Final answer:
An alarm signal from an exterior photoelectric detector is activated when its light beam is interrupted or attenuated to a point that indicates an intrusion or other irregularity, based on the principles of the photoelectric effect.
Step-by-step explanation:
When using exterior photoelectric detectors, an alarm signal shall be initiated only when the light beam, which is part of the detector system, is interrupted or attenuated to a level that the system recognizes as an indication of an intrusion or other irregularity. Photoelectric detectors typically employ a light source and a light sensor, positioned such that an object breaking the light beam will disrupt the light reaching the sensor, resulting in a change in the electrical current and triggering the alarm. This technology is based on the photoelectric effect, where photons of sufficient energy hitting a metal surface will cause electrons to be ejected, creating a current. If smoke is present, it scatters the light, altering its intensity at the receiver and therefore triggering the alarm if it falls below a certain threshold.