Step 2: Ok, but really what is so special here?
well, basically there are a couple of things we do to get better efficiency and output than anything else you can buy/borrow/steal:
1) use cyan or green LED's. these will give 2.5x their rated lumen output using your night vision, because they are rated based on your day vision. (see http://hyperphysics.phy-astr.gsu.edu/hbase/vision/bright.html )
2) use the latest high-power LED's. new LED's such as the Luxeon's are rated about 50 lumens/watt for cyan and green (this is the day-vision rating). this is equal to the high-end HID lamps and at least twice as efficient as a halogen lamp. The Luxeon LED's used to be pricey, but now they are about $3 each.
3) use the latest optics custom-designed for the Luxeon LED's. several companies are now making low-cost lenses for LED's based on the TIR (total internal reflection) principal. These lenses do not have a reflective mirror, and achieve 85%-90% transmission. non-LED lamps lose much more of the light because some of the light shines backwards where it can't be used, and because mirror reflectors are less efficient. typical transmission for reflector-based systems is only 65%.
4) don't over-power the LED's. looking at the LED datasheet, we notice that the LED efficiency goes down somewhat with higher power: 45 lumens at 1W power, but only 80 lumens at 3W power. so we get the best efficiencies with less power. this indicates the importance of having a brightness-switch on the unit. in practice i have found the 1/2 power setting to be nearly as bright as the full power setting.
1) use cyan or green LED's. these will give 2.5x their rated lumen output using your night vision, because they are rated based on your day vision. (see http://hyperphysics.phy-astr.gsu.edu/hbase/vision/bright.html )
2) use the latest high-power LED's. new LED's such as the Luxeon's are rated about 50 lumens/watt for cyan and green (this is the day-vision rating). this is equal to the high-end HID lamps and at least twice as efficient as a halogen lamp. The Luxeon LED's used to be pricey, but now they are about $3 each.
3) use the latest optics custom-designed for the Luxeon LED's. several companies are now making low-cost lenses for LED's based on the TIR (total internal reflection) principal. These lenses do not have a reflective mirror, and achieve 85%-90% transmission. non-LED lamps lose much more of the light because some of the light shines backwards where it can't be used, and because mirror reflectors are less efficient. typical transmission for reflector-based systems is only 65%.
4) don't over-power the LED's. looking at the LED datasheet, we notice that the LED efficiency goes down somewhat with higher power: 45 lumens at 1W power, but only 80 lumens at 3W power. so we get the best efficiencies with less power. this indicates the importance of having a brightness-switch on the unit. in practice i have found the 1/2 power setting to be nearly as bright as the full power setting.
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