We consider N identical two-level systems coupled to a cavity, which is coherently driven by an external field. In the limit of small excitation, the reflection and transmission coefficients for both fields and intensities are calculated analytically. In addition, the frequency content of the cavity field and hence also the emission spectrum of the cavity are determined. A discussion is presented regarding how, in particular, individual collisional dephasing and common energy-level fluctuations prevent the cavity field from being in a coherent state, which in turn affects the outgoing fields.