Roughness as distribution along a random straight line drawn on the examined surface is depicted in Fig. 3. Taking the normal surface at the median value, roughness of PI needle felt is 240–0+120 μm, and that of membrane needle felt is 45–0+20 μm. Surface roughness of heat treated needle felts is higher than that of membrane coated needle felt. Relatively deeper pores exist on PI needle felt resulting in a skewed distribution. Roughness of PPS needle felt is closer to that of PI needle felt which is not surprising because both are heat treated on dust side.