Optical band gaps around electron volts account for the strong absorption of ultraviolet radiation by TiO2 with wavelengths shorter than 380 nanometers. Irradiation with UV laser permanently changes the TiO2 particles from white to blue/black. The same effect occurs when these particles are embedded in the substrate. Ideally, the laser radiation does not react with the substrate, but passes freely through the substrate surface. In contrast, pigment particles within the substrate interact with a laser beam, which changes the particle’s structure and appearance, including color. For example, thin PTFE films are almost transparent to UV light, while small (~0.3u) particles of TiO2 randomly distributed in the insulating layer strongly absorb light and change color.