Flashing visual signal lights for emergency notification have usually used xenon strobe light sources, which produce very short flashes of light. Photometric performance specifications for these signal lights use the effective intensity as the primary metric. Effective intensity appears to be a suitable metric for characterizing detection of signal lights when directly viewed at or near the line of sight. For the indirect detection of xenon strobe light sources, when the light source is in the far periphery or outside of the field of view and the primary cue of the signal’s flashing is the increase in brightness of adjacent room surfaces, an effective intensity of 15 cd seems to be sufficient. When signal lights produce longer flashes of light than xenon strobe sources, much higher effective intensities are needed for indirect detection, suggesting that effective intensity is poorly correlated with detection performance. A series of human factors experiments confirmed this suggestion. Based on the experimental results, a modified metric for characterizing the indirect detection of visual signals, termed the indirect effectiveness quantity (IEQ), was developed. The IEQ metric may be particularly useful at predicting indirect detection of visual signals with flash durations longer than those of xenon strobe light sources, such as some configurations using light-emitting diodes. Experimental data from the present study suggest that under ambient room illumination of 500 lx, an IEQ value of 750 cd will elicit indirect detection percentages of about 90%, independent of the flash duration.
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Bullough, J.D., Skinner, N.P. & Zhu, Y. Indirect Detection of Visual Signals for Emergency Notification. Fire Technol 52, 1427–1444 (2016). https://doi.org/10.1007/s10694-015-0488-8