An apparatus and method are provided for differentiating multiple detectable signals by excitation wavelength. The apparatus can include a light source that can emit respective excitation light wavelengths or wavelength ranges towards a sample in a sample retaining region, for example, in a well. The sample can contain two or more detectable markers, for example, fluorescent dyes, each of which can be capable of generating increased detectable emissions when excited in the presence of a target component. The detectable markers can have excitation wavelength ranges and/or emission wavelength ranges that overlap with the ranges of the other detectable markers. A detector can be arranged for detecting an emission wavelength or wavelength range emitted from a first marker within the overlapping wavelength range of at least one of the other markers.