The dual-band filter sets were designed for simultaneous viewing of two fluorochromes. Each excitation filter contains two passbands for simultaneous excitation. Concurrently, each emission filter contains two passbands for simultaneous viewing. The polychroic beamsplitters contain complementary reflection and transmission bands specific to the excitation and emission filters. These sets were originally designed for FISH (fluorescence in situ hybridization). During their history, their performance has been relied upon in many other applications.