A method of determining degradation of a thermoplastic when exposed to light and heat includes illuminating the thermoplastic with a desired wavelength of light at a desired irradiance while maintaining the thermoplastic at a desired temperature. The method is useful to measure the discoloration rate of transparent, translucent and opaque thermoplastics such as polycarbonates, the discoloration rate being determined by transmission or reflectance spectra of transmitted or reflected white light through or from the thermoplastic.