This article presents results from a two-semester field test of the Light and Spectroscopy Concept Inventory (LSCI). Statistical analysis indicates that the LSCI has the sensitivity to measure statistically significant changes in students' understanding of light-related topics due to instruction in introductory astronomy courses and to distinguish the relative effectiveness of traditional (primarily lecture) and active engagement treatments in introductory college astronomy courses. (Contains 1 table and 2 figures.)