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. 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?
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!