The photo-sensing device is shielded from the light source. When smoke enters the air chamber, it refracts some of the light toward the photo-sensing device. When sufficient light is received, an alarm is initiated. Light obscuration detectors use an opposite approach. Light is normally directed toward a receiving element within a smoke chamber.
Smoke entering the chamber blocks the light.
When a sufficient amount of the light is blocked, an alarm is initiated. Since photoelectric detectors function on visible smoke, these detectors are more suitable for detecting large smoke particles produced by smoldering fires. Beam Smoke Detectors Beam detectors use a similar approach to light obscuration detectors, however, a laser beam is typically projected. Since a laser beam is used, these detectors can span large areas. A beam approach can be much more accurate, therefore they are also used in highly protected spot-type applications.