jasonSWIR Calibration of Spectralon Reflectance Factor

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 1475nm to 1625nm obtained using an indium gallium arsenide detector and a tunable coherent light source. The sample was a 2 inch 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 6deg directional/hemispherical spectral reflectance factors from 900nm to 2500nm. 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 1475nm to 1625nm at an incident angle of 0deg and at viewing angles of 40deg, 45deg, and 50deg. 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. Keywords: BRF, BRDF, Calibration, Spectralon, Reflectance, Remote Sensing

Preliminary Results of BTDF Calibration of Transmissive Solar Diffusers for Remote Sensing

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 on-board 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

Spectralon BRDF and DHR Measurements in Support of Satellite Instruments Operating Through Shortwave Infrared

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 NIST Spectral Trifunction 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 deg and viewing angle of 45 deg. 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 Joint Polar Satellite System (JPSS) Radiation Budget Instrument (RBI) and Visible Infrared Imaging Radiometer Suite (VIIRS) of and other current and future NASA remote sensing missions operating across the reflected solar wavelength region

Characterizations of a KHz Pulsed Laser Detection System

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

Characterizations of a KHz Pulsed Laser Detection System

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

Development and Performance of a Filter Radiometer Monitor System for Integrating Sphere Sources

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 re-calibration against the National Institute of Science and Technology (NIST) 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 2400nm region. Sphere output change mechanisms include lamp aging, coating (e.g. BaSO4) deterioration, and ambient water vapor level. The Filter Radiometer Monitor System (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

Measuring Linearity of Detector Spectral Responsibity at Ultra-Low Incident Powers

A LED-driven integrating sphere uniform light source within a light-tight enclosure was used to characterize the linearity of spectral responsivity for detectors at Pico-watt and sub Pico-watt incident power levels over a wavelength range from 405 nm to 1550 nm. A UV-enhanced Si detector, a Std-InGaAs detector and an Ex-InGaAs detector were used with a pre-amplifier SR-570 and a lock-in amplifier SR-830. The results of spectral responsivity linearity and measurement uncertainty at low incident power levels will be presented and analyzed. Results from the different detectors will be compared, and the measurement methodology will be discussed

Preliminary Results of Solar Diffuser BRDF Measurements Using a Table-Top Goniometer at NASA GSFC

We report the development of a table-top goniometer at GSFC, NASA, and the preliminary results of solar diffuser BRDF measurements for satellite instrumentation in the reflective solar bands. The table-top goniometer is able to conduct both in-plane and out-of-plane BRDF measurements, which was built up using commercial stages and opto-mechanics to minimize the need for extensive machining. Various light sources with the power stabilities of \u3c 0.5 % cover the 300 nm to 2500 nm wavelength region. These include a laser diode driven plasma broadband light source, such as a Xe lamp, a supercontinuum laser, and single wavelength lasers. The broadband sources can be used either with dispersive elements to generate monochromatic light, or as built. Si and extended InGaAs detectors cover the spectral range from 300 nm to 2500 nm and are used in absolute and relative BRDF measurements. Two mini-spectrometers operating in the UV-NIR and NIR-SWIR measure the spectra of scattered light from the broadband sources in a measurement of relative BRDF. Two NIST traceable Spectralon standard samples, a white diffuser and black diffuser, are used to validate the system and to make comparison measurements. We present the spectral coverage of light sources and their stabilities, the detector linearities, and signal to noise ratios. The BRDF results at specific wavelengths are also shown in the different configurations to verify the BRDF reciprocity of diffuser at the angles of interest. We also present the methodology of how to complete the BRDF measurements beyond the spectral and angular coverage of NIST traceable standards. The uncertainty budget of BRDF measurements will also be discussed

Development of a Collimated Large Area Uniform Light Source for the Measurement of Solar Diffuser BRDF in Support of NASA Satellite Instrument Programs

We report the development of a collimated large area uniform light source, which is used to acquire diffuser BRDF measurements in support of the pre-launch calibration of NASA Earth observing satellite instrument. In accordance with the goal of “testing as you fly,” this large area light source permits the measurement of diffuser BRDF using illumination geometrically similar to that realized on orbit. In the design and testing of this source, several approaches using different light sources and collimating optics were examined with the overarching goal of producing a monochromatic, unpolarized large area, uniform, collimated beam with sufficient throughput power to enable BRDF to be measured. The major components of the collimated large area uniform light source employ a series of high-power LEDs from UV-VIS to SWIR with or without an integrating sphere. Light from this source is coupled to either a 30.48 cm diameter size off-axis parabolic mirror (OAP) or a 45.72 cm diameter spherical concave mirror. A large beam uniformity evaluation system employing a scanning detector was used to measure light source uniformity. In this presentation, we describe our approaches to produce large area uniform solar diffuser illumination, and discuss potential technical difficulties. Since technical challenges exist in achieving the 1% uniformity in a collimated large area light source, we also propose a correction method to mitigate non-uniformity using a laser scan method. The characterization of the collimated large area uniform light source and preliminary BRDF results are presented