According to the basic characters of vehicle license plate, this paper presents a method to switch the color of gray image in the ready-to-recognize region into two-color image (black character with white background or the opposite).

Though fluorescent whitening agents can improve significantly the whiteness of black-and-white photographic paper's image picture, this effect may be reduced owing to the shift of whitener molecules during the photographlc processing and induced by acids, alcalis and washing.