Unlike an ionization smoke alarm which uses a small radioactive particle in an ionization chamber to detect the presence of smoke, a photoelectric smoke alarm uses a beam of light. In general, photoelectric smoke alarms are better at detecting highly visible fire particles, while ionization smoke alarms can detect invisible particles. Both types have their uses, though, and may be used together in a home to ensure all types of fire are quickly detected.
How a Photoelectric Smoke Alarm Works
Inside of a photoelectric smoke alarm is a light source beaming down a small chamber. At a 90-degree angle to this chamber is another, connected chamber, at the other end of which is a sensor. When no smoke is present, the light beams uninterrupted through the first chamber. When smoke enters the smoke alarm, however, it enters the chamber, reflecting a portion of the light into the perpendicular chamber. With enough smoke, the sensor at the far end of the perpendicular chamber is hit with light, at which point the alarm sounds.
While photoelectric smoke alarms function well, they tend to work best when there is thick smoke present in a room. Fire with a lighter plume of smoke might evade detection from a photoelectric smoke alarm. This is why it is a good idea to install an ionization alarm as well.