Photo sensor lacks blind spot for shiny objects

July 12, 2001
Conventional photoelectric sensors have a hard time "seeing" reflective objects like chrome lipstick cases and automotive trim.

Conventional photoelectric sensors have a hard time "seeing" reflective objects like chrome lipstick cases and automotive trim. By using polarized filtering, designers at Banner Engineering Corp., Minneapolis, have built a small sensor that eliminates blind spots for reflective objects.

The sensor emits a beam of light that polarizes as it travels through a vertical filter. A beam splitter redirects the light 45 so it travels out through the collimating lens and toward the objects being sensed. If there is no object, the beam hits a reflector, becomes naturally depolarized, and bounces back through the collimating lens and splitter. A horizontal filter polarizes the returning beam and some light reaches the receiver, energizing the output. When an object prevents the beam from hitting the reflector, no light reaches the receiver and the output is de-energized.

If the beam hits a shiny object, however, the beam is still reflected back down the collimating lens. But because it is not depolarized by the reflector, the beam remains vertically polarized. The horizontal filter completely blocks the beam, light never reaches the receiver, and the output is correctly de-energized.

The unit is housed in black ABS with an acrylic lens and withstands vibrations, shock, and washdowns. It operates in temperatures from -20 to 55C. Sensors are CE approved and meet IEC IP67 and NEMA-6 standards.

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