A scanning detection system is provided in which emissions from locations in a flow cell are detected. In some embodiments, the system can include an excitation source, a photocleavage source, and modulating optics configured to cause an excitation beam generated by the excitation source to irradiate a first group of the fixed locations and to cause a photocleavage beam generated by the photocleavage source to irradiate a second group of the fixed locations, which is separate and apart from the first group of fixed locations. Methods of detecting sequencing reactions using such a system are also provided.