Aspherics/Reflector/Throw?

[gcbryan]

I'm not interested in getting into in detail optics theory here but rather just practical observations...

What effect does using a reflector with an aspheric lens have on the beam appearance?

There are lights designed for throw that use only an aspherical lens and there are others that use a reflector and an aspherical optic (the new Wolf Eyes Krait is one and I think Led Lenser does this as well).

What does adding a reflector to an aspherical design do or add that wouldn't occur without the reflector?

 

[Popsiclestix]

First, let's explain how an aspheric lens work. If you remember your high school/college Physics, there are two types of lenses: concave and convex.

Convex lenses take light that is aligned along its axis and focus that light down to a point (this location is known, unsurprisingly, as the focus of the lens). The reverse is also true. A point light source at the focus will be turned into a parallel beam of light. Not all of it though, only the light that hits the lens. Hence, why incandescent mods usually seem dim when used with aspherical lenses.

Aspherical lenses are just a sub-category of convex lenses which correct for several abberations by deviating from a sphere shape, hence aspherical (meaning: not sphere). You can look on wikipedia for a more detailed explanation of lens abberations if you wish.

Now, let's explain a parabolic reflector. A parabolic reflector is the same idea as an aspherical lens. It will take light at the source and project it into a beam. However, the key difference here is that a parabolic reflector does not collimate light that is emitted forward from the source, but will do so for light that hits the reflector surface. Since most parabolic lenses have a focus that is deep inside the reflector, more light can be caught inside a parabolic reflector than an aspherical lens if you are using a source that emits light in all directions. Most of an LEDs light is projected forward, so parabolic reflectors do not help as much there as with other light sources.

Now, here's why the ordinary parabolic reflector + aspherical lens combination doesn't increase throw (which is really: how well you can turn the light of the point source into a beam?)

A parabolic reflector takes all the light that is behind the source and turn it into a parallel beams of light. However, since that beam of light isn't at the focus of the aspherical lens, it doesn't get projected forward.

However, there are elliptical reflectors. These reflectors have the property that light at one source is projected into a second focus (actually, you can think of aspherical lenses and parabolic reflectors as having 2 foci too. Just that the 2nd focus is infinitely far away).

Anyways, I think you can see where I am going with this: Using an elliptical reflector, put your source at the first focus. Then, put the aspherical len's focus point at the 2nd focus of the elliptical reflector. This is in essence how car headlight projector system works. However, even this system is not 100% efficient. Any light that is projected forwards from the source which does not hit the elliptical reflector will inevitably exit as spill. You can play with the distance between the foci and the size of the aspherical lens, but long story short, some applications demanded that there be no spill and thus the laser was invented.

 

[gcbryan]

Thanks. That was a good explanation. So for the companies that are using both reflectors and aspherical lens they are using elliptical reflectors and they are doing this rather than just using an aspheric because it's more efficient (captures more/wastes less light).

Now for an even more practical question...if you use an aspherical lens primarily (set at its focal length from the emitter) but leave a parabolic reflector in place just as a spacing necessity as in drop ins where the emitter isn't fixed will this hurt the beam quality?

I understand that it will not help but will it hurt? If so, would painting the reflector black and leaving it in as a spacer be an option?

 

[Popsiclestix]

I cannot speak for other companies as I have not studied their optics. Perhaps they are not using reflectors which are conic sections. But if the reflector were not elliptical, I suspect they might be just there to capture some light as spill.

As for your suggestion of using a parabolic reflector as a spacer, this is what I currently do for my incand Mag mod. The resultant beam has a hotspot that is similar to the one without the reflector, surrounded by a dark area with very little light, then a rathar ugly spill pattern surrounding that.

See this post. I run a very similar setup (SMO reflector, same bulb, 16x Elite 1500s)

If you wish to remove the ugly spill (an application of playing "lightsaber" for example), you may paint the reflector black and it be as though it were not there. I would caution against this option if you are using higher-powered bulbs though, as whatever you paint black will absorb all the light and heat up very severely.

 

[gcbryan]

If I paint the reflector it would just be used with an led (XR-E R2).

I would really like a custom designed optic that wasn't so ugly looking but I'm just using a cheap, generic, plastic 28mm (16fl) aspheric from DX for $2.00.

It also may not be at the right focal length. I'm going to just replace the front glass lens with the aspheric. It may be too far away. I have an extra reflector so if I paint it and decide the aspheric is just too ugly or not focused enough I can just take out the aspheric, put plain glass back in and use a regular reflector and call it a day!

 

[Popsiclestix]

If you are using this, it is glass btw. And I can't be sure, but reading the comments make me believe it is the correct focal length for an LED. If you are planning to drop this into a Surefire C/P host, you can always adjust the distance between LED and lens by screwing and unscrewing the head?



I'll grab one the next time I hit DX up for stuff. (Just ordered 2x drivers and 2x XRE-R2 for an aspheric Mag mod from there myself actually, then Malkoff finally replenishes his drop-ins :mecry:, so I guess I'm going to go about modding my U2 now and if I still have an emitter and driver left when I'm done, I'm going to build another aspheric LED Mag and perhaps someone will be the lucky one to own the first piece of my work that I will let out to the public )

Sorry about the derailing =P

 

[gcbryan]

If you are using this, it is glass btw. And I can't be sure, but reading the comments make me believe it is the correct focal length for an LED. If you are planning to drop this into a Surefire C/P host, you can always adjust the distance between LED and lens by screwing and unscrewing the head?



I'll grab one the next time I hit DX up for stuff. (Just ordered 2x drivers and 2x XRE-R2 for an aspheric Mag mod from there myself actually, then Malkoff finally replenishes his drop-ins :mecry:, so I guess I'm going to go about modding my U2 now and if I still have an emitter and driver left when I'm done, I'm going to build another aspheric LED Mag and perhaps someone will be the lucky one to own the first piece of my work that I will let out to the public )

Sorry about the derailing =P

You're right! It is glass.

I'm so used to ordering things from DX that say glass when they are in fact plastic that I assumed this was the same but I see the reviews that say it is actually glass!

There may be some adjusting but I'm worried that I can't get it close enough not far enough away so we'll see when the parts come in.

Right now I'm assuming it's about like the plastic version that I took out of a $10 DX flood-to-throw. It seems to work in other's clones so I'm probably OK there.

 

[Midnight Oil]

gcbryan and Popsiclestix,

Now we've got a crowd :laughing:.

I've read in several threads that say, in an identical reflector, the XPE R2 out-throws the XRE-R2, because the XPE's emission angle is 120 degrees, thus sending more light to the reflector that can be projected into the hotspot. This statement however, contradicts the observations of other members. Based on their beamshots, the XRE hotspot throws a much longer way.

Popsiclestix, is it correct to say that, in a parabolic reflector, most of the hotspot is made up of light that is reflected near the bottom end of the reflector, while light that is reflected above that region contributes mostly to the corona and spill? In other words, the regions contributing to the hotspot, corona if there is one, and spill are separate.

If that is correct, what I'm getting at is, unless the light from the other 30 degrees of emission angle on a XPE R2 is delivered to the region on the reflector that is responsible for producing the hotspot, the light just becomes corona or spill, right?

I'm doing some homework for my next Nailbender drop-in purchase. I want a thrower and am trying to decide among several setups between the XPE R2 and XRE R2. I'm not considering the XPE R3 because it's too cool , and based on my previous experience with a Nailbender XPG R3 in a smooth reflector, I'm sticking with the OP reflector from now on.

My options are the XPE and XRE in an OP or XRE with Cree 8 degree optic only. The optic will provide the most throw right? My only concern with the optic is whether it leaves enought spill for use at mid range. I still can't choose between the XPE and XRE based on what I've read.

Thanks.

 

[saabluster]

Popsiclestix, is it correct to say that, in a parabolic reflector, most of the hotspot is made up of light that is reflected near the bottom end of the reflector, while light that is reflected above that region contributes mostly to the corona and spill? In other words, the regions contributing to the hotspot, corona if there is one, and spill are separate.

Assuming you mean the "bottom end" is the end closest to the LED then you would be incorrect. The outer portion or the part farthest away form the LED creates the tightest part of the beam.

 

[PlayboyJoeShmoe]

All I can really add to this is this:

I have a 3D Mag with an XR-E (probably P4) in it. It came with a mop reflector and also with an aspherical. I had put the Aspherical somewhere and did not find it for a long time. I was never very impressed using the reflector.

When I found and installed the Aspherical it was amazing! It will focus to a perfect projection of the die of the led. It will focus to an almost regular flashlight type spot/spill beam. And it will de-focus to a flood.

Only thing I dislike is that the light can't stand face down.

 

[gcbryan]

gcbryan and Popsiclestix,

Now we've got a crowd :laughing:.

I've read in several threads that say, in an identical reflector, the XPE R2 out-throws the XRE-R2, because the XPE's emission angle is 120 degrees, thus sending more light to the reflector that can be projected into the hotspot. This statement however, contradicts the observations of other members. Based on their beamshots, the XRE hotspot throws a much longer way.

Popsiclestix, is it correct to say that, in a parabolic reflector, most of the hotspot is made up of light that is reflected near the bottom end of the reflector, while light that is reflected above that region contributes mostly to the corona and spill? In other words, the regions contributing to the hotspot, corona if there is one, and spill are separate.

If that is correct, what I'm getting at is, unless the light from the other 30 degrees of emission angle on a XPE R2 is delivered to the region on the reflector that is responsible for producing the hotspot, the light just becomes corona or spill, right?

I'm doing some homework for my next Nailbender drop-in purchase. I want a thrower and am trying to decide among several setups between the XPE R2 and XRE R2. I'm not considering the XPE R3 because it's too cool , and based on my previous experience with a Nailbender XPG R3 in a smooth reflector, I'm sticking with the OP reflector from now on.

My options are the XPE and XRE in an OP or XRE with Cree 8 degree optic only. The optic will provide the most throw right? My only concern with the optic is whether it leaves enought spill for use at mid range. I still can't choose between the XPE and XRE based on what I've read.

Thanks.

This is only my opinion and I know nothing about Nailbender drop-ins... I would chose the XRE with the optic. I assume it is a TIR optic and not an aspheric.

This is for a light that throws but has some spill. In other words a practical general purpose light that also throws well.

If you are only going for maximum throw you need a large head with a very wide reflector or a larger aspheric lens. I don't think Nailbender is in that business (correct me if I'm wrong).

Regarding the earlier comments with others saying that their XR-E throws further than a XP-E that's usually apples and oranges. They are comparing different flashlights that just happen to have different emitters.

If everything is the same...current draw, bin, reflector, battery charge, driver efficiency, front lens, etc. then the XP-E R2 should throw a little further than the XR-E R2 in a reflector. All those things usually aren't the same when people are making comparisions.

The XR-E R2 should throw further than the XP-E R2 in an aspheric set-up if everything else is the same and everything usually is not the same.

That's my understanding anyway. Sasbluster is correct (of course) regarding the area of the reflector that provides the throw. That's why the larger the diameter of the reflector the greater the throw. It's because the light beam that is being collimated at that point is getting more and more like a point source and therefore will be brighter when focused and throw further.

 

[Midnight Oil]

So the hotspot consists mostly of light bouncing off of the reflector and a little bit of light directly from the LED, whereas the spill consists of only light that doesn't hit the reflector?

 

[gcbryan]

So the hotspot consists mostly of light bouncing off of the reflector and a little bit of light directly from the LED, whereas the spill consists of only light that doesn't hit the reflector?

Yes. The hotspot comes from the reflector and the spill comes from that part of the emitter beam that doesn't hit the reflector.

Try covering up the center portion of the glass lens on your flashlight if it's using a reflector and you'll see it doesn't affect the hotspot.