A novel broadband transmission method to determine polymer film thickness during manufacturing is proposed, and a measurement system is developed based on this method. The relationship between broadband optical power and film thickness is deduced according to the Lambert–Beer law. The system is composed of a halogen light and an optical power meter. Results show that the measurement error of this method is approximately 1 \mm m, and the resolution of the system is below 0.4 \mu m for polymer films with less than 100-\mm m thickness.