Moire fringe method for the measurement of distortions of hot-embossed polymeric substrates

We present a way to identify distortions of transparent micro-patterned substrates using a desktop document scanner and a set of image processing routines. The method requires neither expensive optical equipment nor precise positioning of the part. It is therefore ideally suited to rapid process monitoring. A 5000-dpi imagesetter is used to print a square reference grid of lines having a known pitch ~100 μm. This reference pattern is used to produce a hard stamp that is subsequently embossed into a sheet of thermoplastic polymer. The pattern transferred to the polymer may include distortions resulting from contraction of the sheet after separation from the stamp. To measure these distortions, the embossed polymeric part is placed on a document scanner. The reference grid is laid on top of the part and rotated by hand until moire fringes are seen. At least two scans are made, each with a different relative reference-part rotation. For each scan, the orientation of the part relative to the reference grid may be arbitrarily chosen within an allowable range of a few degrees. These orientations may be extracted from the captured images, and, together with the moire fringes' orientations and spacings, provide enough information to obtain the part's distortions. Estimates of part strains may be improved, at the expense of measurement time, by capturing more images. The approach can detect isotropic shrinkage of the part with a strain resolution ~10-3.