Fresnel lens vs aspheric

Flashlight Modding and DIY Parts
1

Hey guys,
Just been having a chat about lighting with the father I.L and he’s pointed me in the direction of fresnel lenses for the purpose of fixed focal length collumation. From my brief reading they have slightly higher light transmittance than aspheric and are basically aspheric lenses with the ‘excess’ material removed.
Do they perform any differently to an aspheric when placed at a fixed focal length.

Thanks

Thom.

2

The Fresnel lenses I have seen and the ones I have tried myself (cheap ones I had laying around) they always had a rainbow effect on the beam, super intense but ugly to look at.

3

Here is a good video I watched, about a year ago. He uses a Fresnel lens, with his homemade light.
Note the beam pattern too, at about 2 feet in front of the light, at the 02:39 mark.

4

That video is kinda cool. Certainly throws a fair way! They are available in 42mm (that’s C8 size right???
Might grab one to check out as it seems an interesting idea for a thrower.

5

There are two types of fresnel lenses.
Typical spherical fresnel lenses, and aspheric fresnel lenses.

The aspheric ones are basically the normal aspheric lenses with the excess material removed.
This is the ones you want for a flashlight.
Don’t waste your time with spherical lenses, they suck for throw and collimation.

The only reason to really use these fresnel lenses is if you want a low cost / low weight solution for large diameter lenses.
Anything below 4” or so you can get a regular aspheric lens for cheap.
When you start going 6-12” then regular lenses get extremely heavy or expensive.
Keep in mind that fresnel lenses will have a reduced projection quality when compared to regular lenses, because of the way it is cut up into sections.

6

My experience is that good fresnel lenses can be similar in performance to traditional aspheric lenses in terms of lux for a given diameter lens. Bad fresnel lenses might have larger spaces between the active concentric parts of the lens which leads to effectively less active area and less lux.

Their main advantage is lighter weight and less space.

7

I’ve tried a number of small fresnel lenses in lights and have compared the results to same-size aspheric.

The results:

  • Cheap ($8) 20mm optical acrylic aspheric fresnel lens - blurry image of emitter. Approximately half the throw of an aspheric.
  • More expensive optical acrylic aspheric fresnel lens with finer lines between more numerous fresnel elements ($30) from Edmund Optics - much sharper image of the emitter, though still not as sharp as an aspheric. Approximately 3/4 of the throw of an aspheric.
  • Optical acrylic Aspheric lens - cheapest of all, but also gives by far the best throw.

Net result is fresnels throw worse than a comparable aspheric. I think the problem is that at the transition of each peak and valley on the fresnel some light is wasted and not focused. This loss is much greater than the amount absorbed by the extra thickness of the aspheric.

If you want max throw in a flashlight aspheric is the way to go. Fresnels are more suited for when you want the absolute thinnest and lightest possible light. I agree with previous posters that fresnels make more sense in large lens flashlights where the weight and cost of an aspheric may become prohibitive.

8

Hence their use in light houses. (That’s where the conversation that sparked this question began)

9

someone here?

made a cool weird light once for an experiment.

they took a fairly standard LED and driver, mounted it in what looked like a stereo speaker box, and fitted a big fresnel where the speaker foam would have been.

i remember it being weird and different and creative, and lighting up stuff a ways off. the only downside was size and the beam was a little colored and ugly, maybe. BUT, big size was the POINT, they wanted to see what a 2 foot wide lens would do.

i thoroughly enjoyed the build thread, though… it was just “different” and “creative” in my book, 2 things you cant get enough of for my moneys worth.

fresnels keep popping up unexpectedly for me. I found an old “make your TV screen bigger” fresnel in a junkpile, and took it home to watch my little old laptop play DVDs on my laptop on my bed… it was originally out of a “high school projector transparency thingy” but i made it be a “bigger TV” lens instead.

then, a buddy took me to some guys house, and this guy had BIG lenses, one of which (several feet wide?) would set a 2x6 on fire almost instantly, guy had been experimenting with solar cooking power for camping.

to me? they have a certain coolness factor…

10

Aliexpress selling a cheap Fresnel model s-136 had never heard of before. Looks impressive.

11

Some guys talk about using a 7 by 10 Fresnel sheet and holding it in front of flashlight. I have a small wallet size one and is interesting. Too small to catch the whole beam.