Which proximity sensor?

I hope you guys can point me in the right direction. I'm working on a project and need a sensor that would sense dirt, mud or water on a piece of glass and would activate a wiper to clean the surface. However its a smaller scale than a full windshield on a car. What kind of proximity sensor would I use for this? Thanks for your help.

Car ones use infrared light, and look for the backscatter from the dirt, I think.

You might find that at capacitive proximity sensor works.