Analysis of threshold values of contrast revealing coefficient in highway tunnel lighting

Commission internationale de l'eclairage (CIE) defined the contrast revealing coefficient q(c) based on the perceived contrast method in highway tunnel lighting. Meanwhile, CIE recommended q(c) as an index in the lighting quality evaluation system. Several studies suggested that q(c) was 0.6 under counter-beam lighting and 0.2 under symmetric lighting in the threshold zone. However, there were no threshold values of the contrast revealing coefficient q(c0) under different lighting systems (q(c0) corresponds to the visual adaptation of human eyes). Thus, this study aimed to determine the threshold values of the contrast revealing coefficient q(c0) based on small target visibility. The relationship between q(c0) and minimum luminance threshold was established in tunnel lighting. Therefore, using the fixed value stimulation approach, the minimum luminance threshold and luminance contrast threshold could be determined under different background luminance and viewing angles. Moreover, based on the luminance contrast threshold, the threshold values of contrast revealing coefficient can be obtained under different lighting environments.