The optical filters on board the JET-X telescope comprise thin foils of aluminum coated Lexan. During ground calibration of the filters, narrow spectral regions of high UV leakage, with peak levels of up to a few percent, were observed in broad band optical measurements in the 1000 to 10,000 angstrom range. Furthermore, transmission values were typically up to two orders of magnitude higher than calculated for the aluminum thickness. Investigation showed that these effects were attributed to a combination of aluminum oxidation, which reduces the opacity, and the use of a double sided aluminum layer in the filter design which behaves as a Fabry-Perot interference filter. These effects were verified by a multi- layer model of the filter UV response. Recent redesign of the filters for the flight program eliminated the UV leakage by adopting a single aluminum layer configuration, thus eliminating interference effects, and increasing the thickness by 30% to compensate for oxidation levels. The integrated x- ray transmission below 1 keV was found to be only reduced by 3%. In parallel with the production of the new Lexan flight filters, a set of qualification model filters was produced by the Luxel Corporation in the USA. These filters use polyimide as a substrate material which has the advantage that it is optically opaque to wavelengths below 3000 angstroms, unlike Lexan which is transparent. These new filters were found to have superior mechanical strength, being able to survive extended qualification vibration without any visible degradation in performance, and had a higher cosmetic quality and attenuation levels. As a result, these filters have now been included in the JET-X flight program. We report on the optical tests results from both Lexan and polyimide filters along with high resolution x-ray transmission results carried out at the BESSY synchrotron facility in Germany. Results of the mapping of the filter edge structures, global transmission values and uniformity are presented.