While optical spectroscopic measurements using ellipsometry may be made in air and are non-destructive, the relatively large (> 2 mm) spot size has limited their use to surface regions greater than 2 mm in lateral extent. Recent developments in focusing instruments have made spot sizes on the order of 20 to 25 mu m possible. The work to be presented explores the use of the 25 gm, spot size to probe non-uniform nanostructured thin films. Measurements were performed on a highly non-uniform film (0 to 2 mu m in thickness across 4 mm in lateral dimension) using such a 25 Pin spot. Further reduction of the spot size is possible using mechanical masking with a slit. Measurements have been made to the range of a few microns in width. The practical resolution limits of beam masking may be decreased by increasing incident light intensity, improving slit alignment, and improving detection methods. (c) 2006 Elsevier B.V. All rights reserved.