The photochromic properties of the films were determined by a color difference meter (SC–80C, Beijing Kangguang Instruments Ltd.). The color differences (ΔC) of the obtained films were characterized after being exposed under a UV lamp device (a sealed box, containing 3 parallel UV tubular lamp in it, each UV lamp possess a wavelength of 365 nm, 3 W) and the films were placed 10 cm away from the lamps. The Commission Internationale de'Eclairage Lab uniform color space for color sheet system was used to figure out the color difference. Ln , an and bn are the three-dimensional rectangular coordinates, which are worked out from the tristimulus values. Ln refers to the transparency index while an and bn refer to the chroma indexes. The color difference (ΔC) is calculated according to the formula below