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This capacitor is the one used at NIST today, although with several improvements. In , Cutkosky reported the first SI value of the U. Legal Farad and the U. Legal Ohm [ 92 ] derived from this calculable capacitor. The realization of the ohm from the calculable capacitor at NBS in terms of the SI units of length and time was evaluated to have a relative standard uncertainty of 0. Shields et al. The relative uncertainties were 0. Legal Ohm and 0. Legal Farad. Jeffery et al. The calculable capacitor consists of four vertical cylindrical bars arranged at the corners of a square in the X - Y plane, placed symmetrically about the central Z -axis see Fig.

The Thompson-Lampard equation [ Eq. Diagram of the calculable capacitor electrodes, with one of the four main electrodes shown in cut-away view. Measurement voltages are applied across opposite pairs of main electrodes, and the central blocking electrode can be moved vertically to change the capacitance. The working length of the calculable capacitor is defined by two cylindrical electrodes on the central Z -axis. Except in the space between the ends of these electrodes, the electric field between opposite capacitor bars is completely blocked.

A change in the vertical position of either of these grounded electrodes effectively changes the length associated with C 1 ' and C 2 '. Measurements are made by comparing a fixed-value 10 pF capacitor to the calculable capacitor at two positions of the moveable electrode, where the values are 0. Displacement of the electrode between these two positions yields a difference of 0. By measuring the displacement of the blocking electrode rather than the absolute length of the capacitor, many problems associated with fringing effects at the ends of the capacitor are eliminated.

A Fabry-Perot interferometer measures the relative displacement of optical flats mounted in the moveable and fixed blocking electrodes. The Fabry-Perot interferometer employs a fringe-locking laser optical system. The system is enclosed in a metal case, which is kept under vacuum. This eliminates the need to apply corrections due to the dielectric constant of air and provides a clean environment for the electrodes. The calculable capacitor apparatus is shown in Fig.

From the Fabry-Perot length measurement and Eq. The present relative combined standard uncertainty for this measurement [ 95 ] is 0. The largest relative standard uncertainty in Table 1 is that due to geometric imperfections. This includes the relative alignment of the axes of the bars to each other and to the blocking electrodes, alignment of the electrical axis of the capacitor and the optical axis of the interferometer, and imperfections in the bars.

The magnitude of the uncertainty attributed to geometrical imperfections is the standard deviation of measurements of 0. The blocking electrode end is presently a cylindrical spike, but recent tests by Jeffery [ 96 ] have shown that a modified cone shape cone with a very short cylindrical spike could reduce the effect of geometric imperfections in the bars. Relative standard uncertainties in the measurement of the 10 pF bank with the calculable capacitor. The last row is the root-sum-square rss of the uncertainties listed in the rows above.

Since , the U. The link between the calculable capacitor and the ohm is made through a sequence of measurements called the calculable capacitor chain. This sequence is shown in Fig. The calculable capacitor measurement chain. Capacitance and resistance standards are represented by red boxes, with QHR representing the quantized Hall resistance. Measurement bridges used to relate the standards to each other are represented by ovals.

The difference in the two results is small but significant, and we believe that the most recent result is more reliable as it is based on a series of measurements and not on one measurement as was the result. Both of the above are in close agreement with recent quantum electrodynamic QED calculations of the anomalous magnetic moment of the electron a e by Kinoshita [ 99 ], which may be combined with an accurate experimental value of a e to derive a. Including all the preliminary calibration measurements, the calculable capacitor realization of the farad requires on the order of 1 month to complete.

A 10 pF reference capacitor, measured against the calculable capacitor, is taken to another laboratory and the unit transferred to a 10 pF reference bank of four fused silica capacitors, which are the representation of the unit between calculable capacitor measurements. The NIST 10 pF capacitor reference bank capacitors are intercompared weekly, along with several other capacitors of the same type, to monitor their behavior over time.

Cutkosky and L. One of the 10 pF fused silica capacitors is shown in Fig. The ICL provides calibrations of nominal-valued capacitors in the range from 0. Customers include aerospace companies, instrumentation companies, the U. The laboratory also provides capacitance calibrations at 1 kHz that are used by the high-frequency calibration laboratories at NIST for their calibrations at frequencies above 1 MHz. The inductance value is found using the Maxwell-Wien bridge [ ], which derives the value of the inductor by comparison against two resistors and a capacitor. Presently the primary laboratory operates only at Hz.

The capacitance unit is transferred from the calculable capacitor to the ICL via fused silica dielectric capacitors whose value has been determined only at Hz. However, the ICL performs customer calibrations at 10 kHz, Hz, Hz, and Hz and the frequency dependence of the fused silica capacitors has a relatively large uncertainty. NIST plans to develop multi-frequency measurement capabilities to better support our customers needs in the frequency range from Hz to 10 kHz.

This will require design and construction of capacitance bridges that work at multiple frequencies as well as an evaluation of the calculable capacitor system at these frequencies. The calculable capacitor is one of the most direct ways to obtain the SI farad and allows the realization of the SI ohm. However, it is a very difficult and resource intensive experiment; thus only a few NMIs in the world have implemented it.

Typical set-up times are from five to ten years including precision machining and detailed evaluation of the system. Furthermore, the present calculable capacitor uncertainty is close to its expected limit due to the macroscopic nature of the experiment. There has been much interest in finding a way to obtain a capacitance unit by other means. An ac determination of impedance based on the QHR is one of these alternatives.


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Since most NMIs already have a dc QHR system for resistance calibrations, it would be very convenient if the quantum Hall effect could be used for impedance determinations as well. If the QHR could be used directly with a quadrature bridge, which relates capacitance to resistance, this chain could be made even shorter. Several national laboratories [ , , , , ] have begun working on ac QHR measurements with the hope that the ac QHR could provide a working unit similar to the internationally agreed upon value of the dc QHR.

Several difficulties have been encountered with the ac QHR measurements, so far producing a much higher limit on the uncertainty than with the dc QHR. This consisted of a constant current source, a potentiometer, and an electronic detector.

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The current source energized the QHE device and a series-connected reference resistor of nominal value equal to the QHR. With the potentiometer balancing out the nominal voltage across either resistance, the detector measured the small voltage difference between the QHE device and reference resistor. Starting in , a QHR laboratory was set up near the resistance calibration laboratory for the routine maintenance of the U.

The QHR device is mounted on a special holder inserted in a 4 He cryostat containing a superconducting magnet and a 3 He refrigeration system. Magnetic fields up to 16 T and temperatures as low as 0. The QHR plateaus that are measured have resistances of Dziuba and R. Elmquist are of the overlapped-tube type [ ] with a commercial SQUID sensor to detect the ampereturn balance condition of the comparator. In order to eliminate leakage currents, the current sources are floating and optically isolated from one another [ ]. A commercial nanovolt detector, D see Fig.

The feedback current is monitored by measuring the voltage drop across R f with an optically isolated digital voltmeter and is a measure of the difference of the resistor corrections. The QHR measurements are carried out only a few times a year, while the scaling measurements are done more frequently to monitor the banks of resistors used for customer calibrations. Since January 1, , the maintenance of the U.

Thomas that are still in use. However, the Thomas resistors need to be operated in a controlled environment oil bath because the curvature of its temperature vs. The resistors are sealed in dry air in the annular space between two coaxial cylinders because the wire is subject to surface oxidation and has a significant pressure coefficient of resistance PCR. Until recently, the Thomas-type resistors were available commercially. In , R. Dziuba and D. The resistance elements of these standards consist of precious-metal-oxide PMO film resistors that are commercially available.

To improve their stability, these PMO film resistors are pre-aged by external heating. These containers are purged with dry nitrogen gas, hermetically sealed, and shock mounted in aluminum enclosures. The shock-mounting technique reduces effects due to transport. This design allows the metallic end fittings to be driven at separate guard potentials nominally equal to the potentials at the resistor terminations and greatly suppresses leakage currents flowing across the glass insulator of the seals.

Significant improvement in stability and the elimination of humidity and pressure effects is achieved by pre-aging and hermetically sealing the PMO film resistors. Hermetically-sealed resistor container assembly with glass-to-metal seals and copper purging tubes soldered to end plates. The best evidence indicates that a Wheatstone bridge manufactured by the Otto Wolff firm in Berlin was used to compare resistance standards during the early years of NIST. It included a method for extending the resistance range by ratios. Wenner designed a bridge for comparing resistors that was built in and continued in service for over 50 years [ ].

It was a combination bridge that could be used as a simple Wheatstone bridge or Kelvin double bridge with a or ratio. This dc current comparator measurement system was automated in and is still in use today for calibrating customer resistors. Scaling to higher resistance decades was achieved through the use of Hamon transfer standards having and ratios [ ]. To achieve low uncertainties, eight measurement systems have been developed that are optimized for the various resistance levels [ ]. Over the years from to , six of the systems, covering the full 19 decades of resistance, have been automated.

The main methods of comparing standard resistors for NIST calibrations utilize direct current comparator DCC bridges and resistance-ratio bridges. For example, Thomas-type resistors five comprising the reference bank, along with two check standards and eight resistors under test are connected in series in the primary circuit of one automated DCC bridge [ ].

The value of an unknown or check standard can be determined by indirectly comparing its voltage drop to the mean of the voltage drops of the reference bank via a stable 0. The relative standard uncertainty of this measurement system is estimated to be 0. The measurement system is based on the War-shawsky bridge, which includes auxiliary or fan resistors at the branch points of the bridge to eliminate first-order errors caused by lead resistances.

The automatic selection of resistors is achieved by a unique, programmable, guarded coaxial-connector panel. A computer-controlled XYZ positioning system shown in Fig. This provides for 18 independent four-terminal channels. The combined relative standard uncertainty of this measurement system is 0. Photograph of the programmable guarded coaxial switching system.

The system is designed to measure the differences among six nominally-equal, four-terminal standard resistors of the Rosa type that are mounted on a stand located in a temperature-controlled oil bath, but it has the flexibility to accommodate resistors operating in the laboratory air environment.

In operation a voltage is applied across opposite corners of the ring A and A ' , which divides the ring into two paralleling branches each containing three resistors. Then, a DVM measures voltages between opposite potential terminals of the resistors V 1 through V 6 for the two directions of current. To complete the sequence of measurements, the applied voltage points are rotated in a clockwise or counterclockwise direction to each other pair of resistor connection points B and B ' and C and C '. Again voltage measurements are taken between corresponding terminals of the resistors that are at nearly equal potentials.

From the three subsets of voltage measurements, one obtains a set of nine linear equations that can be solved using a least-squares technique. Values of the resistors can be calculated if the value of at least one of the resistors in the ring is known. The system is operated with two reference standards, one check standard, and three unknowns. Connection of resistors for measurements by the ring method, showing the three subsets of connections for voltage measurements.

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This innovative bridge differs from the conventional Wheatstone bridge in that two of the ratio arms are replaced by programmable voltage sources. The low output impedances of the voltage sources along with the active guard network reduce errors caused by leakage currents. As shown in Fig. NIST maintains a bank of reference standard resistors at each decade level of resistance. An unknown standard resistor is indirectly compared to a reference bank of the same nominal value using the substitution technique, where the unknown and reference resistors are sequentially substituted in the same position of a bridge circuit.

This technique tends to cancel errors caused by ratio non-linearity, leakage currents, and lead and contact resistances. All measurements are done with the resistors in situ by means of shielded cables between oil baths in the two laboratories. The automated CCC bridge system is similar to those described in Sec.

The main advantage of these transfer standards is that they are calibrated at one resistance level and are then used with equal accuracy at different resistance levels as short-term reference standards. Each junction has two current and two potential terminations, and the four-terminal resistance of each junction is adjusted to be negligible compared to the resistance of a main resistor. The transfer standard can be connected with the ten resistors in a parallel mode using special fixtures.

The major difficulties are with the adjustment and stability of the resistances associated with the tetrahedral junctions and fan resistors. The ratio accuracy and linearity of a DCC is self-checking with a resolution of better than 0.

The ratio accuracy can also be checked using two higher-valued standard resistors whose values are based on CCC or Hamon scaling techniques. The quantum-effect standards on which the volt and ohm are now based, the thermal voltage converter, and the calculable capacitor are some of the most fundamental advances of modern metrology.

There have also been innumerable incremental advances in electrical metrology over the past hundred years, which have brought about changes that are no less significant. The 21st century may also provide startling new physics and metrology, and metrologists at NIST are continuing to redesign and improve the structure of basic electrical measurement standards. Meanwhile, technology is changing rapidly and improvements in telecommunications, information processing, and instrumentation are being explored as vehicles for delivering measurement services more effectively.

The Internet and new communication technologies will influence metrology in this century, much as the telephone, fax, and email did in the last. The early stages of this influence can be seen in telemetrology projects that began at NIST in In the s, SIM was expanded to include most of the countries in the Americas now 32 members. One of the main objectives of SIM is to harmonize the basic measurement standards in each country in the hemisphere. SIM provides a framework for international comparisons that support this objective. SIM has sponsored international comparisons in mass, pressure, volume, and electricity in the latter half of the s.

These comparisons use traveling standards that are calibrated at each of the participating laboratories. Each of the pivot labs then circulated the traveling standards to the NMIs within their region. Communications between metrologists in SIM were done by fax and email, which was just becoming available at most of the NMIs. Dubbed SIMnet [ , ]; it is a network of computers devoted to video and data conferencing through the Internet to facilitate international comparisons, foster collaboration between metrologists, promote exchange of information, standardize test procedures, and share software and data.

Early experience with the Internet-based video conferencing software demonstrated how sensitive performance was to different hardware, software drivers, and operating systems. As a result, dedicated SIMnet video conferencing stations were designed. The stations include a desktop computer, digital camera, headset, and software to compress and transmit audio and video.

The camera provides real-time video and high-quality still images, allowing small hardware details and instrument connections to be examined remotely via the Internet. In addition to providing video conferencing capability, the station has an important advantage in international comparisons where computer-controlled instruments are often used as traveling standards. Control software is used to program instrument parameters such as range, settling time, and averaging. However, it can be difficult to verify that different control software is implementing the agreed upon test procedure.

Video conferencing tools available on the SIMnet station include the following see Fig. Chat is a text communication tool that can be viewed and used by all participants in the meeting. When Internet traffic reduces audio quality, a back-up form of communication is needed. Chat is also useful in a multipoint conference for questions or comments. Whiteboard is an important communication and documentation accessory. Participants can paste graphs, data, photographs, and pictures from other applications.

This preserves session information in an electronic notebook available to all participants in the meeting. Unlike the real-time video, images pasted on the whiteboard are the same quality for all participants. Share is a useful tool in data conferencing. One participant can share with the others the window of a currently running program, like a spreadsheet.

All participants see the same window, as shown in Fig. Collaborate is an extension of sharing; the participants can not only observe the screen of the running application but also control it. Typical video conference screen with video images of the participants on the right in the main panel, a shared spreadsheet in the upper left, the chat window in the lower left, and the whiteboard electronic notebook highlighted in the center.

The main task of the server is to provide audio and video to all participants in a multipoint conference. This feature is not presently available without a special server. Since its inauguration, SIMnet has been continuously tested, and in March it was used for multipoint video conferencing during the final phase of the SIM International Comparison of Electrical Units. The standard and customer test data are then returned to NIST where a follow-up calibration and data analysis are performed.

Communication between NIST and the customer during the test is by telephone or email. Since the data are returned with the standard, if something is done incorrectly, the usefulness of the calibration is diminished and the standard may have to be returned to the customer for repeat measurements.

It employs the Internet to improve communications, so the customer can transfer test data, download test procedures, and use NIST control software for system evaluation. Based on experience with this project and SIMnet, almost all measurement services at NIST could utilize the capabilities of the Internet at some time in the future.

Of course, most traveling electrical standards will not be replaced by code traveling on a digital network i. Absolute experimental determinations of units are now known as SI realizations, and the uncertainty of the SI values of the electrical units are limited by the uncertainty of their realizations in terms of the kilogram, meter, and second.

Results from the calculable capacitor experiment Sec. The Josephson constant K J is based both on its direct measurement by voltage balances and by combining R K with a value of the Planck constant, the latter obtained by realizing the watt in a special way. This realization of the SI watt is achieved by the moving-coil watt balance, which is a modern version of the absolute ampere experiment.

The NIST watt balance [ 82 ] has been designed to measure the ratio of mechanical to electrical power, linking the artifact kilogram, the meter, and the second to the practical realizations of the ohm and the volt derived from the QHE and the Josephson effect, respectively. Based on the equations given earlier, the Josephson voltages U J and quantized Hall resistances R H i are linked to the atomic constants by. The experimental method of the moving-coil watt balance, as first proposed by B.


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  • Kibble [ ], consists of two measurement modes. The equation,. This can be rewritten as. The utilization of two separate modes of measurement is the reason that this equation can be realized with a small uncertainty. Thus this experiment uses Eq. Schematic of the s NIST watt balance experiment. The wheel, both magnets, and the fixed induction coil are rigidly connected. A cryostat is between the superconducting magnet and the induction coils.

    Rewriting Eq. We next show that the value of certain fundamental constants are also obtained from the moving-coil watt balance experiment. Using Eqs. The first results from the NIST watt experiment, sometimes called an ampere experiment, were published in [ 30 ], giving a relative standard uncertainty for K J of 6.

    That experiment was a prototype for the next version in which the magnetic field was increased a factor of fifty using a superconducting magnet, resulting in similar increases in the force and voltage. During the next decade many improvements were made [ , ]. In the latest results were published [ 82 ] by E. Williams, R. Steiner, D. Newell, and P. This experiment provided the most accurate measurement of this quantity to date, and is described in the next section.

    A superconducting magnet with turns, consisting of two solenoid sections wound in opposition, produces a 0. Two induction coils, each with turns, are located in the radial field. The lower induction coil is fixed to the support structure and acts as a position reference. The upper induction coil is suspended from a balance made from a pivoting wheel located above the cryostat. Sensors monitor the five rotational, tilting, and translational modes of motion for the induction coil, other than vertical.

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    By using data on the coil motion along with some mutual inductance techniques, one is able to align the experiment and to estimate the alignment errors [ ]. Also, the unwanted motion can be actively damped using data from these sensors. One group of recent measurements recorded values of the SI watt over a 4 month period. The total uncertainty is dominated by Type B uncertainty components, that is, components that have to be evaluated by means other than statistical analysis of repeated measurements.

    Of the possible Type B error sources [ ] that contribute to the uncertainty, the three largest components arise from the following: 1 the index of refraction of air; 2 the present alignment procedures; and 3 residual knife-edge hysteresis effects during force measurements. Using the data discussed above Williams et al.

    By connecting the macroscopic unit of mass the kilogram to quantum standards based on the Josephson and quantum Hall effects, this result provides a significant improvement in the Josephson constant as well as many other constants. Figure 26 compares recent measurements of the Plank constant h , which can be derived directly from this work with a relative standard uncertainty of 8.

    Comparison of recent electrical measurements of the Planck constant h. At that level of measurement uncertainty, the watt-balance experiment becomes a very good means of monitoring the mass artifact that is used in the weighings.

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    Since the kilogram is the last artifact SI base unit defined in terms of a material artifact, a quantum standard of mass founded on electrical measurements would complete the modern trend of removing all artifacts from the definitions of SI units. The largest uncertainties in the s NIST watt experiment arose from operating in air, which required that the changing air buoyancy and refractive index be calculated from many readings of pressure, temperature, and humidity sensors.

    Almost every part of the balance assembly is being rebuilt to operate inside a specially constructed vacuum system consisting of two chambers, schematically represented in Figure The upper chamber houses the balance section. A toroid-shaped chamber houses the inductive coils, located 3 m below and centered about the liquid helium cryostat containing the superconducting magnet. Schematic representation of the electronic kilogram apparatus. The vacuum chamber and support tripod are shown in cut-away view.

    One of the powerful aspects of the relationships in fundamental constants is that we can often derive one from a combination of others. This redefinition of mass is likely to be the last major change in the SI for many decades. Of interest here is the calculation of the mass of an atom from measurements of the moving coil watt balance. From Eqs. The relative combined uncertainty in the group of constants inside the square bracket in Eq.

    At present, this watt balance experiment is the most accurate determination of m 12 C and any improvement will provide a corresponding improvement in m 12 C. Thus, its measurement connects the macroscopic kilogram to the atomic mass scale. Scientists of the Electricity Division wish to further improve the accuracy of the watt balance. If it proves possible to connect the mass of the artifact kilogram to m 12 C with accuracy equal or better than the accepted long-term stability of the artifact, then it would be time to replace the current definition for the kilogram with one based on defining an atomic mass.

    By defining mass in a way similar to the way time and length are now defined, relative to a Cesium hyperfine frequency atomic clock and the speed of light respectively, this will eliminate the last artifact standard in the SI. Contributions from NIST have helped to provide absolute determinations of the values of the ampere and the ohm, and to develop quantum standards that are universal, allowing electrical quantities to be determined in units that do not change with time and that are reproducible in any laboratory.

    The broader achievement has been success in providing electrical standards and measurements of the finest quality. The authors would like to recognize the contributions of many not already mentioned by name, including F. Silsbee, F. Harris, B. Taylor, N. Belecki, W. Phillips, and all those who have built and maintained experiments, standards, and services at NIST. About the authors: Randolph E. Elmquist, Marvin E. Kinard, Jr. Dziuba, Nile M. Oldham, and Edwin R. They are actively involved in advancing fundamental electrical measurements and developing standards and methods for the maintenance of electrical units.

    Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. National Center for Biotechnology Information , U. Published online Feb 1. Randolph E. Elmquist , Marvin E. Dziuba , Nile M. Oldham , and Edwin R. Author information Copyright and License information Disclaimer. Elmquist: vog. Cage: vog. Dziuba: vog.

    Oldham: vog. Williams: vog. Copyright notice. The papers are in the public domain and are not subject to copyright in the United States. More information about this seller Contact this seller 2. Published by U. Department of Commerce About this Item: U. Department of Commerce, Condition: Good.

    Shows some signs of wear, and may have some markings on the inside. Seller Inventory GRP More information about this seller Contact this seller 3. Department of Commerce, Washington DC Department of Commerce, Washington DC, Soft Cover. Wraps have rubbed edges. Pages are clean, text has no markings. Seller Inventory M. More information about this seller Contact this seller 4. About this Item: Condition: Good. May contain highlighting, handwriting or underlining through out the book. Book may show some wear. Used books may not contain supplements such as access codes, CDs, etc. Every item ships the same or next business day with tracking number emailed to you.

    Get Bombed!!. The precision aperture is the limiting aperture of the receiver and defines the collection solid angle for BRDF measurements.

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    Implementation of a high-voltage primary standard method using a capacitance bridge

    The lens has two functions in this system. First, it condenses the nearly collimated 14 mm diameter beam onto the detector such that the detector is underfilled. Second, the lens images the sample plane of the goniometer onto the detector, giving the STARR receiver a well-defined field of view. A 10 mm by 10 mm silicon photodiode is used for the spectral range of nm to nm. A 1 mm diameter thermoelectrically cooled photovoltaic indium arsenide photodiode coupled to an averaging sphere is used for the spectral range of nm to nm.

    The receiver system was constructed in such a way that the baffles, limiting aperture, and fused silica focusing lens remain fixed on the detector arm, while the detectors are interchangeable. This design permits the interchanging of detectors without the need to realign the system. A low noise transimpedance amplifier TIA is used with both detectors.

    When using the silicon photodiode, the output of the TIA is measured with a seven and one-half digit digital voltmeter DVM. When using the indium arsenide photodiode, an optical chopper is used in conjunction with a lock-in amplifier. The output of the lock-in amplifier is measured with the DVM. The linearity of the detectors was measured using a combination of multiple apertures and neutral density filters. A double aperture mechanism consisting of two adjacent semicircular apertures, each with its own shutter, was placed in the sample holder and positioned such that both apertures were overfilled by the incident beam.

    The signal due to light 1 Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. Finally, the signal due to light passing through both apertures simultaneously was measured and designated VAB.

    A neutral density filter was then used to attenuate the beam, and the above measurement sequence was repeated. This process continued using successively higher density filters until the attenuated beam could no longer be measured. These measurements were repeated at several wavelengths. For both detectors, it was found that the above relationship was satisfied until the noise floor of the detector system was reached. That is, no measurable nonlinearity was found at signal levels above the noise floor.

    Table 1 shows the nonuniformity of this detector at four wavelengths. Due to the imaging characteristics of the receiver system, this nonuniformity would only contribute significantly to the measurement error when measuring a sample with a significantly nonuniform spatial distribution of reflected energy, such as measuring near-specular BRDF of a mirror. This is not the type of measurement that this system was designed to perform. The nonuniformity of this detector will not contribute significantly to the measurement uncertainty for the types of measurements to be performed.

    SRM used in industry to calibrate the photometric scale of spectrophotometers; 2 a porcelain enamel on steel tile which is used for colorimetric measurements of chromatic British Ceramic BCRA tiles; 3 an NG-9 black glass, representative of SRM , which is also used to calibrate the photometric scale of spectrophotometers; and 4 a GL-2 black glass which was used to expand the dynamic range of the photomultiplier tube detector system used in the previous generation instrument for low level measurements in the ultraviolet region.

    All of these samples were chosen for their long history of stable, repeatable measurements. The SRF is measured by setting the angle of incidence the angle between the incident radiation and the sample normal to 68 and measuring the specularly reflected flux at 68 on the other side of the sample normal. The incident flux F i and the reflected flux F r are measured, and the ratio of these measurements gives the specular reflectance factor. Silicon photodiode response non-uniformity 5. Wavelength Detector response non-uniformity nm nm nm nm 0. This yielded a relative standard uncertainty in the solid angle of detection of 0.

    To check the angular accuracy of the goniometer system, the receiver was centered on the incident beam and the receiver angle counter was set to A first surface mirror was then mounted in the sample holder and positioned so as to retro-reflect the incident beam. The sample angle counter was then set to The sample holder was rotated in steps and the receiver was positioned to the appropriate theoretical angle to intercept the specular reflection of the beam.

    The distance from the center of the receiver to the center of the reflected beam was measured for each angular setting. This procedure was repeated several times. The largest measured displacement of the beam from the receiver center was 0. Because the center of the beam could be estimated to 0. At a distance from sample to receiver of The incident and reflected flux measurements each have a relative standard uncertainty of 0.