Bryon S. Faust

Uncertainties in Small-Angle Measurement Systems Used to Calibrate Angle Artifacts

We have studied a number of effects that can give rise to errors in small-angle measurement systems when they are used to calibrate artifacts such as optical polygons. Of these sources of uncertainty, the most difficult to quantify are errors associated with the measurement of imperfect, non-flat faces of the artifact, causing the instrument to misinterpret the average orientation of the surface. In an attempt to shed some light on these errors, we have compared autocollimator measurements to angle measurements made with a Fizeau phase-shifting interferometer. These two instruments have very different operating principles and implement different definitions of the orientation of a surface, but (surprisingly) we have not yet seen any clear differences between results obtained with the autocollimator and with the interferometer. The interferometer is in some respects an attractive alternative to an autocollimator for small-angle measurement; it implements an unambiguous and robust definition of surface orientation in terms of the tilt of a best-fit plane, and it is easier to quantify likely errors of the interferometer measurements than to evaluate autocollimator uncertainty.

Minimizing error sources in gauge block mechanical comparison measurements

Error sources in gage block mechanical comparisons can range from classical textbook examples to a completely counter- intuitive example of diamond probe tip wear at low applied force. Fortunately, there are methods available to metrologists that can successfully be applied to minimize these and other effects. Techniques such as statistical process control, use of check standards, thermal drift eliminating measurement algorithms, improved sensor calibration, and well-tested deformation modeling are used at the National Institute of Standards and Technology to minimize errors. These same methods can be applied by anyone making mechanical comparison gage block measurements.

Minimizing errors in phase change correction measurements for gauge blocks using a spherical contact technique

One of the most elusive measurement elements in gage block interferometry is the correction for the phase change on reflection. Techniques used to quantify this correction have improved over the year, but the measurement uncertainty has remained relatively constant because some error sources have proven historically difficult to reduce. The precision engineering division at the National Institute of Standards and Technology has recently developed a measurement technique that can quantify the phase change on reflection correction directly for individual gage blocks and eliminates some of the fundamental problems with historical measurement methods. Since only the top surface of the gage block is used in the measurement, wringing film inconsistencies are eliminated with this technique thereby drastically reducing the measurement uncertainty for the correction. However, block geometry and thermal issues still exist. This paper will describe the methods used to minimize the measurement uncertainty of the phase change on reflection evaluation using a spherical contact technique. The work focuses on gage block surface topography and drift eliminating algorithms for the data collection. The extrapolation of the data to an undeformed condition and the failure of these curves to follow theoretical estimates are also discussed. The wavelength dependence of the correction was directly measured for different gage block materials and manufacturers and the data will be presented.

Case against optical gauge block metrology

The current definition of the length of a gage block is a very clever attempt to evade the systematic errors associated with the wringing layer thickness and optical phase corrections. In practice, most laboratories wring to quartz or fused silica reference plates, and in addition there are very large systematic operator and surface effects. We present quantitative data on these effects and how that the current definition of gage block length is a primary source of measurement uncertainty.

New Capabilities at NIST In Dimensional Metrology

Abstract: A number of new or revised services in dimensional metrology are presented. Included are: a lower cost, high accuracy calibration for sphere diameter; reduced uncertainty in roundness calibration; a new instrument for measurement of the thermal expansion coefficient of gauges and other material, extended capabilities of ring gauge calibration on the M48 CMM, and a system for in-situ calibration of deformation of gauge blocks in mechanical comparison.