Aspheric flashlight design question -- reflector or no reflector?

With the holidays coming up I thought I'd gift something homemade, so I'm going to make an flashlight (of course). I'll be doing this from scratch on my lathe, with an aspheric lens to increase throw. I'm a total noob when it comes to optics though, as I've never even held an aspheric flashlight in my hands before. I've got a couple of questions:

How far should I mount the lens from the LED emitter? Or is this determined by the focal length of the lens? What happens if I mount it too close, will my beam width still get tightened up a little bit?

Can I just leave the space between the emitter and the lens empty, or do I need to add a reflector as with a traditional flashlight? If a reflector is required, can I use a simple conical shape rather than a parabolic?

Thanks in advance for any advice...

I have been out of the flashlight scene for quite a while, but yes it all depends on the focal length of the lense. They will all be different. If the lense is to close, you will not get the nice tight sqaure emmiter beam. If the lense is to far away the same thing will happen. The best thing to do is to have an adjustabe head that way you can focus on the object you are pointing at.

LED4LYF said:

I have been out of the flashlight scene for quite a while, but yes it all depends on the focal length of the lense. They will all be different. If the lense is to close, you will not get the nice tight sqaure emmiter beam. If the lense is to far away the same thing will happen.

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Fuzzy is fine, I'm not looking to throw to the moon, just tighten up the beam width/angle a bit.

The best thing to do is to have an adjustabe head that way you can focus on the object you are pointing at.

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Hm, this would make the design overly complicated. Let's say I do fixed-focus at about 10 feet. If I shine the light on objects that are closer or further, will I get black holes in my beam or will I simply get a fuzzy beam?

If I get black holes with a fixed-focus aspheric, then maybe aspheric is not the way I should go...

Here check out my mag bfl build, my head is adjustabe so I can focus on something a mile away. Or 30ft away.

http://www.candlepowerforums.com/vb...ith-a-better-beamshot-from-almost-a-mile-away!

It will be fuzzy because if the lense is fixed then it is stuck at the distance you set at.

LED4LYF said:

Here check out my mag bfl build, my head is adjustabe so I can focus on something a mile away. Or 30ft away.

http://www.candlepowerforums.com/vb...ith-a-better-beamshot-from-almost-a-mile-away!

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What's the diameter of the front lense in the module thing from the tv?



@The OP, I wouldn't use a reflector but you can if you want, it doesn't increase throw but it adds spill, without it you will have very little spill.
Go for the biggest lense you can find, when you are making a LED thrower bigger is almost always better.
I would try to make it so you can adjust the distance between the lense and the emitter, surely it wouldn't be too difficult to make the head and the body threaded so you can focus it like on maglites. I advise using a Cree XR-E R2 Led, it has very high surface brightness making it one of the best LEDs for aspherical torches.

If the led is out of focus you won't get donut holes the square will just be fuzzy or at the most extreme it will be a large circle instead of a square.

What tv thing

JacobJones said:

@The OP, I wouldn't use a reflector but you can if you want, it doesn't increase throw but it adds spill, without it you will have very little spill.

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Now that's counterintuitive! But easier for me if I don't have to make a reflector.

surely it wouldn't be too difficult to make the head and the body threaded so you can focus it like on maglites

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Well, yes I could, but this is going to be an EDC for a non-flashaholic so a simple interface is a primary consideration.

If the led is out of focus you won't get donut holes the square will just be fuzzy or at the most extreme it will be a large circle instead of a square.

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Cool. Fuzzy I can live with, donut holes I can't. :thumbsup:

LED4LYF said:

What tv thing

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Sorry, I wasn't clear enough.

This is your message from your thread about the mag bfl

LED4LYF said:

You might need to try a few lenses, I got the lens system from an old projection tv. You might not find the same one, It has two lenses in the enclosure. Good Luck

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I'm asking about the thing you took from a projection tv with two lenses in. In the photos the front lense looks really big, what's the diameter of it?
Thankyou

The front lense is around 117mm and the inside lense is about 80mm.

tylernt said:

Now that's counterintuitive! But easier for me if I don't have to make a reflector. . . . . :

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The Lens will focus all the light coming directly off the LED into an image of the LED Die - provided the LED is at the focus of the lens.

It's easy to work out what this distance should be - get a bright light e.g. streetlight and focus the light onto a bit of paper. Light rays travel equally in the reverse direction !

If you add a reflector, the light bouncing off the reflector will follow a different path that the lens cannot focus, i.e. it will be scattered around the focussed LED image.