Single-light-source dual-light-path transmissometer is a kind of transmissometers based on digital photography. The method of segmenting the light spots from the image is one of the key processes in the derivation of visibility. Based on the formation of the image, two methods of segmenting the light spots are presented. The results computed with the two methods above are matched with the results of Vaisala LT31 very well, even without pre-processing of the images and post-processing of the data. In order to find the parameters involved in the two methods above, weighted mean squared error (WMSE) is introduced as a cost function. As a cost function, WMSE performs better than some other frequently used criterion. At the same time, WMSE can be adopted to assist the judgment of the performances of the methods above.