Flame Detectors Working Principle


Most fire detection technology focuses on detecting heat, smoke (particle matter) or flame (light) – the three major characteristics of fire. All of these characteristics also have benign sources other than fire, such as heat from steam pipes, particle matter from aerosols, and light from the sun. Other factors further confound the process of fire detection by masking the characteristic of interest, such as air temperature, and air movement.

In addition, smoke and heat from fires can dissipate too rapidly or accumulate too slowly for effective detection. In contrast, because flame detectors are optical devices, they can respond to flames in less than a second. This optical quality also limits the flame detector as not all fires have a flame. As with any type of detection method its use must match the environment and the risk within the environment.

Typical applications for optical flame detectors are;

    • Wherever highly combustible materials are involved
    • Where there is a need for instantaneous response to flame
    • Wherever unsupervised areas require automated fire protection
    • Where there is a large capital investment to be protected

Examples of actual installations are;

    • Gasoline transport loading terminals
    • Pipeline pumping stations
    • Refineries
    • Aircraft hangers
    • Automotive paint spray booths
    • Munitions production facilities
    • Jet engine test cells
    • Offshore drilling and production platforms

There are three types of flame detectors currently available.

They are Infrared (IR), Ultraviolet (UV), and a combination of UV and IR.


Infrared detectors have been available for many years, however, it has only been in recent times that technology has allowed for stable, accurate detection to occur. There are two types of Infrared detectors, single frequency and multi spectrum.

The basic principle of operation for a single frequency IR detector is;

The detector is sensitive to a narrow band of radiation around the 4.4 micron range which is a predominant emission band for hydrocarbon fuelled fires. Additionally, the sun’s radiation at this band is absorbed by the earth’s atmosphere, making the IR flame detector solar blind. Single frequency detectors use a pyroelectric sensor, which responds to changes in IR radiation intensity. In addition they incorporate a low frequency band pass filter, which limits their response to those frequencies that are characteristic of a flickering fire. In response to a fire signal from the sensor, electronic circuitry in the detector generates an output signal.

Strengths of the single frequency IR detector are;

  • Highly immune to optical contaminants like oil, dirt, and dust
  • High speed response under 30 milliseconds for some brands
  • Insensitive to solar, welding, lightning, X-rays, sparks, arcs and corona

Limitations of the single frequency IR detector are;

  • Generally not suitable for non-carbon fires
  • Some brands will respond to modulated infra-red sources
  • Rain, ice and water vapour on the detector lens will inhibit detection


The basic principle of operation for a multi spectrum IR detector is;

The detector has three sensors, each sensitive to a different frequency of radiation. The IR radiation emitted by a typical hydrocarbon fire is more intense at the wavelength accepted by one sensor than the other two. Electronic circuitry in the detector translates the difference in intensity of the three sensors to a ratio, that along with a synchronous flicker must be present before a fire signal is produced. This allows the detector to reject high intensity flickering black body radiation sources since these sources will not meet the proper ratio criteria.

Strengths of the multi spectrum IR detector are;

  • Virtually immune to false alarms
  • Fire response in the presence of modulated infra-red black body radiation with some brands
  • Long detection range (60 metres to some fires)

Limitations of multi spectrum IR detector are;

  • Typical response time is longer when compared to single frequency detectors

IR detectors are sensitive to most hydrocarbon fires (liquids, gases, and solids). Fires such as burning metals, ammonia, hydrogen and sulphur do not emit significant amounts of IR in the detector’s sensitivity range to activate an alarm. IR detectors are suitable for applications where hydrocarbon fires are likely to occur and high concentrations of airborne contaminants and / or UV radiation sources may be present. The detector should be used with caution when the presence of hot objects and the potential for ice build up on the detector are likely.


A UV detector uses a sensor tube that detects radiation emitted in the 1000 to 3000 angstrom (one ten billionth of a metre) range. It is important to note that ultraviolet radiation from the sun that reaches earth starts at 2800 angstrom. If the detector’s sensor has a wide range then it will be triggered by the sun’s rays, which means it is only suitable for indoor use. There are sensors available with a range of 1800 to 2500 angstroms. Virtually all fires emit radiation in this band, while the sun’s radiation at this band is absorbed by the earth’s atmosphere. The result is that the UV flame detector is solar blind. The implication of this feature is that the detector can be used indoors and outdoors. In response to UV radiation from a flame that falls within the narrow band, the sensor generates a series of pulses that are converted by the detector electronics into an alarm output.

Strengths of the UV detector are;

      • Responds to hydrocarbon, hydrogen and metal fires
      • High speed response – under 10 milliseconds
      • Solar insensitive

Limitations of the UV detector are;

      • Will respond to welding at long range
      • May respond to lightning, X-rays, sparks, arcs, and corona
      • Some gases and vapours will inhibit detection
      • Some UV sensors have a wide detection range resulting in solar false alarms

UV detectors are sensitive to most fires, including hydrocarbon (liquids, gases, and solids), metals (magnesium), sulphur, hydrogen, hydrazine and ammonia. The UV detector is the most flexible general purpose optical fire detector available. They are fast, reliable, have few false alarm sources and respond to virtually any fire.


A UV/IR detector consists of an UV and single frequency IR sensor paired to form one unit. The two sensors individually operate the same as previously described, but additional circuitry processes signals from both sensors. This means the combined detector has better false alarm rejection capabilities than the individual UV or IR detectors.

Strengths of the UV/IR detector are;

        • Virtually immune to false alarms
        • High speed response – under 500 milliseconds
        • Solar, welding, lightning, X-rays, sparks, arcs, and corona insensitive

Limitations of UV/IR detector are;

        • Not recommended for non carbon fires
        • Some gases and vapours will inhibit detection due to blinding of the UV sensor

Since the UV/IR detector pairs two sensor types, it will typically only detect fires that emit both UV and flickering IR radiation. UV detectors will respond to virtually all fires including hydrocarbon (liquids, gases, and solids), metals (magnesium), sulfur, hydrogen, hydrazine and ammonia. IR detectors typically only respond to hydrocarbon fires. Since the IR detector is not sensitive to burning metals, ammonia, hydrogen and sulfur the combined unit will not respond to these fires.

The detector is suitable for applications where hydrocarbon fires are likely and other sources of radiation may be present (X-rays, hot surfaces, arc welding). They maintain constant protection while arc welding takes place. The UV/IR detectors are highly reliable with fast response times and low propensity to false alarms.


Flame detectors can be connected in 4 different ways to provide varying degrees of information.

    1. Stand Alone – the detector is fitted with internal relays that provide alarm and fault outputs. When the detector senses a fire it activates warning devices and some method of fire suppression. This is the simplest method of connection and while the detector does have LED status there is not any remote indication in the event of a fire or if the detector fails.
    2. Fire Alarm Panel – the detector is connected to a Fire Alarm Panel (FAP) as part of an overall site detection system. Warning devices and suppression systems can be operated, the advantages are that the power supply to the detector is monitored, and indication of the detector status is centralised.
    3. Control Panel – the detector is connected to a dedicated flame detector control panel, this is used when the site does not have a Fire Alarm Panel. This system offers the same advantages as a FAP.
    4. Monitoring System – the detector provides a 4-20mA output that connects to a site monitoring system. The output provides multiple alarm and fault conditions. The advantage of this system is that the flame detectors can be incorporated into a system that is monitoring other functions on the site such as air conditioning.


As with all fire detectors the placement of flame detectors is determined by the environment that they will be operating in. What appears to be a good place to locate a flame detector on paper may be a poor location in reality. Some of the factors to consider are;

  • The viewing angle of the detector
  • The detection range
  • Obstructions such as girders, beams, supports, hoists, air conditioners and other solid objects will block the cone of vision and / or hinder access for service
  • All high risk fire ignition areas must be covered by at least one detector
  • Adequate detector coverage will ensure that ‘voids’ in the optical coverage do not occur
  • Optimum detector mounting height is a function of the height of the most likely point of fire ignition

When designing a system we recommend that a manufacturer be contacted as details can be provided on previous installations of a similar nature. This will ensure that the correct number of detectors is provided to ensure the most suitable detection.


When selecting which type of flame detector to use there are 6 questions to be answered;

  • What is the area that I’m protecting (aircraft hanger, storage tank, turbine enclosure etc)?
  • What are the dimensions of the area that I’m protecting?
  • What are the anticipated sources of fire?

Each type of fuel, when burning produces a flame with specific radiation characteristics. The detector must be chosen for the type of fire that is probable. For example, a UV detector will respond to a hydrogen fire but an IR detector will not.

What other sources of radiation will be present?

Radiation sources other than fire are present in many applications. For example, arc welding is often performed in an industrial area. IR or UV/IR detectors will ignore arc welding where a UV will false alarm. Each application must be assessed to determine if any such sources are present before choosing a detector.

What will prevent the detector from detecting a fire?

Industrial environments often contain elements that inhibit the ability of a detector to ‘see’ a fire. For example, a build up of ice on an IR detector will reduce the detector’s range. A build up of oil on a UV detector will reduce its range. Other obstructions such as pipes, partitions, air conditioners etc will block the optical viewing area. If a fire started on the other side of a partition it would not be detected.

How fast must the detector respond to a fire?

UV detectors can respond to a fire as fast as 10 milliseconds. Other detector types such as IR and UV/IR typically take between one and five seconds to respond.

Once these questions have been answered the type of detector required will become evident. As previously stated we recommend that the manufacturer be contacted for verification and further site specific information.

It should be noted that not all flame detectors available offer the same features and level of protection, important considerations are;

    • Frequency Band – a wide band will initiate more false alarms
    • Range – at what range will the detector detect a fire
    • Viewing Angle – at what distance and angle will a fire be detected
    • Cone of Vision – will the detector have the same range over a 90 degree span
    • Optical Integrity – how does the detector monitor the sensor and lens
    • Serviceability – can the detector be serviced on site or does it need to be returned to the manufacturer
    • Construction / Mounting – is the detector construction suitable for hazardous areas, is there sufficient movement in the mounting bracket to ensure the sensor will be aimed at the source
    • Indication – does the detector have on-board visual indication
    • Outputs – does a fault condition over ride an alarm trigger
    • Heating – does the detector have heated optics to prevent ice build up
  • Discrimination – does the detector have electronic capacity to distinguish between black body emission, flickering phenomenon, and flame
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2 thoughts on “Flame Detectors Working Principle”

  1. Flame detectors are required at AirCraft hangers for fire detection and alarm system. Pl fwd technical specs and price for UV IR Flame detector

  2. The model fl 3101 is a flame detector which contains an ultraviolet uv photo tube which responds to uv radiation in the 185 to 260nm region


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