The polarization sensitivity of the Visible/NearIR (VISNIR) bands in the Joint Polar Satellite Sensor 1 (J1) Visible Infrared Imaging Radiometer Suite (VIIRS) instrument was measured using a broadband source. While polarization sensitivity for bands M5-M7, I1, and I2 was less than 2.5 %, the maximum polarization sensitivity for bands M1, M2, M3, and M4 was measured to be 6.4 %, 4.4 %, 3.1 %, and 4.3 %, respectively with a polarization characterization uncertainty of less than 0.38%. A detailed polarization model indicated that the large polarization sensitivity observed in the M1 to M4 bands is mainly due to the large polarization sensitivity introduced at the leading and trailing edges of the newly manufactured VISNIR bandpass focal plane filters installed in front of the VISNIR detectors. This was confirmed by polarization measurements of bands M1 and M4 bands using monochromatic light. Discussed are the activities leading up to and including the two polarization tests, some discussion of the polarization model and the model results, the role of the focal plane filters, the polarization testing of the Aft-Optics-Assembly, the testing of the polarizers at the National Aeronautics and Space Administration’s (NASA) Goddard center and at the National Institute of Science and Technology (NIST) facility and the use of NIST’s Traveling Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (T-SIRCUS) for polarization testing and associated analyses and results.
Improving the precision of ground-based astronomical observations is an objective of both current (e.g. PanSTARRS1) and future (e.g. Dark Energy Survey and the Large Synoptic Survey Telescope) sky surveys. An important element of this effort is to determine the optical attenuation imposed by the atmosphere. We have obtained atmospheric extinction observations from narrowband photometry (typically 10 nm bandwidth) at central wavelengths of 380 nm, 488 nm, 500 nm, 585 nm, 656 nm, 675 nm and 840 nm. The passbands were selected to measure the continuum component (predominantly from Rayleigh and aerosol scattering) of atmospheric attenuation, and to avoid molecular absorption features in the atmosphere. We compare these atmospheric extinction observations with predictions from MODTRAN5, a commonly used computer model of atmospheric optical transmission. The MODTRAN5 calculations were informed by a satellite-based determination of atmospheric ozone on the night of observations. We also adjusted the MODTRAN5 predictions of Rayleigh scattering to account for the difference between the default pressure and that measured at the observatory on the night of observations. We find excellent agreement across all passbands between the pressureadjusted MODTRAN5 extinction model and the observations, within our typical extinction uncertainty of 0.013 mag/airmass, but only if we exclude any aerosol scattering component in the MODTRAN5 model. Even though this is a very limited test, with observations of a single star for a single night, the fact that we obtain excellent agreement between extinction measurements and the MODTRAN5 model, with no adjustable fit parameters, bodes well for exploiting MODTRAN5 to increase the precision of ground-based flux measurements.
We describe a new apparatus for measuring the spectral irradiance of the Moon at visible wavelengths. Our effort builds upon the United States Geological Survey’s highly successful Robotic Lunar Observatory (ROLO), which determined a precise model for the time-dependent irradiance of the Moon from six years of observations obtained with an imaging telescope equipped with a set of narrow-band filters. The ROLO Irradiance Model allows the Moon to be used as a radiometric reference for tracking changes in the absolute responsivity of near-infrared to visible satellite sensors as a function of time to better than 1 %. The goal of the present effort is to improve the absolute radiometric accuracy of the ROLO model, presently estimated at 5 % - 10 %, to better than 1 %. Our approach, which uses an integrating sphere at the focal plane of a telescope to direct light from the integrated lunar disk into a stable spectrograph, also eliminates the need to interpolate between the 32 visible and near-infrared bands measured by ROLO. The new measurements will allow weather, climate, land-surface, and defense satellites to use the Moon as an absolute calibration reference, potentially reducing the impact of disruptions in continuous long-term climate data records caused by gaps in satellitesensor coverage.
Changing atmospheric transmission accounts for the largest systematic errors limiting photometric measurement
precision and accuracy for ground-based telescopes. While considerable resources have been devoted to correcting the
effects of the atmosphere on image resolution, the effects on precision photometry have largely been ignored. To correct
for the transmission of the atmosphere requires direct measurements of the wavelength-dependent transmission in the
same direction and time that the supported photometric telescope is acquiring its data.
We describe a multi-wavelength lidar, the Facility Lidar for Astronomical Measurement of Extinction (FLAME) that
observes the stable upper stratosphere, and the Astronomical Extinction Spectrophotometer (AESoP), a
spectrophotometer that creates and maintains NIST absolute standard stars. The combination of these two instruments
enables high photometric precision of both the stellar spectra and atmospheric transmission. The throughput of both
FLAME and AESoP are calibrated to NIST radiometric standards.
This inexpensive and replicable instrument suite provides the lidar-determined monochromatic transmission of Earth’s
atmosphere at visible and near-infrared wavelengths to better than 0.25% per airmass and the wavelength-dependent
transparency to better than 1% uncertainty per minute. These atmospheric data are merged to create a metadata stream
that allows throughput corrections from data acquired at the time of the scientific observations to be applied to
broadband and spectrophotometric scientific data. This new technique replaces the classical use of nightly mean
atmospheric extinction coefficients, which invoke a stationary and plane-parallel atmosphere and ultimately limit
ground-based all-sky photometry to 1% - 2% precision.
NIST-calibrated detectors will be used by the ground-based 100mm diameter Astronomical Extinction
Spectrophotometer (AESoP) to calibrate the spectral energy distributions of bright stars to sub-1% per 1nm spectral
resolution element accuracy. AESoP will produce about a hundred spectroradiometrically calibrated stars for use by
ground- and space-based sensors. This will require accurate and near-continuous NIST calibration of AESoP, an
equatorially mounted objective spectrophotometer operating over the wavelength range 350nm – 1050nm using a CCD
detector.
To provide continuous NIST calibration of AESoP in the field a near-identical, removable 100mm diameter transfer
standard telescope (CAL) is mounted physically parallel to AESoP. The CAL transfer standard is calibrated by NIST
end-to-end, wavelength-by-wavelength at ~ 1nm spectral resolution. In the field, CAL is used in a near-field
configuration to calibrate AESoP. Between AESoP science observations, AESoP and CAL simultaneously observe clear
sub-apertures of a 400mm diameter calibration collimator. Monochromatic light measured simultaneously by AESoP and
CAL is dispersed by the objective grating onto the AESoP pixels measuring the same wavelength of starlight, thus
calibrating both wavelength and instrumental throughput, and simultaneously onto a unique low-noise CAL detector
providing the required throughput measurement. System sensitivity variations are measured by vertically translating the
AESoP/CAL pair so that CAL can observe the AESoP sub-aperture.
Details of this system fundamental to the calibration of the spectral energy distributions of stars are discussed and its
operation is described. System performance will be demonstrated, and a plan of action to extend these techniques firstly
into the near infrared, then to fainter stars will be described.
Improving the precision of observational astronomy requires not only new telescopes and instrumentation, but
also advances in observing protocols, calibrations and data analysis. The Laser Applications Group at the National
Institute of Standards and Technology in Gaithersburg, Maryland has been applying advances in detector
metrology and tunable laser calibrations to problems in astronomy since 2007. Using similar measurement techniques,
we have addressed a number of seemingly disparate issues: precision flux calibration for broad-band
imaging, precision wavelength calibration for high-resolution spectroscopy, and precision PSF mapping for fiber
spectrographs of any resolution. In each case, we rely on robust, commercially-available laboratory technology
that is readily adapted to use at an observatory. In this paper, we give an overview of these techniques.
Verification of the Visible Infrared Imager Radiometer Suite (VIIRS) End-to-End (E2E) sensor calibration is
highly recommended before launch, to identify any anomalies and to improve our understanding of the sensor onorbit
calibration performance. E2E testing of the Reflective Solar Bands (RSB) calibration cycle was performed
pre-launch for the VIIRS Flight 1 (F1) sensor at the Ball Aerospace facility in Boulder CO in March 2010.
VIIRS reflective band calibration cycle is very similar to heritage sensor MODIS in that solar illumination, via a
diffuser, is used to correct for temporal variations in the instrument responsivity. Monochromatic light from the
NIST T-SIRCUS (Traveling Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources)
was used to illuminate both the Earth View (EV), via an integrating sphere, and the Solar Diffuser (SD) view,
through a collimator. The collimator illumination was cycled through a series of angles intended to simulate the
range of possible angles for which solar radiation will be incident on the solar attenuation screen on-orbit. Ideally,
the measured instrument responsivity (defined here as the ratio of the detector response to the at-sensor radiance)
should be the same whether the EV or SD view is illuminated. The ratio of the measured responsivities was
determined at each collimator angle and wavelength. In addition, the Solar Diffuser Stability Monitor (SDSM), a
ratioing radiometer designed to track the temporal variation in the SD Bidirectional Reflectance Factor (BRF) by
direct comparison to solar radiation, was illuminated by the collimator. The measured SDSM ratio was compared
to the predicted ratio. An uncertainty analysis was also performed on both the SD and SDSM calibrations.
Traditionally, satellite instruments that measure Earth-reflected solar radiation in the visible and near infrared wavelength
regions have been calibrated for radiance response in a two-step method. In the first step, the spectral response of
the instrument is determined using a nearly monochromatic light source, such as a lamp-illuminated monochromator.
Such sources only provide a relative spectral response (RSR) for the instrument, since they do not act as calibrated
sources of light nor do they typically fill the field-of-view of the instrument. In the second step, the instrument views a
calibrated source of broadband light, such as a lamp-illuminated integrating sphere. In the traditional method, the RSR
and the sphere spectral radiance are combined and, with the instrument's response, determine the absolute spectral radiance
responsivity of the instrument. More recently, an absolute calibration system using widely tunable monochromatic
laser systems has been developed. Using these sources, the absolute spectral responsivity (ASR) of an instrument can be
determined on a wavelength-by-wavelength basis. From these monochromatic ASRs, the responses of the instrument
bands to broadband radiance sources can be calculated directly, eliminating the need for calibrated broadband light
sources such as integrating spheres. Here we describe the laser-based calibration and the traditional broad-band sourcebased
calibration of the NPP VIIRS sensor, and compare the derived calibration coefficients for the instrument. Finally,
we evaluate the impact of the new calibration approach on the on-orbit performance of the sensor.
It is standard practice at many telescopes to take a series of flat field images prior to an observation run. Typically the
flat field consists of a screen mounted inside the telescope dome that is uniformly illuminated with a broadband light
source. These flat field images are useful for characterizing the relative response of CCD pixels to light passing through
the telescope optics and filters, but carry limited spectral information and are not calibrated for absolute flux.
We present the results of performing in situ, spectroradiometric calibrations of a 1.2 m telescope at the Fred Lawrence
Whipple Observatory, Mt. Hopkins, AZ. To perform a spectroradiometric calibration, a laser, tunable through the
visible to near infrared, was coupled into an optical fiber and used to illuminate the flat field screen in situ at the
telescope facility. A NIST traceable, calibrated photodiode was mounted on the telescope to measure the spectral flux
reaching the aperture. For a particular filter, images of the screen were then captured for each laser wavelength as the
wavelength was tuned over the filter bandpass. Knowledge of the incident flux then allows the relative responsivity of
each CCD pixel at each wavelength to be calculated.
Earth's atmosphere represents a turbulent, turbid refractive element for every ground-based telescope. We describe the
significantly enhanced and optimized operation of observatories supported by the combination of a lidar and
spectrophotometer that allows accurate, provable measurement of and correction for direction-, wavelength- and timedependent
astronomical extinction. The data provided by this instrument suite enables atmospheric extinction correction
leading to "sub-1%" imaging photometric precision, and attaining the fundamental photon noise limit. In addition, this
facility-class instrument suite provides quantitative atmospheric data over the dome of the sky that allows robust realtime
decision-making about the photometric quality of a night, enabling more efficient queue-based, service, and
observer-determined telescope utilization. With operational certainty, marginal photometric time can be redirected to
other programs, allowing useful data to be acquired. Significantly enhanced utility and efficiency in the operation of
telescopes result in improved benefit-to-cost for ground-based observatories.
We propose that this level of decision-making will make large-area imaging photometric surveys, such as Pan-STARRS
and the future LSST both more effective in terms of photometry and in the use of telescopes generally. The atmospheric
data will indicate when angular or temporal changes in atmospheric transmission could have significant effect across the
rather wide fields-of-view of these telescopes.
We further propose that implementation of this type of instrument suite for direct measurement of Earth's atmosphere
will enable observing programs complementary to those currently requiring space-based observations to achieve the
required measurement precision, such as ground-based versions of the Kepler Survey or the Joint Dark Energy Mission.
Ground-based telescopes supported by lidar and spectrophotometric auxiliary instrumentation can attain space-based
precision for all-sky photometry, with uncertainties dominated by fundamental photon counting statistics. Earth's
atmosphere is a wavelength-, directionally- and time-dependent turbid refractive element for every ground-based
telescope, and is the primary factor limiting photometric measurement precision. To correct accurately for the
transmission of the atmosphere requires direct measurements of the wavelength-dependent transmission in the direction
and at the time that the supported photometric telescope is acquiring its data. While considerable resources have been
devoted to correcting the effects of the atmosphere on angular resolution, the effects on precision photometry have
largely been ignored.
We describe the facility-class lidar that observes the stable stratosphere, and a spectrophotometer that observes NIST
absolutely calibrated standard stars, the combination of which enables fundamentally statistically limited photometric
precision. This inexpensive and replicable instrument suite provides the lidar-determined monochromatic absolute
transmission of Earth's atmosphere at visible and near-infrared wavelengths to 0.25% per airmass and the wavelengthdependent
transparency to less than 1% uncertainty per minute. The atmospheric data are merged to create a metadata
stream that allows throughput corrections from data acquired at the time of the scientific observations to be applied to
broadband and spectrophotometric scientific data. This new technique replaces the classical use of nightly mean
atmospheric extinction coefficients, which invoke a stationary and plane-parallel atmosphere. We demonstrate
application of this instrument suite to stellar photometry, and discuss the enhanced value of routinely provably precise
photometry obtained with existing and future ground-based telescopes.
In this work, development of a fiber-optically coupled, vacuum-compatible, flat plate radiometric source applicable to
the characterization and calibration of remote sensing optical sensors in situ in a thermal vacuum chamber is described.
Results of thermal and radiometric performance of a flat plate illumination source in a temperature-controlled vacuum
chamber operating at liquid nitrogen temperature are presented. Applications, including use with monochromatic tunable
laser sources for the end-to-end system-level testing of large aperture sensors, are briefly discussed.
The feasibility of developing a network of telescopes to monitor the composition of the nighttime atmosphere using
stellar spectrophotometry is explored. Spectral measurements of the extinction of starlight by the atmosphere would
allow, for instance, quantification of aerosol, cloud, water-vapor, and ozone levels over the full range of elevation and
azimuth. These measurements, when combined with data from solar spectrophotometry derived from other instruments,
would provide continuous day/night monitoring of the atmospheric composition from the ground. The foundation for
such an effort would be a set of stable standard stars with known top-of-the-atmosphere spectral irradiances traceable to
international standards based on the SI system of units. Fully automated, reliable, easily maintained and highly costeffective
replicas of the spectrophotometric telescope used to calibrate the standard stars can be deployed worldwide at
sites such as atmospheric and astronomical observatories.
Photomultiplier tubes (PMTs) are used in biological detection systems in order to detect the presence of biological
warfare agents. To ensure proper operation of these biological detection systems, the performance of PMTs must be
characterized in terms of their responsivity and long-term stability. We report a technique for PMT calibration at the
Synchrotron Ultraviolet Radiation Facility (SURF III) at the National Institute of Standards and Technology (NIST).
SURF III provides synchrotron radiation with a smooth and continuous spectrum covering the entire UV range for
accurate PMT measurements. By taking advantage of the ten decade variability in the flux of the synchrotron radiation,
we studied properties of commercial PMTs such as the linearity, spatial uniformity, and spectral responsivity. We
demonstrate the degradation of PMTs by comparing new PMTs with PMTs that were used and operated in a biological
detection system for a long period of time. The observed degradation is discussed.
The NIST role in supporting our Nation's climate research is described. The assembly of climate data records over
decadal time scales requires assimilating readings from a large number of optical sensors deployed in space and on the
Earth by various nations. NIST, in partnership with NASA and NOAA, develops and disseminates the calibration tools
and standards to ensure that the measurements from these sensors are accurate, comparable, and tied to international
standards based on the SI system of units. This effort helps to provide confidence that the small decadal changes in
environmental variables attributed to climate change are not an artifact of the measurement system. Additionally, it
ensures that the measurements are physics based and thus comparable to climate models.
Deployment of compact hyperspectral imaging sensors on small UAVs has the potential of providing a cost-effective
solution for rapid-response target detection and cueing based on time critical spectral information collected at low
altitudes. To address this goal, a new compact hyperspectral imaging sensor is being developed with an anamorphic
optical system that partially decouples image formation along both the spatial and spectral axes found in conventional
push-broom hyperspectral imagers. This design concept benefits from a reduction in complexity over standard highperformance
spectrometer optical designs while maintaining excellent aberration control and spatial and spectral
distortion characteristics. The anamorphic optical system has the advantage of removing the spectrometer slit focus
along the spatial axis and in turn eliminates nearly all aberrations in the front-end optics, regardless of field angle or
aperture size. This paper presents results from the first prototype anamorphic imaging spectrometer, which weighs 4
pounds and is designed for operation in the Short Wave InfraRed (SWIR) spectral band over a wavelength range of 1
μm to 1.7 μm dictated by the uncooled InGaAs focal plane array used as the detector. The anamorphic system design will be discussed and results from characterization and field measurements will be presented.
Improperly imaged, or scattered, optical radiation within an instrument is difficult to properly characterize and is often
the dominant residual source of measurement error. Scattered light can originate from the spectral components of a
"point" source and from spatial elements of an extended source. The spectral and spatial scattered light components are
commonly referred to as stray light and can be described by an instrument's spectral line spread function (SLSF) and
point spread function (PSF), respectively. In this paper, we present approaches that characterize an instrument's
response to scattered light and describe matrices that have been developed to correct an instrument's response for this
scattered light. Examples are given to demonstrate the efficacy of the approach and implications for remote sensing
instruments are discussed.
Absolute stellar photometry is based on 1970s terrestrial measurements of the star Vega calibrated by using
the Planck radiance from a Cu fixed-point blackbody. Significant advances in absolute radiometry have
been made in the last 30 years that offer the potential to improve both terrestrial and space-based absolute
stellar photometry. These advances include new high-temperature blackbody standards, absolute cryogenic
radiometry, solid-state optical radiation sources, improved atmospheric transmittance modeling, and laser-based
radiometric calibration. We describe the possible use of these new technologies for ground-based
calibration of standard stars and their impact on stellar photometry, including present efforts to achieve
highly accurate measurements from the ultraviolet to the near infrared for cosmological applications.
Onboard diffuse reflecting plaques are carried to orbit as radiometric reference standards for Earth-observing satellite
instruments. For many instruments the reflectance properties of the plaque are characterized independently of the instrument,
and the effects of scattering by the diffuser housing are determined through mathematical modeling. The pre-launch
laboratory calibration of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) included a system-level calibration
of the onboard diffuser using an external lamp and a reference plaque at the sensor's Earth-view port. The calibration
of the onboard diffuser was made relative to the reference plaque using SeaWiFS as a transfer radiometer. Recent
developments in laboratory light sources enable a significant improvement to the SeaWiFS calibration technique. These
include sets of fiber-optically coupled tunable lasers that illuminate integrating spheres or the prime focus of a collimator,
to produce uniform, high radiant flux Lambertian or collimated sources, respectively. In addition, newly developed,
spectrally tunable supercontinuum-based light sources can provide sets of radiance spectra for the collimator to validate
the laser-based diffuser calibration. An absolute calibration of the diffuser system in the laboratory also provides the
first step in a two-part transfer-to-orbit experiment, in which the second step uses the illumination of the diffuser on orbit
by the Sun. For hyperspectral instruments, the laboratory calibration must account for spectral artifacts in the diffuser
material. For on-orbit measurements, the calibration must account for the effects of Earth-shine as a contaminating
source of irradiance illuminating the diffuser.
Spectrographs are used in a variety of applications in the field of remote sensing for radiometric measurements due to the benefits of measurement speed, sensitivity, and portability. However, spectrographs are single grating instruments that are susceptible to systematic errors arising from stray radiation within the instrument. In the application of measurements of ocean color, stray light of the spectrographs has led to significant measurement errors. In this work, a simple method to correct stray-light errors in a spectrograph is described. By measuring a set of monochromatic laser sources that cover the instrument's spectral range, the instrument's stray-light property is characterized and a stray-light correction matrix is derived. The matrix is then used to correct the stray-light error in measured raw signals by a simple matrix multiplication, which is fast enough to be implemented in the spectrograph's firmware or software to perform real-time corrections: an important feature for remote sensing applications. The results of corrections on real instruments demonstrated that the stray-light errors were reduced by one to two orders of magnitude, to a level of approximately 10-5 for a broadband source measurement, which is a level less than one count of a 15-bit resolution instrument. As a stray-light correction example, the errors in measurement of solar spectral irradiance using a highquality spectrograph optimized for UV measurements are analyzed; the stray-light correction leads to reduction of errors from a 10 % level to a 1 % level in the UV region. This method is expected to contribute to achieving a 0.1 % level of uncertainty required for future remote-sensing applications.
Infrared radiometers for irradiance measurement have been developed at the National Institute of Standards and Technology (NIST). These high performance irradiance meters are used to realize and maintain the spectral irradiance responsivity scale between 1000 nm and 5000 nm. They are also working standards that disseminate the infrared irradiance responsivity scale to other institutions and facilities. Both design considerations and responsivity scale transfer to the irradiance meters are discussed. The radiometers were calibrated for spectral irradiance responsivity on the new NIST Infrared Facility for Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (IR-SIRCUS). The spectral irradiance responsivity calibrations described are derived from absolute cryogenic radiometers.
Detectors have historically been calibrated for spectral power responsivity at the National Institute of Standards and Technology (NIST) using a lamp-monochromator system to tune the wavelength of the excitation source. Silicon detectors can be calibrated in the visible spectral region with uncertainties at the 0.1 % level. However, uncertainties increase dramatically when measuring an instrument's spectral irradiance or radiance responsivity. In addition, the uncertainties are even larger in the UV and IR ranges. We will discuss a new laser-based facility for Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) that was developed to calibrate instruments directly in irradiance or radiance mode with uncertainties approaching those available for spectral power responsivity calibrations. In this facility, high-power, tunable lasers are introduced into an integrating sphere using optical fibers, producing uniform, quasi-Lambertian, high radiant flux sources. Reference standard irradiance detectors, calibrated directly against national primary standards for spectral power responsivity, are used to determine the irradiance at a reference plane. Knowing the measurement geometry, the source radiance can be readily determined as well. The radiometric properties of the SIRCUS source coupled with state-of-the-art transfer standard radiometers whose responsivities are directly traceable to primary national radiometric scales, result in typical combined standard uncertainties in irradiance and radiance responsivity calibrations less than 0.1 % in the visible (larger in the UV and IR). Details of the facility are presented and examples of unique calibrations possible in the facility are given, including system-level responsivity calibrations in support of the National Aeronautics and Space Administration's (NASA's) remote sensing activities.
We describe the development of a mechanically simple, radiometrically stable transfer radiometer designed for both radiance and irradiance measurements. The filter radiometer consists of a six-element Si trap detector, a temperature stabilized filter wheel with up to 5 filters, and two precision apertures in a Gershun tube arrangement. With the Gershun tube installed, the instrument operates in radiance mode; with the front aperture removed, in irradiance mode. Two trap detector filter radiometers have been designed and built by the National Institute of Standards and Technology (NIST) for use in remote sensing applications. The filter radiometers have been characterized for optical and electrical performance, and have been calibrated for responsivity using both narrow-band, tunable-laser-illuminated and broad-band, lamp-illuminated integrating sphere sources. This paper describes the filter radiometer design, characterization, and deployments for two remote sensing projects.
Sun photometers are used to characterize the radiative properties of the atmosphere. They measure both the incident solar irradiance as well as the sky radiance (from scattered incident flux). Global networks of sun photometers provide data products such as aerosol optical thickness derived from these measurements. Instruments are typically calibrated for irradiance responsivity by cross-calibration against a primary reference sun photometer and for radiance responsivity using a lamp-illuminated integrating sphere source. A laser-based facility for Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) has been developed at the National Institute of Standards and Technology. Sensors can be calibrated in this facility for absolute spectral irradiance and radiance responsivity with combined expanded (k = 2) uncertainties ranging from 0.15% to 0.25%. Two multi-channel filter radiometers used in the Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) program of the National Aeronautics and Space Administration (NASA) at the Goddard Space Flight Center (GSFC) were calibrated for radiance and irradiance responsivity using conventional approaches and using laser-illuminated integrating spheres on SIRCUS. The different calibration methods are compared, the uncertainties are evaluated, and the impact on remote sensing applications is discussed.
We are developing a technique for accurately measuring spectral responsivity functions of infrared cameras using tunable lasers. We present preliminary results for uniform scenes where tunable infrared lasers illuminate an integrating sphere, diffusing the light to fill the imaging system optics. A commercial camera based on a liquid nitrogen-cooled InSb focal plane array was tested in the 1.4 micrometer to 4.7 micrometer spectral range using a continuously-tunable periodically-poled lithium niobate (PPLN) optical parametric
oscillator. Another commercial camera based on an uncooled
microbolometer array was tested using a discrete-tunable CO2 laser in the 9 micrometer to 11 micrometer spectral range. Results from these tests show that signal-to-noise ratio, uniformity, stability, and other characteristics are favorable for use of this technique in the characterization of infrared imaging systems. We also propose a generalization of this technique, to include scenes with arbitrary, controlled spatial content such as bar patterns or even pictures, by illuminating a commercially-available digital micromirror device (DMD). Dependence on irradiance level, exposure time, and polarization can also be measured. This technique has an inherent advantage over thermal-emitter based methods in that it measures absolute spectral responsivity directly without requiring knowledge of the spectral emissivity or temperature of the source.
We have developed a tunable laser-based facility for the absolute radiometric calibration of digital imaging system such as CCD cameras, spectrographs, and microscopes. Several types of silicon-based digital imaging systems have been calibrated in this new facility, including a commercially available camera equipped with a removable photopic filter, a custom-designed digital microscope, and a CCD spectrography. We present result of the CCD camera calibration in detail and discuss relevant aspects of the microscope and spectrograph calibrations. During the radiometric calibration, the pixel-to-pixel uniformity, linearity, and absolute spectral responsivity of each system were determined over the visible spectral range. Each of these aspects of the CCD camera calibration will be presented, along with a discussion of the measurement uncertainties.
We have made direct measurements of the internal quantum efficiency and the reflectivity of UV-damped silicon photodiodes in the spectral range of 125 nm to 320 nm. The above quantities, coupled with absolute spectral responsivities, may yield unique information leading to the identification of the mechanisms responsible for the degradation of performance of the silicon photodiodes in the ultraviolet. The measurements were made using synchrotron radiation from the NIST synchrotron ultraviolet radiation facility and an absolute cryogenic radiometer as a primary standard detector.
Resonantly enhanced multiphoton ionization (REMPI) laser spectroscopic and molecular beam-surface scattering techniques are coupled to study inelastic and reactive gas-surface scattering with state-to-state specificity. Rotational, vibrational, translational and ang ular distributions have been measured for the inelastic scattering of HCI and N2 from Au(1 1 1 ). In both cases the scattering is direct-inelastic in nature and exhibits interesting dynamical features such as rotational rainbow scattering. In an effort to elucidate the dynamics of chemical reactions occurring on surfaces we have extended our quantum-resolved scattering studies to include the reactive scattering of a beam of gas phase H-atoms from a chlorinated metal surface M-Cl. The nascent rotational and vibrational distributions of the HCI product are determined using REMPI. The thermochemistry for this reaction on Au indicates that the product formation proceeding through chemisorbed H-atoms is slightly endothermic while direct reaction of a gas phase H-atom with M-Cl is highly exothermic (ca. 50 kcal/mole). Details of the experimental techniques, results and implications regarding the scattering dynamics are discussed.
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