NEW WEB RESOURCES FOR DIMENSIONAL METROLOGY.

NIST has launched two new web pages (http://patapsco.nist.gov/mel/div821/) that perform calculations commonly required by engineers and metrologists who are involved in ultrahigh accuracy length measurement. One web page calculates the index of refraction of air, which is needed to determine the wavelength of laser light in air. (This wavelength serves as the basic unit of length when performing high-precision distance measurements based on interferometry.) The second web page calculates the magnitude of elastic deformation of surfaces in contact under force. This deformation must be taken into account in all high-accuracy length measurements when surfaces are probed mechanically.

The refractive index of air is computed from atmospheric conditions (air temperature, pressure, and humidity) using either the Edlen or Ciddor equations (the two equations most commonly used to determine air refractive index). The usual version of the Edlen equation was modified to make it more accurate for non-laboratory conditions, such as high air temperature and humidity as might be encountered on the shop floor.

The deformation calculation web page will allow a user to correct for elastic deformation at the point of contact between a mechanical probe and part. This correction is essential when submicrometer accuracy is desired. Currently the web page performs the computation for the seven most common types of contact geometry (for example, a sphere in contact with a plane or two crossed cylinders). Additional geometric cases will be added in the future.