Problem-based learning (PBL) is an instructional approach whereby students learn course content by collaboratively solving complex real-world problems and reflecting on their experience. Research shows that PBL improves student knowledge and retention, motivation, problem-solving skills, and the ability to skillfully apply knowledge in new situations. One of the challenges with PBL, however, is that real-world problems are typically open-ended with more than one possible solution, which poses a challenge to educators with regard to assessing student performance. In this paper, we describe an approach to assessing student performance in PBL developed by the Photon PBL Project, a three-year National Science Foundation Advanced Technological Education (NSF-ATE) project in which eight interdisciplinary multimedia PBL "Challenges" were created in collaboration with photonics industry and university partners for use in high school and college math, science and technology courses. Assessment included measures of content knowledge, conceptual knowledge, problem-solving skills, motivation, self-efficacy, and metacognitive ability. Results from pilot testing at four community college photonics technology programs are presented.