switch lever exhaust

[Jeff Rodriguez]

I'm working on patent for an air filter that I designed during my freshman year at Drexel. I'd tell you more but you'd steal it,
I am looking for a material that air can pass through easily, but light cannot. I have thought of either AC filters (I'm not sure if they're opaque enought, though) and this stuff at work.

At work, there is a pneumatic system setup for switching between rails, etc. (details not important) On the switch levers, or whatever you call them, the levers used to actuate the cylinder, there is an exhaust. The exhaust is covered (to protect it from foreign objects) by a hard white foam-like material.

I'm sorry I don't know the proper terminology, but does anyone know what I'm refering to? Does anyone know what this foam like material is?
I'll bring my digital camera to work tomarrow and get a picture of it, to clarify for you.