Precise Length Measurement Research Articles

Non-linearity in length measurement using heterodyne laser Michelson interferometry

Heterodyne laser Michelson interferometry is widely used in precision length measurement but the accuracy is often limited by the unknown size of the periodic non-linearity due to imperfect separation of the two optical frequencies. A method for measuring this non-linearity with an accuracy of about 0.15 nm using pressure scanning of the interferometer is described and results are given for three configurations of a commercial measuring system.

Modifications to the AHRTAG child length measurer. Appropriate Health Resources and Technology Action Group.

Great accuracy is needed in measuring a childs length both for individual assessment of growth and for studies of groups to monitor their nutritional status. A small error in length measurement has greater effect than a similar error in weight. This is because a child in its 1st 2 years puts on a much greater % of weight than length. A precise length measurement is also more difficult to obtain because of the crucial element of the childs posture. Good technique is of paramount importance and the more accurate the equipment the better. Working with a carpenter it was found that simple changes could be made on the basic AHRTAG (Appropriate Health Resources and Technology Action Group) design which improved precision. These changes also made for greater durability and a more readily transportable measurer. The degree of precision achieved with the modified measurer was demonstrably better than for the original form and is acceptable for field conditions compared with standards achieved elsewhere. Plans are provided for local construction of the measurer.

Volt balance realization of the volt at ETF in Zagreb

A volt balance having plate electrodes operating in a temperature-controlled atmosphere of dry nitrogen is described. The voltage is measured by a special procedure of substitution of a weight for the electrostatic force, with automatic control of the balance equilibrium. This, together with the application of very sensitive capacitive transducers for measuring the balance equilibrium and electrode displacements enables a weighing reproducibility of less than 0.1 ppm to be achieved. The earthed electrodes are suspended over the beam arms by means of two thin strips separated one from the other in such a way that optimal sensitivity to the existence of horizontal electrostatic forces is achieved. Through the remote adjustment of the horizontal position of the high-voltage (H-V) electrodes, and by measuring the displacement of the suspended and H-V electrodes, it is possible to eliminate horizontal electrostatic forces. The displacement of the H-V electrodes by 51 mm (corresponding to a capacitance change of 100 pF) enables highly precise capacitance and length measurements to be made. The third improved version of the Zagreb Faculty of Electrical Engineering (ETF) volt balances is now being completed. The mean result of six measurements obtained with two earlier balances indicates that the difference between the International System (SI) volt determined by the ETF volt balances and volt BI-76 is +7.1 ppm with a random one-standard-deviation (1σ) uncertainty of 0.4 ppm and a systematic 1σ uncertainty of 1.6 ppm.

Fresnel-Beugung für die Präzisions-Längenmeßtechnik / Fresnel diffraction for precision length measurement

Article Fresnel-Beugung für die Präzisions-Längenmeßtechnik / Fresnel diffraction for precision length measurement was published on October 1, 1987 in the journal tm - Technisches Messen (volume 54, issue 10).

Precision length measurement by computer-aided coordinate measurement

Gives a brief overview of computer-aided coordinate measurement as a method for testing the geometry of solid workpieces or components in mechanical engineering and similar fields. In contrast to conventional workpiece testing equipment, one can measure most kinds of workpiece with the coordinate measuring instrument as the only universal inspection centre. The principle is to measure a sample of surface points as space point coordinates, to evaluate them by means of mathematical models and computer software and to compute the desired sizes by using the computer's internal digital model of the workpiece. A brief description of the various measuring instruments, the best-fit evaluation algorithms according to Gauss and Tsebyshev, the uncertainty and the software for the new workpiece inspection method oriented on flexible manufacturing systems are given.

A history of the International Association for Dental Research Wilmer Souder Award in Dental Materials, with a short biography of Wilmer Souder.

Abstract In the 39 years that Dr. Wilmer Souder was associated with the National Bureau of Standards, he accumulated many awards in several disciplines. The principal areas of his recognition, criminology and dental materials research, were founded in his interest in and capability of making precise length measurements. In 1919, the army dental corps asked the Bureau to assist in formulating a specification for the purchase of alloys for dental amalgam. Dr. Souder agreed to take on the assignment. From that beginning the Dental Materials Section grew to encompass all dental materials and some equipment. His dedication to precision in all of this endeavors led to Dr. Souder's being recognized as the “Father of Dental Materials Research”. He became president of the International Association for Dental Research and an honorary member of the American College of Dentists and the American Dental Association. A short biography covers his early years and his education in physics at the University of Chicago. It also includes his many contributions to dental materials research and some of his many successes in criminology.

Swelling of polyacetylene when doped by iodine or sodium

Measurement of swelling of cis polyacetylene during doping with iodine or sodium, and the relative compensations, have been carried out by an original technique. The use of a tensile micromachine has allowed in situ conductivity and precise length measurements. A linear dependence of volume with iodine concentration per carbon atom y I , is observed over a wide range of concentration (5% > y I > 13%). A 1% y I increase causes a 3% volume expansion. This value is very close to the one deduced from the structural model of Baughman et al. Similar results are obtained with sodium doping. During doping, compensation, and redoping, a continuous volume increase is observed, up to a 40% augmentation. Thus the volume effects of the dopants in the sample are cumulative.

An unmodulated bi-directional fringe-counting interferometer system for measuring displacement

A bi-directional fringe-counting Michelson interferometer is described that is used in conjunction with a frequency-stabilised laser for precise length measurement. The two counting signals, in phase quadrature and sinusoidally related to path difference, are produced by a novel system that does not employ any form of modulation. Under optimum conditions, the instrument is capable of measuring displacement to a precision of 0.1 μm and the correct fringe-count is maintained even with a signal attenuation corresponding to a 99% loss of intensity in one arm of the interferometer. This performance can be further improved by the application of an automatic gain control system. The interferometer is simple in concept, cheap to produce and easy to set up and align.

Electron microscopy of niobium and tantalum hydrides

Nb, Ta and V are prototype substances for the study of the endothermic reactions of H with metals. Such metal-hydrogen reactions have gained increased importance due to the application of metal-hydrides in hydrogen- und heat storage devices. Electron microscopy and diffraction were demonstrated to be excellent methods in the study of hydride morphologies and structures (1). - Figures 1 and 2 show the NbH and TaH phase diagrams (2,3,4). EM techniques have contributed substantially to the elucidation of the structures and domain configurations of phases β, ζ and ε (1,4). Precision length measurement techniques of distances in reciprocal space (5) recently led to a detailed understanding of the distortions of the unit cells of phases ζ and ε (4). In the same work (4) the existence of the new phase η was shown. It is stable near -68 °C. The sequence of transitions is thus below 70 %.

Über den Einfluß der Beugung auf die interferentielle Längenmessung

In interferometry ideal plane or spherical waves are supposed to propagate between the reflecting surfaces. Diffraction effects however, due to the finite diameter of the beams, cause distortions of the wavefronts, changing fringe position and contrast of the interference pattern. Corrections for length measurements with a two beam interferometer have been calculated. If for example the wavelength of a He-Ne-laser, stabilized by saturated absorption of methane at 3.39 μm, is determined by a precision length measurement in a Michelson interferometer with a beam diameter of 25 mm and a path difference of 400 mm a correction of Δλ/λ = 5.10-9 was found.

New Devices for Precise Angle and Length Measurement Using Special Capacitors and Voltage Dividers

The angle and length transducers ("Circap" and "Lincap") utilize a circular or linear capacitor which consists of four rings or four cylinders, respectively, divided into two segments. A rotating or laterally moving shield increases the capacitance in one segment and decreases it in the other. The position of the shield is indicated by the voltage drop across the unshielded parts of these segments. This voltage is measured in an ac excited bridge using a precision inductive voltage divider. Prototypes of the Circap and Lincap have been constructed and tested. Results show that angle can be measured with a standard deviation of about 0.1" and length with a standard deviation of about 0.2 μm.

Precise length measurement technique under hydrostatic pressure: Isothermal bulk modulus of PMMA

An ultrasonic technique for precise measurement of sample length changes under pressure has been developed. The sample is immersed in mercury, and the length of the mercury column below the sample is measured by an ultrasonic interferometric method to give the length change of the sample relative to the steel container. The length change has been measured and the bulk modulus obtained for PMMA (polymethylmethacrylate) up to a pressure of 10 kilobars. An error analysis indicated that the principal sources of error are temperature changes during the experiment, and nonreproducible acoustic coupling effects at the mercury boundaries.

The Role of Interferometry in Long Distance Measurement

This paper reviews various techniques of very precise distance measurement and in this context presents the possibilities of optical interferometry. It is concluded that the latter will play the primary role for highly precise measurements of lengths up to 100 m and well beyond that in special cases.

A new theorem in electrostatics with applications to calculable standards of capacitance

The paper presents what is believed to be a new theorem in electrostatics, showing that the direct capacitance of a very general type of cylindrical 3-terminal capacitor is a constant, namely (log? 2)/4?2. The presentation and proof of this theorem is followed by a detailed analysis of a cylindrical 3-terminal capacitor of rectangular cross-section, in which the effects of various practical limitations are evaluated in quantitative terms. The results of similar analyses of cylindrical 3-terminal capacitors of other cross-sections are stated. As a consequence of this work, it appears that such cylindrical 3-terminal capacitors are extremely attractive as precise calculable standards of capacitance, only one precision length measurement being necessary to enable their capacitance to be computed to a high order of accuracy.

Atomic Length Standards

Although the world's official primary standard of length is still the one‐meter distance between two lines on a metal bar, practically all precise measurements of length requiring accuracies greater than one part in ten million are made and will continue to be made with light waves. Only recently, however, has it become possible for most research organizations to have an ultimate primary standard of length in their own laboratories. In March, the National Bureau of Standards and the Atomic Energy Commission issued an announcement stating the availability to science and industry of primary standards of length in the form of spectroscopic lamps containing a single pure isotope of mercury, thus enabling any laboratory possessing the necessary auxiliary optical equipment to become its own bureau of length standards. The mercury is sealed in a small glass tube and when subjected to high frequency radio waves it emits light waves with very sharply defined wavelengths uniquely characteristic of the mercury, which, with the help of interferometric techniques, can give highly precise length measurements.