How should a flat target be oriented with respect to a retro-reflective photoelectric sensor's beam to trigger its contacts?

• A. Perpendicular to beam
• B. At an angle to beam
• C. Parallel to beam
• D. Behind the sensor

Answer: (B)

In retro-reflective sensing, the system relies on light returning from the reflector to indicate a clear path. To trigger the contacts with a flat target, you want to interrupt that return by deflecting the beam away from the sensor.

Placing the flat target at an angle to the beam does just that: the light that hits the target is reflected away rather than back toward the sensor, so the sensor no longer sees the reflected beam and responds as if the path is blocked. If the surface were perpendicular to the beam, the light would tend to reflect straight back toward the sensor, which can fail to trigger. If it were parallel to the beam, the target would present little effective obstruction to the beam. Behind the sensor isn’t in the beam’s path at all.