Flame Detector Types: UV, UV/IR and Triple IR (IR3)

A flame detector is an optical fire-detection device — it senses the ultraviolet and infrared radiation a flame emits, rather than the smoke or heat a fire produces. That makes it the right choice where smoke detection would be too slow or unsuitable: large, open or outdoor spaces, and high-risk industrial environments where a fire can develop fast. The main types differ in which part of the radiation spectrum they watch: ultraviolet (UV), combined ultraviolet and infrared (UV/IR), and triple-infrared (IR3). This guide explains how each works and where each fits. It is written by Midland Fire Direct, a BAFE SP203-1 certified fire detection supplier.

UV flame detectors

A UV flame detector responds to the ultraviolet radiation given off in the first instant of most fires. Because that radiation appears almost immediately, UV detectors are among the fastest-reacting flame detectors. The trade-off is false alarms: other sources emit UV too — arc welding is the classic example, along with some forms of lightning and certain lamps — so UV detectors are best suited to indoor, controlled environments where those sources can be excluded or managed. In the Honeywell FSL100 range, the FSL100-UV is the single-band UV unit, with a 25 m detection range and a Zone 2/22 rating for hazardous areas. Where welding or similar UV sources are present, a dual- or multi-band detector is usually the better call.

UV/IR flame detectors

A UV/IR detector watches two parts of the spectrum at once — ultraviolet and infrared — and only alarms when it sees the signatures of a real flame in both bands together. Requiring two independent confirmations sharply reduces false alarms from single-band sources: a welding arc that would trigger a UV-only unit is rejected because the matching infrared signature is not present in the way a flame produces it. That immunity makes UV/IR a common choice for environments a UV-only detector would struggle with, while keeping fast response. The FSL100-UVIR covers this with a 25 m range and the same Zone 2/22 rating as the UV unit. It is the middle option in the FSL100 range — more false-alarm immunity than UV alone, without the longer range and outdoor bias of triple-IR.

Triple IR (IR3) flame detectors

A triple-infrared, or IR3, detector looks at three separate infrared wavelengths and compares them. A real flame flickers and radiates across those bands in a characteristic pattern; hot surfaces, sunlight and other infrared sources do not. By analysing the ratio between the three bands, an IR3 detector distinguishes genuine flame from a hot, radiating background — which is what makes it suitable for outdoor and sun-exposed locations that defeat simpler detectors. IR3 also tends to offer the longest range of the three types. In the FSL100 range, the FSL100-IR3 provides a 35 m detection range with a Zone 2/22 rating — longer reach than the 25 m UV and UV/IR units. Honeywell's FS24X is a triple-IR detector too, supplied in aluminium and stainless-steel housings for the environment it sits in: see the FS24XP-AMGXX (aluminium) and FS24XP-SMGXX (stainless steel). The wider Honeywell FS20X multi-spectrum family covers further options where the application calls for them.

Choosing between them, and typical applications

Picking a type comes down to the environment and the false-alarm sources in it, not just the fire risk. UV suits clean, indoor spaces without welding or other UV sources. UV/IR adds immunity where single-band false sources are present. IR3 is the choice for outdoor, sun-exposed or hot-background locations, and where the extra range helps. Typical applications for optical flame detection include fuel and solvent storage, warehousing and waste or recycling facilities, aircraft hangars, and turbine or generator halls — places where a flammable fire can develop faster than smoke detection can catch, or where the space is too large or open for it. The Zone rating matters too: all three FSL100 units carry a Zone 2/22 rating for the relevant hazardous areas. If you are matching a detector to a site, browse the industrial flame detectors range, or start at the flame detection overview and send us the environment — we will help pick the type and range.

Testing flame detectors

Flame detectors are function-tested with a broadband test lamp that emits radiation across the wavelengths the detector watches, prompting it to respond as it would to a real flame — without lighting anything. For the FSL100 range that is the FSL100 test lamp; the flame detector testers and accessories page covers the lamps and mounts. Functional testing should follow the manufacturer's instructions and the site's maintenance regime; for the wider routine — the weekly test, servicing and the logbook — see our commercial fire alarm testing guide.