any plane parallel to and intersecting the longitudinal axis of the pore are straight lines for at least 50 percent and typically preferably at least 75 percent of the length of the regions of intersection these membranes may be made with pores having an average 34 characteristic dimension 34 of desired size as used herein 34 characteristic dimension 34 means the largest cross sectional dimension of the pore in a plane perpendicular to the longitudinal axis of the pore two or more arrays of pores having different properties of size shape orientation or distribution throughout the membrane also may be utilized an advantage of using the membranes described herein is that the membrane may have within a given array of pores a pore size distribution of less than about 10 percent and sometimes less than 5 percent as used herein 34 pore size distribution 34 means the standard deviation of the average characteristic dimension the membranes of the invention may be made with pores having a variety of shapes i e cross sectional profile of the pore as the region of a plane perpendicular to longitudinal axis of the pores bounded by the pore walls for instance by selection of a suitable mask pores may have a cross sectional profile that is circular ovate square rectangular triangular some other polygon or irregular shapes the orifices or pores may also be provided in a polka dotted pattern with relatively large areas of densely.