree radiation pattern, negative slug (underside), and easily soldered "legs." Lux Vs are similar, but with higher Vf and larger emitting surface. Luxeon K2s are also similar, but can be driven at higher power levels. XR-Es have a narrow radiation pattern (about 120 degrees) that projects a very faint gridlike pattern (bare, without optics) which lends itself well to collimation by aspheric lenses, a floating acrylic dome, and tiny pads that are very difficult to solder. The SSC P4 has a soft gummy dome to which dust sticks, very similar appearance to a Lux III except for tiny metallic portions visible surrounding the dome, smooth 180-degree radiation pattern, positive slug, easily soldered legs, and the tendency to tint-shift toward a cool blue when it heats up (if improperly heatsinked). The KLC8 has a positive slug, 180-degree radiation pattern, acrylic dome, legs, slightly less luminous flux when compared to the other EZ1000 packages, and collimation by a reflector results in a yellow-green ring around the hotspot. The Luxeon Rebel has a rectangular board with the emitter surface located off to one side, a neutral slug, and tiny pads that are very difficult to solder.
