The AXAF CCD imaging spectrometer (ACIS) consists of ten CCDs arranged in two arrays, one for imaging astronomical fields and one to be used in conjunction with transmission gratings for spectroscopy of astrophysical sources. ACIS uses Lexan/aluminum meshless films placed above these two CCD arrays to filter by mapping their soft x-ray transmission on fine spatial scales, so that the filter response can be removed from the CCD data and a more accurate estimate of the true sky recovered. We measured engineering and flight versions of these filters at the University of Wisconsin Synchrotron Radiation Center between June 1995 and April 1996. For all data, better than one percent accuracy in transmission as a function of energy was maintained over the entire filter area. The resulting transmission maps reveal spatial non-uniformities in the filters of about 0.5 percent to 2 percent. These transmission maps provide the finest spatial calibration ever achieved on such filters.