Fresnel lens equations

[spikespiegal]

I'm building some custom theatrical / stage / DJ projectors from LED's, and due to the costs of glass optics I'm looking more and more at Fresnel lenses. The type used for basic magnification, which are rather inexpensive.

What I'm not clear on is if Fresnels use the typical thin lens formulas.

What I need to know is something like; if I have a light source 4" in height, and I have a 10" Fresnel lens with a focal length of 200mm, and if that light source is exactly 200mm on one side of the Fresnel (yes, the ridged side), how far away would you have to be on the other side of the Fresnel for that light source to project 2x as wide (8").

400mm seems a bit too obvious.....

 

[serpretetsky]

I want to bump this thread sense no one has responded.

I am not too familiar on the subject, but from what i remember in physics class, yes, you can use the exact same thin lens formula. In fact, if what i remember is correct, fresnel lens's are actually based on the idea that the thickeness of the lens is not taken into account in the thin lens equations, so the fresnel lens is a good approximation while using far less material

Sorry, I can't help you with the actual number crunching.

 

[Doublejr]

Not sure if this will help, but here is a small paper on fresnel lenses

www.wiley-vch.de/berlin/journals/op/07-04/OP0704_S52_S55.pdf

Wiki has some formulas too, I used Schlick's approximation in my computer graphics class but that was for rendering images so I do not think that would help you in your case.

 

[Daedalus685]

You can use the same thin lens equations. They work just like regular lens but with the radius compacted into segments. The only difference is the quality and the difficulty in eyeballing the power as you cant see the curvature.

The optical quality is quite poor though. If you are using a lot of light from more than one point source it won't matter but you can get odd looking rings if you use them for spot lights with just one source.

I've worked a lot with polymer lenses on single or clusters of LEDs, most these days are just molded lenses, but many have a fresnel lens top to add a bit more optical power. They work pretty well as low profile flood lighting for inspection work.