Since fire produces heat, a rise in temperature can mean a fire is present. Heat detectors use either a fixed temperature level or a rise in temperature to produce an alarm. Ionization detectors use an electrical grid within the detector to sense a fire. When particles from a fire pass through the grid, the electrical characteristics change and cause an alarm to be produced by the detector. Ionization detectors are useful when trying to sense a hotter fire that produces smaller particles of combustion.
Photoelectric detectors use a light beam within the detector to sense a fire.
When particles enter the detector's chamber, the light beam is obscured. These types of detectors are useful when trying to sense a colder fire that produces larger particles of combustion. When addressable fire alarm systems were first introduced, they incorporated device types similar to hardwired systems, only with addressable elements. Smoke detectors still used the same technology to detect a fire. Once addressable technology became standard in the industry, manufacturers began to improve the detector's ability to sense a fire and provide more information to system operators and responding personnel.