Detectors come in two common types--the ionization and the photoelectric variety. Ionization smoke detectors feature a harmless radioactive source within a dual detection chamber. Ionization detectors respond to invisible by-products of combustion. They operate by sensing for a change in the electrical conductivity across the detection chamber. The advantage of the ionization detector is that the smoke can be invisible to the human eye, while remaining very much visible to the ionization detector.
Photoelectric smoke detectors work on an entirely different principle. Photoelectric detectors look for the presence of visible by-products of combustion in the detection chamber. When a sufficient density of visible combustibles fill the detection chamber, the detector sounds an alarm condition.
