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 advancing
the use of optical technology. Through
research and development, we 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 participating in international
comparisons of measurements
with other national metrology institutes.
They are organized 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
highest accuracy optical power measurements
by performing a direct comparison
of 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 better
than 0.02 %.
The improvements realized by tying the
radiometric measurements to cryogenic
radiometry are significant. Recently the
Division's spectral irradiance scale, as
disseminated by FEL-type lamps, was
converted to a detector-based scale
traceable to the HACR. The improved
scale has led to a reduction of spectral
irradiance uncertainties by a factor of
two in the ultraviolet and visible, and
greater in the infrared. The spectral
radiance and radiance temperature
scales will also be detector based in the near future.
These advances 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 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
light. The SURF III effort is complemented
by programs to develop
improved blackbody sources.
Accomplishments
High-Temperature Radiometric Standards
To address the lack of high-temperature
standards in the International Temperature
Scale of 1990 (ITS-90), the
Division is researching the suitability of
blackbody-based standards tied to the
fixed points of the phase transitions in
metal-carbon and metal-carbide-carbon
eutectics. Such fixed points offer the
potential for temperature standards as
high as 3034 K (using the TiC-C eutectic),
over a factor of two greater than
the present high-temperature limit of
1358 K (fixed at the Cu freezing point).
The availability of high-temperature
standards would allow routine measurements
to be made between 1358 K and
3034 K by interpolation rather than
extrapolation from the 1358 K standard,
resulting in a reduction of temperature
measurement uncertainties by a factor
of five near 3000 K.
To accurately measure the thermodynamic
temperatures of these fixed
points, the Division is developing
absolute pyrometers calibrated against
a cryogenic radiometer using SIRCUS.
The first generation of these absolute
pyrometers, denoted AP1 and operating
at 650 nm, has a noise figure of approximately
2 mK and a one-year stability
of approximately 100 mK at 1300 K.
Efforts are directed at improving the
stability and extending the operating
wavelength into the ultraviolet near
365 nm, to achieve reduced uncertainties
near 3000 K.
In assessing the appropriateness of
these eutectics for high-temperature
standards, the Division recently participated
in a measurement comparison
with the National Physical Laboratory
(UK). The measured phase-transition
temperatures were compared for five
metal-carbon eutectics (Co-C, 1597 K;
Pd-C, 1765 K; Pt-C, 2011 K; Ru-C,
2226 K; and Re-C, 2748 K) relative to
the ITS-90 temperature scale. The
measured temperatures agreed to within
0.5 K, demonstrating the utility of these
eutectics for international temperature standards.
New Infrared Calibration
Facility
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(© Denease Anderson).
Figure 1. Spectral irradiance responsivity for an
InSb radiometer measured on IR-SIRCUS. |
The increased need for highly accurate
infrared radiation measurements for
homeland security, national defense, and
remote sensing applications has led the
Division to develop, with support from
the Air Force through the Department
of Defense Calibration Coordination
Group (CCG), an infrared SIRCUS facility.
When completed, IR-SIRCUS will
extend the visible-to-ultraviolet capabilities
of the present SIRCUS into the
1 µm to 20 µm wavelength region of
the infrared, including the critical long-wave
and mid-wave infrared atmospheric
windows, coincident with the peak
spectral responsivities of HgCdTe and
InSb single-element detectors and focal
plane arrays. (See Fig. 1.)
The facility will allow the characterization
and calibration of a variety of
infrared imaging and camera systems,
including hyperspectral imagers, at a
level of detail and accuracy not possible
with conventional broadband, blackbody radiation sources.
Challenging the development of IR-SIRCUS
is the lack of availability of
commercial high-dynamic-range transfer
detectors and broadly and continuously
tunable high-power infrared lasers.
Therefore, the IR-SIRCUS team has
constructed a 1 µm to 5 µm infrared
laser system based on optical parametric
oscillators [LiB3O5 (LBO-OPO) or periodically
poled LiNbO3 (PPLN-OPO)]
pumped by a mode-locked Nd:Vanadate
laser. For a transfer-standard detector
the team is using a NIST-developed,
liquid-helium cooled, electrical-substitution bolometer.
With this laser system and detector,
infrared detectors, radiometers, and
focal plane arrays can be calibrated on
IR-SIRCUS for spectral power, spectral
irradiance, or spectral radiance responsivity
between 1 µm and 5 µm with a
relative standard uncertainty of 1.5 %.
Novel LED Sources for Radiometry
| |
Figure 2.Solid-state integrating sphere
source pumped by light emitting diodes,
showing some of the different colors that
can be generated by varying the input
current to the individual LEDs.
|
The Division is developing new radiometric
applications for light emitting
diodes (LEDs) to take advantage of their
unique properties, which include narrow
bandwidth, high spectral brightness,
compact size, high efficiency, and low cost.
A prototype LED-illuminated integrating
sphere source has been successfully
developed using 40 LEDs operating in
10 distinct bands between 380 nm and
780 nm. Varying the light intensity in
the individual bands allows variation
of the spectral output or color, as shown
in Fig. 2. A second-generation LED
source, now under development, will
use 150 diodes grouped into 40
distinct bands to achieve a source that
can match a conventional source
within 2 % for luminance and 0.002 in chromaticity.
A compact 660 nm LED source, consisting
of a tube containing 36 diodes
illuminating a volume-diffuser output
coupler, has also been developed to
allow the routine monitoring of the size-of-source
error in optical pyrometers.
The size-of-source error provides a rapid
assessment of the stability of the optical
alignment of the pyrometer and the surface
cleanliness of the optics. It can be
an important correction to the measurement
and an important component of
the measurement uncertainty, depending
upon its magnitude.
Following a similar design, a compact,
blue LED source has been developed to
model ocean color for the calibration of
instruments measuring ocean optical
properties. By matching the spectral
output to the ocean waters of interest,
many potential sources of calibration
error in the instrument under test, such
as stray light, can be reduced or eliminated.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2004" - Table of Contents |
