Optical Measurement Services:
to disseminate optical radiation measurements and
standards to industry, government, and academia.
INTENDED OUTCOME AND
BACKGROUND
The Division builds and maintains world-class optical-radiation
measurement facilities to meet the continued and emerging needs for standards
and specialized measurements by government and industry. These facilities are
available to government and industry customers through formal calibration
services, special tests, and standard reference materials offered through NIST
Technology Services or through collaborative research efforts.
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Figure 5. Charles Gibson preparing a blackbody-radiation source for
calibration of spectral irradiance. |
We maintain facilities for optical-properties measurements of reflectance,
transmittance, and gloss; for photometric measurements including luminous
intensity and color temperature; and for radiometric measurements, including
spectral radiance, spectral irradiance, spectral responsivity, and radiance
temperature. The Division has highly specialized facilities for performing
low-background radiometric measurements, for characterizing remote-sensing
instruments, for measuring the area of precision radiometric apertures, and for
determining the absolute optical power, radiance, and irradiance spectral
responsivities of instruments. New measurement facilities under development
include reflected color, emittance, retro-reflectance, and ultraviolet spectral
irradiance.
The Division strives to ensure the quality of these programs by publishing our
research and measurement methodologies as NIST Special Publications or in
outside peer-reviewed archival journals, by participating in measurement
comparisons with other laboratories, and by maintaining a measurement-quality
program. The Division also aids the dissemination of good optical-radiation
measurement practice by publishing training documents and offering formal
short-courses in Photometry, Spectroradiometry, and Radiation Temperature
Measurements.
Accomplishments
High-Accuracy, Deep-Ultraviolet, Index of Refraction Measurements at
SURF III
A new beamline (BL-5) was completed at the NIST Synchrotron Ultraviolet
Radiation Facility (SURF III), equipped with a high-resolution,
deep-ultraviolet, Fourier-transform spectrometer (DUV-FTS) to measure the index
of refraction of materials, with high accuracy. In contrast to the traditional,
prism-goniometer measurement of the refracted angle, the DUV-FTS analyzes the
fringe pattern in the transmittance spectrum of an etalon made from the sample
material.
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Figure 6. Index of refraction of CaF2 measured with the
DUV-FTS at SURF III. The solid circles show the measured values, and the
solid line shows a three-term Sellmeier fit, whose residuals are shown in the
inset. |
Using independent measurements of the sample thickness and fringe order, we
determined the index of refraction with uncertainties
( 10-5)
similar to that of the classical prism method. The interferometer approach
offers significant advantages over the prism method, including greater speed,
continuous wavelength coverage, and the ability to measure thin samples such as
absorbing fluids.
Initial results at room temperature on a 1 mm thick CaF2 sample
at wavelengths between 175 nm and 600 nm are shown in Fig. 6.
The solid circles indicate the measured index values, while the solid line is
the result of a three-parameter Sellmeier fit. The residuals from the fit,
shown in the inset, are approximately 5 × 10-6. Comparison
to our previous prism measurements on CaF2 at 193 nm shows
agreement within 1.3 × 10-5, consistent with the estimated
combined uncertainties in the two measurements.
Plans are underway to extend the range to below 157 nm for measuring
optical materials important to the deep-ultraviolet photolithography industry.
Measurements are also planned on deionized water at 193 nm, which is under
consideration for immersion photolithography.
New Reference Colorimeter
We have constructed and are presently testing a new reference instrument for
measuring the reflectance color of materials. This project was undertaken in
response to industry and government demands for improved color measurements and
standards, as articulated in the 6th and 7th Reports of
the Council for Optical Radiation Measurements (CORM). Improved color standards
are required to ensure better color matching of products manufactured at
different sites. Because the color of a product often plays a major role in its
acceptability, color measurements and standards are extremely important to
industry.
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Figure 7. Maria Nadal prepares the NIST reference colorimeter. |
Our new instrument performs measurements for all possible combinations of
illumination and viewing angles, including the standard 0º/45º and 6º/diffuse
geometries, allowing the complete characterization of the reflectance
properties of an object. The popular 0/45 measurements are highly automated
through a sample wheel with a capacity for 20 samples, as shown in Fig. 7.
We did a complete uncertainty analysis, considering wavelength accuracy,
stray-light rejection, reproducibility of sample positioning, and signal noise.
A critical aspect of the analysis was the inclusion of the statistical
correlation between signals at the same wavelength and between reflectance
factors at different wavelengths. The estimated measurement uncertainties are
 Eab < 0.4,
where Eab = 1
is generally taken as the threshold for visual discrimination between two
colors.
Thermal-Infrared Transfer Radiometer Verifies Radiance Scales Used in
Earth Remote Sensing
To establish the traceability of radiometric measurements performed by the
remote-sensing community to NIST’s radiometric scales, and to perform on-site
calibration verifications of critical remote-sensing instruments, we have
developed the transportable Thermal-Infrared Transfer Radiometer (TXR). Such
traceability helps ensure that the measurements undertaken by this community
are of the highest accuracy and can be compared to similar measurements
performed by other countries or at other times.
The TXR was deployed at the University of Miami's Rosenstiel School of Marine
and Atmospheric Sciences, in collaboration with NASA-Goddard, to verify the
radiance scales of several blackbody sources used by the sea-surface
remote-sensing community to calibrate ship-based radiometers. These, in turn,
validate satellite measurements of sea-surface temperature. The radiometer was
then sent to ITT Industries in Ft. Wayne, Indiana, to measure the radiance
of two calibration targets used to verify the calibration of the National
Oceanic and Atmospheric Administration (NOAA) Geostationary Operational
Environmental Satellite (GOES) imagers. A third deployment was made to Los
Alamos National Laboratory, to measure the radiance of blackbody sources in a
cryogenic vacuum chamber previously used to calibrate a space-flight instrument
for the Department of Energy.
The success of these deployments has led to new efforts to improve the
calibration of the TXR and to extend its operational range to temperatures
below 288 K by performing the measurements in a low-infrared-background,
liquid-N2-cooled, vacuum test chamber. To do these measurements, the
NIST Medium-Background Infrared (MBIR) facility was used to calibrate the TXR
against a high-accuracy cryogenic blackbody in conditions that simulate outer
space. The facility allowed the extension of the TXR radiance scale down to
200 K and enabled the evaluation of measurement uncertainties due to
room-temperature infrared background radiation and atmospheric infrared
absorption and emission.
Wire-Contrast Measurement Standards
We developed a new facility to measure the contrast of ultraviolet-laser-written
markings on electrical wires used in aerospace and military vehicles, in
response to a request by the Navy for improved standards in this area. Although
the ultraviolet-laser-written markings are immune to fading with age, the gray
color of the writing reduces the visual contrast relative to black-ink markings.
A high contrast level is desirable for these markings to ensure the correct
identity of a wire during installation, repair, or maintenance.
Writing-contrast values of 60 % or higher are recommended by industry
standards. However, the measured values for the same wire sample often vary by
as much as 10 % to 20 %.
Our new reference instrument consists of a well-characterized luminance meter
with a microscope to measure the luminance on a circular spot of 12 µm
diameter or larger. The wire is illuminated by a fiber-optic source with a
known spectral distribution. A three-axis translation stage under computer
control automatically scans the wire surface. The computer also acquires and
analyzes the data. The instrument characterization yields an expanded relative
measurement uncertainty of better than 2 % for typical wire colors and
sizes.
A formal calibration service is being established for both standard wires and
contrast-measuring instruments.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2002" - Table of Contents |
