A KHz Pulsed Laser Detection System was developed employing the concept of charge integration with an electrometer, in the NASA Goddard Space Flight Center, Code 618 Calibration Lab for the purpose of using the pulsed lasers for radiometric calibration. Comparing with traditional trans-impedance (current-voltage conversion) detection systems, the prototype of this system consists of a UV-Enhanced Si detector head, a computer controlled shutter system and a synchronized electrometer. The preliminary characterization work employs light sources running in either CW or pulsed mode. We believe this system is able to overcome the saturation issue when a traditional trans-impedance detection system is used with the pulsed laser light source, especially with high peak-power pulsed lasers operating at kilohertz repetition rates (e.g. Ekspla laser or KHz OPO). The charge integration mechanism is also expected to improve the stability of measurements for a pulsed laser light source overcoming the issue of peak-to-peak stability. We will present the system characterizations including signal-to-noise ratio and uncertainty analysis and compare results against traditional trans-impedance detection systems.
Satellite instruments operating in the reflective solar wavelength region require accurate and precise determination of the Bidirectional Reflectance Distribution Functions (BRDFs) of the laboratory and flight diffusers used in their pre-flight and on-orbit calibrations. This paper advances that initial work and presents a comparison of spectral Bidirectional Reflectance Distribution Function (BRDF) and Directional Hemispherical Reflectance (DHR) of Spectralon*, a common material for laboratory and onorbit flight diffusers. A new measurement setup for BRDF measurements from 900 nm to 2500 nm located at NASA Goddard Space Flight Center (GSFC) is described. The GSFC setup employs an extended indium gallium arsenide detector, bandpass filters, and a supercontinuum light source. Comparisons of the GSFC BRDF measurements in the shortwave infrared (SWIR) with those made by the National Institute of Standards and Technology (NIST) Spectral Tri-function Automated Reference Reflectometer (STARR) are presented. The Spectralon sample used in this study was 2 inch diameter, 99% white pressed and sintered Polytetrafluoroethylene (PTFE) target. The NASA/NIST BRDF comparison measurements were made at an incident angle of 0° and viewing angle of 45° . Additional BRDF data not compared to NIST were measured at additional incident and viewing angle geometries and are not presented here. The total combined uncertainty for the measurement of BRDF in the SWIR range made by the GSFC scatterometer is less than 1% (k = 1). This study is in support of the calibration of the Radiation Budget Instrument (RBI) and Visible Infrared Imaging Radiometer Suit (VIIRS) instruments of the Joint Polar Satellite System (JPSS) and other current and future NASA remote sensing missions operating across the reflected solar wavelength region.
Satellite instruments operating in the reflected solar wavelength region require accurate and precise determination of the optical properties of their diffusers used in pre-flight and post-flight calibrations. The majority of recent and current space instruments use reflective diffusers. As a result, numerous Bidirectional Reflectance Distribution Function (BRDF) calibration comparisons have been conducted between the National Institute of Standards and Technology (NIST) and other industry and university-based metrology laboratories. However, based on literature searches and communications with NIST and other laboratories, no Bidirectional Transmittance Distribution Function (BTDF) measurement comparisons have been conducted between National Measurement Laboratories (NMLs) and other metrology laboratories. On the other hand, there is a growing interest in the use of transmissive diffusers in the calibration of satellite, air-borne, and ground-based remote sensing instruments. Current remote sensing instruments employing transmissive diffusers include the Ozone Mapping and Profiler Suite instrument (OMPS) Limb instrument on the Suomi-National Polar-orbiting Partnership (S-NPP) platform,, the Geostationary Ocean Color Imager (GOCI) on the Korea Aerospace Research Institute’s (KARI) Communication, Ocean, and Meteorological Satellite (COMS), the Ozone Monitoring Instrument (OMI) on NASA’s Earth Observing System (EOS) Aura platform, the Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument and the Geostationary Environmental Monitoring Spectrometer (GEMS).. This ensemble of instruments requires validated BTDF measurements of their onboard transmissive diffusers from the ultraviolet through the near infrared. This paper presents the preliminary results of a BTDF comparison between the NASA Diffuser Calibration Laboratory (DCL) and NIST on quartz and thin Spectralon samples.
A Light-Emitting Diode (LED)-driven integrating sphere light source has been fabricated and assembled in the NASA Goddard Space Flight Center (GSFC) Code 618 Biospheric Sciences Laboratory’s Calibration Facility. This light source is a 30.5 cm diameter integrating sphere lined with Spectralon. A set of four LEDs of different wavelengths are mounted on the integrating sphere’s wall ports. A National Institute of Standards and Technology (NIST) characterized Si detector is mounted on a port to provide real-time monitoring data for reference. The measurement results presented here include the short-term and long-term stability and polarization characterization of the output from this LED-driven integrating sphere light source. As an initial application, this light source is used to characterize detector/pre-amplifier gain linearity in light detection systems. The measurement results will be presented and discussed.
A tunable, intensity-stabilized, quasi-continuous wave (CW) laser system, patterned after the Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) system at the National Institute of Standards and Technology (NIST) 1, has been installed and is being tested in the NASA Goddard Space Flight Center (GSFC) Code 618 Biospheric Sciences Laboratory’s Calibration Facility. This system is referred to as SIRCUS-G (SIRCUS-Goddard). The tunable output of the laser system is fiber-fed to a 76.2 cm diameter integrating sphere lined with Spectralon. The uniform radiance light emitted from the integrating sphere is used in system-level radiometric responsivity characterizations and wavelength calibrations of remote sensing instruments. The primary radiance reference standards in the responsivity characterizations are a three-element Si trap radiometer for the visible and near infrared and a radiometer employing an InGaAs detector. Both radiometers have been calibrated by NIST. These radiometers are located at the exit port of the Spectralon coated integrating sphere. In addition, a set of three radiometers are mounted on the 76.2 cm integrating sphere’s wall ports to monitor source radiance and to provide real-time sphere radiance data during the calibrations of remote sensing instruments. These monitor radiometers provide spectral coverage from 300nm to near 2500nm. This paper presents the results of our characterization of the performance of these monitor radiometers. Results are presented and discussed on monitor radiometer short- and long-term system stability, noise level, and total measurement uncertainty.
The NASA Goddard Space Flight Center (GSFC) Radiometric Calibration Laboratory (RCL) maintains several large integrating sphere sources covering the visible to the shortwave infrared wavelength range. Two critical, functional requirements of an integrating sphere source are short- and long-term operational stability and repeatability. Monitoring the source is essential in determining the origin of systemic errors, thus increasing confidence in source performance and quantifying repeatability. If monitor data falls outside the established parameters, this could be an indication that the source requires maintenance or recalibration against the National Institute of Science and Technology irradiance standard. The GSFC RCL has developed a Filter Radiometer Monitoring System (FRMS) to continuously monitor the performance of its integrating sphere calibration sources in the 400 to 2400 nm region. Sphere output change mechanisms include lamp aging, coating (e.g., BaSO4) deterioration, and ambient water vapor level. The FRMS wavelength bands are selected to quantify changes caused by these mechanisms. The FRMS design and operation are presented, as well as data from monitoring four of the RCL's integrating sphere sources.
Satellite instruments operating in the reflective solar wavelength region require accurate and precise
determination of the Bidirectional Reflectance Factor (BRF) of laboratory-based diffusers used in their pre-flight
and on-orbit radiometric calibrations. BRF measurements are required throughout the reflected-solar spectrum from
the ultraviolet through the shortwave infrared. Spectralon diffusers are commonly used as a reflectance standard for
bidirectional and hemispherical geometries. The Diffuser Calibration Laboratory (DCaL) at NASA's Goddard Space
Flight Center is a secondary calibration facility with reflectance measurements traceable to those made by the
Spectral Tri-function Automated Reference Reflectometer (STARR) facility at the National Institute of Standards
and Technology (NIST). For more than two decades, the DCaL has provided numerous NASA projects with BRF
data in the ultraviolet (UV), visible (VIS) and the Near InfraRed (NIR) spectral regions. Presented in this paper are
measurements of BRF from 1475 nm to 1625 nm obtained using an indium gallium arsenide detector and a tunable
coherent light source. The sample was a 50.8 mm (2 in) diameter, 99% white Spectralon target. The BRF results are
discussed and compared to empirically generated data from a model based on NIST certified values of 6°directional-hemispherical spectral reflectance factors from 900 nm to 2500 nm. Employing a new NIST capability
for measuring bidirectional reflectance using a cooled, extended InGaAs detector, BRF calibration measurements of
the same sample were also made using NIST's STARR from 1475 nm to 1625 nm at an incident angle of 0° and at
viewing angle of 45°. The total combined uncertainty for BRF in this ShortWave Infrared (SWIR) range is less than
1%. This measurement capability will evolve into a BRF calibration service in SWIR region in support of NASA
remote sensing missions.
The Ocean Radiometer for Carbon Assessment (ORCA), currently being developed at Goddard, is a hyperspectral
instrument with a spectral range extending from 350nm to 880nm in the UV and visible wavelength. Its radiometric
measurement accuracy will depend, in part, on the extent to which it is insensitive to linearly polarized light. A wedge
type depolarizer is used to reduce ORCA's polarization sensitivity over its entire spectral range. The choice for this
approach is driven by the large spectral range and to a certain extent is also influenced by the currently orbiting SeaWifs
instrument's use of a wedge depolarizer and its low polarization sensitivity. The wedge depolarizer's design, its modeled
and measured depolarization characteristics are presented.
Optical spectroradiometers used to measure and monitor the radiance output of uniform sources must be thoroughly
characterized. The viability of the use of an instrument for such purposes is based upon the establishment of knowledge
of its radiometric responsivity characteristics. The NASA Goddard Space Flight Center Radiometric Calibration
Laboratory (RCL) has commissioned a new spectroradiometer for use in measurements of irradiance and radiance
sources. The spectroradiometer is comprised of a commercial scanning grating, Czerny-Turner double monochromator.
This spectroradiometer has been used to make measurements on a number of irradiance and radiance sources over the
wavelength range of 300 to 2400 nm. Instrument characterization included determination of stability, functional
wavelength calibration and scattered light performance. Comparison measurements were also made with other
radiometers. The data gathered from these measurements is presented, analyzed, and discussed.
The NASA Goddard Space Flight Center (GSFC) Radiation Calibration Facility (RCF) maintains several large
integrating sphere sources covering the visible and near infrared wavelength range. Two critical requirements of an
integrating sphere source are short and long-term operational stability and repeatability. Monitoring the source is
essential in determining the origin of systemic errors, thus increasing confidence in source performance, and quantifying
repeatability. If monitor data falls outside the established parameters, this is an indication that the source requires
maintenance or re-calibration against the National Institute of Science and Technology (NIST) irradiance standard. The
GSFC RCF has developed a Filter Radiometer Monitoring System (FRMS) to continuously monitor the performance of
its integrating sphere calibration sources in the 400-2400nm region. Sphere output change mechanisms include lamp
aging, coating (BaSO<sub>4</sub>) deterioration, and ambient water vapor level. The FRMS wavelength bands are selected to
quantify changes caused by these mechanisms. The FRMS design and operation is presented, as well as data from
monitoring three of the RCF's integrating sphere sources.