Visibility performance thresholds of exit signs in smoky indoor environments

Abstract

Exit signs are critical for effective evacuations, but building fire smoke can obscure their visibility. This study evaluates the visibility of internally illuminated, photoluminescent, and reflective exit signs under different smoke extinction and ambient light conditions, building upon Jin's (1970s) visibility experiments. The internally illuminated sign outperforms non-illuminated signs, remaining visible at extinction coefficients up to 1.4 m 5 m distance, with dimensionless number σ 1 at V (or K factor) of 3 to 7, aligning with Jin's findings. Photoluminescent and reflective signs are visible only up to 0.4 m-1 in bright indoor environment with V values of 0.5 to 2, lower than Jin's values for reflective signage (2 to 4). Visibility declines with distance, approximately 50% per 5 m, with all signs visible blow 0.2 m σ 1 at 15 m. Smoke scattering and absorption reduce illuminance by 5–30%, while back-scattering slightly lowering perceived luminance. Larger, high-contrast signs enhanced visibility, but small details are indistinguishable in dense smoke or beyond 10 m. Human behavior during fire evacuations, including stress and visual acuity variations, may further impact effectiveness. For fire safety engineering, exit sign design (size and type), ambient light, and the selection of an appropriate K factor for visibility calculations should be considered for the design performance.