Monday, May 16, 2011

Cleaning Lenses and Optics Part II: Field Shortcuts

Here's the second post from Guest Blogger Spencer Luster of LightWorks ...

(To my fellow optical engineers: Avert your eyes!)

So it's late at night in Borneo and you're servicing a vision system. The protective windows are covered in grease, but all you have to work with is your shirt sleeves and your lunch debris. What do you do? Pack it in and go back to your hotel room!

If you insist on working, however, there are a few field expedients you can try. The following are actually some general tips for cleaning when you don't have all the right materials. You needn't be desperate in Borneo.

Spit isn't so bad! I made a strong point in Part I that blowing off dust and debris is the essential first step in cleaning optics. This will always be true, but if you don't have a squeeze bottle, blowing on the optics with your mouth is acceptable. Cleaning up stray spit is better than scratching mirrors.

"Duster" cans work. Sometimes they're great, as long as you keep them vertical and don't shake them. Otherwise, liquid difluoroethane can land on your optics. Best case is staining. Worst case is fracturing a lens from thermal shock.

Soap and water. If you can remove the window, mirror, or un-mounted lens, swishing warm tap water and liquid soap isn't a bad option. Be sure to rinse well, and shake off any excess water. This is also an excellent way to remove dust and debris prior to "real" cleaning.

Ketchup. Seriously. The vinegar will do a decent job dissolving grease and oil, allowing you to then wipe away the ketchup with water. Your Borneo lunch debris is useful after all!

Do nothing. Sometimes this really is the best choice. Light from a single target point diverges toward and eventually fills the entrance pupil of the lens. A speck of dirt on a window or mirror between the target and the lens entrance pupil will likely be very defocused. It's also often too small to block more than 0.001% of the light. Even if there are a thousand specks of dust or dirt, their total contribution to image degradation amounts to almost nothing. Not a great thought for the self-esteem of the speck, but good for you. Small particles far from an object or image plane are very often best left alone, especially when field cleaning might make things worse.

An exception to the above is when you operate the lens at a very high F-stop, or small aperture. The cones of light are quite skinny, and a small speck might block a lot of light.

copyright © 2011 Spencer Luster, LIGHT WORKS LLC

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