Flood to throw lights - why do we need aspheric lenses?

Thick aspheric lenses usually reduce light output by a considerable amount (I remember something about 30-40%?). Maybe multicoated high grade grade glass lenses give better output.

But a led with a shallow reflector gives flood light, a led with a deep reflector gives good throw, both do that with flat lenses.

So why not make a flexible (or collapsible) reflector and use a flat lens? Lose less light, have both flood and throw, and everyone will be happy...

Just a thought. :)

Making something that flexible, in that it could stretch 20%-50% or more, and highly reflective, would be very difficult. Any type of coating would crack and flake off, unless it was so thin it let much of the light through it.


Interesting idea. Here are some of the zoom mechanisms I've seen or read about:

1. Aspheric lens

I think the reason why aspherics are common is that they are cheap and easy to build. The body of a typical aspheric lens flashlight is very simple with a main body section and a sliding section. The lens can be ordered from a lens manufacturer in a standard size so no additional engineering is needed for the optics.

Most aspheric lens lights don't even come with reflectors probably making them even easier to engineer than conventional lights. The only one I've read about that comes with a manufacturer installed reflector behind the aspheric is the very non-budget Wolf-eyes Krait.

In addition to being cheap, it's also possible to make aspheric lens flashlights very small. This makes them good for EDC carry even with the inefficiency. A well-designed aspheric lens lights can also produce a fairly nice beam: a perfectly smooth even flood with no noticeable hotspot (if no reflector is used) that narrows down to a laser-like projection of the emitter die in spot mode.

The downside of aspheric lens flashlights is that only light that goes through the lens is outputted. In zoom mode, the lens is moved further away from the LED. Especially in smaller aspherics, this results in a ton of light lost as it gets absorbed into the side of the zoom section's casing rather than going through the lens. You can immediately see the loss of output in a ceiling bounce. Turn off all the lights, take a Sipik SK68 and point it at the ceiling. Look at the floor. Move the bezel to zoom mode and watch the floor immediately dim.

2. Moving reflector:

This is what the classic maglite incandescent lights had. A few early LEDs also use this method. The idea is to have the emitter mounted onto the flashlight body. The head contains a conventional reflector and non-aspheric lens. The reflector can be screwed or unscrewed taking the emitter off the reflector's focal point which should move the light from spot to flood.

I haven't seen this method used effectively with a modern LED light. To get a really effective zoom, the LED would need to be mounted on a narrow post, which means a conventional star cannot be used, and the LED might overheat due to lack of heatsinking. The few lights I have seen using this method tend to produce a very ringy beam in flood mode, and their range of angles going from spot to flood is far weaker than an aspheric.

The biggest advantage of this method is that in theory much less light would be lost switching modes than with an aspheric. The reflector would be there to catch any light emitted to the sides of the die.

3. Moving TIR (Total Internal Reflection).

One of the 18650 Romisen lights uses this method I think. Basically the same as the moving reflector method except that instead of a conventional reflector a TIR optic is used. A typical TIR optic consists of a solid chunk of plastic in the shape of a reflector, possibly with a vertical hole in the center for the emitter. Light passing into the TIR from the emitter bounces off the inner edge of the reflector and then passes forward through the front of the TIR. The principle is the same as how a typical automotive or bicycle reflector works even though they do not have reflective silver coatings.

By mounting the LED on a post and then moving the TIR up and down around it, the LED can be moved off of the TIR's focal point, thus widening the beam for flood mode. Since the TIR is catching light that is being emitted sideways, in theory much less light should be caught into the sides of the casing than with a traditional aspheric.

4. Multiple emitters

This method is used on one of the Ultrafire recoil throwers. In a recoil thrower the LED is mounted on an arm in front of a reflector. The emitter faces backwards into the reflector, allowing for a much more focused beam than with a traditional light where the emitter is mounted at the base of the reflector. The manufacturer chose to mount a second emitter on the front of the arm. The second emitter is right behind the lens and has no reflector at all, producing an incredibly wide even flood.

Activate one emitter for flood, and the other for spot mode. There is no in-between halfway mode.

The downside of this method is that any heatsinking to the emitter has to go down the narrow arm so the light cannot be driven very hard or it will overheat. Also to make an effective recoil thrower typically means a larger light. So this method wouldn't work for making an EDC light.

5. Reflector lens

This method is used by LED lensers. A small plastic aspheric lens in the center of a flat lens is mounted on a moving bezel. The area near the base of the emitter below the lens has no reflector. However, just beyond that area is a small conventional reflector designed to catch some of the sidespill and reflect it forward through the flat non-aspheric portion of the lens.

Output should be a combination of conventional reflector flashlight with some zooming from the aspheric lens. I don't own one myself, but have read that LED lensers have a very smooth beam, even if they are underpowered by today's standards and for the most part lack proper drivers.


Your idea for a deformable reflector is an interesting one, but not one I'd expect to see anytime soon, if ever. Conventional reflectors typically rely on a small coating of evaporated silver on the face of the reflector to provide the actual reflections. This coating isn't very flexible. It put on a flexible plastic there might be issues of it cracking or pealing rather than bending. Flexible reflective surfaces are probably either more expensive or much less efficient (think of the silver Mylar in a party balloon).

Engineering a piece of rubber into the shape of a reflector that is precise enough to give a smooth focused beam would also be very difficult. Especially if that piece of rubber is then constantly deformed as the light is cycled between modes.

Another option might be rather than having the reflector made from flexible material, it could instead be made out of pie-pieces of conventional material mounted on small hinges. Here's one idea I just thought of: Have a reflector focused for throw assembled in wedges that are hinged around the base of the emitter and spring loaded to stay in position. As you screw the light into flood mode, the lens will move closer to the reflector. When the reflector wedges hit the lens, the pressure of the lense would then cause them to slide apart on their hinges creating a flatter reflector suited for flood. A wider shallower reflector would be mounted behind the wedges to catch any light falling through the gaps. It might even have recesses for the reflector wedges.

However, this would be pretty expensive, would require lots of parts, and would probably produce an incredibly ugly beam even with an aggressive orange peal texturing. The head would also be quite wide.

Any of these options would likely require some very unusual engineering expertise. You wouldn't be able to do it with off the shelf parts like you can with a budget aspheric.

I like combination reflector + aspheric... it's not perfectly efficient, but it's much better than just an aspheric. I don't know why it's not used in production lights - I can only think the manufacturers think we care more about not having some ring artifacts than we do about all the missing light? Or it's just cheaper...

You could probably pull off a moving reflector/TIR if you mounted the LED to a heat pipe, but that's getting expensive.

I've thought of a couple designs using fresnel lenses, mostly to save space and weight in complex multi-emitter setups, but the efficiency and beam quality would both be poor.


I have a question.. I was thinking the other day, since so much light gets lost when zoomie lights are extended, would it make any sense to coat the inside of the extending part with a reflective finish? That way much less light would be absorbed by the sides of the extended part, correct? Would this result in an ugly beam pattern? Not very familiar with optical physics..

I've contemplated sticking a ring of mylar inside the light to see what happens... My guess is it results in a lot of flood even when in zoom mode, but it's free light, not dimming the spot at all. Try it and let us know? :)


Coating the inside of the aspheric's bezel with a more reflective material (mylar, white paint, etc.) would probably yield a bit of extra spill in spot mode. It wouldn't dim the output at all.

In my own EDC aspheric, I took a small plastic reflector from another light. In flood mode, the reflector goes from the base of the LED to within a mm of the back of the lens, so almost everything in that mode should be reflected out the front. The effect is pretty nice: Flood mode is much brighter, has no rings, and has a wide diffuse hotspot.

I still lose a lot of output in spot mode, but the reflector adds some spill, which I find to be very useful inspot and halfway modes.

I haven't tried coating the inside of the bezel with reflective paint or material. In my modifed SK58 the side of the pill slides across this area. I'm worried that it would scrape off or cook any paint or relective material I tried to apply.

Still, I should go get some Mylar and give it a shot.

Cree should do something like the XP-G or XM-L with primary optics. Just as they did with the XR-E.

Coast uses moving TIRs for their flood-to-throw lights.

I have a dorcy 1xAAA aspheric zoomable stainless steel light. The inside of the zoomable portion is threaded stainless steel so is quite shiny. Unfortunately, when zoomed every single thread appears as a ring around the spot, producing the world's ringiest beam!

I do think it would be worthwhile having a reflective surface on the inside of the bezel if the inside were smoother. Or if a more diffuse reflective surface were used such as white paint.

Part of it depends on how close you can get the LED to the lens If the LED isn't right next to the lens in flood mode some of the light will be lost sideways especially if using an XM-L or XP-G emitter with their wide angle output.

With many cheap aspherics it's not practical to get the LED right up against the lens because the place for mounting the star is recessed slightly into the pill, and the pill itself prevents the lens from moving closer to the LED. However, there is room to mount a small reflector, which can make a very noticeable difference in the beam's output even in flood mode.

In spot and halfway mode, the small reflector adds a usable amount of spill that is otherwise completely lacking. The spill in those modes may be ugly, but I've found it produces a much more useful beam.

my led lenser h7 also uses a movable tir, it is losing far less light when zoomed than aspheric (ceiling test zoomed in/out against nf-500)

That's it! You could use a spinning cup of mercury around the emitter. The faster you spin it, the steeper the resulting parabola, giving you a nice spot. Slow it down, parabola flattens out, you get a nice flood. Of course, the light would have to point straight up all the time for this to work...


What about something like the recoil throwers with a bigger mirror and then the ability to move the led in and out relative to the mirror?

aspheric+reflector: You have seen the images in this post?

Be-Seen Triker: -properly- reflecting the lost light back into the aspheric is tough. I will be working on the reflection angles for this problem sometime soon and will post up the data.

You can't "reroute" the "lost spill" to make it contribute to the spot. It's not possible by laws of optics.

Well, there is a way to indirectly and partially achieve that though: Reflect the side beams directly back onto the LED itself like the 'Wavien collar'. That back reflected light increases the die's luminance ("surface brightness") and thus throw.

Haggai: But a led with a shallow reflector gives flood light, a led with a deep reflector gives good throw.

Not really, the main reason for throw is diameter (and SMO), not depth; here's some info.

Fortunately that specific one won't have any effect as it's too flat and shallow, all the rays hitting it do so in a very flat angle and be reflected to the walls...

I say fortunately, because if it did hit the lens, artifacts like in the images here are unavoidable. There'll always be an ugly gap between spot and reflector-'spill'.

Those artifacts are, however, free light. They don't use any extra battery power nor dim the spot any. Other than objecting to their appearance, there's no reason not to have them.



What I've found is that in flood mode all artifacts blend away into the beam and are invisible. The beam looks perfect with a noticeably brighter wide diffuse hotspot in the center.

In spot mode, you can see every artifact and blemish in the reflector. This appears in a wide halo around the spot. But as the previous poster pointed out this is free light. And its bright enough to serve as usable spill ... something an aspheric without a reflector doesn't have.

Also with a properly chosen reflector and a big die like an XM-L, the reflector's halo only separates from the image of the die at full spot mode. At halfway mode, they still blend together producing an image that looks similar to that of a ringy smooth conventional reflector like setup for throw.

Here's some pics:

Modified Sipik SK58 with XM-L and reflector taken from old underwater kinetics 2xAAA incandescent. Lens is off:

Here's a pic down the business end of the light in full flood mode with the reflector and lens on:

Here's a beamshot in full flood mode. The reflector eliminates all rings in this mode while adding a wide diffuse spot in the center of the beam. Unlike other zoomies there also are no rings outside of the spill in this mode. I love how flood mode looks. Much floodier than the beam from a Xeno E03. And much brighter too with the driver I'm using. Note that the actual light output is not green-tinged at all. My cell phone pictures don't do it justice.

Halfway mode. This looks a lot uglier, but on the other hand the reflector is providing usable spill. Without the reflector everything outside of the bright spot in the center is gone:

Full spot mode. Note the halo and artifacts from the reflector. The beam is pretty ugly, but in real-world use this halo is bright enough to serve as usable spill, without affecting the light's throw:

Note that all modifications on this light were performed using my own rather amateurish do-it-yourself methods. Nothing professional quality about this mod.

I'm in the other camp anyway...I do care about beam quality. I don't like or use flood to throw lights anyway but regarding aspheric throwers I'm quite OK with it being like a laser with no spill rather than having the throw effect spoiled (for me) by having a disco light show going on around my head :)

As Dr. Jones has explained (and as I also understand it) you can't reflect the light back into the aspheric anyway. It has to take it from the emitter through it's focal point...anything else doesn't make the trip (which is why you lose some lumens).

Multi-purpose anything usually doesn't fair as well as task dedicated gear. It's easier to take two small lights than to try to make one that does it all (poorly) IMO.