As the welding robot’s arm swings into position, a beam of light flashes across the gap between the two sheet-metal halves of a car door. The moment the beam is interrupted—when the door panel is perfectly aligned—the sensor signals the controller to trigger the weld. This fiber-optic sensor system, consisting of a light emitter and receiver, is mounted just millimeters from the moving parts, immune to the sparks and electromagnetic noise that would blind conventional proximity switches. In body-in-white lines, these sensors verify part presence before each weld, detect misaligned panels before they jam the fixture, and confirm that robotic grippers have released the component before the next cycle begins. Their slim profile allows them to be embedded inside welding guns or along conveyor side-guides where space is measured in single-digit millimeters. Because the fiber itself carries the light signal, the electronics stay safely outside the heat and vibration zone, reducing false triggers and extending service intervals. On final assembly lines, the same sensors monitor the precise insertion of door seals, ensuring the rubber lip is seated before the door is closed for the first time.