|A direct density measurement technique was developed for high-velocity, compressible flow fields using acetone planar laser induced fluorescence (PLIF). Motivation for this work was mainly aero-optics. A 4” x 4” transonic wind tunnel (M∞≈0.78) was designed/fabricated to investigate the shear layer and wake region of a small-scale hemisphere. The flow was seeded with acetone vapor and the acetone vapor was excited by a 266 nm laser sheet. Resulting fluorescence was converted to density field data. The centerline was the main area of interest, however other parallel planes of the flow were also investigated. Large density drops and distinct vortex cores were seen in the shear layer and wake region, along with shocks at the point of separation. Schlieren and shadowgraph imaging and PIV experiments were also performed. Aero-optic distortion was calculated for the density field data and connections were made between flow structures, velocity, vorticity, density, and aero-optic distortion.