- Cryogenic Radiometry.The Division maintains an absolute High
Accuracy Cryogenic Radiometer (HACR) with a combined relative standard
uncertainty of 0.02% as the foundation for a radiometric measurement chain to
maintain scales of spectral radiance and irradiance, photometry, and absolute
detector responsively. A second, high-sensitivity cryogenic radiometer is the
basis for the Low Background Infrared (LBIR) facility, which provides
calibrations, research and development for high-sensitivity infrared sensors.
At the Synchrotron Ultraviolet Radiation Facility (SURF), a new
monochromator-based cryogenic radiometer will be used to establish the SURF
spectral radiance scale and to serve as a calibration facility for
transfer-standard detectors. Development of new radiometers incorporating
superconducting technology and high-TC materials is an important
component of the cryogenic radiometry program. A second generation HACR is
being developed to improve utility and sensitivity.
Transfer-standard detectors used throughout the Division are calibrated with
the cryogenic radiometers. The Division also develops transfer-standard
detectors to enable the high-accuracy radiometric scales to be propagated to
other laboratories. Transfer standards are being developed at near-infrared
and ultraviolet wavelengths that will substantially improve the calibration
uncertainties in these areas.
- Synchrotron-Radiation-Based Radiometry. NIST is developing
SURF III (Synchrotron Ultraviolet Radiation Facility), an advanced
radiometric light source of unprecedented accuracy and spectral range. When
commissioned in 1998, the SURF III electron storage ring will be an
extremely bright, absolutely predictable source of light spanning the
electromagnetic spectrum from soft x-rays through far infrared. The radiation
emitted by SURF III will be critical for applications such as: setting new
standards for electromagnetic radiation power measurements; calibrating optical
detectors and instrumentation for use in the semiconductor industry and in space
research; verifying high-temperature radiation thermometry scales; and opening
new research opportunities in longer wavelength regions (infrared and
terahertz). The Optical Technology Division, Electron and Optical Physics
Division, and Atomic Physics Division are collaborating to develop the new
facility.
SURF has served for many years as the national standard for radiation power
measurements in the far ultraviolet. The major facility upgrade to SURF III
will improve the magnetic field uniformity of the storage ring by two orders of
magnitude, thereby reducing the uncertainty in SURF III as a radiometric
source. SURF III will be used for both source-based and detector-based
radiometry. Source-based radiometry exploits the predictability of the power,
spectral, and spatial distributions of radiation emitted by the electrons
accelerated in the storage ring, based on fundamental physical principles.
Detector-based radiometry will use SURF III as a bright light source,
particularly in the ultraviolet and soft x-ray spectral regions, basing
measurements on absolute detectors including the newly developed cryogenic
electrical-substitution radiometer optimized for short wavelengths. The
monochromator-based cryogenic radiometer system will significantly improve the
detector spectral responsivity measurements in the ultraviolet and infrared
spectral ranges.
- Temperature. The Division has the institutional responsibility to
maintain temperature scales above the freezing point of silver (1234.96 K)
and radiation-temperature scales at all temperatures. The pyrometry scale is
based upon the spectral-radiance scale and hence is inferred from the absolute
detector scale based upon the HACR. A wide range of blackbody sources is
maintained for calibration purposes, spanning temperatures from approximately
100 K to 3000 K. The Division pursues a vigorous program in
thermal-source research and development to provide the highest quality
measurement assurance for temperature-scale calibrations for a variety of
industrial and scientific applications. Two NIST Competence programs are
underway to remove uncertainties from the International Temperature Scale and to
improve heat-flux measurement techniques. New methods using synchrotron
radiation and blackbodies to establish an independent temperature scale are
being pursued in conjunction with the SURF III upgrade.
- Photometry, Colorimetry and Appearance. Photometry, the science of
measuring light with the response function of an "average" human
observer, is integral to the detector metrology program. The SI unit of luminous
intensity, the candela, is maintained using a set of well-characterized,
filtered detectors. This provides a direct link between the HACR and the
candela, and provides an alternate method, other than conventional lamps, for
transferring calibrations of this unit to customers. While the practice of lamp
dissemination will be continued, depending upon lamp availability, the Division
can offer photometric detector characterization to customers as a more direct
and more stable calibration procedure. The Division has developed a
total-luminous-flux scale based upon the new candela and hence directly upon the
HACR. This new technology should result in lower costs and better calibrations
for the Division’s customers.
The physical measurement of appearance quantifies attributes of an object’s
interaction with light. Appearance is generally categorized into spectral
(color) and spatial (gloss, texture, etc.) properties of reflected light.
Physical measurements of source, object and reflected light are weighted by CIE
tristimulus functions or by standard illuminates for the computation of visual
appearance and color. A goniophotometer is under development for the measurement
of 20, 60, and 85 gloss, and research is underway to develop primary gloss
standards. In the area of colorimetry, the Spectral Tri-function Automated
Reference Reflectometer (STARR) is being used to develop a measurement-assurance
program with industry standard color tiles, and to perform research into the
instrument attributes necessary for highly accurate colorimetry for future
calibration services. The primary goals of the program are development of
reference instruments and standards for current appearance-measurement
technologies and eventual development of new measurements and standards to more
accurately capture visual appearance.
- Optical Properties of Materials. The Division provides materials
optical data, measurement methods, and measurement support for the Nation’s
advanced manufacturing efforts. The Division maintains scales of transmittance
and reflectance with absolute reference spectrophotometers that provide
intrinsic uncertainties of a part per ten thousand in the 200 nm to
2500 nm wavelength range. These instruments support a range of calibration
services and preparation of SRM materials. For the spectral range from 2 µm
to 25 µm, a high-accuracy prism-grating monochromator for transmittance
measurements has been developed, and research is being pursued to develop
Fourier Transform Infrared (FTIR) instrumentation for quantitative
spectrophotometry. Transmittance, spectral reflectance, and diffuse reflectance
measurements are being performed at close to 1 % uncertainty using FTIR
instrumentation. Laser-based spectrophotometry is being developed to supplement
these measurements, and cryogenic capabilities are being added to perform
measurements for a wide range of sample temperatures. The Division also
maintains laser-based instruments for hemispherical reflectance, Bi-directional
Scattering Distribution Function (BSDF), and Bi-directional Reflectance
Distribution Function (BRDF) for high-accuracy, low-level transmitted and
reflected scattering measurements.
These instruments and related ones, such as the infrared beamline and microscope
at SURF, establish a capability to characterize optical properties of materials
over an extended wavelength range. In collaboration with other Physics
Laboratory Divisions, we are also developing theoretical tools to predict
optical properties of materials. In addition, the Division is developing
advanced optical measurement methods for materials characterization, such as
Near-field Scanning Optical Microscopy (NSOM), nonlinear spectroscopy of
interfaces, and femtosecond spectroscopy for materials characterization.
- Environmental and Remote Sensing. In response to the U.S. Global
Change Research Program (USGCRP), NASA, NOAA, EPA, and other government agencies
are supporting a wide range of space-based and terrestrial research programs to
ascertain the effects of human activities on the biosphere. These programs
envision long-term monitoring and survey activity, which require consistent
calibration of instruments distributed around the world. NIST and NASA fund an
effort to provide a comprehensive calibration base for radiometric instruments
referenced to the Division’s cryogenic radiometers. In addition to calibrations,
NIST manages round-robin calibration comparisons among instrument manufacturers
and provides cross calibration with other national laboratories. Activities
include work on Earth Observing System (EOS) platforms to ensure calibration
quality prior to launch and accuracy after launch, NIST’s establishment of a
solar UV monitoring station in Gaithersburg, and calibration support for a
worldwide UV monitoring program assessing the impact of stratospheric ozone
depletion. The Division also provides spectroscopic measurements of atmospheric
molecules and chemical processes for refinement of atmospheric models needed for
the remote sensing and global change efforts.
- Optical-Scattering Metrology. Mechanisms by which material properties
and surface topography affect the distribution and polarization of light
scattered from surfaces are studied. The goal is to develop measurement methods
and standards for use in industry, and to provide a basis for understanding
scattering distributions so that industry can optimally use optical-scatter
methods. Applications include evaluation of highly polished optical surfaces,
bulk optical materials, surface residues and coatings, and diffuse scattering
materials. Optical-scattering metrology is also used to assess quality of
patterned materials and periodic structures, such as pits on compact discs,
exposed photoresist films, and microcircuitry on semiconductor wafers.
Experiments and theoretical modeling are underway to use optical scatter to
distinguish and characterize features such as surface microroughness,
particulate contamination, and subsurface defects that affect performance of
these materials and devices.
- Near-field Scanning Optical Microscopy (NSOM). NSOM is being
developed as a quantitative technique for noninvasive optical measurements. Its
resolution is not limited by the wavelength of light, as in traditional
diffraction-limited microscopes, but by the size of the sub-wavelength aperture
or tip used as a probe. Well-characterized microscopes and small light sources
are being constructed, and methods to determine resolution are being developed.
This requires fundamental understanding of contrast mechanisms and modeling the
fields around small light sources as they interact with materials and surface
features. The Division collaborates with other NIST programs applying near-field
microscopy to problems in chemical, biological, optical, and semiconductor
technology.
- Spectroscopy and Dynamics at Interfaces. Femtosecond lasers are used
to measure rates for technologically important interface processes, such as the
coupling and relaxation of excited carriers, phonons, and surface electronic and
vibrational states (particularly molecular states) at metal, semiconductor, and
dielectric solid and liquid interfaces. A laser pulse excites a system and the
subsequent evolution is measured with time-delayed probe-laser pulses.
Surface-sensitive probe techniques include infrared absorption and sum-frequency
generation (SFG). Nonlinear optical diagnostics such as SFG are uniquely
sensitive to interface structure at the surfaces of materials, in thin-film
systems, or buried in layered materials. Spectroscopic measurement applications
include characterization of electronic structure at buried epitaxial interfaces,
assessment of the structure and quality of thin films, and vibrationally
resonant SFG of organic films such as self-assembled monolayers, molecules at
liquid interfaces, and biological interfaces.
- Femtosecond Condensed-phase Transient Spectroscopy. Unique
femtosecond infrared spectroscopic techniques have been developed to study
highly excited vibrational states, energy transfer, photochemical reactions, and
the dynamics of hydrogen bond formation and rupture in the condensed phase. New
instrumentation is being developed to expand the application of these
techniques, including extension of the wavelength range to the far infrared, and
addition of imaging capabilities. The measurements identify transient species
and determine energy-transfer rates that serve to improve models of
condensed-phase processes. Efforts include measurements on photocatalytic
processes, charge transfer in hybrid semiconductor-chromophore systems,
conductivity of colossal-magnetoresistance materials, and infrared and THz
probes of biological systems.
- Laser Studies of Elementary Chemical Reactions. Laser pulses are used
to observe and manipulate fundamental molecular transformations, such as bond
breaking and bond formation, in order to develop an atomic-level understanding
of reactions important in combustion and propulsion chemistry, in the chemistry
of the upper atmosphere, and in orbital environments. Current efforts emphasize
elementary reactions of O-atoms with H2, H2O, HCN,
CH4, and SiH4. The experiments use state-resolved
nanosecond and time-resolved femtosecond spectroscopic techniques to produce
data to test quantum-chemical models of these benchmark systems, and to provide
input for predictive models of industrial and atmospheric chemical processes.
- Analytic Spectroscopy. Spectroscopic technology is increasingly
important for applications in chemical analysis and detection, including
atmospheric remote sensing, emissions monitoring, catalysis, industrial process
control, forensic science, medical diagnostics, chemical manufacturing, and
materials development. The Division has a vertically integrated research and
development effort to support this technology. The effort includes:
(1) establishment and dissemination of spectroscopic databases to
facilitate choices of monitoring frequencies and inversion of measurements to
extract concentrations; (2) development of quantum-mechanical Hamiltonians
which provide convenient and concise representations of spectroscopic data and
their validation; (3) laboratory spectroscopic measurements to provide
accurate frequency and intensity information for instrument calibration;
(4) development of new optical chemical-sensor technology in the microwave,
infrared, and visible/UV spectral regions; (5) working with other
government agencies to solve novel and important chemical analysis and detection
problems; and (6) working with industry to transfer these technologies and
to assess needs for new optical chemical analysis technologies, standards, and
data.
- Consultation and Calibration Services. The Division staff provides
SRMs and radiometric calibrations for use by a variety of industrial and
academic customers. The staff has an active and vital role in consulting with
other government agencies such as NASA, NOAA, EPA, USDA, FAA, and DoD to develop
calibration programs appropriate for their demanding missions. The Division has
completed an ambitious program to implement ISO Guide 25 standards in all
the calibration services during the past several years and has developed a
protocol for annual assessment of the quality program.