Improved method for nighttime visibility measurement based on digital photography

In order to reduce the uncertainty caused by background light noise when measuring nighttime visibility using digital photogrammetric method, the system based on double light sources was improved. An effective method was adopted to limit CCD field angle by using the shading cylinder. For obtaining the grayscales of the double LED backlights at different distances taken by CCD, computerized image processing was applied for captured images. Then the visibility was derived from extinction coefficient acquired by Allard-s law. The contrast test was done to analyze the gray information in three conditions: in dark background, with or without shading cylinder in light background. Taken the visibility measured in dark background as the standard value,compared with the results from using shading cylinder ,the correlation coefficient and average relative errors are 0.911 3 and 6.87%, while they are 0.322 7 and 23.38% without shading cylinder.It can be seen that, the ability of restraining interference from background noise is enhanced by using shading cylinder, thus more accurate results can be got with measuring nighttime visibility.