Fire Friday #2: Spatial Resolution

Is a smaller spatial resolution always better?

Fiber optic Linear Heat Detection (LHD) systems measure the temperature along a fiber optic cable. The performance of the measurement is directly affected by three interdependent parameters, which are:

  • the length of the fiber (maximum monitoring distance)
  • the measurement time, and
  • the spatial resolution.

While the fiber length is typically defined by the project design and cannot be adjusted flexibly, measurement time and spatial resolution are configured during system commissioning. Changing these two parameters affects the temperature resolution, which is the main factor for reliable fire detection and at the same time for elimination of false alarms.

Spatial Resolution

The spatial resolution of a fiber optic LHD system is defined by the slope width of a measured step temperature profile from 10 to 90 % of the actual step temperature levels.

A smaller spatial resolution typically results in higher noise levels, while a wider spatial resolution smoothens a temperature trace and reduces statistical uncertainty of measurements, hence improving the temperature resolution and consequently false alarm resilience. Considering the real world and real fire situations, normally a spatial resolution as low as one meter is more than sufficient, often a larger resolution of two or even four meters is preferential.

Taking a fire in a tunnel caused by an accident as an example, such a fire would quickly have a spatial distribution of multiple meters, therefore rendering a measurement resolution of 0.5 - 1 m has no benefit. In addition, wind effects further widen the thermal effect of a developing fire.

Even more, a larger spatial resolution is often advisable, even if the technology would enable a smaller setting. In the example of a road tunnel, a small spatial resolution of 1 m would undesirably detect punctual hotspots created by hot fumes from the exhaust of standing/ slow moving trucks or heat release of cleaning machines. Regulations request quick and reliable detection of real fire. A larger spatial resolution is consciously configured to eliminate the risk of nuisance alarms caused by other, extraneous events which should not trigger the fire detection system.
AP Sensing’s LHD system allows a freely configurable spatial resolution in order to adapt the system to actual project demands, also considering other parameters. In some cases, a very fine spatial resolution of down to 50 cm is preferred, in other cases, a resolution of 2 - 4 m is more appropriate. 

For further information about configuring the spatial resolution of an LHD to your project and its specific requirements, contact us at

Response time, spatial resolution, and alarm zone configuration: what is important when selecting a Linear Heat Detection system? Find out more in our weekly series.

AP Sensing Fire Friday