Length and Dimensional Measurements at NIST.

This paper discusses the past, present, and future of length and dimensional measurements at NIST. It covers the evolution of the SI unit of length through its three definitions and the evolution of NBS-NIST dimensional measurement from early linescales and gage blocks to a future of atom-based dimensional standards. Current capabilities include dimensional measurements over a range of fourteen orders of magnitude. Uncertainties of measurements on different types of material artifacts range down to 7×10(-8) m at 1 m and 8 picometers (pm) at 300 pm. Current work deals with a broad range of areas of dimensional metrology. These include: large-scale coordinate systems; complex form; microform; surface finish; two-dimensional grids; optical, scanning-electron, atomic-force, and scanning-tunneling microscopies; atomic-scale displacement; and atom-based artifacts.

Sizing of individual optically levitated evaporating droplets by measurement of resonances in the polarization ratio.

Resonances observed in the polarization ratio of light scattered at 90 degrees from single optically levitated evaporating droplets are shown to provide a means for continuous high-resolution monitoring of droplet size. Due to the distinctive character of the individual features in the polarization ratio, each experimentally measured feature could be clearly identified with a specific calculated one. For evaporating droplets of glycerol from 6.6 to 11.5 microm in diameter, the sharp features which appeared in the calculations at ~0.03-microm intervals allowed measurement of droplet diameter to a resolution of 0.003 microm.

Inverse-fourth apparatus for photometric calibrations.

A new photometric device is described which can perform direct, as opposed to comparison, measurements of optical transmittance, without bootstrapping, over a range of nearly six orders of magnitude. According to the demonstrated principle of operation, the amount of light flux received at a detector is proportional to the inverse-fourth power of the effective source-to-detector distance. As the device is configured, the adjacent source and detector are stationary while a flexible light guide, which collects light from the source and re-emits it toward the detector, moves on a carriage. The transmittance, T, of a filter is measured in terms of the ratio of carriage positions, for constant detector output, with and without the filter in the light path; the optical density is then D=-logT. The position of the carriage, relative to the source-detector plane, is measured over the 3 m range of travel with a resolution of 0.01 mm. Operated with a 1000 W tungsten-halogen lamp as a source, a photomultiplier as a detector, and either an integrating sphere or opal glass for diffuse collection at the detection plane, the system is designed to attain an overall accuracy of +/-0.5% for ANSI PH2.19 diffuse transmission densities up to 6.0 density units, corresponding to +/-0.1% for transmittances above 0.5000 transmittance units and to +/-3% for transmittances between 0.000001 and 0.0001 transmittance units.