For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 Calculation of the Whole Number of Fringes Calculation of Block Length from the Observed Data 25 SINGLE WAVELENGTH INTERFEROMETRY John S. Beers The interferometric transfer of the length unit from its defined wavelength to NBS reference standard gage blocks is basic to the gage block calibration program at NBS. The interferometric measurement process using a laser light source and a Kosters type gage block interferometer is described here. Continuous evaluation and refinement of the process is aided by statistical treatment and control chart techniques. All error sources, both random and systematic, are evaluated and the process is maintained in a state of statistical control. Key words: Calibration; gage blocks; interferometry; laser; length; measurement process; uncertainty. Gage block calibration at NBS depends on the interferometric measurements by which the unit of length is transferred from its defined wavelength of light to the NBS gage block reference standards used for the intercomparison process [1, 2]1. Although the interferometric process has undergone changes, the new process does not differ fundamentally from previous gage block interferometry at NBS except for using a laser light source. It employs some new techniques and some techniques successfully used in the past coupled with an analysis program designed to reveal random and systematic errors. The analysis program fosters refinements aimed at reducing these errors. A practical limit is eventually reached in making refinements, but the analysis program is continued to assure the reliability of the measurement process. Briefly, static interferometry is employed to compare NBS gage block reference standards with a laser wavelength. The blocks are wrung to a platen and mounted in an interferometer maintained in a temperature controlled environment. The fringe pattern is photographed and at the same moment those ambient conditions are measured which influence block length and wavelength. A block length computed from these data is a single point in a collection of measurements. By least squares fitting, this collection gives an estimate of process precision, a rate of change of length with time and an accepted value for the correction to the block length at any given time. 1. Figures in brackets indicate the references at the end of this paper. pendicular distance from a gaging point on the top face of the block to the plane surface of identical material and finish wrung to the bottom face. This definition has two advantages. First, specifying a platen identical to the block in material and surface finish minimizes phase shift effects which may occur in interferometry. Second, it duplicates practical usage where blocks of identical material and finish (from the same set) are wrung together to produce a desired length. The NBS gage block reference standards are commercially produced gage blocks and possess no unusual qualities except that they do have a history of calibration from frequent and systematic comparison with wavelengths of light. Although this description is for blocks from 5 to 20 inches long, a similar process is being applied to blocks less than 5 inches in length. A Kosters type gage block interferometer, shown schematically in figure 1, is used. The light beam from laser A passes through polarization isolator B and spatial filter B', is diverged by lens C, converged by lens D through aperture E, where it diverges again to fill the field of the interferometer and is collimated by lens F. Entrance aperture E is at the principle focus of lenses D and F. The collimated beam is directed by constant deviation prism G* to beam splitter H where the beam is divided into two beams of equal intensity by the semi-reflecting upper surface. One beam (the measuring beam) continues through to gage block surface K, and optical flat L, and the other beam (reference beam) is reflected through compensating plate I to plane mirror J. Now the beams are reflected by mirror J, gage block surface K, and optical flat L, and are recombined at the beamsplitter. The remaining portions of these beams (some light is lost at each reflection) are visible through lens M and aperture N as an interference pattern created by the phase relationships of the wavefronts in the recombined beams. Interference in this instrument is most readily explained by assuming that an image of reference mirror J is formed at J' by the beamsplitA small angle, controlled by adjustment screws on table P, between the image and the gage block-platen combination creates a Fizeau fringe pattern. When this wedge angle is properly adjusted for reading a gage block length, the interference pattern will appear as in figure 2. Table P has its center offset from the optical axis and is rotatable by remote control. Several blocks can be wrung to individual small platens or a single large platen and placed on the table to be moved, one at a time, into the interferometer axis for measurement. This prism separates the spectral lines of quasi-monochromatic light sources such as Hg198 for multiple wavelength interferometry. |