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

Nereus

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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:

P8010505.jpg


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:

PA080216.jpg


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:

PA080217.jpg


It is indeed perfectly flat:

PA080219.jpg



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... :) ):

PA080215.jpg


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.

PA080220.jpg


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

PA080225.jpg


Glo-powder glows through the fresnel... :D

PA080224.jpg


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

PA080221.jpg


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

PA080222.jpg


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! :D

-N
 
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DM51

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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.
 

PhantomPhoton

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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:)
 

DM51

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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.
 

f22shift

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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.
 

jashhash

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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.
 

VidPro

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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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Nereus

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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! :D

-N
 
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VidPro

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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.
 

Nereus

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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).

#5 could this be done small too
Yes, it can - you can find examples of small fresnel lenses on the page quoted above.

#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.

#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

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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?
 

VidPro

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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.
 

yellow

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