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Measurement of Flashing Light Sources

Flash Photometry

Photometric Testing provides measurements of the luminous exposure and effective intensity of flashing lights and emergency beacons. The effective intensity of Visual Alert Devices (VADs) is tested in accordance with BS EN 54-23.

What Can We Measure?

Applicable Standards

  • Luminous exposure [lux•seconds]
  • Peak luminous intensity [candelas]
  • Effective luminous intensity [candelas]
  • BS EN 54 part 23

The measurement of flashing light sources is a specialised area and Photometric Testing is one of very few laboratories with the expertise and specialist equipment necessary to undertake this kind of measurement. A flashing light source appears “brighter” to the human eye than a steady-state source of the same average luminous intensity. For this reason, flashing lights are used to attract attention. As examples, think about the beacons used on emergency vehicles, aircraft marker lamps, bicycle lights, turn indicators on cars and maritime light houses. All of these are instances where flashing lights are used to attract our attention.

What makes measuring a flashing light source so challenging is because most commercial illuminance photometers are deliberately designed with a relatively slow sampling rate, meaning that they cannot capture a single flash and instead average over a number of flashes. Measuring luminous exposure requires that we integrate the reading over a known number of flashes, whereas for peak and effective intensity, we have to measure the time-varying illuminance or luminous intensity. Therefore, for flash photometry we need a special kind of photometer, one that is capable of integrating flashes and has a high speed sampling rate that can record the time-varying light levels.

The Gigahertz photometer that we use here at Photometric Testing has a 100 microsecond integration time and can measure luminous exposure from 1ms to 999 seconds. We can measure both photometric illuminance (lux) in the 380-780nm as well as UVA radiometric irradiance (W/m2) in the 315-400nm bands. It was chosen to allow us to measure almost any kind of flashing light source and in particluar to measure the effective luminous intensity of visual alarm devices in accordance with BS EN 54 part 23 ("Fire detection and fire alarm systems. Fire alarm devices. Visual alarm devices"). The Effective Luminous Intensity, IEFF of a flashing source is defined as “the luminous intensity of a fixed light, of the same relative spectral distribution as the flashing light, which would have the same luminous range as the flashing light under identical conditions of observation.” Depending upon the application, the effective luminous intensity of a flashing light source can be calcluated with reference to three well-established formulae, the modified Allard method, the Blondel-Rey method or the Schmidt-Claussen method.

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