We demonstrate a remote sensing method, based on an imaging polarimetric spectrometer, to determine the complex refractive index of materials. The approach represents an adaptation of a technique used in semiconductor ellipsometry. Our experimental demonstration setup comprises a Sagnac-type LWIR spatial interferometer (8 - 12 micron) designed for hyperspectral imaging in emission and reflection. The method facilitates direct measurement of the complex refractive index and the thickness of a layer. Presented work focuses on SF96, a liquid surrogate for toxic chemicals, but the method is generally applicable to solids, including soils, and to any substrate with a general bidirectional reflectance distribution function.
We produced a one-dimensional model of the signal to noise performance of Fourier Transform
hyperspectral imagers using microbolometer array detectors. Our prior work could predict the peak
response of such sensors, but not a measured fall-off in SNR at short and long wavelengths. We improved
the fidelity of the model by including necessary wavelength dependent terms. We then tested the model
using the measured performance of an FTIR, and critical measurements of the relative wavelength
responsivity and NEDT of the microbolometer camera, and the wavelength response of the beamsplitter
modulation. The model agrees with measurement with 15%, and the difference is consistent with the lack
of spatial nonuniformity in the model. We also present similar analysis using the interferometer and a
cooled mercury cadmium telluride camera showing that high performance can be obtained if dark currents
can be reduced relative to commercial broadband IR cooled cameras.
Airborne surveillance presents challenging target-detection opportunities for optical remote sensors, especially under the constraints of size, weight, and power imposed by small aircraft. We present a spatial-frequency dependent figure-of-merit, called the Detector Quantum Efficiency (DQE), by first tracing its origins in single pixel photon multiplication detectors, where it is shown to be yield (quantum efficiency or QE) divided by the noise factor. We then show the relationship of DQE to several well-known figures-of-merit. Finally we broaden the definition of DQE to include the spatial-frequency dependence on the MTF of the system and the noise power spectrum (NPS) of the detector. We then present the results of the application of this DQE to a hyperspectral camera under development at BAE Systems Spectral Solutions LLC.
The design, calibration, and performance of the high purity germanium (HPGe) solid state detectors (SSDs) used in the calibration of the Advanced X-ray Astrophysics Facility high resolution mirror assembly (HRMA) is discussed. The focal plane SSD was used with various apertures to measure the point response function, as well as the effective area of the mirror. The good energy resolution of the detector allowed the effective energy of the mirrors to be measured with a single exposure using a continuum source. The energy resolution was also exploited in measuring the molecular contamination on the mirror surfaces. The SSDs are the transfer detector standards for the HRMA calibration over the energy range from 700 eV to 10 keV. The calibration of the SSDs was performed mostly at the PTB radiometry laboratory using the electron storage ring BESSY. The spectral and spacial distribution of the undispersed synchrotron radiation can be calculated from first principles using the Schwinger Equation. With the electron storage ring being run in a reduced current mode of a few electrons, uncertainties in the calculated flux are below 1%. A comparison of the measured and calculated flux made it possible to determine the detector efficiency with an uncertainty of typically 1%. Electronic effects such as pile- up, count rate linearity and deadtime have been investigated.
We discuss details of the spectral fitting procedures and algorithms used in deriving line count rates for the calibration of AXAF (the Advanced X-ray Astrophysics Facility) during end-to-end testing in the winter and spring of 1996/1997. An approach involving simultaneously fitting both detector and source parameters was implemented within XSPEC, a standard x-ray spectral fitting package (Arnaud 1996). Theoretical and practical difficulties in fitting spectra taken with flow proportional counters (FPC) and solid state detectors (SSD) are discussed, including both effects incorporated into the numerical model, and those which must be estimated outside the model. Sensitivity of the parameter of interest, the counts in a strong line in the spectrum, to changes and errors in the other fit parameters is explored. The impact of uncertainties on the overall absolute AXAF calibration is discussed.
We describe here the computer program for modeling the FPCs and SSDs used in the ground calibration, relative and absolute, of AXAF. The program is called as a subroutine by XSPEC. The design is hierarchical: at the lowest level are models for single electron spectra, upon which are based the computation of moment generating functions (mgfs) for the detector response functions. Any number of discrete lines and a continuum are allowed. The continuum is the source x-ray spectrum as modified by intervening filters. The overall mgf is numerically transformed into the count spectrum via an inverse fast Fourier transform. The component mgfs have unit normalization, and final fitted normalizations give the count rate for each component.
Focused ion-beams have proved to be a superior means of fabricating holes between 5 and 20 microns in diameter, through gold and tungsten sheets over 40 microns thick. These holes are milled with an FEI FIB800 Focused Ion Beam Workstation using a Ga liquid metal source, with and without an enhancement gaseous etchant. They will be used as apertures for detectors that probe the point response function of the X-ray optics (Wolter Type-I mirror pairs), which form part of the NASA Advanced X-ray Astrophysical Facility. It is found that both pure milling and gas-assisted etching produced micron-sized holes of a quality superior to those produced by laser beam sputtering.