The recent development of serial crystallography has popularized time-resolvedcrystallography as a technique to determine the structure of protein-reactionintermediate states. However, most approaches rely on the availability ofthousands to millions of microcrystals. A method is reported here, usingmonochromatic synchrotron radiation, for the room-temperature collection,processing and merging of X-ray oscillation diffraction data from <100 samplesin order to observe the build up of a photoreaction intermediate species. Usingthis method, we monitored with a time resolution of 63 ms how the populationof a blue-light photoreceptor domain in a crystal progressively photoconvertsfrom the dark to the light state. The series of resulting snapshots allows us tovisualize in detail the gradual rearrangement of both the protein andchromophore during this process.