Optical Radiation Standards:
to develop and provide optical radiation standards
based on the SI units.
INTENDED OUTCOME AND
BACKGROUND
The Optical Technology Division plays a fundamental role in
promoting the use of optical technology by undertaking research and development
to advance the measurement science needed to maintain the Nation's primary SI
standards for the candela and kelvin and associated photometric, colorimetric,
pyrometric, and spectral radiometric quantities. These standards affect a host
of industries, from aerospace to lighting, by ensuring the accuracy and
agreement of measurements between and within organizations.
A significant part of the Division's activities includes participation in
international comparisons of measurements with other national metrology
institutes through the Consultative Committees on Temperature, and on
Photometry and Radiometry, of the International Committee of Weights and
Measures. These comparisons provide an important assessment of measurement
quality and help guarantee the international acceptability of our Nation's
optical radiation measurements.
To ensure unsurpassed measurement accuracy, the Division's radiometric scales
are increasingly being based on stable, low-noise, highly linear detectors,
radiometers, and photometers. Their absolute responsivities are traceable to
optical-power measurements performed using cryogenic radiometry and a
state-of-the-art laser facility, SIRCUS (Spectral Irradiance and Radiance
Calibrations with Uniform Sources). Cryogenic radiometry provides the most
accurate optical-power measurements by directly comparing optical and
electrical power. The Division's High-Accuracy Cryogenic Radiometer (HACR), the
Nation's standard for optical power, achieves a relative standard uncertainty of
less than 0.02 %. This will be reduced to 0.01 % with the completion
of HACR 2.
The improvements realized by tying the radiometric measurements to a cryogenic
radiometer are significant. Recently the Division's spectral-irradiance scale,
as disseminated by FEL lamps, was converted to a detector-based scale traceable
to optical-power measurements performed by HACR. The improved detector-based
scale has led to a reduction of the spectral-irradiance uncertainties by a
factor of two for ultraviolet and visible radiation, and a more significant
reduction for infrared radiation.
These activities in detector-based radiometry are complemented by new research
in source-based radiometry, i.e., radiometry based on a source of radiation
whose spectral output is absolutely known. This new research uses synchrotron
radiation from an electron-storage ring, the recently upgraded NIST Synchrotron
Ultraviolet Radiation Facility (SURF III), as a source of continuously
tunable, intense ultraviolet radiation.
Accomplishments
Detector-Based Radiance and Radiance-Temperature Scales
We have reduced the uncertainties of the present, source-based,
spectral-radiance and radiance-temperature scales by converting to
detector-based scales.
These scales are tied to an absolute imaging pyrometer (AP1). We calibrated its
absolute radiance responsivity at the SIRCUS facility, using a tunable,
laser-illuminated integrating sphere as the source of spectral radiance. The
spectral radiance is traceable to silicon trap detectors calibrated for
absolute power responsivity against HACR and a set of apertures of known area
to define the geometry.
Measurements performed with AP1 of the freezing point of a high-emissivity
blackbody cavity in contact with molten gold reveal a noise-equivalent
temperature of 2 mK (k = 2) at 1337 K. The total
uncertainty of the measurements, about 120 mK (k = 2) at
this temperature, is due to the combined uncertainties arising from the
radiance-responsivity calibrations, the size-of-source effect, and the
long-term stability of the pyrometer.
In concert with the AP1 development, we have performed radiance-temperature
measurements of a 2950 K high-temperature blackbody (HTBB), comparing both
conventional source-based and newer detector-based measurement methods. The
source-based approach uses measured radiance relative to a gold-point,
fixed-temperature blackbody to assign the temperature to the HTBB as prescribed
by the International Temperature Scale of 1990. The detector-based approach
uses a set of filter radiometers to assign a temperature to the HTBB. Their
spectral-irradiance responsivities are traceable to the HACR optical-power
scale through the Division's Spectral Comparator Facility. The
spectral-irradiance responsivities are converted to spectral-radiance
responsivities by an appropriate choice of precision apertures and measurement
geometries. The aperture areas required in the analysis are determined using an
optical coordinate-measuring machine. The net result: we demonstrated a
detector-based temperature uncertainty of 0.21 K (k = 2)
at 3000 K, more than a factor of six better than the source-based approach.
SURF III as an Absolute Source of Spectral Irradiance
An experiment was undertaken to verify that the absolute spectral irradiance
from the NIST Synchrotron Ultraviolet Radiation Facility (SURF III) can be
predicted using the Schwinger relativistic electrodynamical model for
synchrotron radiation and knowledge of the electron-beam energy, current, and
radius.
The study was performed on Beamline 3, recently developed for absolute,
source-based, ultraviolet radiometry at SURF III. The measurements
consisted of characterizing the angular spread of the radiation from
SURF III in the direction perpendicular to the orbital plane of the
electron beam. Because of the highly relativistic speed of the electron beam,
the angular spread is narrowly confined to within a fraction of a degree of the
orbital plane. Narrow-band filtered radiometers, with spectral responsivities
measured to 0.1 % relative uncertainty (k = 2) at the
SIRCUS facility, were used to directly measure the radiation emitted from a
tangential source point of the ring, from the near ultraviolet to the infrared.
The experiment demonstrated agreement with theory to within 0.5 %. Such
excellent agreement not only confirms the Schwinger theory, but it also
connects the detector-based spectral-irradiance scale, based on SIRCUS and
cryogenic radiometry, to the source-based scale of SURF III. The study
further validates Beamline 3 as a broadband standard of spectral
irradiance for the absolute calibration of optical instruments and for the
calibration of deuterium and FEL incandescent lamps as secondary or transfer
standards.
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(© Denease Anderson).
Figure 1. Jeanne Houston and Joe Rice preparing to calibrate a
silicon-photodiode trap detector using our second-generation High Accuracy
Cryogenic Radiometer, HACR 2. |
Second-Generation High-Accuracy Cryogenic Radiometer (HACR 2)
A second-generation cryogenic radiometer is under development to further reduce
the uncertainty in our optical-power measurements. The expected factor-of-two
reduction in the power-measurement uncertainty will affect many of the
radiometric and photometric scales maintained in the Division and presently
tied to the first-generation HACR.
The new instrument is designed to have a greater dynamic range, from 1 µW
up to 70 mW in power, faster response time, lower noise figure, and
improved modular construction. It will be installed in the SIRCUS facility,
providing ready access to a variety of lasers. These lasers allow a broad range
of wavelength and power levels to be selected for scale transfer to silicon
trap detectors, further reducing the uncertainties in the measurement chain.
Additionally, the modularity of the detector section permits new detector
modules to be designed and built, optimized for specific wavelengths and power
levels.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2002" - Table of Contents |
