to develop and provide optical radiation standards
based on the SI units.
INTENDED OUTCOME AND BACKGROUND
The Optical Technology Division provides the optical radiation measurement science and standards
to aid the advancement and application of optical technology. In particular,
the Division advances, maintains, and disseminates standards for the candela and kelvin base SI units,
and associated photometric, colorimetric, pyrometric, and spectral radiometric quantities.
These standards benefit industries from aerospace to lighting, by ensuring the accuracy and
consistency of measurements between and within organizations.
The Division helps maintain the quality and international comparability of our Nation’s
optical radiation measurements and standards by participating in international measurement
comparisons with other national metrology institutes (NMIs).
These comparisons are organized through the Consultative Committees on
Temperature (CCT) and on Photometry and Radiometry
(CCPR) under the auspices of the International Committee of Weights and Measures (CIPM).
Accomplishments
New Spectral Irradiance Standards
from the NIST Synchrotron Ultraviolet Radiation Facility, SURF III
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Figure 1. Spectral irradiance responsivity for an
InSb radiometer measured on IR-SIRCUS.
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To reduce uncertainties in the ultraviolet spectral irradiance scale, to meet the increasingly
stringent demands of the climate remote sensing, semiconductor manufacturing,
and health and safety communities, the Division in collaboration with Electron and Optical Physics
Division has developed a highly accurate method for calibrating deuterium lamps.
The lamps are now calibrated in air from 200 nm to 400 nm using synchrotron radiation from the NIST
Synchrotron Ultraviolet Radiation
Facility, SURF III.
The absolute spectral irradiances are calculated
from the Schwinger equation using experimental knowledge of the SURF III electron beam current and beam
energy and accurate measurements of the area of the aperture used in the specification of the irradiance
geometry. The storage ring is operated at low flux levels to minimize radiation damage to optical components.
The total expanded uncertainty of the spectral irradiance from 200 nm to 400 nm is 1.3 % (k = 2).
A comparison of the SURF III–based calibrations with past NIST calibrations shows agreement within the combined uncertainties. The availability of a new type of deuterium lamp with relighting reproducibility of better than 0.1 % allows dissemination of a UV spectral irradiance scale with lower uncertainties, approximately
1.5 %.
Total Spectral Radiant Flux Standards Developed
In response to a request from the Council
for Optical Radiation Measurements (CORM), the Division has developed a new calibration service to disseminate
standards for Total Spectral Radiant Flux, an important fundamental attribute of light sources.
A Total Spectral Radiant Flux standard allows industry to improve their measurements of the efficacy of
non-incandescent light sources, such as solid-state light sources now under development, for which the amount
of output radiation varies significantly with wavelength.
We have realized the scale of total spectral radiant flux (W/nm) in the 360 nm to 800 nm region by using a
specialized reference goniospectroradiometer to map the lamp output over the full sphere, as shown in Fig. 2.
The use of a spectroradiometer reduces the typically dominant measurement error associated with the imperfect
matching of the spectral response of a photometer to the standard visual response of the eye.
The use of a goniometer for mapping the angular output of the sources provides
higher measurement accuracy and increased information about the angular
dependence of the lamp output not available if an integrating sphere is used instead,
as standard within the lighting industry.
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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.
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The NIST Total Spectral Radiant Flux scale is tied to the NIST spectral irradiance
scale for the relative spectral distribution
and to the NIST total luminous flux unit or lumen for the absolute spectral photometric output.
Two types of lamps standards have been made available: 75 W and 60 W quartz-halogen lamps. Higher (200 W) and
lower (20 W) power lamps will be offered in the near future. Customer-provided lamps can also be calibrated.
This new service provides national traceability for total luminous flux, as well as for color quantities of
light sources measured by integrating-sphere-based spectroradiometer systems. The uncertainty
in total luminous flux is approximately
0.5 % (k = 2), and the uncertainty in total spectral radiant flux ranges from 1.5 % at 360 nm. to 0.7% at 550 nm.
LBIR Facility Improves the Accuracy of Missile Defense Sensors
The Division continues to collaborate with the Missile Defense Agency (MDA) to ensure that infrared sensors used
in missile defense, such as for the Exo-atmospheric Kill Vehicle or EKV, have the accuracy necessary to
discriminate the infrared signature of an incoming missile from space thermal background or from thermally
emitting decoys intentionally released by or with the missile during its trajectory.
This work utilizes the Division’s state-ofthe-art Low Background Infrared (LBIR) Facility, which offers a
low-temperature thermal background that mimics the 3 K thermal background of space.
The LBIR facility is used to directly calibrate MDA and NIST infrared source and detector
standards against the LBIR Absolute Cryogenic Radiometer (ACR). The NIST source and detector standards are
deployed to MDA for calibrating test facilities.
The primary NIST detector standard deployed to MDA facilities is the BXR. Recently, the calibration uncertainty
of the BXR was lowered by a factor of two to 3 % (k = 1). This reduction was achieved through the use of a
specially fabricated and calibrated silver chloride filter to effectively
eliminate out-of-band filter infrared radiation leakage affecting the accuracy of the calibration transfer
from the ACR to the BXR.
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Figure 3. A blackbody standard being calibrated by the nIST BXR in the Exoatmospheric
Kill Vehicle (EKV) missile sensor cryogenic test chamber.
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The reduced uncertainty helped resolve a 5 % discrepancy between NIST measurements
and MDA test chamber optical models. The discrepancy was shown to be dominated by inaccurate measurement of the
reflectivity of a mirror in the test chamber and incomplete modeling of the effects of diffraction in the
optical system. Improved mirror reflectance measurements and more accurate diffraction modeling reduced the
discrepancy to 2 %, in line with the overall measurement uncertainty. This level of agreement satisfies present
EKV program needs, and will help ensure reliable warhead discrimination in the presence of sophisticated decoys.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents |
