Radiometric Physics Division
name changed to
Technical Highlights
- High Accuracy Cryogenic Radiometer. The High Accuracy Cryogenic
Radiometer (HACR) links the measurement of optical power to the watt, and
functions as the primary standard upon which all the Division's scales are
based. The HACR can determine the radiant power in a beam of light with an
unprecedented uncertainty of 0.01 %; it is currently operating and is a
key factor in improving the Division's scales in absolute spectral
responsivity, spectral radiance and irradiance, and photometry.
The thrust in the last year has been to automate the calibration of transfer
detectors with the HACR, and to perform calibrations at a series of
wavelengths from 400 nm to 900 nm. These measurements will allow us
to improve our scale of absolute spectral responsivity. A scale realization is
in progress using silicon photodiode trap detectors, whose response has been
very stable over three years of measurements. The quantum efficiency of three
photodiode traps measured with the HACR is shown in Figure 1.
Figure 1: Quantum efficiency of three photodiode traps measured
with the HACR.
The major thrust for the coming year will be to extend our measurements to
infrared wavelengths, beginning with the CO2 laser wavelength
(10.6 µm). This requires replacing the silicon quadrant photodiodes (used
to detect radiation scattered out of the beam) with large area pyroelectric
detectors. Measurements will be extended into the UV with frequency doubled
lasers.
The Division has also investigated the feasibility of using mode-locked lasers
with the HACR. Application of the HACR is currently limited to wavelengths
available from CW lasers. A much larger wavelength range is available via
nonlinear optical processes using pulsed lasers. Mode-locked lasers generate a
continuous train of picosecond duration pulses at a repetition rate of
100 MHz. The output is quasi-CW and can be amplitude stabilized while
still allowing for efficient harmonic generation. The application to the HACR
requires study of potential saturation problem in the Si photodiode transfer
detectors. Commercially available Ti-sapphire lasers with harmonic generation
and optical parametric oscillators can produce wavelengths from the UV to the
IR (190 nm to 13 µm). (T.R. Gentile, J.M. Houston,
J.E. Hardis, and C.L. Cromer)
- Bidirectional Scattering Metrology. The objectives of the
Bidirectional Scattering Metrology Project are: to study the physics of the
interaction between surfaces and incident electromagnetic waves; to
investigate measurement methodology considerations; and to develop standard
reference materials. Instrumentation for the measurement of the bidirectional
reflectance distribution function (BRDF) has been developed for this research
and is currently being characterized. The capabilities of the instrument
include wide dynamic range, high angular accuracy, very low instrumentation
scatter contributions, and multiple wavelengths. This system can be used to
study the optical scattering properties of highly polished surfaces at any
angle as well as to measure surface roughness and sub-surface properties of
materials. Candidate materials and processing techniques are also being
developed for the production of standard reference materials.
This research serves many applications in optical instrumentation and
materials development communities. Among these are remote sensing instruments;
astronomical telescopes; medical imaging optics; consumer optical devices such
as cameras and binoculars; and textiles, paper, paints and finishes. For
example, the performance of diffraction-limited optical systems is limited by
scatter from the optical components. Optical systems designers would benefit
from accurate BRDF data to accurately predict system performance levels.
This research could be exploited by a field of particular interest to U.S.
technological competitiveness in the world market: semiconductor
manufacturing. As feature sizes shrink on integrated circuits, the role of
contamination, defects and surface microroughness becomes more critical. The
BRDF of a surface can be analyzed to reveal microroughness, contamination, and
subsurface features on silicon wafers if the instrument used and the theory
employed are well understood.
The BRDF instrumentation has been used to verify the measurement limit of
10-9 sr-1, which is the limit imposed by Raleigh
scattering in air (see Figure 2). Prototype standard reference materials
have been produced using a technique of rough grinding and partial polishing
to obtain reproducible BRDF levels, and the Division staff is collaborating
with Dr. E. Church from the U.S. Army Picatinny Arsenal to develop a
theoretical model of these surfaces. C. Asmail has recently been involved
in a task force sponsored by SEMATECH on wafer evaluation and cleaning
technology. Our involvement in these programs is critical for the success of
our role in supporting the surface evaluation efforts of the semiconductor
industry. (C.C. Asmail, J.E. Proctor, J.J. Hsia, and
C.L. Cromer)
Figure 2: Rayleigh scattering limitation on low-level BRDF
measurements.
- The NIST Low-Background Infrared Calibration Facility. The
Low-Background Infrared (LBIR) facility had a very active schedule with
measurements of blackbodies for the aerospace contractors of DOD and DOD
calibration laboratories. The LBIR completed a series of broadband flux
measurement for MIT/Lincoln Laboratories to be used in the calibration program
for the Midcourse Space Experiment (MSX) spacecraft. The test fixture for this
calibration is shown in Figure 3. The spacecraft will release reference
spheres, at a known initial temperature, to be viewed by the on-board
radiometer for calibration purposes. The measurements at NIST are to be used
as part of a modelling program to characterize the emission of the reference
spheres in orbit around the earth. The sources under test consisted of a
2 cm diameter sphere and an elongated disk mounted at 45 degrees to
the optical axis.
Figure 3: The text fixture used in the calibration program for the
Midcourse Space Experiment (MSX) spacecraft.
The expansion of the facility to provide spectral calibrations for sources,
detectors and optical materials is almost complete. The spectral instrument is
undergoing final alignment and testing by Talandic Research Corporation of
Irwindale, CA. and will begin service in early 1994. A schematic drawing of the
experimental arrangement to be used in calibrations is shown in Figure 4.
A new calibration chamber has been contracted to GNB Corporation of
Hayward, CA. This new chamber will provide a vacuum environment with a
cryo-shroud which can be varied in temperature from ambient down to 20 K.
(S.R. Lorentz, S.C. Ebner, M. Navarro, and R.V. Datla)
Figure 4: Side view of LBIR spectral instrument in relation to NIST
blackbody and the Absolute Cryogenic Radiometer (ACR).
- Photometry. The Division is responsible for the realization of the
candela, the SI base unit of luminous intensity. The candela represents a unit
of measure of the apparent brightness of a light source as observed by the
human eye. The NIST candela scale is currently realized using standard
detectors which are constructed to emulate the CIE spectral luminous
efficiency function for photopic vision.
Requests for photometric calibration services have been increasing, and the
Division is currently providing illuminance (lx), luminous intensity (cd), and
luminous flux (lm) measurements on incandescent sources from 1000 W to 1/4 W.
The Division has re-evaluated the opal glass standards for luminance, and is
issuing them as an inexpensive alternative to sphere sources (for those who use
irradiance standards). In the long term, a transmissive diffuser will be used
to replace the opal glass. Intercomparisons between PTB and NIST for luminous
intensity and luminous flux are in progress.
The Division is encouraging customers who are involved in photometer and
colorimeter calibrations to be traceable to NIST through a calibrated
reference radiometer, instead of a candela lamp. The Division can provide
calibrations of meters for illuminance and luminance for specific source
types, and thus improve the accuracy of measurements provided in industry.
In the coming year the Division will develop a new scale of luminous and
spectral flux based on measurements in a large integrating sphere. The
Division is planning to begin providing spectral flux measurements for
fluorescent, high-intensity discharge, and UV sources this year. A temperature
controlled reference photometer has been developed and will be used in future
candela realizations. The new design includes specially designed apertures,
temperature controlled filters, and trap detectors calibrated with the
HACR.
The Division is also developing new capabilities to meet the need to make
accurate spatially and temporally resolved photometric and colorimetric
measurement of color displays. This effort will improve process control for
display manufacturers, and improve the accuracy of systems which use color
displays for CAD/CAM, advertising and printing, textiles, and medical
applications. (Y. Ohno, G.P. Eppeldauer, J.E. Hardis, and
C.L. Cromer)
- Apertures. Accurate measurements of optical flux usually involve
one or more apertures to define the geometrical extent of the optical beam.
The measurement of the effective area of the aperture is in many cases the
limiting factor in the overall accuracy of optical radiation measurements. The
purpose of this project is to provide state-of-the-art measurements of
aperture area for radiometric applications to the division and industry, and
to improve the accuracy of optical radiation measurements involving
geometrical flux transfer.
The Division has built and tested instrumentation that can compare the
effective areas of apertures with a standard uncertainty better that
0.05 %. This instrument can be used to measure the area of an aperture by comparison to
reference apertures with known area. The unknown aperture can be of any shape
or configuration. Reference apertures have been fabricated using diamond
turning techniques.
The Division is also in the process of acquiring a high accuracy 2D coordinate
measuring machine with a microscope and CCD camera. This machine will be used
to determine the quality and area of the reference apertures traceable to NIST
length standards. (J.B. Fowler, J.E. Hardis, and
C.L. Cromer)
- Radiation Temperature Research. The Division is responsible for
realizing the radiation temperature scale that maintains the Kelvin above
1234.96 K. A project to relate the radiation temperature scale to the High
Accuracy Cryogenic Radiometer (HACR) was started this year and will continue
next year. This project involves developing an absolute pyrometer calibrated
against the HACR. A prototype pyrometer was built using a germanium detector
and was used to characterize two heat-pipe blackbodies whose temperatures were
between 400 °C and 1000 °C. Although this pyrometer was not
calibrated against the HACR, the result showed that a stable detector system
for measuring temperatures between 0 °C and 1000 °C, with
instrumental uncertainty on the order of a few mK, can be realized.
This project also involves an intercomparison with Japan. On May 15,
1993, a research plan and cooperative agreement entitled "Research on
Radiation Thermometry in the Ultra-High Temperature Range" was signed by
the National Research Laboratory of Metrology (NRLM, Japan) and the Physics
Laboratory at NIST. This three-year project was initiated by NRLM;
Dr. F. Sakuma has made similar agreements with several other
national standards laboratories. The object of the research is to conduct
joint studies so that the temperature scales in the interval from 700 °C
to 2800 °C can be intercompared.
In May 1993, NIST calibrated an NRLM transfer radiometer. This device is a
filtered, radiance radiometer. The center wavelength is 665 nm and the field of
view is 0.39 degrees. The detector is a silicon photodiode. NRLM uses transfer
radiometers of this type to establish and verify radiance scales at industrial
metrology laboratories. The preliminary results show that there is a small
offset at 1000 °C and a larger offset at 2500 °C. This trend
was also observed at NPL (UK) and IMGC (Italy). (B.C. Johnson,
R.J. Bruening, T.R. O'Brian, and B.K. Tsai)
- Remote Sensing. The Division is continuing the collaboration with
NASA's Earth Observing System (EOS) and NASA/NOAA's Sea-viewing Wide
Field-of-view Sensor (SeaWiFS) environment monitoring programs. NIST
participated in the Sixth General Meeting of the EOS Calibration Panel and
provided written input on the nature and purpose of radiometric
intercomparisons for a proposal on cross-calibration to NASA management. NIST
also hosted a workshop on cross-calibration that was attended by 16 off-site
personnel and seven NIST personnel. These attendees represented various
universities, aerospace corporations, national standards laboratories, and
government facilities from the U.S., Canada, Japan, and the U.K. The various
instrument calibration requirements, ground support equipment for radiometric
calibration, and measurement approaches were summarized and documented by
NIST. Two transfer radiometers are being designed, built, and characterized
for the EOS program so that the spectral radiance of various integrating
sphere sources can be verified and monitored.
For the SeaWiFS program NIST developed and characterized a portable
multichannel spectroradiometer (SeaWiFS Transfer Radiometer, or SXR).
The device was used during the second SeaWiFS Round Robin in June 1993
(SIRREX-2). NIST also provided an uncalibrated commercial spectroradiometer
and a diffuse plaque for use at SIRREX-2. The first round robin (July 1992)
demonstrated that: the methods and equipment were inadequate to measure the
spectral irradiance of standard FEL-type quartz-halogen lamps with
uncertainties that reflected the assigned uncertainty in the lamp irradiance;
the technique used by NASA/Goddard Space Flight Center (GSFC) to transfer the
spectral irradiance of an FEL lamp to the spectral radiance of an integrating
sphere source could be used during the round robin, but the 3 % scatter
in the measurements was unexplained; and that the calibration of the
electronic equipment from the various laboratories was in some cases
inaccurate. The scatter in the measurements of the spectral irradiance lamps
was of the order of 1 %.
The spectral radiance of six integrating sphere sources was measured by NIST
and two other laboratories. The sphere sources are used to maintain the
SeaWiFS spectral radiance and irradiance scales at NASA/GSFC or to calibrate
submersible spectroradiometers at four other participating organizations. The
preliminary results demonstrate that there are still problems in measuring
these spheres at the desired uncertainty level of 1 %. The SXR will be
used by NIST during field measurements in February at the Lanai buoy site in
Hawaii. (B.C. Johnson, T.R. Gentile, and J.B. Fowler)
- Monochromator-Based Bidirectional Reflectance Distribution Function
Metrology. A monochromator-based instrument has been designed to measure
the BRDF and directional hemispherical reflectance of diffusers. In the past,
the BRDF instrument was used for about 50 of these measurements a year, and
the process was very labor intensive. The new instrument will make about the
same number of measurements a year but it will require only half as much
labor.
The new state-of-the-art instrument will also improve the accuracy of the BRDF
and directional hemispherical reflectance measurements. Its capability has
been increased over the old instrument to include raster scanning of
300 cm × 300 cm samples; the
10 cm × 10 cm size was a severe limitation of the old
instrument. The spectral range can now be extended to 200 nm with the
proper source and detector system.
The f-number of the monochromator has been decreased, thus increasing the
throughput by almost a factor of 10. This increase in power will permit
measurements down to 200 nm for both the BRDF and the directional
hemispherical reflectance determinations. An advantage of this new instrument
is that it can make either BRDF and directional hemispherical measurements
without requiring that the instrument be reconfigured. This design will allow
one to compare the BRDF and the directional hemispherical measurements quickly
before the material properties change. This new instrument is a valuable
component in the Division's program on the optical properties of materials.
(J.E. Proctor and P.Y. Barnes)
- Solar Radiometry. The proposed global environmental problems of
stratospheric ozone depletion and the greenhouse effect are intimately
involved with changes in the transmission of the atmosphere, resulting in
changes in the spectral irradiance of solar radiation reaching the Earth. The
Division is committed to assisting the agencies involved in the Global
Climatic Change Program and believes the program's mission to be of pressing
national and international interest. The determination of whether these solar
spectral irradiances are actually changing will require extremely accurate,
longterm measurements. This means that the measurement base must be stable
over this time period and necessitates a strong commitment by NIST to support
this mission. At present there are no NIST traceable solar measurement
standards for this potential growth area.
Currently NIST is involved with the EPA and USDA efforts to establish a
National UV Monitoring Network. NIST's responsibilities in this network will
include characterizing instruments and developing standard calibration
procedures and physical transfer standards. The Division is in the process of
developing laboratory characterization procedures to assess the performance of
solar UV instruments, having already characterized sixteen broadband solar UV
radiometers and a reference solar UV spectroradiometer for the USDA and EPA.
This plan is currently under review by a Global Change subcommittee and will
be incorporated into the "U.S. Interagency In-Situ UVB Monitoring
Network Plan." The Division is also developing an instrument
intercomparison protocol. In 1994 NIST, with EPA support, will establish a
high accuracy reference solar measurement site in Gaithersburg, which will be
the EPA's Washington UV monitoring site and will focus on the radiometric
uncertainty of solar measurements. In addition, the site would be a testbed
for the development of new solar instrumentation and calibration strategies.
(E.A. Thompson)
- Imaging Radiometry. The Division is collaborating with Los Alamos
National Laboratory in the calibration of an advanced infrared thermal imaging
telescope with a focal plane array detector to be used for remote (orbital)
measurements of earth surface temperatures.
The Division staff has evaluated the performance of several commercial thermal
imaging systems (using either focal plane array or scanned single element
detectors) for accurate measurement of near-ambient temperatures, including
detailed modeling of the expected imaging performance based on measurement of
optical parameters such as modulation transfer function, etc. The relative
strengths and deficiencies of different representative imaging technologies
have been compared and critically evaluated for use in radiometric
applications.
The Division is aggressively pursuing internal and external funding for
developing an advanced facility for characterization and utilization of
infrared focal plane array thermal imaging systems. The proposed facility will
explore such applications as: exploiting the high thermal and spatial
resolution of thermal imaging systems, thermal imaging technology to monitor
and control rapid thermal processing (RTP), and characterization of large
scale resistively heated arrays of microscopic emitters (thermal scene
generators). (T.R. O'Brian and R.J. Bruening)
- Infrared Diffuse Reflectance Spectrophotometry and Standards.
Accurate knowledge of the optical properties of materials with non-specular
surfaces in the infrared (IR) spectral region is impeded by a lack of
available standard reference materials (SRMs), as well as a lack of accurate
and reliable commercial measurement equipment. A program to develop absolute
techniques and measurement equipment as well as SRMs for IR spectral diffuse
reflectance and transmittance is in progress.
Candidate materials for consideration as diffuse reflectance SRMs have been
examined. The primary requirement for such an SRM is a near-Lambertian
scattering characteristic. Scattering measurements of these samples performed
at 10.6 µm wavelength demonstrated that a plasma sprayed metal coated
sample had the best diffuse scattering characteristics. A comparison of the
relative bidirectional distribution function (BRDF) of the plasma-sprayed
material and another standard integrating sphere coating is shown in
Figure 5. The horizontal axis shows the viewing angle relative to the
sample normal.
Figure 5: Comparison of relative BRDF of two IR diffuser
materials at 10.6 µm for normal incidence light.
An integrating sphere system for measurement of absolute diffuse reflectance
of samples in the infrared has been designed and constructed. The sphere
employs the coated plasma sprayed metal discussed above as a wall coating for
improved performance. A schematic of the sphere, highlighting the most
important components, is shown in Figure 6. A Monte Carlo based
integrating sphere simulation program has been used to analyze the effects of
non-Lambertian sphere wall behavior on diffuse reflectance measurements. The
results of the analysis were combined with the requirements and limitations
set forth by previous analytical work to design the sphere. This integrating
sphere incorporates several features to allow characterization and correction
of the deviations of the sphere wall scattering characteristics from the ideal
Lambertian character. (L. Hanssen and J.J. Hsia)
Figure 6: Schematic views of integrating sphere design for
absolute diffuse reflectance measurements.
- Fourier Transform Infrared (FT-IR) Spectrometer Methodology and
Calibration. Infrared (IR) spectroscopic measurements are required in a
variety of applications in U.S. industry and government. Recently these
measurements are being made almost exclusively by Fourier Transform
instrumentation. FT-IR instruments have many advantages over grating or prism
instruments, such as better signal-to-noise ratio and faster spectral
acquisition, that have led to their wide applications. However, FT-IR
instruments do have sources of potentially significant error. Physical
standards are required to reduce measurement uncertainties.
The Division has an ongoing comprehensive program to calibrate FT-IR
spectrometers. This involves studies of all the major sources of error as well
as standards development. As part of the investigation into error sources in
FT-IR measurements, several have been examined and demonstrated to be
significant and easily overlooked by the user. These error sources include
detector non-linearity, detector non-equivalence, inter-reflection between
sample and other system components, and beam deviation due to sample tilt and
wedging.
The Division has recently completed the production and characterization of
polystyrene SRMs for calibration of the wavelength scale. Currently,
calibration techniques as well as SRMs for the photometric scale (e.g.,
transmittance, reflectance, etc.) calibration are being developed. Neutral
density filters have been examined for development as calibration SRMs. A set
of filters having a range of transmittance values from 0.00001 through 0.5
were measured and found to have sufficiently neutral character to be promising
as candidate SRMs for calibration of the linearity of an FT-IR system.
(L. Hanssen and Z. Zhang)
- Correlated Photon Radiometry. The Division is developing the
capability to measure the absolute responsivity of photon counting detector
using the parametric down-conversion method. This process employs a nonlinear
medium which allows photons from a pump beam to, in effect, decay into pairs
of photons under the restrictions of energy and momentum conservation. Since
the two "decay" photons are born at the same time, the detection of
one photon indicates with high certainty the existence of the other photon of
the pair. In addition, the conservation of energy and momentum allow the
wavelength and direction of one photon to be determined from the other. The
responsivity of photon counting detectors can be determined using these pairs
of photons by positioning two detectors to intercept each of the pair of
photons. The counting rate of each detector is recorded along with the
coincidence rate between the two detectors. The ratio of the coincidence rate
to the single rate of one detector is the absolute quantum efficiency of the
other detector and vice versa. Put another way, the output pulses of one of
the detectors can be thought of as a trigger which indicates the existence of
a second photon headed for the other detector. The quantum efficiency of the
detector is then just the fraction of the time that a photon is detected at
the second detector in conjunction with a trigger from the first.
To test this method, a parametric down-conversion source producing 702 nm
photon pairs has been set up. The goal is to determine the ultimate accuracy
that may be achieved with this method. It is expected that uncertainties near
0.1 % are possible. The method will be verified using independent
calibration methods available within the division. Avalanche photodiodes and a
photomultiplier (PMT) are now being measured with this method. Measurements to
map the spatial variation of the PMT response are underway.
Longer range plans include arrangements to make the method continuously
tunable over the visible and then to extend the spectral range into the
infrared. The Division will also investigate the extension of the method
beyond the photon counting regime into analog detector measurements.
(A.L. Migdall)
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