Optical emission was used to determine absolute Cl-2 number densities in a Cl-2 high-density, helical resonator plasma that was used to etch Si. Emission from Cl-2 at 306 nm was monitored, and rare-gas actinometry was used to correct for changes in the electron energy distribution. This emission band of Cl-2 is ascribed in this study to a state at 8.4 eV, or possibly to one at 9.2 eV. A state of Xe at 9.8 eV, emitting at 823.2 nm, was chosen as a suitable match to the energy of the emitting state of Cl-2. Absolute number density measurements were obtained by extending the measurement to extremely low powers where little dissociation occurs. This simple self-calibration method has advantages over other more complicated techniques such as laser induced fluorescence that rely on external calibrations. It is also much easier to carry out than mass spectrometry and is more sensitive than optical absorption. It is applicable to feed gases with optical emission from states with energies above about 8 eV In this study it was found that a Cl-2 helical resonator discharge is nearly completely dissociated (95%) at a power density of 0.4 W/cm(3) in the glass tube source region. The percent dissociation drops to 85% below the source and above the Si substrate. Near the stainless-steel walls, fast Cl-atom recombination reduces the dissociation to similar to 10% (determined by line-of-sight mass spectrometry). The high degree of dissociation in the source and near-downstream region is in agreement with recent model predictions for high-density discharges.