Which one of the following is not detected by fire detectors ?

Fire detectors are designed to sense the presence of a fire by monitoring specific physical or chemical changes in the environment. They are generally categorized into three main types based on what they detect:

• Smoke Detectors: These sense particles produced by combustion using ionization or photoelectric (light-scattering) sensors.
• Heat Detectors: These respond to a significant increase in temperature, either through a fixed-temperature threshold or a rapid rate-of-rise in heat.
• Flame (Radiation) Detectors: These are optical sensors that detect electromagnetic radiation, specifically in the ultraviolet (UV) or infrared (IR) spectrums emitted by flames.

While flame detectors are optical, they are specifically tuned to non-visible radiation or specific flicker frequencies. General visible light is not used as a primary detection parameter because it would cause constant false alarms from ambient lighting, torches, or sunlight.