Self-cleaning window glasses based on TiO2 photocatalysis are becoming popular products, and the importance of quantitative measure of glass contamination is increasing in order to evaluate its self-cleaning performance. Contaminated glass samples were rated for visual impression of dirtiness. Then the dirtiness ratings were compared with transmittance, haze value, change in lightness, change in color, intensity average of digital photograph, standard deviation of intensity of digital photograph. It was found that the last parameter gives the best correlation with the dirtiness rating.