  We measure the fluorescence spectrum of broadband emitters in an open optical
microcavity with radius of curvature R = 17.7(3) um and finesse F ~= 1000. This
geometry enables a combined measurement of emission spectra versus cavity
length, which has several benefits over measurements at fixed wavelength or
fixed cavity length alone. We demonstrate the role of the optical penetration
depths on the cavity modes and provide practical working equations for its
analysis. Furthermore, we show the ability to measure the coupling of cavity
modes within a small scan range of the cavity length. By measuring these cavity
emission spectra as a function of cavity length, we thus obtain a rich and
complete picture of the optical microcavity.
