Exit-sign image degradation in smoke: A quantitative simulation

Abstract

A physical model developed for atmospheric imaging is used to predict the effects of smoke on exit-sign luminance. Images of exit signs in smoke were recorded and used to calculate the modulation transfer functions (MTFs) representing the effect of the smoke. Parameters for the particle size, absorption coefficient and scatter coefficient of the smoke were obtained from the measured MTFs. MTFs derived from the measured smoke properties were applied to images of exit signs taken in a clear atmosphere to predict the luminance distribution of exit signs in smoke-filled atmospheres. Comparison of the predicted luminance distributions with the actual luminance measurements of the same signs in the same smoke demonstrates that the model can be used to predict the luminance distribution of an exit sign in smoke. The accuracy of the prediction varies with the format of the sign and the optical density of the smoke. The model offers an improvement in accuracy for calculating the luminance distributions of exit signs seen in smoke compared with calculations of luminance based on Lambert's law alone. The model only applies when the exit sign is the sole source of illumination.