Measures the number of smoke particles in the air. Photoelectric & Ionization smoke detectors are used here
Heat detection
Measures the rise of temperature within a given interval and the maximum temperature. This group includes rate-of-rise temperature detectors and fixed-temperature heat detectors.
Flame/ Spark detection
Measures the radiation stemming from the fire (infrared, visible, ultraviolet). Infrared and ultraviolet flame detectors are mainly used in this instance.