The 976 two-wire, stainless-steel proximity sensor from Honeywell's Micro Switch Div. (Freeport, IL) is suitable for sensing needs in harsh environments, including factory automation, machine tools, transfer lines, food processing equipment, parts positioning and presence/absence.
The Dynamic Optical Window from Balluff (Florence, KY) detects and reacts to moving objects as small as .03" dia. Suited for counting small objects air-conveyed in a transparent tube or dropped from a chute, such as drug capsules, wire trim scrap or electronic components.
Photoelectric and proximity sensors from Boston Gear (Quincy, MA) can be used as system status indicators, providing input to PLCs. Available in infrared, visible-beam, polarized-light and fiber-optic models.