Area measurement of knife-edge and cylindrical apertures using ultra-low force contact fibre probe on a CMM

Several radiometric and photometric measurements depend on high accuracy area measurement of precision apertures. Some apertures have sharp edges and are generally measured optically. At the Precision Engineering Division of the National Institute of Standards and Technology (NIST), we have developed a contact fibre (fiber) probe for diameter and form measurement of micro-holes (holes of size 100 mu m or larger). This probe exerts extremely small forces, under 5 mu N, and can therefore be used on knife-edge apertures without causing edge damage. We have measured the diameter and roundness of three knife-edge and one cylindrical apertures with this probe. The uncertainty in diameter ranges from 0.06 mu m (k = 1) to 0.17 mu m (k = 1). The uncertainty contributions from the probing system and machine positioning are together only 35 nm (k = 1). The largest contributors to the diameter uncertainty are the overall form (sampling uncertainty) and surface finish (mechanical filtering due to finite probe size) of the aperture.