This four slot filter-wheel was originally designed for the US military to work in a 10^(-7) Torr vacuum environment. Today, a standard non-vacuum version and the original vacuum version are available. The vacuum version can be manufactured in a Class 10,000 clean room, fully baked out, and meets Class A cleanliness standards, at a small additional cost. Please note that the optional external control electronics is not designed to operate in a vacuum. It must operate in ambient pressure environment.