Evaluation of the DQE should be made under well-defined X-ray beam condition in order to assure intercomparison among different facilities. For this purpose, IEC61267 requires the use of high purity (at least 99.9% or 3N) Al attenuation filter, while IEC62220-1-1 requires lower purity of 99.0% (or 2N) filter since high purity metals are prone to kinds of non-uniformities including unexpected NPS increase in lower spatial frequencies <0.32 mm-1. The purpose of this study was to explore a possibility to adopt high purity Al filter without sacrificing NPS degradation in the low frequency region. To this end, we evaluated several types of high purity Al filters with different processing methods: casting, forging and rolling. Since the beam quality of RQA5 requires the use of 21 mm thick Al filter, we compared the following 4 types of 5N purity Al filters with 2N5 purity Al of 21mm thick: A. casting (5N) with 21mm x 1 sheet, B. forging (5N) with 21mm x 1 sheet, C. rolling (5N) with 7mm x 3 sheets, D. rolling (5N) with 1mm x 21 sheets. The comparison was made in terms of the Normalized Noise Power Spectrum (NNPS) exposure product in order to eliminate the effect of exposure variability. As a result, thin rolling sheets (D) showed no meaningful difference with 2N5 Al, though casting and forging sheets showed an observable NPS increase throughout the whole frequency range, suggesting that high purity thin rolled sheets could be used as beam attenuating material without suffering from non-uniformity problem.