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):
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 :-)