Comparison of some instruments for measuring mirrorlike surfaces

Surface roughness measurements of pitch-polished surfaces by surface profilometers that have nanometer-order resolution are reported. The specimens used are made of fused quartz or silicon wafer. Two different polishing conditions are adopted. The four specimens obtained are measured by means of surface profilometers employing four stylus methods and four optical methods. In addition, the effects of stylus tip radius, measuring pressure and so on are examined. The specimens are adequate for surface roughness standards because of their flatness and their insensitivity to stylus tip radius or measuring pressure. In the eight measurements, the values of the surface roughness are scattered. The values obtained by optical methods are apt to be bigger than those obtained by stylus methods. The optical methods are inappropriate for measurements on transparent specimens. Ra or RRMS are more suitable for representing the surface roughness of ultraprecision machined surfaces than Rmax. The estimate that the value of Rmax is 10 times Ra applies well.