Planar radio frequency (rf) inductively coupled plasmas (ICP) are currently being investigated as sources for materials processing. We have characterized an Ar plasma ICP discharge using spatially resolved optical emission spectroscopy. Abel inversion of line-integrated intensities of Ar* emission yields two-dimensional (r, z) profiles of emission in a cylindrical volume, driven at one end by a spiral antenna with rf power at 13.6 MHz. Measurements were made over a pressure range of 10-100 mTorr, and power levels of 100 and 200 W. In all cases emission intensity was found to peak in a ring-shaped region at one end of the cylinder adjacent to the location of the antenna, where the rf field is strongest. A local maximum is also observed in most cases near the center of the volume where the electron density peaks [L. J. Mahoney, A. E. Wendt, E. Barrios, and C. J. Richards, J. Appl. Phys. 76, 2041 (1994)].