Flame detectors watch for the radiation a flame emits rather than for the particles a fire produces. That makes them fast on fires that flame early and blind to fires that smoulder, which is the opposite profile to a smoke detector and is why they are chosen for specific spaces rather than as a general solution.
How they sense a fire
An optical sensor looks for radiation in bands characteristic of flame: ultraviolet, infrared, or combinations of infrared bands. Multi band detectors compare the signal across bands and look for the flicker frequency of a real flame, which is how they discriminate against sunlight, hot machinery and welding arcs. Response can be very fast, because the detector sees the fire directly rather than waiting for products to travel to it.
Where they earn their place
Large volume spaces where smoke has to travel a long way before reaching a ceiling detector: warehouses, atria, hangars. Spaces with high air movement that dilutes smoke. Outdoor and semi outdoor areas where smoke detection is unusable. Places where the credible fire is a flammable liquid or gas that flames immediately rather than smouldering. Turbine halls, fuel handling, spray booths.
Their blind spots
They need line of sight. Anything between the detector and the fire hides it, so stacked goods, racking changes and new plant can silently reduce coverage in a warehouse that was surveyed when it was empty. They respond poorly to smouldering fires, which is what an electrical fault in a cable tray tends to produce. And contamination on the lens reduces sensitivity, which is why self test and contamination monitoring features matter.
Designing with them
Flame detection is part of a designed system under the fire detection standard, with a stated category and a documented cause and effect. It is normally combined with other detection types rather than used alone, and the coverage has to be recorded against the layout it was designed for so that a change in the racking is recognised as a change to the fire system.
Questions people ask about flame detection
Are flame detectors better than smoke detectors?
They are different instruments. Flame detection is faster on flaming fires and blind to smouldering ones, so the choice follows the credible fire in that space.
Can sunlight cause false alarms?
Single band ultraviolet and infrared detectors are more vulnerable to it. Multi band detectors with flicker discrimination are designed to reject it, and siting still matters.
Do they need maintenance?
Yes. Lens cleaning, self test verification and confirmation that the line of sight is still clear, which in a warehouse is the item most likely to have changed.