Is this cheating? ->350mm diameter fresnel lens test

Fresnel lenses every now and then pop up in discussions as a way to project a led for a thrower flashlight. Fritz t. Cat has done some impressive mods using a fresnel lens instead of an aspheric, but he did not have the means to measure the spot brightness. An internet search including BLF and CPF did yield some attempts at using big fresnel lenses, but those attempts were not very persistent, I found. So here's my go at it :-)

Thanks to a recent hype called 'solar cooking' there's quite a few bigger fresnel lenses for sale at Ebay. I had no idea if they were any good, but to my utter pleasure some guy hobbying in optical communication had done a great job at testing a bunch of cheap and available fresnel lenses: http://modulatedlight.org/optical_comms/fresnel_lens_comparison.html , and that looked like they were not that crappy at all apart from the shortcomings that fresnel lenses show in general (i.e. a huge chromatic abberation at these short focal lengths, scattering at edges, and all sorts of pretty distortion patterns).

So for GBP27.95 (40 dollar?) I bought a 330mm focal length 39.5x39.5cm square fresnel lens from a Ebay-shop in England (this one). Upon arrival the focal length appeared to be ~230mm (there was a 220mm focal length one as well in their shop, they sent me the wrong lens!), but since I had no definite idea what the ideal focal length should be I just went on with it. A big thin plastic lens needs a rigid frame (with any bending the image has huge extra distortions) so I made one from two pieces of flat plywood with the lens screwed in between. With the edge of the lens disappearing under the frame I had 35x35cm of lens leftover, the light cone that the lens will catch now is about 80 degrees (diagonally a bit more), that sounds good, although a bit less than the stock aspheric of the 1405 (~90 degrees at zoom in). This focal length is perhaps better than the 330mm one after all (but this is depending on the lens quality at steeper angles!).

The first quick test shining a led through the thing looked quite promising, so I went on with a proper test. I screwed off the lens of my UF-1405 shorty and used that as the light source. In theory the surface area of the fresnel lens (1225cm sqr.) is 42 times that of the UF-1405 aspheric lens (29.2 cm sqr.), so with the dedomed XM-L2 throwing 335 kcd with the aspheric lens, with the fresnel lens, if the optical quality would be similar a throw could be expected of 14 Mcd. But of course a Fresnel lens is worse than that, what is the real number? The test set-up looked like this:

The distance from the lens to the (Mobilux A) luxmeter sensor was 5 meters. The light in the hotspot was very yellow, apparently the blue region of the lens is relatively more scattered by this lens than other wavelengths (you can see that around the hotspot). After charging the battery, targeting the hotspot at the meter at low setting, and blasting at full throttle for 30 seconds, the brightness of the hotspot appeared out of reach of the luxmeter. The meter measures up to 200,000 lux, so I was at least over 5 Mcd. I topped up the battery again and measured now at 5.5 meter distance (there was no length leftover for more in my living room), and now the luxmeter was within range: 183,000 lux at 30 seconds after switch-on, so that is 5500kcd (= 5.5 million lux when calculated back to 1 meter, or 4.5km throw).

Hmm, so using this fresnel lens made me loose 60% of the theoretical throw if the fresnel lens was as good as the aspheric. But 5500 kcd is still a nice number, so if I have some time for a little project, I will build a nice wooden box around this lens with a dedomed XM-L2 mounted on the inside, this will make a fun spotlight :-)

Thanks for reading!

Interesting. Kind of curbs my desire to test a fresnal though.

You need more surface area to get the same throw with a fresnel, compared to an aspheric lens, but... at the bigger sizes, weight and lens price is going to play a huge role! Find me an aspheric lens, or even an aray of aspheric lenses, that can do this for under 40 dollars? And this 40x40cm fresnel lens weighs about the same as the 6cm 1405 aspheric lens.

Somewhere around here I have a high-quality 26 inch (none of those dinky millimeter thingies used around here, thank you) fresnel lens… hmmm…

Pretty cool. Thanks for reporting on that. I want to make one of those now. Maybe make the housing out of fiberglass.

Speaking of cheating. Did your search happen to include those overhead projectors that use to be commonly used in classrooms? Seems like they would have like a half a meter focal length (guessing off memory). Would you consider that too long?

I ask because they sometimes show up in a surplus warehouses.

I have one of those overhead projector lenses at work, pretty scratched, that is what I looked at first, did not seem to work so well, but it perhaps desrves another look at.

how much light do you loose cause it misses the lens?

you could try to use a adidtional lens to collimate all the emitted light to a circle within your frame.
I am sure losses due to the 1st lens are less than what passes the fresnell unused.

That’s a pretty Cree rainbow.

Really cool project.
Thanks for sharing djozz!