Experiment: can an aspherical lense be substituted with a Fresnel lense?

[Nubo]

Excuse my ignorance - AGAIN...

Is there such a thing as a glass Fresnel lens?:help:

Sure! Fresnel invented his lens for use in lighthouses long before the era of plastic


http://www.lighthouseratings.com/Lens/

 

[lctorana]

Ahhh

yes, of course. I can picture that.

An optical version of a horn.

 

[LukeA]

PhantomPhoton: Even though the focal length is 5mm less in the fresnel lense, it seems that it has to be set *further* away from led... weird? :thinking: I guess that when you couple this to the comment of DM51, you get the explanation for the center beam brightness difference.

The focal length of the lens is measured (if I'm not mistaken) from the focus of the parabola that is the non-flat part of the lens. This focal point is (should be, I think) several mm inside the lens. The focal point of the fresnel lens is very near the lens's surface, so even though the lens has a shorter focal length, the focal length is measured from nearer the outside of the lens.

 

[VidPro]

hmmm
from: http://www.anchoroptics.com/catalog/product.cfm?id=402

High groove density allows higher quality images, while low groove density yields better efficiency (as needed in light gathering applications). In infinite conjuate systems, the grooved side of the lens should face the longer conjugate.

i was trying to find a very clean magnifyer (high density), like what is on film cameras $$$ and learned that. might be usefull?
that could be why the stuff on lights like marine lights and all, is HUGE grooves.

and Because the lens is thin, very little light is lost by absorption.
from: http://www.anchoroptics.com/catalog/product.cfm?id=404
which might indicate that thinner is better?

 

[Nereus]

nice one!
except for the part that the grooves have to be mounted to the outside + front glass.

What happens if they (or the aspheric) are mounted the other way?
Shouldn't it work the same?

I tried installing the fresnel lense grooves facing in and the results were poor. The led die was projected like before but lot of lumens were lost.

-N

 

[Nereus]

The focal length of the lens is measured (if I'm not mistaken) from the focus of the parabola that is the non-flat part of the lens. This focal point is (should be, I think) several mm inside the lens. The focal point of the fresnel lens is very near the lens's surface, so even though the lens has a shorter focal length, the focal length is measured from nearer the outside of the lens.

Ah, that explains my findings - thanks LukeA!

-N

 

[neilp1]

Hi there

great use of the fresnel lens instead of an aspherical. I had a quick question about the mod.

You mentioned ( in the original thread, with the aspherical lens) that the hotspot was about 5 cm from a distance of 1m, and this is similar in the fresnel lens. What happens as you increase the distance? have you looked to see how wide the spot gets at, say, 5 metres? and how easy would it be to re-focus the spot for this distance? is it a matter of moving the lens a little further away?

Im looking at a P7 mod, and was hoping to use a fresnel lens to get a roughly 15-20 cm tight spot from around 5 metres or so. Not so interested in throw ( at least not the 100's of feet seen in this forum) itself, but the lumens have to be tight for my application.

thanks in advance!

 

[Blue72]

Wow great thread!!!


Thanks for bumping this one up, otherwise I would have missed it.

 

[Furrballz]

Great information!

 

[Nos]

ive tried a 500x500 mm overhead projector lens........but the results were disappointing

same with a brandnew biconvex 120mm glas lens, it did only throw ~10% more than the KD lens but with a much smaller die :sick2:
-----> i guess the reflections from the backside caused the loss of light, because i nearly got blinded by it

after testing every type of lenses in my oppinion its most important that the lens is made of optical glass and plano convex. and furthermore i think that a AR coating is more important than the aspheric form..... :shrug:

 

[Nereus]

Hi there

great use of the fresnel lens instead of an aspherical. I had a quick question about the mod.

You mentioned ( in the original thread, with the aspherical lens) that the hotspot was about 5 cm from a distance of 1m, and this is similar in the fresnel lens. What happens as you increase the distance? have you looked to see how wide the spot gets at, say, 5 metres? and how easy would it be to re-focus the spot for this distance? is it a matter of moving the lens a little further away?

When you increase the distance, the beams behave in a very similar way. At 5 metres,the hotspot is roughly 20 cm wide. Re-focusing is very easy, just twist the flashlight head.

WBR,

-N

 

[neilp1]

When you increase the distance, the beams behave in a very similar way. At 5 metres,the hotspot is roughly 20 cm wide. Re-focusing is very easy, just twist the flashlight head.

WBR,

-N

That would put the beam angle at about 3 degrees. That is VERY interesting!

Im looking at a fixed focus application, and getting as tight a hotspot as i can at 5 metres before setting the focus. But ill have it adjusted for more of a round pattern as opposed to the square emitter shape. Sounds very doable.

Thanks for the info!

 

[neilp1]

If anyone is interested, i have a few of these left over from my order, and am selling them in the marketplace.

Thanks!

 

[russthetoolman]

I posted a reply in Marketplace to purchase your last lense. A question about the mounting of the LED and lens. Do you have measurements and or pics of how and where you place the lense in behind the bezel and what height the led sits at on the heatsink. Also, is this used with or without the stock reflector? I have a lathe and can make a sink and make two if you want one, or need one. Would you be willing to share your measurements?
Thanks
Russ

 

[Hmmm]

I know its been a while since this has last been posted on but I was wondering, could you use a fresnel lens and then put the normal plastic lens over it to protect it?

 

[saabluster]

I know its been a while since this has last been posted on but I was wondering, could you use a fresnel lens and then put the normal plastic lens over it to protect it?

Or glass- yes. It will not effect the beam any other than the small amount of reflection losses.

 

[busylifemeto]

Hi Just reading this thread. Having played a little just recently with fresnels I think you need a shorter focal length to get the efficiency, many Leds are 90 degree beam angle therefore requiring the focal length to be half the diameter. With Luminous and new CREE XML there up to 110 degree so the shorther the focal length the more light you should capture

 

[jspeybro]

Hi guys, sorry to dig up an old thread.
I just finished a mod of a maglite 2D using a fresnel lens which was already round and 50mm diameter so it fitted perfectly in the maglite. There was no need to modify the maglite or the lens, except for cutting off the tube that holds the incandescent bulb and soldering the driver to the contacts. The focal length was shorter than what the OP used, so should capture more light.
Edit: just measured the beam. at about 2m away, the image of the Led is only 8-9cm so that's a full angle of not even 3° ;-)

All in all, a very nice upgrade with minimal cost and minimal modifications. The beamshots look almost identical to the beamshots in this thread, although I have some additional glow around the focussed LED due to light reflecting off my heatsink and not going propperly through the lens. I see this a goodthing because otherwise you loose this light (it is light that exits the led at angles greater than the acceptance cone of the lens).

I found the lens through some chinese guy (also had some quality issues as you can expect from chinese guys, but all lenses are good now). Due to the high shipping charges, I bought a few more than I needed so if some of you are interested to play with them a little, PM me (shipping in Europe is cheap, for other parts of the world I'll need to check). I'll see if I can post a picture here in the coming days to show my results.

Johan

 

[nikosb]

Hi guys, sorry to dig up an old thread.
I just finished a mod of a maglite 2D using a fresnel lens which was already round and 50mm diameter so it fitted perfectly in the maglite. There was no need to modify the maglite or the lens, except for cutting off the tube that holds the incandescent bulb and soldering the driver to the contacts. The focal length was shorter than what the OP used, so should capture more light.
Edit: just measured the beam. at about 2m away, the image of the Led is only 8-9cm so that's a full angle of not even 3° ;-)

All in all, a very nice upgrade with minimal cost and minimal modifications. The beamshots look almost identical to the beamshots in this thread, although I have some additional glow around the focussed LED due to light reflecting off my heatsink and not going propperly through the lens. I see this a goodthing because otherwise you loose this light (it is light that exits the led at angles greater than the acceptance cone of the lens).

I found the lens through some chinese guy (also had some quality issues as you can expect from chinese guys, but all lenses are good now). Due to the high shipping charges, I bought a few more than I needed so if some of you are interested to play with them a little, PM me (shipping in Europe is cheap, for other parts of the world I'll need to check). I'll see if I can post a picture here in the coming days to show my results.

Johan

I am trying to do the same thing. Are you sure the glow around the LED image is from light reflecting off the heatsink or from the limitation of the fresnel lens to collimate the light?

 

[HarryN]

I am pretty sure that when he posted this, the light had either a 1 or 3 watt Luxeon in it. A lot of the LEDs in that era struggled to have perfectly uniform color output across the entire emitting area, especially right at the edges. Early Cree parts were especially notorious for this - emitting blue at the edge, and more yellow in the middle.

On average and with diffuse reflectors, the effect didn't matter, but with highly focusing optics, there was color separation in the beam. You can see the yellow and blue color separation in the beam in various fringe areas. That used to be a huge challenge, even with higher end LED, when trying to match the Lux performance of incan filaments.

I did a lot of work with 12mm diameter reflector + fresnel lens setups - around $4K worth of parts, plus my time. It can work, but in order to really work, the assembly precision becomes really critical. I figured out a way to do laser aligned, epoxy bonded assemblies in my garage to make it all work. A few years later I was visiting a precision optical assembly place in Germany, and learned that for many years, virtually all precision optics are laser aligned, in multiple dimensions, and epoxy bonded, in temperature controlled environments.

If only I had hired an optical engineer in the first place, I could have saved so much time and money.