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



## Nereus (Oct 8, 2007)

I finished my aspherical Mag (see this thread) some time ago and I have been very satisfied with it. However, there is one weakness in it, the protruding glass lense which is very fragile:







In principle aspherical lense can be substituted with a flat fresnel lense and of course I had to try that...

I found a suitable fresnel lense in the Rolyn Optics catalog, namely this product. Here you can see a photo of it:






The diameter of the actual lense is some 5 cm so it is perfect for a D size mag. Focal length is 32 mm. Here I hold the lense in my hand for a size reference:






It is indeed perfectly flat:







Next I had to cut the square edges away, otherways it will not fit in the Mag head. Intact lense on the left, cut-down on the right (yes, I bought two of them...  ):






Here is a closer look on the cut-down lense. Acryl is surprisingly hard and brittle. That's why the edges are quite rough but you can't see them after the installation.






Here you can see the fresnel lense installed - perfectly flat lense guarded by the bezel 






Glo-powder glows through the fresnel... 






Here is a close distance beamshot with as narrow beam as possible:






The beam at its widest (you can see my kitchen ceiling here...  ):






The beam projected by fresnel lense is very similar to the beam projected by aspherical lense - based on the beam profile, you can not tell which optics is used.

So far this fresnel lense experiment has been a success story - all the benefits of an aspherical lense without its vulnerability. However... when I measured center beam brightness (narrow beam) I noticed that aspherical lense performs slightly better: 57 600 lux @ 1m for aspherical and 40 000 lux @ 1m for the fresnel lense.

So, to answer my own question in the topic of this thread: yes, the aspherical lense can be substituted with a fresnel lense but there is a trade-off here between robustness and performance of the flashlight.

On the other hand this was my first fresnel lense experiment and the performance might be improved with slight changes in the fresnel lense specs. I guess I have to order a few more... :thinking:

Thanks for looking! 

-N


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## f22shift (Oct 8, 2007)

great find!


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## datiLED (Oct 8, 2007)

Nereus,

That is a very cool experiment. I like the way you think!


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## TorchBoy (Oct 8, 2007)

:twothumbs


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## DM51 (Oct 8, 2007)

It is an excellent solution to the problem of the bulky aspheric lens, and it will reduce weight, but a fresnel lens is never going to be as good optically as a true optic, because of all the refraction rings. With each ring you get a slight drop in optical efficiency due to slight scattering at the ring margins.


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## stitch_paradox (Oct 9, 2007)

great experiment!


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## PhantomPhoton (Oct 9, 2007)

Nice idea.
Aside from the enlightening input that DM51 gave, also theres ~ a 5 mm difference in focal length between the Aspheric and the Frensel. (I saw someone put up the math here on CPF once but I have since forgot) I s this going to have a significnat impact on lux as well? I don't think 5mm is going to account for over 17000 lux but it probably counts for part if it.
Mmmm Frensel Mag. Must have one!
(Beautiful *Green* Mag there by the way :rock


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## DM51 (Oct 9, 2007)

The focal length will dictate the correct distance from the lens to the emitter, if the objective is to produce a perfectly collimated (i.e. almost parallel) beam. A shorter focal length lens would be placed nearer the emitter than one with a long focal length. 

Given lenses of the same physical diameter, the angle from the emitter to the lens would be smaller in a longer focal length lens, which would therefore collect a smaller proportion of the light emitted. 

However lenses of a longer focal length are likely to be more accurate and more tolerant of emitter characteristics, so there is no hard and fast rule about the optimal diameter / focal length ratio.

The ideal would be a large-diameter lens with a proportionately long focal length. I think Long_John made something with a large lens like that, and the result was pretty impressive IIRC, but in the lights we are discussing here, the diameter and focal length are governed by the size of the host (Maglite) bezel. 

These considerations are the same for fresnel and true optical lenses.


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## f22shift (Oct 9, 2007)

the acryl is a plexiglass type material? but you mentioned brittle so is it like glass? what did you use to cut it?maybe you can sand the edges?
where did you buy it? link?

this could be a good alternative to someone who bangs up their flashlights in daily use.


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## jashhash (Oct 9, 2007)

Im not sure if you could find this, but wouldnt a glass lense be much better than accrylic since it's more transparent. Great experiment.


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## lctorana (Oct 10, 2007)

Excuse my ignorance - AGAIN...

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


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## Aircraft800 (Oct 10, 2007)

Nice Find! :candle:


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## VidPro (Oct 10, 2007)

He Did it :twothumbs
i was hoping somone would, i really needed to know if that was possible very interesting results

which leads to stupid question #1, now could this be done Removable?
flip in, snap in, swing in, and then what could you have as "normal" behind there.

question #3 
*Focal length is 32 mm *
*~ a 5 mm difference in focal length* 
this is ~5mm FURTHER away from the led right?

because Nereus has his heat sinc hotlips or whatever shoved down pretty far, i still have the lip on mine, and its a beech to move, so it sits up about 3mm further. 
How many "threads" or turns are left to your head falling off, or going past the o-ring

then reverse that, nereus (and others) also have some space removed from the mags Head threads, which puts the aspherical down closer to the led.


some Others from what i saw are still applying cam rotation ( of reflector) for led drop-in movment, i have not seen one of those in person or pictures yet to completly understand it.

question #2, is the actual focus of the projection harder to "maintain" at different distances.
the oversized freznel that i played with here unsucessfully, has to be moved (focused) to a different position if i hit the wall at 8 feet, or if i hit the tree 200 yards away.
the asperical lens thing (basically) stays a tight beam near or far, does this frenel lens work the same for focus as the aspherical, not the zoom aspect, but actual focused projection of the led die?

i will get to questions 4-100 later  this is really interesting.


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## DM51 (Oct 10, 2007)

lctorana said:


> Is there such a thing as a glass Fresnel lens?


Schott make glass ones, and I'm sure there will be other manufacturers too. Glass would be needed where very lot bulbs are used.


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## Nereus (Oct 10, 2007)

Thanks for the positive feedback - wow!  I guess it's time to answer the questions...

DM51: Yes, the optical "imperfectness" (is that a word?) of the fresnel lense might be the explanation to the weaker performance when compared to a true optic.

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.

f22shift: The material is harder than regular plexiglass but softer than pure glass. When I cut it with pincers it fragmented in small sharp chips.

jashhash & Ictorana: I guess DM51 answered your questions.  And yes, glass might be better material for fresnel lense.

Vidpro: Thanks...  About the 5mm difference in the focal lenght, see my answer to PhantomPhoton above. I think this is reversible mod because you can remove the fresnel lense and the led+heatsink combo. And with the fresnel lense, you do not need to remove material from the flashlight head. The need to re-focus at different distances is similar with fresnel and aspherical lense. Waiting for your questions 4-100... 

Ok, next a few more further considerations... It is good to put the brightness (lux @ 1m) difference in perspective (literally?): the ability to throw does not depend linearily on the brightness. In order to double the throw in metres, you have to multiply the brightness by four. This means that there is square root relationship between the brightness and throw (See this PDF file for further info). Let's put the numbers in:

(57600-40000)/40000=0,44. So aspherical lense has 44% greater brightness than fresnel lense.

sqrt(57600/40000)=1,2. In other words aspherical lense can throw 20% further than fresnel lense. The difference here is smaller than the difference in brightness.

One more comment that I forgot to mention in the first post: the fresnel lense has to be installed grooves facing out. And you have to install a UCL lense in front of the fresnel lense in order to prevent it from rattling and in order to protect the fragile grooves. So there are two flat lenses "sandwiched", UCL being the outer one.

BTW guys, I still haven't decided if I prefer aspherical or fresnel lense. Max throw or robustness, that's the problem! 

-N


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## VidPro (Oct 10, 2007)

Q#4 , are all fresnel lenses basically made with a similar focal point?
(like how did you figure out which one to get)

#5 could this be done small too

#6 is there fresnel magnifyers for just normal magnifying stuff that arent so nastily ridged? i want a pocket magnifyer but i find that viewing through (the cheap stuff) is not anywhere near as good as a normal magnifyer.

#7 the point focus is not as bright, but did you lose "overall lumens", or are those lost lumens spilling all about the place as usual.


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## Nereus (Oct 11, 2007)

VidPro said:


> Q#4 , are all fresnel lenses basically made with a similar focal point? (like how did you figure out which one to get).


 
There is lot of variability in the focal lengths (and other specs) of fresnel lenses, just like in that of aspherical lenses, see e.g. this Rolyn Optics catalog page. I found my own fresnel lense simply by searching the one having the specs closest to the specs of the original aspherical lense (52mm diameter and 37mm focal length).



VidPro said:


> #5 could this be done small too


 Yes, it can - you can find examples of small fresnel lenses on the page quoted above.



VidPro said:


> #6 is there fresnel magnifyers for just normal magnifying stuff that arent so nastily ridged? i want a pocket magnifyer but i find that viewing through (the cheap stuff) is not anywhere near as good as a normal magnifyer.


It's inherent that fresnel lenses are ridged (or grooved). Of course you can increase the density of grooves which makes the grooved surface look more smooth.



VidPro said:


> #7 the point focus is not as bright, but did you lose "overall lumens", or are those lost lumens spilling all about the place as usual.


 Hard to say - I do not have integrating sphere with which I could measure the total lumens. But like said, the beams are very similar when comparing fresnel and aspherical lense. So I guess that there is not very much difference in lumens.

-N


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## yellow (Oct 11, 2007)

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?


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## VidPro (Oct 12, 2007)

yellow said:


> 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 have mounted an asperical reversed and it "worked" then i punched it into a cree dome and destroyed the dome  oops, i almost had it.


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## yellow (Oct 12, 2007)

doh,
didnt think about this. 
Sure the dome will now point inside where there is not enough space


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## Nubo (Oct 12, 2007)

lctorana said:


> 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/


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## lctorana (Oct 12, 2007)

Ahhh

yes, of course. I can picture that.

An optical version of a horn.


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## LukeA (Oct 12, 2007)

Nereus said:


> 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.


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## VidPro (Oct 12, 2007)

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?


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## Nereus (Oct 13, 2007)

yellow said:


> 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


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## Nereus (Oct 13, 2007)

LukeA said:


> 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


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## neilp1 (Jun 25, 2008)

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!


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## Blue72 (Jun 25, 2008)

Wow great thread!!!


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


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## Furrballz (Jun 27, 2008)

Great information!


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## Nos (Jun 28, 2008)

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:


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## Nereus (Jul 3, 2008)

neilp1 said:


> 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


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## neilp1 (Jul 3, 2008)

Nereus said:


> 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!


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## neilp1 (Aug 26, 2008)

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

Thanks!


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## russthetoolman (Sep 15, 2008)

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


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## Hmmm (Mar 30, 2011)

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?


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## saabluster (Mar 30, 2011)

Hmmm said:


> 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.


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## busylifemeto (Dec 26, 2011)

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


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## jspeybro (Jan 16, 2013)

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


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## nikosb (Mar 16, 2016)

jspeybro said:


> 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° ;-)
> 
> ...



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?


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## HarryN (Mar 16, 2016)

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.


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## jspeybro (Mar 17, 2016)

HarryN said:


> 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 used a Cree XML or XPG at that time. Given the 8-9cm at 2m I'm guessing it was the XPG. The blue/yellow distribution of LEDs is caused by how the phosphor is applied and not the issue I was describing. This color change is usually a problem when using extreme refraction angles. In my case you get a sharp image of the LED die with some white glow around it. I also have this with the Ahorton lens, so it is not caused by the rings of the fresnel, although it is possible that they contribute a tiny bit to this.

Regards,
Johan

I used Cree XM


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## Blitzwing (Mar 17, 2016)

An interesting and I'd say successful experiment!


Hmmm, fresnel lense zoomie...


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## HarryN (Mar 17, 2016)

A zoom is not out of the question, but honestly, substantially more challenging than a fixed setup. It was "non trivial" to design and align the two stage optics for even a fixed setup. There is a very narrow set of alignments that produce a nice looking beam, high flux (throw), reasonably uniform color, etc. 

I would think that to do a quality zoom, it would take at least 3-5 optical elements. A good example are comparing a rifle scope with fixed vs zoom capability.

When I did my own work, I found that I tend to use light in either close up situations (really smooth, broad, very spectrally consistent - virtually no optics) , or really want some throw. A light with a lot of throw tends to have relatively little useful spill.


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## eh4 (Mar 21, 2016)

Would it be possible to have a glass or even a sapphire lens cut with a Fresnel lens array on the inside, and a flat, easily cleanable surface on the outside? 
-or would TIR situations arise?


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