Exciters are designed with necessary blocking to reduce/eliminate any signal which matches the fluorophore's emission signal back through the system. Emitters are designed with adequate blocking to eliminate any signal from the excitation source. The edges of both types of filters are optimized for the excitation and emission spectra of the excitation source and fluorophore.
For some set ups, this design criteria is not as important as others. In this case, exciters and emitters can be interchanged with little to no effect on the resulting response.
If you already have Semrock filters, you may check the performance specifications online to determine if they are exciters or emitters. All blocking and transmission data is available for our filters. If your filter data is not available, please contact Customer Support.