Reverse aspheric with sapphire cover

[purduephotog]

To give you a bit of context, I just lost my father, and I just returned from a baseball game with my son.

I am wondering if anyone has done any builds within an asperic lens element that has been installed in the reverse direction. While watching the game with my five-year-old it occurred to me that I might be able to build a "statue of light" for my father.

While there still is a lot of planning and testing to do, if I could install an aspheric over a high intensity LED and optically bond a sapphire top element to it it ought to survive a significant time before degrading or being no longer able to maintain a tight beam into the sky.

I have only recently begun experimenting with fresnel lenses 4 as an aspheric substitution, but I do not believe Optical bonding them with UV curable epoxy would provide the necessary refraction or robustness from acrylic/pmma.

Help?

 

[Harold_B]

If I understand correctly you want to flip an aspheric lens so that the high curvature surface will face the source? If that's the case then you will likely miss out on a lot of output from the high angle rays being emitted from the LED. Full Width Half Max (FWHM) of most LEDs is around 60 degrees meaning the 50% of the light is within 30 degrees of perpendicular to the emitter surface. The critical angle for thermoplastics is around 40-45 degrees depending on refractive index and coating so you want to have an entrance angle of much less than that. The closer to perpendicular the better. You might consider a lens tube where you can collimate the light with whatever lens (or lenses) you choose and then put the SiO2 window over that.

 

[HarryN]

You are right that the PMMA Fresnels won't hold up in an open environment. Even more annoying, they are usually designed so that the ridges face "out".

It is a little bit surprising, but when I was experimenting with them, it mattered less than I expected to reverse the lens direction.

Sapphire is great stuff, and not that expensive in optical flats. Another option is to use an AR coated quality glass, which might transmit a bit better than the sapphire since the reflections are reduced.