please can someone explain LED and OFT

Hi
Could someone explain the difference between LED lumen and OFT lumen in a simple way.
I have seen sites advertise the lights as both of these and am unsure which to take when comparing lights.

Thanks

LED lumens are often calculated (not measured) based on the specs of an LED and drivers and this is often the theoretical amount of light an LED should generate at a given driver level.

But there is a fair bit of variance in LEDs and drivers and the actual light generated by a particular LED and driver may be considerably different than the theory. Also not all the light generated by the LED will make it out of the light as there will be losses from each optical surface (things like the reflector or lens or glass). So the concept of OTF (out the front) lumens was created to indicate a more realistic number - these are usually measured to be a more realistic representation of what a light produces (this can be quite an expensive process however).

A good quality light that specifies say 750 LED lumens may only actually generate 600 OTF lumens (and lower quality lights may vary even more). As long as you are just using it as a relative comparison it doesent much matter which you use, but if you want a more absolute measurement the OTF lumens will be more realistic. A particular way of measuring OTF lumens gets you ANSI lumens. But even most real measurements are done based on small samples in a large batches so a particular light may vary significantly from that.

Thanks, that makes sense. Some sites list both and some just give one number. I was using my lights as a base line to estimate a comparison but want to make sure they are all roughly OFT

easy view:

each media transition = - 3-5 % of light
... that were: going into glass, "leaving" glass, into optic, "leaving" optic,


each reflection= - 10-20 % of light
... being reflected somewhere (reflector/optic) ...



extremely good materials (coated glass, special optical grade thermoplast) ... possibly even lower numbers
cheapo stuff ... higher numbers

Moved to LED Flashlights - Norm

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