For fire detection systems, such as for engine fire detection, how does the infrared flame detector work?
Infared flame detectors have an imaging sensor that "sees" infared light.
When a flame ignites the various types of materials burn at different strengths, intensities, and -what we care about- wavelengths. In the visible spectrum we can describe a flame as red or blue which can be simplified flames discrete wavelengths.
The IR sensor can detect the wavelengths given off outside that visible light spectrum. This helps eliminate sources like a technicians flashlight from setting off a pure visible optical sensor.
An example from Wikipedia shows a distribution of this emission.
There a several types of IR sensors. The simplest measure a point and radiometrically read the object temperature. Other sensors like room occupancy sensors read the ambient temperature but don't care about the ambient temperature. Rather they have lenses which create a series of apertures which interrupt the sensing as a warm body moves around the sensor field of view. That interruption rate range is thresholded to sense the expected periodicity of the event (eg a human moving in a room).
Both types are employed in aircraft fire detection and in some other systems like gensets.
If there is a particular sensor model of interest, the manufacturer specs on that sensor will tell you if it employs a lens to detect motion.
A third variant is to sense the ambient in an area with one wide view sensor and use a second sensor to read a localized temperature in an area of interest. If the sensed differential exceeds a threshold then the sensor system shows a triggered event.
Fire detection methods such as ionization measurement don't work well in aircraft applications as they are suspectable to airflows changing concentrations and therefore the sensing threshold.
Optical sensors, often referred to as flame detectors, are designed to alarm when they detect the presence of prominent, specific radiation emissions from hydrocarbon flames. The two types of optical sensors available are infrared (IR) and ultraviolet (UV), based on the specific emission wavelengths that they are designed to detect. IR-based optical flame detectors are used primarily on light turboprop aircraft and helicopter engines. These sensors have proven to be very dependable and economical for these applications.
When radiation emitted by the fire crosses the airspace between the fire and the detector, it impinges on the detector front face and window. The window allows a broad spectrum of radiation to pass into the detector where it strikes the sensing device filter. The filter allows only radiation in a tight waveband centered on 4.3 micrometers in the IR band to pass on to the radiation-sensitive surface of the sensing device. The radiation striking the sensing device minutely raises its temperature causing small thermoelectric voltages to be generated. These voltages are fed to an amplifier whose output is connected to various analytical electronic processing circuits. The processing electronics are tailored exactly to the time signature of all known hydrocarbon flame sources and ignores false alarm sources, such as incandescent lights and sunlight. Alarm sensitivity level is accurately controlled by a digital circuit. [Figure 17-9]