Bombardment of thin (1–10 μ) single crystal targets with energetic ion beams has been found to result in macroscopic distortion of the thin film in the bombarded region. This effect, which has been euphemistically termed a ‘dimple’, is readily observed with the naked eye even at relatively low particle fluence. A useful first-order model has been developed which interprets the dimpling as an expansion of the bombarded region. For very thin samples, this expansion can be accommodated by bowing of the crystal out of the original crystal plane. For this simple model, the fractional expansion is proportional to (δ/d)2where δ is the maximum displacement from the original crystal plane anddis the diameter of the bombarded area. This measurement allows expansioh to be determined with a sensitivity comparable to or better than the most sensitive existing methods.