achieved. This was accomplished by experimental and theoretical verification of the magnitude of an important error, usually referred to as the "rf-de" substitution error, which must be accounted for in bolometric measurements. The reduction in the rf-dc substitution error by 0.7 percent permits an equivalent improvement in several calibration accuracies applied to both waveguide and coaxial bolometric powermeasuring devices.

Reflection Coefficient Measurements.-Microwave calibration systems were developed for the measurement of reflection coefficient magnitudes in WR284 waveguide (2.6-3.95 GHz) and WR42 waveguide (18.0-26.5 GHz) using a modified reflectometer method of measurement. The development of the WR284 calibration system required the construction of an unusually large section of precision waveguide. Weighing more than 300 pounds, this component was constructed in NBS machine shops with specially-developed techniques and tolerances on the internal dimensions better than 100 microinches throughout its 24-inch length.

Phase Shift Calibration System.—A microwave phase shift calibration system has been developed for measurements on variable phase shifters in WR90 waveguide (8.2–12.4 GHz) utilizing a modulated subcarrier method of measurement. The estimated limits of the total measurement error for this new service are reported as approximately ±0.25 degree including the error contribution from the reflection coefficient magnitude of the phase shifter being calibrated.

Electromagnetic Field Standards

Field-Strength Calibration.-A calibration service for field-strength and other precision receivers has been developed for the 1 to 10 GHz range. This new service includes calibration of the receiver as a twoterminal rf voltmeter, calibration of the signal attenuators, and calibration of the overall linearity of the receiver. The voltage measurements are made in terms of a coaxial power standard and are valid for rms CW measurements only. The attenuation and linearity measurements are made in terms of calibrated rotary vane attenuators. The calibration uncertainties vary from 2 to 10 percent, depending on frequency.

Antenna Field Measurements.-Significant progress has been made in the development of techniques for computing antenna characteristics (absolute gain and patterns) from near-zone data. In one NBS method, the radiation pattern of an antenna is expressed in terms of an angular spectrum of vector plane waves. The spectrum is obtained

from a two-dimensional, spatial Fourier-transform of the transverse components of the complex electric field in an "aperture plane.” The antenna pattern can then be calculated for any distance beyond the aperture plane.

Dielectric and Magnetic Standards

High-Temperature Dielectrometer.—An accurate high-temperature (to 815 °C) microwave dielectric measurement service was activated and compared with previous results of five other laboratories. This service utilizes a measuring system developed in response to needs of industry for refined experimental techniques for evaluating high temperature dielectrics for radio-electronic applications. The technique uses a resonant cavity and a rod specimen.

Millimeter-Wave Dielectrometer. The mm-wave interferometric dielectric measuring system was further evaluated and improved. Accuracy, defined in terms of "goodness" of fit of the theoretical to the experimental interference curve, is now of the order of 1/2 percent for the real part and 1 percent for the imaginary component.

Survey of Dielectric Measurements.-A critical survey and literature review of dielectric and magnetic measurements was undertaken from the viewpoint of radio-electronic and communications applications. Of special interest was the way in which these measurements depend upon the state of development of basic radio frequency-attenuation, impedance, etc.—and on how the measurements relate to the geometrical structure of the specimen and the electromagnetic environment. Dimensional gaging and small departures of the system from the form assumed in the ideal boundary value problem are often the limiting factors, rather than the electrical observations.

Narrow Linewidth Measurements.-A measuring service for the linewidth of single-crystal ferrimagnetic materials was activated. A nonresonant shorted waveguide method for the ferrimagnetic resonance linewidth is employed. Measurement of the linewidth was examined in detail by orienting the rf magnetic field relative to the static magnetic field on a given spherical specimen. It was concluded that the unloaded (intrinsic) linewidth can be correctly obtained from the measured linewidth and the magnetic resonator model of the specimen.

Tensor Permeability Measurement. It was demonstrated that a cylindrical TM110 mode cavity is very satisfactory for measuring the tensor permeability of rod samples to as low as 1 GHz. It is possible to obtain good linewidth measurements on cylindrical samples with standard rectangular waveguide techniques normally used in studies of

ferrimagnetic resonance. A previously unreported ferrimagnetic absorption was also observed in this study.

Vibrating-sample Magnetometer.-Saturation magnetization is an important index or "figure of merit” of a ferrite for use in designing nonreciprocal microwave devices. The vibrating-sample magnetometer is a practical and accurate means for determining this quantity. Two of the most widely accepted methods for calibrating such magnetometers were examined in detail and the sources of error delineated. The study included nickel and pure iron as standard reference specimens.

THERMAL QUANTITIES

High-Temperature Heat Content Standard.-Standard materials for high-temperature heat content (enthalpy) measurements are essential for comparing different methods and apparatus and ensuring consistency of tables of standard reference data. The present standard material is aluminum oxide-synthetic sapphire-which melts around 2300 °K and is therefore an unsatisfactory standard material for higher temperatures. As part of a program to develop a standard material for higher temperatures, the enthalpy of tungsten has been measured up to 2700 °K with new apparatus developed at NBS. The precision of these measurements is higher than any previously reported.

Thermal Expansion Measurements at High Temperatures.—To meet the increased demands of high-temperature technology in both industry and defense, apparatus for measuring thermal expansion of solids at temperatures between 800 and 1600 °C has been developed. Length measurements made with this apparatus have an accuracy of 30-50 ppm, and are the most accurate available in this temperature range. Platinum, single crystal aluminum oxide, magnesium oxide, tantalum carbide, and pure tungsten are being investigated as possible reference standards for thermal expansion at high temperatures.

PHOTOMETRIC AND RADIOMETRIC QUANTITIES

Freezing Point Blackbodies as Primary Standards of Special Radiance.-Freezing-point blackbodies having efficiency of better than 0.999, and whose operating temperatures are known to better than one part in 50,000, were constructed and delivered to the Department of Defense Calibration Laboratories for use as primary standards of spectral radiance. Each furnace comprised a cavity, machined from high-purity graphite, immersed in a bath of liquid metal, contained in a crucible of high-purity graphite. As the metal freezes, the walls of the cavity are maintained at a very uniform temperature. High

purity zinc, freezing point 692.745 °K, and tin, freezing point 505.093 °K, were used as the working metals. The crucible-cavity assembly was enclosed inside an insulated resistance heater, equipped with guard heaters to reduce conduction losses from the crucible. The duration of a freeze could be extended to several hours.

Spectral Irradiance Measurements. Two specroradiometers, one based on a conventional prism monochromator and the other on a system employing a set of 36 narrow-band-pass interference filters, have been set up and independently used in the determination of the spectral irradiances of a number of sources commonly used in solar simulation. The wavelength range of measurement was 0.25 μm. The method of measurement for each system consists of comparing the spectral irradiance of the source under investigation to that of an NBS standard of spectral irradiance. The results obtained with each system on a number of continuous sources agree to about 1 percent, whereas the differences in the spectral irradiances obtained with the two setups on a number of line sources range up to several percent.

Spectral Radiance of the Graphite Arc.-Graphite arcs have long been employed as sources of spectral radiance, but their usefulness has been largely limited to wavelengths near 650 nanometers because of large differences or uncertainties in the results at other wavelengths. The spectral radiance of a commercially available low-current graphite arc has now been determined throughout most of the 210 to 850 nanometer region, with a standard deviation uncertainty varying from about 2 percent at the longer wavelength to 5 percent at the short. The investigation involved a direct determination of spectral radiance using a high accuracy spectroradiometer, which includes a stable high temperature blackbody and a photoelectric pyrometer for determining its temperature.

Laser Energy and Power Measurements.-The energy emitted by pulsed ruby lasers at 694.3 nm has been measured at both the Gaithersburg and Boulder NBS laboratories. At the former a calibration system was developed that uses phototubes and is based on the NBS blackbody traceable radiometric standard. This has been used to measure the output of a 1-joule laser. At the Boulder laboratory, calorimeters have been developed for use with lasers, and energies of 1 to 100 joules have been measured. Measurements by the two methods now agree to within 9 percent. Both methods are believed to have considerably higher pecisions-about one percent. Efforts are continuing to refine the techniques, determine an accurate value, and establish instruments that may serve as the basis for a national standard of laser energy.

Laser Beams Measured Using NBS Lamp Standards.-Two helium-neon cw laser beams, one of 0.83 milliwatt and one of 60 milliwatts, have been measured successfully on the setup normally used for the calibration of NBS lamp standards of spectral irradiance. No adverse effects, due to either the monochromaticity or the structured pattern of the laser beam, were present in the results.

Photo- and Cathodoluminescence Studies.-Relative spectral-energy emission characteristics and buildup and decay characteristics have been measured for most of the NBS standard phosphor samples by using ultraviolet excitation at either 253.7 nm or 365.0 nm. The time response of the zinc sulfide type phosphors was found to be a function of the magnitude of the excitation pulse used. A depressed cathode, oil-pumped, demountable system is being established to be used for measuring cathodoluminescence properties of phosphor powders and screens. A study of the coulomb aging mechanism of cathodoluminescence materials is also planned.

Calibration of Fluorescent Lamps.-Fluorescent lamps are now being calibrated for lumen output and spectral power distribution by direct comparsion, wavelength by wavelength, with incandescent lumen and color-temperature standards. This will make possible the issuance of fluorescent-lamp reference standards of known spectral distribution as well as known luminous flux and chromaticity coorinates.

New Low Spherical-Candlepower Standards.-A development has recently been completed on new low spherical-candlepower standards to extend downward the currently available luminous flux range and to supplement or replace the present low spherical-candlepower standard. The new standard are of four wattages, with spherical candlepowers about 0.34, 0.15, 0.05, and 0.03 candela at 5 volts. The standard lamps were developed primarily for the aerospace industry because many lamps of low light output are used in aerospace vehicle cockpit illuminated displays. A need for the new standards has also been expressed by the computer industry.

Artificial Star Source.-At the request of the National Aeronautics and Space Administration, a prototype of a low-intensity, uncollimated light source was developed as a standard for calibrating startracking equipment. Light from this source simulates the illuminance and "color temperature" of a visual second magnitude type AO star. The illuminance from a second magnitude star is 3.1× 10-8 footcandle and the "color temperature" of a type AQ star has been defined as 10,700 °K. This standard has been used to calibrate a group of similar units constructed by NASA for use in the field.