The WT4 is SICK Optic Electronic's (Eden Prairie, MN) smallest photoelectronic proximity sensor. It measures about 1 sq" and weighs 20 g. The WT4 has a variable scanning distance of 20 to 80 mm (0.788" to 3.152") to allow sensing of any color object against any reflective background.
Banner Eng.'s (Minneapolis, MN) Omni-Beam® Q45 photoelectric sensors operate in a retroreflective mode. A built-in Class II laser diode allows long sensing ranges up to 225', or 130' in the polarized version, with a 670-nm beam.