37.5cm fresnel lens spotlight build

For a medium or decent quality fresnel lens I usually just assume about 35% loss and maybe with good fresnel (lower groove/mm count) only about 25% throughput loss against a typical un-coated lens of the same size. I only use that as an approximation for known fresnels, like I said if you haven’t held the model series it is totally unpredictable. That is basically what you pay for between thin/thick lens; all the ring valleys which do not amount to real usable area. Good luck on getting to ten million. :wink:

The unavoidable loss is due to diffraction. The diffraction blurring is important with telescopes but not with LED lights, because of the size of the LED. The diffraction loss is from an area around each corner of width comparable to the wavelength. So for 500 nm. light and 100 zones per mm., the loss is a few percent. That is comparable to the loss in the material, so a perfectly shaped Fresnel lens may have more or less loss than a comparable thick lens.
500 * 10-9 / (1/100 * 10-3 ) = 0.05 = 5%
I don’t know why LED Lenser only uses two zones. Maybe because it is prettier that way.

I think you’re saying one thing that makes sense to you, that I’m not exactly following to the fullest extent. There’s all kinds of diffraction occurring in there, not just wavelength restriction at angular problem areas like groove transitions. Each valley is “spraying” photons all over the place into the next grooves begin, and on and on it goes. 5% loss? OKAY, when NASA is building one, maybe. :wink:

A fresnel lens suffers from higher losses because of the obvious I would say; you have hundreds or potentially thousands of plastic grooves molded and the area cannot be used with the same average resolution (not even close). Due to the size of the grooves, things like arc centering highly change per groove peak and valley; not to even highlight on arc centering like that’s its primary downfall, but it’s now like 1000s of little tiny lenses are managing the light diffraction, not one smooth ground surface. It’s much harder to make a high resolution lens that is 0.5mm diameter (times a few million of them like a bug eye) than to make one lens at 200mm diameter that’s ground by CNC from a glass type. I think that’s the part I see sticking out as obvious to me with fresnel lenses.

For a point source, it is possible, and not hard with fairly simple software, to design a Fresnel lens that is perfect, to a geometric optics approximation. The Wikipedia “Fresnel lens” article shows ray diagrams. The finite size of the LED causes some shadowing of off axis rays that don’t line up with the steps, but I think that is a small effect. The precision needed is similar to that needed for an aspheric, but the process of making the mold may be more difficult.

But light is a wave and does not go exactly in straight lines, as shown here.

Diffraction pattern from a slit of width equal to five times the wavelength of an incident plane wave. From Wikipedia “Diffraction”.

But the light that is lost to the main beam is only that which comes through within a wave length or so of the edge of the step.

Nice box. Dah da da Dah dah, Dah da da DAH dah, Dah da da DAH! dah, Dah da da dah. It loses something in print. :bigsmile:

It never ceases to amaze how little response some truly interesting threads get compared to whine and cry threads.

I'm sure you know the answer to your own question but wont answer it here due to repercussions. Either that or your to polite.

I read what is going on here but dont understand a lot so dont respond. The same in a lot of technical threads. Keep up the good work guys.

That's clearly die Walküre von Wagner!, what else? :bigsmile:

This is pretty awesome.

For the next one, you going to use the Fresnel from an old big screen TV?

Like MRsDNF said: I read everything in this thread with great interest but I do not understand “some” of it.
“some” being quite a lot, actually. :wink:

If I cannot add anything usefull, I do rather not post at all.


Same here, i subscribe to interesting threads like this but if i can’t add anything of value i often don’t post.

But i have to say good work djozz :slight_smile: It looks great both the beam & box.

I wish i had the willpower to keep modding when the world is so beautiful outside :bigsmile: but i haven’t modded a single light since may, It is just too bright & nice outside in summer here, and so few dark hours at night i hardly get to use them at all :frowning:
I can’t wait until the darker months so i can find time to mod again :slight_smile:

In that case: use the long summer days to be prepared for the long winter nights! :wink:


Makes perfect sense, go modding when the light's out. Looking forward to the new batch this winter :-)

About me saying 'smart' things, as a biologist and not into physics I only know some basic optics (unlike a few here on BLF who are way more knowledgable than me), but I'm fairly good in thinking those basics through and find a practical use for it in this hobby. And I can not help myself boring the BLF community with that. (As long as it comes with beamshots I think I can get away with that ;-) )

Now now Djozz, no fals modesty please :wink:

I personally think you are one of the members truly contributing to this foruym so pls keep it up!


Djozz, I think you have a lot more fans than you think. Like others have said, sometimes I very much enjoy a post, but fail to comment. Usually because I can’t think of something constructive (or witty) to say — other than maybe the overused “awesome.”

So……. “Awesome”, it is.

Oh wait. I just thought of something almost constructive. Did you know almost everybody mispronounces “Fresnel”?

(at least here, in the backcountry.)

“Augustin-Jean Fresnel (French pronunciation: [ɔgystɛ̃ ʒɑ̃ fʁɛnɛl]; pronounced freɪˈnɛl fray-NELL in English) (10 May 1788 – 14 July 1827), was a French physicist who contributed significantly to the establishment of the theory of wave optics. Fresnel studied the behaviour of light both theoretically and experimentally.”
BTW he was born at Broglie. Not many french people know how to pronounce correctly “Broglie”

Two outside beamshots in this thread:


Saw that thread! Wow! :steve:

I did not seen this thread before. This is very nice experiment with extreme kcd numbers and very nice rainbow beam :)


Why was I snoozing on this thread? My perspective on making throwers or search lights completely changed upon reading this. I have to admit though that there are a lot of things I don’t understand also. :stuck_out_tongue:

I think I’ll copy this concept shamelessly when I try to build a ‘portable’ ultra long range search light for our rescue team. :bigsmile:

A few tips: measure the focal length of the lens carefully, at a distance as large as possible, and then very carefully make the box with the corect dimensions. My box allows a focal correction of a cm, but that appeared only just enough, more focal travel than that is not a luxury. I was surprised about the focal difference between 20 meter and infinity. Mount the lens as flat as possible (a rigid frame) and stress-free. The finning on the back appeared completely unneccessary for an XM-L2 at 5A.