An excitonic cluster model is proposed to describe the optical responses of strongly correlated electronic systems, e.g. antiferromagnetic insulators of parent crystals of high-temperature superconductors. This model can take into account both the strong correlation effect of 3d-electrons and the itinerant nature of electronic excitations. Consequently both the electron-hole bound and unbound states can be described on the same footing. As a result, we can resolve the mystery of resonance enhancement of 2-magnon Raman scattering, and remove inconsistencies in the understanding of the absorption spectrum, the photoconductivity spectrum, and large-shift Raman signals.