A system and method for transmission electron microscopy (TEM) of a photocatalyst sample exposed to UV and/or visible light at irradiance levels comparable to those provided by irradiation with sunlight or at least 1,000 W/cm.sup.2 while maintaining the spatial resolution of interrogation of at least 0.14 nm. Light is delivered to the sample substantially transversely to the sample's surface from an external broadband source through an optical fiber with an output facet formed at an acute angle with respect to the fiber axis. The light delivery system is adapted to not interrupt an operation of auxiliary TEM systems responsible for changing the TEM-chamber environment.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
 This invention was made with government support under DE-SC0004954 awarded by the Department of Energy. The government has certain rights in the invention.