37.5cm fresnel lens spotlight build

Not many responded to my test of a big cheap plastic fresnel lens for use in a thrower light ( https://budgetlightforum.com/t/-/34066 ), and if reply count ( 8 ) is a measure of the interest I think it deserved a bit better: even though the light loss caused by the mediocre lens quality was huge (~70% scattering), the hotspot was yellow, there was rainbows everywhere, and it is overall a too big ugly thing: Hey: the throw number was over 5 million candela, 12 times the throw of my best UF-1504 build!!

Perhaps it helps if I build a box around it . Too big and ugly, but throwy.

I started out with this lens (picture taken from the test thread):

The build.

I wanted the led on an aluminium plate for cooling, that would also be at the back of the box. This plate needed some way to be focussed. so I made a back construction of wood, with 4 small bolds sticking out:

(inside and outside)

The panel was painted black, over the 4 bolds, 4 small springs (I sacrified four ballpoint pens for this ) were slided, and over that, the aluminium plate was bolted with 4 nuts. There's my focussing system, with a travel of about a cm. It was quite a measuring challenge to get the dimensions of the box so that the correct focus was within this cm. The light should focus at infinity, but also somewhere within 12m, to be able to measure the throw inside my house. Between those focus points the travel of the led was already almost a cm.

Some middle stage in the building process:

The lens was screwed onto the plywood with three screws per side, the screws were just touching the lens plastic so no deformation of the lens took place and it remained as flat as possible. That did not really help a lot, this lens just is not flat, I made a video showing that, look at the wobbling reflection in the lens :sick: :

The aluminium plate was painted black on the inside, with an unpainted circle where the ledboard was going to be. A MCU-cooler on the back (appeared a bit overkill for the dedomed XM-L2 that I was going to use, nothing got even warm), and a hole for the battery holder.

The battery holder is a spare Supfire S2 flashlight that I bought cheap a while ago. I chopped the front part of the head off, I used the battery tube with switch, and the pill. The driver was only used as the contact board for the battery, the battery was going to be directly connected to the led. The ledwires was 3A twin wire from an old hobbylamp.

The head of the Supfire uses the same threading as the tail, so I used a slice of the chopped-off head to screw the battery tube tight against the aluminium backplate. There was just enough tail threading leftover that after some sanding down the tail up to where the threads began, the tail could be adequately screwed tight. Tail spring and driver spring were bypassed with silicon wire. A piece of brass was soldered on top of the tail spring to correct the length of the battery tube for all types of 18650's (without it, unprotected batteries did not fit).

A last peek inside before closing up:

Closed. I cut off some small corners off the side panels to create holes for heat to escape, but I found it never got anything near hot inside.

The plastic lens is very prone to dirt and scratches, so I made a cover for it, and some wooden sliding guides on the front of the box.

Some varnish to make it look more slick (ok, this big thing will never be slick ), and a handle to finish it (suddenly it looks like a speaker box). Done!


I thought I had ordered some U4 binned XM-L2's from intl-outdoor recently, but either I never did that or it is still somewhere in the mail (I can not find back the order, I still do not have an acount at intl-outdoor due to the silly leaking of all buyers information on the internet a few years ago, sorry Hank ;-) ). So I had to make do with an XM-L2 I had leftover from my Olight S10 Baton that I swapped the led for a more pleasant one. Don't know the exact tint or bin of this led, it was some cool white before I dedomed it for this build. The battery was a Sony VTC5, fully loaded. Measured at a distance of 12 meter, 30 seconds after switch-on, I measured 5340kcd :-)

So how does that look? Well, very bright and ugly! I do not dare to shine this spotlight on the square where I live, mainly because of, apart from the hotspot, the huge percentage of the light that is just scattered in all directions, it just attracts attention. I will have to take the spotlight somewhere out to do some proper beamshots, but here are two quick ones that I dared to take, first one into a very clear sky (next to the moon), the other on the roof on the other side (~50meter).

More beamshots to come when I have the time to go outside the city.

Edit: a few more beamshots in post #10 and post #36

What else with this spotlight?

I did not exactly use use the latest XM-L2 led at the best current (it was under 4 amps in the throw test, I will have to use thicker led wires), with improvements I could get it to 7 or 8 million candela. And because I can just screw out the aluminium plate it is easy to work on the light engine of this spotlight. It could handle a XHP70 or a 100W COB, the light will sacrifice throw but I doubt that it will be less impressive :-)

Thanks for reading!

PS: this spotlight can be used for selfies, I bet M.C. Escher had a fresnel lens too :-)

Darn, there goes my backup plan for the competition build. I wish I was kidding. :_(

Nice numbers, that’s one heck of a thrower. Not real pocketable though. :bigsmile:

At 5mcd you’ve got nearly 4.5 kilometers of throw. It would be fair to say that lights like this definitely have a use, and city life ain’t it. I live in a city and feel your pain on that. I don’t have anything more than maybe 300m from my property that I can hit because there’s no line of sight to anything further than that.

Sorry about ruining the back-up plan, that was not the intention. BTW, it would my back-up plan too, but the use of an almost complete flashlight in this build (even though as a battery carrier) would make it a tough one to judge :bigsmile:

I guess now we both have to stick to our beautiful first plan for the competition ;-)

Wowsa. Does the wife or girlfreind need a new bigger handbag to carry this light around for you?

Or parhaps it is cool to carry this around on my shoulder, like the beat boxes of the eighties. And why not build an big array of speakers in the side panels and play Wagner while shining this light around in the dark, woohahaha!!!

Can’t wait to see some cloud lighting shots!! COOOOL!

It produces an amazing beam. Nice work.

Yep, definitely more interesting with the magical box ;)

Nice out of the box thinking :bigsmile:

I love it. Looks steampunkish. I bet it would be a lot of fun to take car camping.

Every once in awhile I find an old projection tv thrown out in the street. I think they have a big fresnel lens in them. Maybe I can harvest one. Im definitely interested in playing with these now.

Thanks for sharing.

I found my 10x optical zoom camera back in a drawer, I took another beamshot, still from the square where I live, but as far as I can project the hotspot from my balcony. The iron fence at the end is at 125m. First wide-angle to give an idea of the situation:

10x zoomed in, fence=@125meter:

Chimney in the rain (@50meter):

Where does that tint end up on this scale? :bigsmile:

More beamshots when I dare to go into the park at night

Its a rainbow in the rain. Nice.

5340kcd !!! :open_mouth: :open_mouth: :open_mouth: Good job! congratulations H)

That sounds like either a lens quality problem or else you have the lens backwards. It could also be that the lens wasn’t designed for point to parallel, but I think a good lens designed as a magnifying glass should also work fairly well.

It sure is a lens quality problem, for one thing the whole thing is wobbly because it is made from thin plastic (see video in the OP), and a fresnel has some inherent scattering at the edges of the grooves. And I have no high expectations about the preciseness of the manufacturing of these budget fresnels. As for if the design is made for point->parallel, I would not know, it was sold as a solar cooking lens which is point->parallel (the other way around actually but that is optically the same), but I doubt that that was the original purpose of the lens when manufactured.

The lens is mounted the correct orientation, the throw is ten times less the other way around.

Hi Djozz,

you can think about glueing the fresnel onto a IR filter glass. This makes the fresnel completely plain without stealing much light. I did this when i buildup a DIY-Beamer which used a fresnel, too. Far more better results with the plain fresnel lens.



Thanks for the info. It is now already flat at the four sides, to get it flat over whole surface using a piece of glass it needs to be glued over the entire surface, right? Which clear glue would you use for that?

I would be a bit worried about the added weight, it is pretty nice lightweight as it is now. But getting it over 10 million cd is also tempting of course :-)

Hi Djozz,

yes, the fresnel has to be covered completely. I used some kind of UV glue but don´t know exactly any more. That was about 10 years ago when beamers with high resolutions were expensive. Today there is no need to build one by yourself.



What(!), we have a yearly build competition and it’s here??? I thought that “lighthouse” build I sold my precious coated aspherics for was another website completely having a “design competition”, or something like that. :open_mouth: Is it all over? Ahh, nvm, I’ll do my thread digging tonight/later for that stuff.

When Mr Kloepper Knife Works visited my home last month I brought out a decent grade of 320mm diameter fresnel lens with around 50-grooves/Inch groove density. I was thinking it was slightly lower groove count like 1 per mm, but I believe it did end up being 2/mm when I counted rings in the light with caliper and loupe.

We were putting a dedomed XP-G2 build with bezel taken off behind that lens and that was driven around 4 amps current I believe (close enough?). Focal length is around 200mm on that fresnel lens (always smooth side faces LED just like aspherical lens). I do say “FREZ-NAL” lens just like it looks and we are not supposed to say. (FRAH-NELL = “proper”, go figure.) The fresnel we were shining the G2 through 200 meters out, looked like a tiny G2, I would say. It was not rainbow colored. When the lens was quickly flipped around so grooves were facing LED, apparent brightness looked like it went down to 50%. If the lens is bending everywhere, I can imagine you’d get prisms all over the place happening. But chances are djozz, you have some very sharp grooves that aren’t rounded well in there, which would split light well into different colors like that. Good, correct grooves should display a good, semi-correct color image with some resolution reduction is all.

Fresnel lenses vary more than any other collimating lens type in their ability. Some of them are odd and have a definite FL you can see is there, but they are really intended to be more of an ‘end point’ for light to be viewed on rather than projected through to a far distance (even though light technically still passes through to be seen by the eye—the image is viewed on the fresnel itself). They are very hit-and-miss lenses; I usually don’t buy them unless I plan to experiment and hope to expect a good lens grade if I’m paying anything for one. In this case I did get a decent grade of lens, but this one’s very thin, about 1.5mm—which is a factor known to reduce throughput (not to mention rigidity). Candela can’t be predicted, as you see djozz can get a reading, but on what color is the cd measurement, green? :wink:

A good fresnel output should look like a very dirty aspheric or lens with a very light diffuser grade behind it, and prism-effect with the extreme aberration should not be there, not that bad at least if the lens will be of good use.

TV lenses are circular groove and horizontal together, so you actually get a short line (more like a bow-tie shape) when an LED is focused down and the Kcd is probably not as high as you think—not even close.

For 5-million cd, there are a few different ways I can think of, but non are as cheap as a fresnel.

Nice old arcade screen, I mean retro center speaker, I mean light box, djozz! :slight_smile:

The competition is still on, just join and you have 'till sept 30th . https://budgetlightforum.com/t/-/33821

It is nice to know that there is better quality fresnel lenses out there, and as Kenji pointed out, ways to make it flatter, so there is a lot of improvement still possible. This light for me was checking out the concept using cheap materials and I even put in more effort in the casing than originally planned (I'm glad I did, it is a nice thing now). I was already surprised that it works as well as it does (the hotspot is bright yellow btw, could be a worse colour :-) ). If there are easy fixes I will surely implement them, but to get it past 10 million candela it will take more, like using a better lens.

Not all aspherics are well made either.

This TrustFire Z5 has a lens that has a shorter focal length around the outside than near the center, resulting in this ringy spot at intermediate zoom. Others with similar problems include some cheap lenses from DX.