Why is there so much lumen-age lost between the emitter and the glass?

I keep reading the specs of all these different lights, and I was just reading another thread here about the FourSevens XM18, and the 2nd poster pointed out, that the FS Maelstrom S18 is rated at 1800 emitter lumens, but only actually puts out 1200 at the front(OTF).

So that means it has lost 33.3% of its brightness in, oh.....about, 1 inch! **I assume its "losing" this brightness because it's being spread out by the reflector in directions other than straight forward like a laser. Thats the only explanation I can figure out as a non lighting expert. Otherwise it wouldnt make sense for an emitter to lose that much energy in that short a space when surrounded by a 100% reflective surface. Maybe if the emitter was surrounded by a black cone shaped object, then some would be absorbed.

##So question 1 is: Am I correct in my assumption above(see asterisk) as to why so much brightness seems to be lost?

##Question 2: Do most LED, or other flashlight manufacturers give you the OTF or 'out the emitter' brightness numbers in their specs? Are there any companies that regularly give OTF specs?

I've seen a couple flashlights where the manufacturer has it stated in the specs that the numbers they are giving are OTE specs, so I guess that means I can subtract about 33% from that number to get the REAL specs.....:thumbsdow

A couple of points:

- Spreading the light out has no effect on the lumen rating. Lumens are a rating of overall output not throw.

- Reflectors aren't used to spread out the light in fact they do the complete opposite. They are used to focus the beam to make it more "laser like"


There are quite a few reasons for this light loss. A typical mirrored reflector is not actually that efficient. The best optical mirrors using a silvered surface are only about 80% efficient. The rest is absorbed. Your average reflector in a flashlight is considerably worse than that (although not all of the light is emitted via the reflector). And then there are internal reflections from the glass lens.
An uncoated surface of a glass lens reflects ~4% of the light and there are 2 surfaces.

TIR optics are potentially more efficient but much more restrictive in their design.

basically no different than rear wheel/wheel horse power vs crank/engine horsepower on cars.
which one is advertised in the car mag/brochure?

emitter/LED/bulb Lumens (engine HP), vs OTF Lumens (wheel HP), vs ANSI lumens (OTF after 3 minutes)

TIR/Optics have 30-40% loss (AFAIK)
Incans average 30-40% loss
LED average 25-35% loss
YMMV

the lens (glass/plastic) is another source - cheap glass/plastic can easily be 10%+ loss
good lens/glass is in single digit loss range.

on inexpensive lights that have removable lens - take the glass out and compare with vs without glass - it's pretty amazing.

EDIT: a reason why flashlightlens.com does good business....

Cheers
Tod

You can SEE the glass, and that means some light is being sent to your eyes from it...which means its not 100% transparent.

Even an imperceptible amount of clouding in the glass will rob OTF lumens...as described above, its possible you would observe it to be "clear" until you removed it and saw the difference. That difference is where your loss is noticeable, and some smaller losses are occurring even when you CAN'T see the difference.

One more thing to add is heat sinking,
Led spec is at a specific temp. In flashlight, they heat up & reduce lumen output.
High power flashlight with inadequate heat sinking will see OTF lumen dropping pretty fast.

BigC used to do OTF measurements with time, pretty telling.

OTF lumens vs emitter lumens?

I was wondering how many lumens are lost from the emitter of a flashlight to the actual OTF lumens in a well made flashlight? Ive heard it can be as much as 50% loss in lumens with cheap flashlights with 35% being more common with average flashlights. How about a well made flashlight with a good reflector?

Re: OTF lumens vs emitter lumens?

kuna said:

I was wondering how many lumens are lost from the emitter of a flashlight to the actual OTF lumens in a well made flashlight? Ive heard it can be as much as 50% loss in lumens with cheap flashlights with 35% being more common with average flashlights. How about a well made flashlight with a good reflector?

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If I remember my optics correctly, the biggest loss is in the glass lens which can be 10% (or more depending on how cheap the lens and coatings are). The reflector usually loses a smaller amount.

OTF lumens vs emitter lumens?

Google Eagletac D25 Clicky specs, they list both emitter and OTF, ~30% loss for the D25A on turbo, for example.

Re: OTF lumens vs emitter lumens?

Aside from reflector quality, you also have to take into consideration the texture of the reflector. Orange Peel textures will be less efficient than Smooth one of the same quality. The heavier the diffusion, the more lumen lost.

Most standard reflectors have 70~80% efficiency. High quality ones with special coatings can take that all the way up to around 98%.

And then there is the lumen loss at the lens. UCL with AR coating can trasmit a lot of light, and is usually the most efficient lens used, but can easily be shattered. Sapphire lens is more scratch resistant but trasmit less light than UCL. Pyrex/Borofloat have excellent physical and thermal resistance, but aren't as efficient as UCL. Plastic lens won't shatter, but will get scratched up faster than any others.

Re: OTF lumens vs emitter lumens?

Along with the reflector and window, otf lumens are reduced by heat buildup, while emitter lumens are measured with a nearly ideal heat sink.

Re: OTF lumens vs emitter lumens?

yes iirc a regular glass lens passes about 88% of light so right off the bat we have 12% loss just with a glass lens. The reflector im assuming could only pass around 70% of the light because the light emitted beyond around 60degrees of the bulb misses the reflector completely and just escapes the front of the flashlight at an angle.(assuming flashlight is on a spot mode). So we end up with only around ~60% of the origal lumens exiting the light. Is this correct?

Re: OTF lumens vs emitter lumens?

The purpose of reflector is to shape the beam into desirable beam profile. If correctly designed and installed, it should be able to capture all aspect of the light emitted, and project it into a beam, which means that, aside from minor output loss due to imperfections on the coating of the surface, it will shape most of the light into a beam.

Let's say that we have a flashlight that can produce 1,000 lumens at the emitter, equipped with a reflector with 90% efficiency and lens with 95% efficiency. The output will be decreased to 900 lumens after hitting the reflector surface (1,000/0.9). From there, that light will have to pass through the lens, which comes out to 855 lumens (900/0.95). That comes out to approximately 15~16% loss.

Keep in mind that this is just OTF measurements, which doesn't account for thermal/voltage sag. ANSI/NEMA FL-1 covers them.

Re: OTF lumens vs emitter lumens?

enomosiki said:

The purpose of reflector is to shape the beam into desirable beam profile. If correctly designed and installed, it should be able to capture all aspect of the light emitted, and project it into a beam, which means that, aside from minor output loss due to imperfections on the coating of the surface, it will shape most of the light into a beam.

Let's say that we have a flashlight that can produce 1,000 lumens at the emitter, equipped with a reflector with 90% efficiency and lens with 95% efficiency. The output will be decreased to 900 lumens after hitting the reflector surface (1,000/0.9). From there, that light will have to pass through the lens, which comes out to 855 lumens (900/0.95). That comes out to approximately 15~16% loss.

Keep in mind that this is just OTF measurements, which doesn't account for thermal/voltage sag. ANSI/NEMA FL-1 covers them.

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Hi, your math needs work, you should multiply them instead (e.g. 1000*.9).

Re: OTF lumens vs emitter lumens?

hellokitty[hk] said:

Hi, your math needs work, you should multiply them instead (e.g. 1000*.9).

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Five hours of sleep in the past 30 hours will do that.

Results are fine. Just the symbols are mixed up.

A polished stainless bezel ring permits higher lumen compared to a black anodized one. It'll make a ring surrounding the spill, but the artifact isn't unpleasant to look upon.

Modding white the color of the o-ring and washer sandwiching the lens, as well as the bezel lip that the glass sits on also helps.

Re: OTF lumens vs emitter lumens?

enomosiki said:

Let's say that we have a flashlight that can produce 1,000 lumens at the emitter, equipped with a reflector with 90% efficiency and lens with 95% efficiency. The output will be decreased to 900 lumens after hitting the reflector surface. From there, that light will have to pass through the lens, which comes out to 855 lumens

Click to expand...

Right but I'm not sure you took into account the spill which basically seems to be all light emmited from the bulb beyond say 50 degrees vertical. In the example you gave, the reflector reflects about 90% of the light that hits it which is resonable for a well made reflector but unfortunately not 90% of the light hits the reflector anyway and is wasted in spill. This is more of a problem for spotlights and not so much for floodlights because a wide angle is desired. However for spotlight type lights like the stanley and maxabeam this limits otf lumens to 70%emmitter but then again otf would still be around 85% but just not in the beam where it counts.