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 available from NIST Technology Services or
through collaborative research efforts.
The Division maintains facilities for measuring optical properties such as reflectance, retroreflectance,
transmittance, color, and gloss; for photometric measurements
such as luminous intensity and color temperature; and for radiometric measurements
such as spectral radiance, spectral irradiance, spectral power, detector 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 have been developed for measuring
the emittance and retroreflectance of materials and for providing standards for ultraviolet spectral irradiance.
Accomplishments
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Hyperspectral Image Projector for the Calibration of Earth Remote-Sensing
Instruments
Present approaches for calibrating multispectral
and hyperspectral imagers and sounders, such as those used in climate-change research and weather forecasting,
use spatially uniform and temporally constant optical radiation sources, such as lamp-illuminated integrating
spheres, to fill the full aperture of the light-collecting telescope in front of the sensor. Consequently,
many systems are not tested prior to deployment with optical signals that mimic the spectrally, spatially, and
temporally varying signals expected during operation. The lack of such testing is of particular concern for
satellite imagers due to their high cost, difficult operating environment,
and inability to be retrieved.
A Hyperspectral Image Projector (HIP) is being developed to enable the realistic test, evaluation, and
calibration of ground and space-based optical radiation sensors. The HIP is based on digital micromirror
devices (DMDs) commonly found in commercial high definition televisions and projectors.
DMDs consist of 768 × 1024 individually addressable, 15 ?m square mirrors.
One DMD is illuminated by a prism or a grating, which disperses broadband light into a spectrum focused onto
the DMD. The DMD allows 1024 narrow spectral bands to be modulated in intensity, thus creating a user-specified
spectral distribution of light. A programmable source with such a high spectral fidelity is essential for
realistically reproducing the spectral content of solar radiation reflected off of the Earth’s surface or of
the thermal emission of a chemical-agent cloud. DMDs used for video displays have color-filter wheels to
generate visual images. However, the HIP generates a spectral match, not just a color match.
A second DMD, optically in series with the first, projects any combination of these arbitrarily programmable
spectra into the pixels of a (768 × 1024) element spatial image, thereby producing temporally integrated images
having spectrally mixed pixels. Calibration of the resulting image using an absolute spectroradiometer provides
the absolute values of the radiance as a function of wavelength, spatial coordinate,
and time. This calibrated scene, chosen, for instance, to mimic the view of the ocean by a satellite in a
geostationary orbit, is then directed into the sensor under test.
Tool Tackles Translucence and Other Color Challenges
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Figure 8. The NIST goniospectrometer measures the intensity of light reflected from the surface of a
sample at 332 points. A plot of these measurements results in a different shape depending on whether the
illumination comes from above (top) or at a 60-degree angle (bottom).
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Special effect paints and coatings, such as some automobile paints, look different depending on illumination or
viewing angles, or both, subtleties that cannot be accounted for by traditional color measurement instruments.
To address this deficiency in measurement capabilities, NIST has developed a goniospectrometer that automatically
measures the color of light reflected from a surface as a function of illumination and viewing angles.
The new goniospectrometer provides more complete data on the reflection of light from a color surface, and will
be used for calibrating similar instruments and for research on exotic-appearing materials and coatings.
A database of measurements of different materials that could be used for modeling surfaces that have complex
visual effects will be developed using this new capability. The work is part of a NIST effort to develop
accurate measurement methods for reproduction and quality control of appearance attributes, including
color matching, by determining the optimal subset of illumination and viewing geometries needed to accurately characterize
the perceived color.
The goniospectrometer, housed in a clean room, illuminates a sample with a range of wavelengths of visible light,
every 5 nm from 360 nm to 780 nm, i.e. from the near ultraviolet/deep blue to red/infrared.
The sample and detector are rotated around three axes, allowing illumination
and viewing in any direction within a hemisphere around the sample.
The intensity of the reflected beam is measured at several hundred locations on a sample surface,
as shown in Figure 8.
Based on these measurements, computer software assigns a numerical value to the color of the reflected light at each illumination-
viewing geometry.
Perceived and Measured Colors of Retroreflective Materials used in
Traffic Signs
In collaboration with the Federal Highway
Administration (FHWA), Division scientists performed a pilot study to compare measured colors with
perceived colors for roadway signs. The study was initiated in response to anecdotal observations that
instrument-measured daytime chromaticities of retroreflective materials do not correspond well with
perceived chromaticities.
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Figure 9. Cameron Miller mounts a STOP sign on the goniometer at the NIST Center for
High-Accuracy Retroreflection Measurements (CHARM)..
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In the pilot study, instrument measured appearances were directly compared with visually perceived
appearances. A hue scaling method was used to measure the colors perceived by the human subjects for
six different types of retroreflective materials, as well as for a diffuse reference reference
material. The test samples had chromaticities within the range allowed by U.S. Federal regulations for
yellow and white traffic signs.
The results of this pilot study were presented at the Quadrennial Session of the International
Illumination Commission (CIE) held in China in July 2007. Overall,instrument measured chromaticities of
retroreflective materials corresponded well with observer reported chromaticities.
Both the instrument measurements and a majority of observer data showed a marked decrease in chromatic
saturation(colorfulness) for yellow and orange retroreflective samples, compared to the yellow and orange
diffuse reference material. To confirm and expand these findings, a larger study involving more colors
is under development at the FHWA laboratories.
Implementation of “Once is Enough” at NIST
To help improve the quality and control the costs of its measurement services, the Division has
implemented a “Once is Enough” measurement strategy to eliminate unnecessary repeat measurements in
calibrations. Spectral Irradiance Lamps and Spectral Transmittance Filters are two of the Division’s
measurement services in which “Once is Enough” is being applied.
To help Measurement Service customers
understand this transition and to aid calibration laboratories in implementing a similar approach in their
own facilities, the Division has documented the “Once is Enough” process for the Spectral Irradiance Lamp
example in an article in the Journal of Research of the National Institute of Standards and Technology.
The implementation strategy has five critical components: automation,
uncertainty budget, measurement process controls, quality system, and peer review, described in detail in
the article. Careful attention to each of these components provides the calibration scientist with the
necessary control over the entire process to confidently eliminate repeat measurements.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents |
