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Thread: Emitter vs OTF Lumens

  1. #1
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    Default Emitter vs OTF Lumens

    Does this still hold true? You multiply emitter output by 0.7 to get flashlight lumens?
    If so with 99% emissivity lenses and high quality reflectors, where is that extra 30% light going?

    Also I recently got a light which drives an sst-20 5k at 1.4A. Now looking at charts I figure that's about 500L. Going by the 70% rule, that's 350L OTF.
    Looking at the new malkoff m61, they're an sst-20 at roughly 1.1A from a fresh 18650 and they're rated at 450 OTF.
    Thats a 5700k vs my 5000k but should be close enough.
    And malkoff I'm sure is claiming the flashlight output.
    The numbers aren't adding up. Now this is arbitrary as the high mode is so high I barely use it so I don't really care about numbers. I'm curious is all

  2. #2

    Default Re: Emitter vs OTF Lumens

    Reflectors and lenses aren't 99% efficient. Transmission through a piece of uncoated but good quality glass is around 90%-92%, and reflector coatings are typically in the 85%-90% reflectivity range (in both cases, the balance gets converted to heat). Since efficiencies multiply, a reflector and lens combo is letting 77%-85% of the light out the front*. You might lose a bit more from other surfaces (like an exposed PCB, or a bezel that cuts off the beam). So the 70% rule-of-thumb is probably a bit pessimistic, although it's probably pretty close for modern LEDs if it's also accounting for thermal droop (when you calculated your LED lumens, you corrected for both operating temperature and operating current, right?).

    Also, don't forget that the Malkoff has a step-down converter in it, so that 1.1A from the cell could easily be 1.4A to the LED (or even slightly higher, depending on the converter efficiency - 1.4A would require an 88% efficient converter based on the 800-mA at 6V rating of the drop in, which seems about right.

    So assuming the emitters have the same output (assuming same drive current and same flux bins), the Malkoff drop-in has about a 90% optical efficiency. As the drop-ins don't appear to have cover lenses, that's just the reflector losses, which is right in line with what's typical.

    *If the lens is right on top of the reflector, the losses from the reflection off the inner surface of the lens can get recirculated, bumping the efficiency up slightly

  3. #3
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    Default Re: Emitter vs OTF Lumens

    Thank you for the explanation. I've seen on flashlight lens.com and they have 98% to 99% transmission. I assumed the reflectors would be better than that

  4. #4

    Default Re: Emitter vs OTF Lumens

    If Gene says 500 lumens he means out the front, not at the LED.
    And I surmize he means out the front through his polycarbonate lens.
    John 3:16

  5. #5

    Default Re: Emitter vs OTF Lumens

    Quote Originally Posted by vicv View Post
    Thank you for the explanation. I've seen on flashlight lens.com and they have 98% to 99% transmission. I assumed the reflectors would be better than that
    Nope. As an example, here's a typical small reflector from one of the various optical vendors: https://www.ledil.com/product-card/?...=C11347_REGINA
    If you look at the datasheet, the efficiency ranges from 84% to 89% depending on the LED used.

  6. #6
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    Default Re: Emitter vs OTF Lumens

    Quote Originally Posted by bykfixer View Post
    If Gene says 500 lumens he means out the front, not at the LED.
    And I surmize he means out the front through his polycarbonate lens.
    A PLASTIC lens? Oh no! Not malkoff! Lol

  7. #7

    Default Re: Emitter vs OTF Lumens

    I think the idea there is ultimate impact and shock resistance. Although I myself still find the idea of a “plastic” lens to be unappealing. I’ve considered a glass lens in my Malkoff head if it’s doable.

  8. #8
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    Default Re: Emitter vs OTF Lumens

    Oh. Plastic doesn't bother me. Polycarbonate makes a great lens material. It was more of tongue in cheek due to the hatred to plastic lenses. Mostly from the ant-maglite crowd. Surefire uses them too. As does streamlight

  9. #9
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    Default Re: Emitter vs OTF Lumens

    Ive kept the acrylic lens on my MD2, although I could replace it with a coated glas one. On my MDC however, Ive replaced it with glas. The problem is that dust tends to stick to plastic lenses due to static electricity, which lowers their light Transmittance. This leads to me wiping off the lens every so often with a cloth, which in turn leads to scratches accumulating. Vicious cycle. Since I dont carry the MD2 regularly, this is not a problem, and I like the increased toughness it gives the light.

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