This study presents an approach to measure the atmospheric visibility of two areas in Taiwan – one located near Taichung and the other at Kaohsiung – by digital image processing. The approach includes the following four steps. First, take digital images of selected targets around these areas. Second, analyze the difference between the brightness (the gray level of the images) of the targets and that of atmospheric background using a computing program. Third, calculate the specific brightness defined as digital visibility. Finally, determine the relationship between specific brightness and atmospheric visibility.
The existence of a limit to human vision can be determined by investigating the visibility of two targets, the Da-du Mountain and the Western Shore, at different distances from Taichung Harbor Weather Station. Furthermore, a telephotographic investigation determines the specific brightness of the skyscraper at Kaohsiung. Its specific brightness (Bg) is closely related to the real-time visibility of the atmosphere (Va) when visual range is from 5 to 10 km: |Bg| = -0.2186 Va + 2.7035, r = 0.9079. Our results show that this new index, specific brightness, can provide visibility measurement by accurate digital data. Furthermore, images of areas of interest can be established for future analysis, without the potential for discrepancy caused by humans.