Frustrated total reflection of light was investigated through the interaction of a steel ball and a plane glass surface. The technique has a theoretical sensitivity of 20 billionths of an inch, and may provide an improved method for measuring wringing-film thickness in gage block calibration (page 22).

The resulting charts will be suited for a variety of scientific and industrial uses. For example, they will serve for approximate color specification wherever the ISCC-NBS color designations are applicable, namely, in descriptions of drugs and chemicals, in qualitative chemical analysis, in dermatology, and in descriptions of mica, building materials, soils, and rocks. They will also form the basis for statistical studies of trends in industrial color usage, and they may be useful in planning lines of merchandise having coordinated colors.

Artificial Daylight Standard.

Since 1931 the standard artificial daylight for color measurements in the laboratory has been based by international agreement on two-cell, liquid filters developed at the Bureau. During the past year a cooperative study was carried out with Corning Glass Works in which a three-component glass filter was developed for converting incandescentlamp light into a closer duplication of the spectral character of natural daylight than has previously been possible. These filters may form the basis of a new international agreement on standard sources for colorimetry. In the meantime they can be used as superior color-temperature-altering filters by science and industry.

Color-Rendering Index Developed. The widespread acceptance of fluorescent lamps of high luminous efficacy poses the problem of how closely object colors are rendered in their natural colors by these sources. Since 1952 joint effort has been made with the Illuminating Engineering Society to solve this problem. During the year a tentative method for specifying a color-rendering index was developed and validated for use when the chro

maticity of the light source to be tested is closely identical to that of the standard against which it is to be compared.

This tentative method has been accepted by the Committee on Color Rendition of the International Commission on Illumination as one of two closely similar methods on which the future work of that committee is to be based. This is one step toward international agreement regarding methods of appraising the merit of the various fluorescent lamps available in world markets.

Specular Reflectance Standard.

In cooperation with the Bureau's enameled metals laboratory and the Army Engineer Research and Development Laboratory, a spectral directional reflectance study was made of a number of metals and evaporated metal films on glass. The work was undertaken to find a suitable standard of specular reflectance in the ultraviolet, visible, and near infrared spectra. It was found that a deposit of rhodium on glass gave the best promise of being a permanent standard. This tentative standard was used in a cooperative test for the University of Wisconsin, in their work on the solar effect on soils.

Color Scale for Vegetable Oils. For many years Lovibond red glasses have been calibrated for use in measuring the colors of vegetable oils (cottonseed, peanut, palm) for commercial evaluation. These calibrations have been based on a Bureau scale (Priest-Gibson) set up in 1927. As a result of negotiations between the American Oil Chemists Society and the makers of Lovibond glasses, and based on NBS color measurements, an AOCS color scale has been established for the vegetable-oil industry in this country. It is anticipated that the manufacturer will be able to supply working standards of color to the American vegetable-oil industry which will agree with current practice and that future Bureau calibrations will not be required.

Refractive Indices Provided. The critical components of most optical instruments are the lenses, prisms, and windows. Designers of infrared and ultraviolet devices must have accurate values of refractive index of all available transparent materials to select optimum materials and designs for such components. In a continuing program to provide such information, the index of refraction of a natural and a synthetic prism of calcium fluoride was measured over a wide range of wavelengths (0.23 to 10μ) at several temperatures. The refractive index of six experimental infrared glasses developed at NBS was also determined. Various components of a vacuum monochromator system, used in extending the measurement of refractive index and other optical properties of transparent materials into the short wavelength region of the spectrum, were installed and tested.

Image Analysis. The evaluation of imagery concept was extended to include the measurement of lens resolution in terms of frequency response using either sine wave or square wave targets. In this method, the aerial image of an infinitely distant target is scanned by a slit and photocell to read out variations in image intensity. The lens is then treated as a low pass filter of spatial frequencies and response is determined by comparing the calibrated with the modulated image.

Ray-Tracing Equations Developed. In the last decade a new principle in optical design, called common path interference, has been introduced and applied to interference microscopes and lens testing interferometers. These devices have double-focus lenses made of uniaxial crystals that divide a beam of light into ordinary and extraordinary rays. The design of a common path interference device is based on the difference in refraction of these two rays. However, because simple ray-tracing equations for the extraordinary ray were not available, few double-focus lenses have been designed.

The Bureau therefore developed equations which are not much more difficult to apply than are those employed for skew rays. Moreover, they make use of data from an ordinary ray-tracing program. These equations are derived from the purely geometric point of view. They presuppose a knowledge of the ordinary ray, obtainable from ordinary ray-tracing procedures together with the normal to the refracting surface. In the final derivation both Huygen's principle and the ellipsoidal indicatrix for a uniaxial crystal are employed.

Interference Microscope Techniques. In recent measurements of very fine surface finishes on prepared steel surfaces, it was found that twobeam interference microscopy did not provide sufficient resolution to distinguish small differences. Hence, commercial metallurgical microscope components were employed to produce multiple-beam interference and achieve the desired resolution. These components consist of cover glass slides, coated for maximum reflectivity on one side with zinc sulfide and coated on the other side for minimum reflectivity with cryolite. The technique was applied to the study of surface finishes on spherical surfaces of 0.5inch radius and on cylindrical surfaces of 0.002-inch radius.

Absolute Testing of Wavefront Shapes. A method was developed for making absolute tests by interferometry. The process compares (1) an unknown wavefront with a sheared image of itself, or (2) one part of a wavefront with one or more different parts of the same wavefront, or (3) different parts of one wavefront with another unknown wavefront. A unique solution is then obtainable by combining simple mathematical operations. This manner of compounding interferometry with mathematical operations eliminates the need for reference standards and thus improves the accuracy of the results obtained. The process has been tested and reports have been prepared on the absolute testing of wavefront shapes that are characteristic of aberrations of lenses and lens systems (entire optical imaging forming units); shapes of optical mirrors; and image quality of simple or compound optical systems.

Calibration of Crash Flight Record. Jet aircraft are required to carry an automatic flight recorder which makes a permanent graph record of such parameters as air speed, altitude, azimuth, and acceleration as a function of time. One of these recorders, retrieved from the crash of an aircraft in New York in December 1960, was submitted to the Bureau for calibration of the record. Such a calibration consists of measuring the co

Experimental one-pan balance designed to investigate the limitations of such an apparatus in experiments where the maximum attainable level of precision is demanded-work with the National Standard Kilogram, for example (page 22).

ordinates at numerous positions of traces made by diamond stylii on a metal foil capable of retaining the record and of maintaining its mechanical strength after exposure to fire, shock, and salt water immersion. The Bureau succeeded in extracting the record from the battered recorder, removing the heavy carbon and polymer deposits from the portion of the foil showing the latest recording from Chicago to New York, measuring the coordinates of the traces, and with the help of the manufacturers and the Civil Aeronautics Board, interpreting the measurements as quantitative and correlated values of the parameters which they represented.

Photographic Density Measurements. Fine photography in science, industry, and art largely depends on the photographic effect on a film of a given exposure. To determine the optical density of these films, photo

graphic step tablets calibrated at the Bureau are made available for calibrating the transmission densitometers used to measure optical density. During the past year, the apparatus and method used for calibrating step tablets were refined so that the uncertainty of measurements previously ranging from 0.02 to 0.09 on the density scale were reduced to 0.01.

2.1.2. MECHANICS

The Bureau's work in mechanics is primarily in the development and improvement of methods of measurement of mechanical phenomena in solids, liquids, and gases; the establishment of required standards in mechanics and the relation of such standards to the prototype standards; the support of these activities by theoretical and experimental researchers into mechanical phenomena; the determination of physical constants of particular importance in mechanics; and provision of assistance to other laboratories in relating their measurements to a common basis (or to established standards) by transfer standards, calibration services, and other means. Measurement areas include sound pressure and intensity, shock, vibration, force, strain, pressure, vacuum, viscosity, and rate of gas and liquid flow.

These measurement areas are of vital importance in the missile and space programs, which require great accuracies over widely extended ranges under extreme temperature environments. Special emphasis therefore is given to research directed toward meeting these needs.

Because of the increasing requirements for measuring mechanical quantities in defense industries and in government laboratories, and because of the requirements of missile and space projects, requests for calibration services continued to increase. For example, during the year, more devices for measuring force and flow were calibrated than in any previous year.

Measurement of Vibration Amplitudes. The calibration of vibration pickups, used for measuring vibrations in machines, missiles, satellites, and aircraft, can now be accomplished by means of a recently developed photometric system. The amplitude of vibration of one plate of a Fizeau optical interferometer is deduced from photometric measurements on the interference pattern. This new technique was used to calibrate pickups over the audiofrequency range and amplitude range 72-4400 Angstroms (0.317.3 microinches), with estimated errors no greater than 2 percent. Vibration amplitudes as small as 5 A (0.02 μin.) can be measured with uncertainties no greater than 10 percent.

Calibration of Microphones. Condenser microphones calibrated by the reciprocity technique serve as the basic standard instruments for measurement of sound pressure over a wide range of frequencies. A simple method was developed for measuring the relative response of a microphone by means of carrier-frequency circuits. First of all, diaphragm motion is brought about by voltage of various frequencies applied to the microphone. Then the carrier-frequency measurement yields relative response over the frequency