Wednesday, July 27, 2011

Imaging in the UV

I was forwarded an interesting email from JAI last week regarding the advantages of using shorter wavelengths of light. The message was that when trying to see really small features it can help to get down into the UV part of the spectrum. And guess what! JAI make cameras that are UV-sensitive!

Okay, please excuse my cynicism because this is a good point. It all boils down to the diffraction limit and the Rayleigh criterion. This little chart shows how wavelength has an influence on the resolving power of an optical system. Shorter wavelengths mean better ability to distinguish small details. And as we like to use red light at 660 nanometers, dropping down to say 350 nm can make a difference (assuming everything else in your system is perfect.)

If you follow the link to the JAI page for UV cameras, and go on to read the spec sheets – make sure to look at QE without glass or micro-lenses – you’ll see that the tradeoff for seeing in the shorter wavelengths is lower overall sensitivity. Your application will determine if this is an issue or not, but it may mean using high powered UV illumination, which brings up another concern: safety.

Longer wave UV light – UVA – is all around us, but that doesn’t make it good. The shorter wavelengths, as you get down to 300 nm and below, can start to get quite nasty: a suntan is the least of your problems! So if you head into the UV end of the spectrum, just exercise a little common sense regarding eye protection.

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