Radiometric Physics Division
name changed to
Technical Highlights
- Irradiance Scale Realization. A central calibration service
provided by the Division is the calibration of lamp standards for spectral
irradiance. At present, the spectral irradiance scale has a 2σ
uncertainty between 0.7 % and 4.3 % in the spectral region extending
from 0.2 µm to 2.4 µm. The goal of ongoing research is to reduce the
uncertainty by a factor of five to ten. This uncertainty reduction will be
achieved by elimination of several steps from the present calibration method
and by using the HACR as the primary calibration standard.
A suite of six filter radiometers calibrated with respect to the HACR will be
used to measure spectral radiance and infer the temperature of a
high-temperature blackbody. The spectral irradiance produced from the lamp
standards can then be determined by comparing the spectral irradiance produced
from the high-temperature blackbody and the lamps using a monochromator.
Characterization of the filter radiometers has already been completed this year
by measuring the absolute spectral response in the Detector Spectral Comparator
and by mapping the point spread response. The absolute spectral response
(Figure 1) was checked by comparing the predicted signals using the filter
radiometer calibration and the measured signals from a quartz-halogen lamp
calibrated on Facility for Automatic Spectroradiometric Calibrations (FASCAL).
The discrepancy between the predicted and measured signals were less than
0.7 % for all six filter radiometers. ["Comparison of Filter
Radiometer Spectral Response with the NIST Spectral Irradiance and Illuminance
Scales," in press for Metrologia]. (B. Tsai, C. Johnson,
R. Saunders, and C. Cromer)
Figure 1. Graph of the absolute spectral response for six filter
radiometers, measured on the Visible/Near Infrared (for filter radiometers #2
through #6) and the Ultraviolet (for filter radiometer #1) Spectral Comparator
Facilities.
- Radiometric Metrology for Environmental Science and Remote
Sensing. Environmental science is a broad term for the
interdisciplinary effort to measure, model, predict, and control the
interaction between natural and anthropogenic perturbations on the
Earth system. Many of the sensors used to measure the environmental
parameters of interest are optical in nature and require highly
accurate radiometry to meet stated science and policy goals.
NIST met with representatives from NASA's Mission to Planet
Earth and the EOS, and developed a plan for establishing a long-term
collaboration between the EOS Project Office and NIST's Radiometric
Physics Division. In addition, the three-year collaboration with the
ocean color science group at NASA, the Sea-viewing, Wide Field-of-view
Sensor Project (SeaWiFS) produced results, and lessons that will
be beneficial to the development of the EOS program. Finally, the
collaboration between NIST and the government agencies that are
tasked to monitor the amount of uv radiation incident on the Earth's
surface (i.e., EPA, FDA, USDA, and NSF) matured into a long-term
program with NIST providing a key role. Highlights in these three
areas are given below.
For EOS, most of the activity this year has centered on program development and
NIST participation on calibration peer review panels for a number of EOS
instruments in area of ocean color, NIST delivered the SeaWiFS transfer
radiometer (SXR) to NASA. Prior to delivery, NIST participated in the third
SeaWiFS Intercalibration Round-Robin Experiment (Figure 2), using the SXR
to measure seven integrating sphere sources and six flat diffuse plaques. In
addition, NIST analyzed and published SXR data from the previous round-robin
(1993).
Figure 2. Comparison of spectral radiance scales of four laboratory
standard integrating sphere sources using the SXR at the third SeaWiFS
round-robin. The environmental science measurement requirement is to determine
the ocean's spectral radiance to within 1 % of the true value.
To promote better terrestrial UVB solar irradiance measurements, NIST hosted a
workshop on uv metrology in May 1994, and has published a conference report and
an executive summary. Through the USGCRP, NIST participated in the drafting of
a U.S. interagency uv monitoring network plan. The plan included the formation
of an interagency quality assurance panel and the establishment of a network
central calibration laboratory.
A permanent site in the EPA-UVB Network was established at NIST. Currently
routine measurements are being made at NIST of uv irradiance and column ozone
with a spectroradiometer and direct and diffuse solar irradiances atmospheric
optical depth in the visible and near infrared are being measured with a DOE
developed multichannel shadow-band radiometer. A second sun photometer deployed
by NASA at NIST makes multichannel direct beam solar irradiance measurements.
Both instruments are used in determining atmospheric optical depths used in
estimating aerosols concentration. This experiment compares the sun photometer
estimates with data from the shadow-band instruments. NIST organized and
conducted the first Interagency Ultraviolet Spectroradiometer Intercomparison
which was held at Table Mountain near Boulder, CO in September, 1994
(Figure 3). The intercomparison was structured to assess the accuracy of
the ordinary instrument calibrations and partially characterize the instruments
by performing several tests, followed by several days of simultaneous solar
observations.
Figure 3. Comparison of measured solar ultraviolet irradiance near local
solar noon on September 23, 1994 at Table Mountain near Boulder, CO.
Instruments were calibrated by NIST personnel with a specially-calibrated
horizontal irradiance standard. The need is to detect the resultant increase in
terrestrial solar uv irradiance from a projected 10 % per decade decease
in stratospheric ozone.
- Monochromator-Based BRDF Measurements. The Monochromator-based BRDF
instrument (Figure 4) was put into operation this year
and was named "STARR" (Spectral Tri-function Automated Reference
Reflectometer). STARR can characterize samples up to 40 cm2
over the spectral region 0.2 µm to 4 µm with improved speed and
accuracy. The present spectral region is limited by the Si and InAs detection
systems but with other detectors the spectral range of the instrument can go to
20 µm. The design of the new instrument will allow quick comparison of the
BRDF and the hemispherical measurements of a same sample thus minimizing the
time for a sample to change its reflectance properties.
Figure 4. Optical layout of the STARR used to measure BRDF of highly
diffuse plaques.
STARR was compared to the old instrument using samples that were characterized
on the old instrument with difference within a few percent. (J. Proctor,
Y. Barnes, and R. Saunders)
- SURF III Synchrotron Radiation Source. The Radiometric Physics
and Electron and Optical Physics Divisions at NIST are collaborating to
establish the new Synchrotron Ultraviolet Radiation Facility (SURF III) as
the world's most accurate absolute radiometric standard source from the extreme
ultraviolet through far infrared spectral regions. SURF III will replace
SURF II, which is presently the nation's far uv standard source.
At least two new beamlines will be developed at SURF III for radiometric
research and calibrations, in addition to the two existing beamlines used for
absolute radiometry and extreme ultraviolet detector calibrations. Initial
research will focus on improving ultraviolet radiometric scales and measurement
techniques in support of industrial needs (such as photolithography for
semiconductor device manufacture) and environmental monitoring (such as solar
ultraviolet irradiance monitoring both on the earth's surface and above the
atmosphere). Absolute cryogenic radiometric techniques will be extended into
the ultraviolet and far ultraviolet spectral regions, and be applied to high
accuracy radiometric measurement of the SURF III electron current. The
precisely known relative spectral distribution of the SURF III synchrotron
radiation will be exploited to reduce the uncertainty in the temperature scales
maintained by the Division. The high brightness, highly polarized synchrotron
radiation can be used for research into optical properties of materials over
broad spectral ranges. SURF III will enable NIST to better support the
scientific and technical needs of industry and strategic national programs.
(T. O'Brian, J. Proctor, A. Parr, and Electron and Optical
Physics Division staff)
- Spectral Calibrations at the NIST Low Background Infrared
Radiometry (LBIR) Facility. The goal of the LBIR facility is to provide
the infrared community with a calibration base which includes total,
as well as, spectral radiant flux calibrations. The LBIR facility began
broadband radiant flux calibrations of cryogenic sources in 1990. A
second cryogenic-vacuum chamber has been brought into service
which houses the spectral instrument, and will be the center for the
spectral calibrations of sources, detectors and materials (Figure 5). The
new chamber, like the existing LBIR chamber, will provide a 20 K
environment to perform experiments. The LBIR spectral instrument
is composed of a prism predisperser, followed by a grating
monochromator. It provides coverage over the spectral range from 2 µm
to 30 µm. This calibration capability is currently being tested
and developed into three areas: spectral calibration of infrared
detectors for use in low-background applications; spectrally resolved
radiation from blackbody sources and characterization of optical
components.
Figure 5. LBIR spectral calibration chamber.
Infrared detectors are not readily available that can provide NIST
traceability for the wavelength range of 2.5 µm to 30 µm.
To meet this detector need of industry Rockwell Corporation has been
contracted to develop a standards quality infrared detectors. The
detectors are of a blocked impurity band design employing arsenic doped
silicon. The range of spectral coverage is 2 µm to 30 µm
at an operating temperature of 12 K. The detectors are suited for use
as transfer standards in the calibration of on-orbit sensor systems.
(S. Lorentz, S. Ebner, and J. Walker)
- Fourier Transform Infrared (FT-IR) Spectrometer Methodology and
Instrumentation. Infrared (IR) spectroscopic measurements are
required in a wide variety of applications in U.S. industry and
government. FT-IR instruments have many advantages over grating
or prism instruments such as a better signal to noise ratio and faster
spectral acquisition which has led to their widespread use. FT-IR
instruments have sources of potentially significant error which require
characterization of the instrument and usage of appropriate standards for
calibration.
Errors in FT-IR measurements are difficult to handle because the direct
result of measurement, the detector signal must be Fourier transformed to
obtain the final spectral information. Thus error
sources such as detector system non-linearity can result in a variety
of errors in the final spectrum, including positive and negative shifts
as well as additional spectral structure.
Infrared transmittance standard reference materials are needed to calibrate
detector non-linearity and spectral responsivity. These should be spectrally
neutral in the wavelength region from 2 µm to 25 µm. Commercially
available neutral-density filters with optical density (OD) greater than 2
exhibit significant variations in OD over this broad wavelength region. A study
of metal thin film filters was performed to develop improved neutral density
filters. The optical constants of the films were obtained from the
transmittance, reflectance, and thickness measurements. Evaporated alloy films
were found that yielded flat transmittance for OD near 3 and 4. A comparison of
spectra of the new filters (solid curves) and a typical commercial filter
(dashed curve) is shown in Figure 6. These new filters are being developed
as SRM's to be provided to FT-IR users for their instrument calibration.
Figure 6. Comparison of new NIST filters and a commercial filter.
In many applications, material optical properties are needed at cryogenic
temperatures. To meet this need a cryogenic optical apparatus, shown in
Figure 7 has been developed, which in combination with an FT-IR
spectrometer will be capable of measuring transmittance and reflectance over a
range of sample temperatures from 6 K to 100 K. It incorporates four
Si bolometer detectors to cover the wavelength range from 1 µm to
1000 µm.
Figure 7. Liquid He cryostat for cryogenic spectrophotometry.
- Correlated Photon Radiometry. The Division is developing the
capability to measure the absolute responsivity of a 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 to a determined direction and wavelength. 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
(Figure 8). The counting rate of each detector is recorded along with the
coincidence rate of 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.
Figure 8. Correlated photon source.
In the highest accuracy tests to date, this method has been verified by
simultaneously measuring the efficiency of a photomultiplier using this
technique and a conventional measurement scheme. The results showed agreement
to about 0.5 %, which is 1σ uncertainty of the measurements. Further
improvements are planned to test the accuracy at the 0.1 % level.
(A.L. Migdall).
- High Accuracy Cryogenic Radiometer. The HACR was developed to
improve the accuracy of all the radiometric scales within the Division, and
link them to a common primary standard using calibrated transfer standards. The
HACR was used in conjunction with silicon photodiode transfer standards to
establish a new scale of absolute spectral response with an accuracy of
0.03 % in the wavelength range between 406 nm and 920 nm. The
capability to do measurements at a wavelength of 10.6 µm are currently
being developed and a new cryogenic bolometer that will serve as the transfer
standard for the Infrared Detector Comparator Facility is being characterized.
A detailed search for systematic errors in the operation of the HACR has
demonstrated an absolute accuracy of 0.02 %. This detailed study has
revealed the important noise sources in the instrument, and thus where
improvements can be made to increase the accuracy of the HACR for lower power
measurements.
The HACR has been used to calibrate silicon photodiode light-trapping detectors
at nine wavelengths between 406 nm and 920 nm, with a typical
accuracy of 0.03 %. The results of this work will soon be the basis for
the working standards of the Detector Comparator Facility, resulting in an
improvement in accuracy for customer calibrations. Other activities planned for
this year include developing and calibrating transfer standards at
near-infrared and ultraviolet wavelengths, and decreasing the noise floor of
the HACR to facilitate measurements with lower power sources. In addition, a
Laser Comparator Facility will be brought into service; the purpose of this
facility will be to allow the calibration of working standards against the
HACR-calibrated transfer standards at laser wavelengths, and also to
characterize candidate transfer standards for use with the HACR.
(J. Houston and T. Gentile)
- Calibration Quality Program: ISO/IEC Guide 25. Calibration
laboratory accreditation has been increasing along with the trend of
companies and organizations in the US seeking quality system
registration (ISO 9000). Accreditation is seen by many laboratories to
be an important step in gaining an edge over their competitors,
especially where international trade is concerned. In response to NIST
customer's requests and to help lead the nation into the future of
laboratory accreditation, an effort is underway by the Radiometric
Physics Division to document its quality system for the calibration
services it offers. The quality documentation is based on the ISO/IEC
Guide 25 and the ANSI/NCSL Z540-1-1994. The calibration services
participating in the effort are: Radiance Temperature Measurements,
Spectroradiometric Source Measurements, Optical Properties of
Materials Measurements, Photometric Measurements, and Spectroradiometric
Detector Measurements. The compliance with ISO Guide 25 Standard, and the
effort to document the quality system will benefit the services offered by the
Division. (S. Bruce and T. Larason)
- Aperture Measurement. High accuracy optical flux measurement
generally involves the use of one or more apertures of known area to
define the geometrical extent of the optical beam. The effective
aperture area in many cases is a limiting factor in the overall
uncertainty of the measurement. This project addresses both direct
absolute area measurement and transfer measurements relative to an
reference aperture of known area. These measurements will support
both activities within the division and the needs of industry.
An instrument has been built and tested in the Division that can
compare effective aperture area relative to an reference aperture with
uncertainty better than 0.037 % including the uncertainty of the
reference aperture, which is currently 0.022 %. An instrument to
measure the absolute area of apertures is also being developed
utilizing a customized commercial 2D coordinate measuring machine.
The machine will measure diameters and/or chords of apertures and
areas will be calculated using various techniques being developed in
the Division. The areas thus determined will be directly traceable to
the NIST length standards. (J. Fowler and C. Cromer)
- Ambient Background Infrared Detector Calibration Facility
(IRDCF). An IRDCF has been developed to provide absolute spectral
response measurement of detectors in the 2 µm to 20 µm
wavelength range. The high flux 10 kW argon arc ir source, is imaged
to the input of a prism-grating infrared monochromator which was
designed and built at NIST. The test detectors are substituted for the
transfer standard bolometer in the imaged output beam of the
monochromator. The detectors will be calibrated for radiant power
response versus wavelength.
The cryogenic transfer standard bolometer, also developed at NIST, is a high
sensitivity thermal detector linear below 10 mW, and with a noise floor of
20 pW. The bolometer is calibrated against the HACR at a number of laser
wavelengths. The relative spectral response of the bolometer was determined
using measurements of the total hemispherical reflectance of the gold black
coating on the bolometer receiver between 2 µm to 17 µm using an
specially designed FT-IR spectrometer. The bolometer response stability and
spatial uniformity are both better than 0.5 % (1σ).
This new facility will allow NIST to disseminate spectrally calibrated infrared
detector standards and provide a basis for research in improved detectors.
(A.L. Migdall, G. Eppeldauer, L. Hanssen, and
J. Rice)
- Photometry Research. The Division is responsible for the
realization of the candela, one of the SI base units, and other
photometric units for luminous flux (lumen), illuminance (lux),
luminance (cd/m2), and color temperature (K). Photometric standards
are critical to many industries including lighting, optical instruments,
visual displays, aircraft, automobile, etc. The photometric units are
based on standard photometers which are traceable to the Division's
HACR. In 1994, calibration services for fluorescent lamp luminous
flux, and the issuing of luminous intensity standard lamps (1000 W
FEL) were started. The Division now accepts various artifacts for
photometric calibration such as illuminance/luminance meters,
photometers, sphere sources, opal glasses, colorimeters, as well as
various types of lamps including miniature lamps. In an effort to
verify the state-of-the-art accuracy of the national standards,
intercomparisons of photometric units with PTB, Germany and OMH,
Hungary have been performed.
The realization of a new luminous flux unit has begun using a
newly-developed integrating sphere method, traceable to the HACR.
The work will be completed, and the new unit will be disseminated by
the end of 1995. The luminous flux calibration facility will be
renovated with a new 2.5 m integrating sphere. In addition, a study
has begun to develop illuminance sources for very high level
illuminance and luminance meter calibrations.
The Division is also developing capabilities for colorimetric characterization
of color displays and other color imaging systems. There is an increasing need
for accurate spatially and temporally resolved photometric and colorimetric
measurements of color displays, including flat panel displays, and also for
precise reproduction of colors in imaging systems using CCD cameras, scanners,
monitors, and color printers, particularly for commercial applications. The
division plans to develop standard procedures for the characterization
measurements to help users of such devices evaluate and compare products, and
to support manufacturers. (Y. Ohno, J.E. Hardis, G. Eppeldauer,
and C.L. Cromer)
- DUV Metrology for Semiconductors. The trend in the
semiconductor industry towards smaller feature sizes increases their
reliance on accurate uv metrology. Deep-uv (DUV) radiation is used to
expose images onto silicon wafers coated with photoresist, which are
then chemically processed to develop the features of the circuit.
Incorrect exposure means that the lines on the chips would be too
narrow or not well-defined, spoiling the process and reducing
manufacturing yield.
The Division, in collaboration with the Atomic Physics Division,
built and delivered to SEMATECH two calibrated spectroradiometers
and a mobile calibration unit (MCU). The MCU contains a third
spectroradiometer that can be returned to NIST periodically to
maintain the calibration on the other two instruments. These
spectroradiometers contain probes that can be mounted on the wafer
plane of a commercial wafer-stepper to receive the same DUV dose as
silicon wafers would. Fiber optics bring the DUV signal out to a
dispersive grating spectrograph, and its spectral image is focused onto
a cooled CCD detector. To make accurate irradiance measurements,
a diffuser is required to make the probe insensitive to the angle of
incidence of the DUV radiation. In the course of this work, several
candidate DUV diffuser materials were tested for their optical
properties (transmission and Lambertian character) and for their
resistance to radiation damage. Sintered aluminum oxide was
determined to be the material of choice.
This work was a step in ongoing collaboration between the
Division and the semiconductor industry. Earlier work included the
analysis of commercial mercury 1-line (365 nm) exposure meters, to
resolve differences in their performance, and the construction of an
earlier generation of spectroradiometer. Future work includes further
research on high-intensity DUV sources for lithography and its
metrology, the development of detector-based calibration systems for
greater accuracy and stability. (C. Cromer and J. Hardis)
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