Do high/low pass lenses exist? For an experiment I will hopefully be soon conducting at Johns Hopkins I need two different lenses.
The first needs to allow all wavelengths above 500 nm to pass (thus a high pass filter) and cut off everything else.
The second needs to allow all wavelengths below 370 nm to pass (thus a low pass filter) and cut off everything else.
My knowledge of optics is middling.  I know that good old glass cuts of UV light, but I was hoping for something more specific.  Does anyone know of the theory necessary to "tune" materials to make such filters?
Truth be told, I'm an experimentalist, so simply giving me a retail source that has such lenses would get me to where I need to go!  But learning the theory would be nice as well.
Thanks,
Sam
 A: You may consider to use a prism to separate the different frequency light (essentially you are performing a Fourier transform). Then physically blocked those light with wavelength higher than 500 nm and lower than 370 nm. Finally, you merge these light ray together. This will be the device you want and you can freely adjust the frequency range.
A: What you're looking for isn't a lens so much as a filter. There are filters for whatever purpose you might want out there in the world, and they are relatively inexpensive. Doing a google search on Optics Filters, I came across a couple of sites that might help you.


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*Edmond's Optics

*Optical Filters USA
A: If you need extremely sharp filters because your wavelengths in question are either close together or need to be sepearated by a high degree, look at filters from http://www.semrock.com (no affiliation). 
Other than that a spectral device (grating or prism based) combined with knife edges as described by hwlau is best, but as he said, you will need a 2nd such device to recombine your desired wavelengths into a single beam and there will be losses (non-brewster reflections on the prism, higher diffraction orders in the grating case, optical surface quality limitations) in the process.
A: There are many ways to do this. Which option you choose depends on what degree of performance you require, and how much money you re willing to spend.
First of all, you should understand that, while you could apply a wavelength selective coating to a lens, this would much more commonly and cheaply be done with a wavelength filter separate from the lens itself.
Now, the cheapest way to do this would be with a glass filter which absorbs short or long wavelengths selectively. There is a wide range of these available from Schott Glass. I use these commonly, and I've never had trouble. Most vendors will be happy to produce custom shapes, thicknesses, etc.
The down side to filter glass is that it doesn't have a particularly sharp cutoff between the bassband and the stopband. For that, you will need a dielectric coating engineered to your specifications. For that, I would look at CVI or Newport, although there are other vendors out there. There may be something you can use in their catalogs, but custom orders are normal for the optical manufacturing industry, so don't hesitate to call up a sales engineer. In my experience, sales people in this industry are very well educated on their products, or will direct you to an engineer who can tell you exactly what they can produce for you.
Again, there are other vendors you could look at, but these are the ones I would go to first. At the very least, looking at their catalogs will give you an idea of what it is you are really looking for.
