Visual alarm devices - their effectiveness in warning of fire

VADs provide visual warnings of fire for the deaf and hard of hearing by producing pulses of light. They can also be used in areas of high ambient noise, such as factories, or where a silent alarm is preferred, such as in operating theatres. There remains, however, a lot to be learnt about what type of visual signal provides the most effective warning for people.

This project investigated the pulse duration of VADs by comparing the responses of a group of participants to flashing Xenon and LED devices of varying pulse durations. One Xenon device, three cool white LED devices (of 40, 20 and 10 millisecond (ms) pulse durations) and two warm white LED devices (of 40 and 20ms pulse durations) were used.

The flashing signals were presented individually to 96 participants who were seated in front of a screen and occupied in a written task. The tests were performed in high and low ambient light level conditions. The devices were flashed one at a time, starting at a distance of 19m, and were gradually brought closer to the screen until the subjects responded.

Analysis of this data revealed that as the pulse durations of LED devices shorten, their attention-drawing effectiveness increases. It also demonstrated no significant difference in responses between warm white LED devices and cool white LED devices. The Xenon and 10ms cool white LED device had similar responses.

This work has led to changes in US and British codes of practice and has influenced manufacturers of VADs to produce devices with shorter pulse durations. Further research work is currently in progress to identity other factors that influence the effectiveness of visual fire warning for people.