Detection range enhancement using circularly polarized light in scattering environments for infrared wavelengths.

We find for infrared wavelengths that there are broad ranges of particle sizes and refractive indices that represent fog and rain, where circular polarization can persist to longer ranges than linear polarization. Using polarization tracking Monte Carlo simulations for varying particle size, wavelength, and refractive index, we show that, for specific scene parameters, circular polarization outperforms linear polarization in maintaining the illuminating polarization state for large optical depths. This enhancement with circular polarization can be exploited to improve range and target detection in obscurant environments that are important in many critical sensing applications. Initially, researchers employed polarization-discriminating schemes, often using linearly polarized active illumination, to further distinguish target signals from the background noise. More recently, researchers have investigated circular polarization as a means to separate signal from noise even more. Specifically, we quantify both linearly and circularly polarized active illumination and show here that circular polarization persists better than linear for radiation fog in the short-wave infrared, for advection fog in the short-wave and long-wave infrared, and large particle sizes of Sahara dust around the 4 µm wavelength. Conversely, we quantify where linear polarization persists better than circular polarization for some limited particle sizes of radiation fog in the long-wave infrared, small particle sizes of Sahara dust for wavelengths of 9-10.5 µm, and large particle sizes of Sahara dust through the 8-11 µm wavelength range in the long-wave infrared.

High birefringence of the yttrium vanadate crystal in the middle wavelength infrared.

A high birefringence of over 0.21 for the yttrium vanadate (YVO4) crystal in the middle wavelength infrared (i.e., 3-5 microm) was measured. A Fourier transform infrared spectrometer was employed in the channel spectra technique to obtain the measurements.

Midwave-infrared snapshot imaging spectrometer.

We report results from a demonstration of a midwave-infrared, nonscanning, high-speed imaging spectrometer capable of simultaneously recording spatial and spectral data from a rapidly varying target scene. We demonstrated high-speed spectral imaging by collecting spectral and spatial snapshots of blackbody targets and combustion products. The instrument is based on computed tomography concepts and operates in a midwave-infrared band of 3.0-5.0 mum. We record raw images at a frame rate of 60 frames/s, using a 512 x 512 InSb focal-plane array. Reconstructed object cube estimates were sampled at 46 x 46 x 21 (x, y, lambda) elements, or 0.1-mum spectral sampling. Reconstructions of several objects are presented.

Optimization of retardance for a complete Stokes polarimeter.

We present two figures of merit based on singular value decomposition, which can be used to assess the noise immunity of a complete Stokes polarimeter. These are used to optimize a polarimeter featuring a rotatable retarder and a fixed polarizer. A retardance of 132 degrees (approximately three-eighths wave) and retarder orientation angles of +/-51.7 degrees and +/-15.1 degrees are found to be optimal when four measurements are used. Use of this retardance affords a factor-of-1.5 improvement in signal-to-noise ratio over systems employing a quarter-wave plate. A geometric means of visualizing the optimization process is discussed, and the advantages of the use of additional measurements are investigated. No advantage of using retarder orientation angles spaced uniformly through 360 degrees is found over repeated measurements made at the four retarder orientation angles.

Mixed-expectation image-reconstruction technique.

An iterative method of reconstructing degraded images is developed from consideration of a mixed-noise imaging situation. Both photon noise in the image itself and postdetection Gaussian noise are combined by use of the standard maximum-likelihood method to produce a mixed-expectation reconstruction technique that demonstrates good performance in the presence of both noise sources. The new algorithm is evaluated through computer simulations.

Demonstration of a high-speed nonscanning imaging spectrometer.

We report results from a field demonstration of a nonscanning high-speed imaging spectrometer [computed-tomography imaging spectrometer (CTIS)] capable of simultaneously recording spatial and spectral information about a rapidly changing scene. High-speed spectral imaging was demonstrated by collection of spectral and spatial snapshots of a missile in flight. This instrument is based on computed-tomography concepts and operates in the visible spectrum (430-710nm). Raw image data were recorded at video frame rate (30frames / s) and an integration time of 2ms. An iterative reconstruction of the spatial and spectral scene information from each raw image took 10s. We present representative missile spectral signatures from the missile firing. The accuracy of the high-speed spectrometer is demonstrated by comparison of extended-source static-scene spectra acquired by a nonimaging reference spectrometer with spectra acquired by use of CTIS imaging of the same static scenes.

Demonstration of a computed-tomography imaging spectrometer using a computer-generated hologram disperser.

We have constructed a computed-tomography imaging spectrometer that uses a phase-only computer-generated hologram (CGH) array illuminator as the disperser. This imaging spectrometer collects multiplexed spatial and spectral data simultaneously and can be used for flash spectral imaging. The CGH disperser has been designed to maintain nearly equal spectral diffraction efficiency among a 5 x 5 array of diffraction orders and to minimize diffraction efficiency into higher orders. Reconstruction of the (x, y, lambda) image cube from the raw, two-dimensional data is achieved by computed-tomography techniques. The reconstructed image and spectral-signature data compare favorably with measurements by other spectrometric methods.

Error-free image compression algorithm using classifying-sequencing techniques.

The development of a new error-free digital image compression algorithm is discussed. Without the help of any statistics information of the images being processed, this algorithm achieves average bits-per-word ratios near the entropy of the neighboring pixel differences. Because this algorithm does not involve statistical modeling, generation of a code book, or long integer-floating point arithmetics, it is simpler and, therefore, faster than the studied statistics codes, such as the Huffman code or the arithmetic code.

Measurement of the index of refraction of the plastic Phenoxy PKFE.

The index of refraction, n of Phenoxy PKFE, a light yellow plastic produced by Union Carbide, is measured at lambda = 588 nm (n(D)), lambda = 656 nm (n(C)), and lambda = 486 nm (n(F)). These values are of interest because they must be known to determine the optical characteristics of a Phenoxy PKFE lens over the visible spectrum. Lenses made of Phenoxy PKFE have an important advantage over lenses made of other plastics. Phenoxy PKFE is chemically resistant to many epoxy glues commonly used to form airtight bonds in hermetically sealed chambers.

Extrinsic silicon photodetector characterization.

A gallium doped silicon (Si:Ga) extrinsic photoconductive detector was tested for sensitivity and quickness of response. The developmental goal for this detector material was high speed operation without compromised detectivity (D*). The high speed, p-type infrared photoconductor, with photoconductive gain less than unity, was tested at 10.5 microm to determine an experimental value for the detectivity-bandwidth product of D*f* = 3.69 x 10(18)cm-Hz(3/2)/W. Subsequently a theoretical model taking into account the optical absorption profile and majority-carrier-transport processes within the detector was developed. It agreed with experimental data and corroborated the theory.

STM imaging of molecular collagen and phospholipid membranes.

The application of STM to biological materials has been limited by poor conductivity, sample geometry and stability of biological materials. In this paper we describe an STM study of the monomeric helical forms of collagen, a stable, conductive and widely prevalent structural protein. We have also used STM to image artificial Langmuir DPE (dipalmitoyl phosphatidyl ethanolamine) phospholipid membranes. Both molecular collagen and the phospholipid membranes were dried in air on highly oriented pyrolytic graphite (HOPG). Our STM images of collagen dried on HOPG reveal strands 15 A in diameter with a periodicity of about 30 A which correlates with that known to occur in collagen. Spikes which periodically protrude from strands in our STM images of collagen appear to represent pyrrolidine ring structures in the amino acids proline and hydroxyproline. Thus, we report the first STM imaging of native biomolecules revealing intramolecular details and what appear to be specific amino acids. STM imaging of phospholipid membranes show a lattice pattern with densities spaced approximately 4.5 A apart. These are thought to represent individual phospholipid molecules in an artificial membrane formed on the HOPG. We believe STM and its related technologies will have great future utility in biomolecular studies.

Bidirectional transmittance distribution function measurements on ZnSe.

The bidirectional transmittance distribution function (BTDF) is used to evaluate the scattering properties of chemically vapor-deposited (CVD) zinc selenide (ZnSe). Conceptually, the BTDF is a logical extension of the bidirectional reflectance distribution function (BRDF) relation. A working equation for BTDF based on the practical limitations of the instrumentation has been developed. The practical limitations inherent in making BTDF measurements are the finite detector size, the two scattering surfaces of the sample, and the linear radiometric responses of the detector. The instrumentation used to measure BTDF and the data collection procedure are described. Plots of BTDF vs angle from the sample normal are presented for CVD ZnSe at room temperature. The plots are for three thicknesses of ZnSe (6, 19, and 25 mm) at three laser wavelengths (0.6328, 3.39 and 10.6 microm).

Derivation of the effect of atmospheric and aerosol emissions on IR imaging device performance.

The authors have previously called attention to the importance of atmospheric and aerosol emission in IR imaging device performance. The present paper derives a more precise and general result than the empirically derived expressions in the earlier paper. The same general conclusion, that atmospheric and aerosol emissions are an important factor in IR imaging performance, is supported, assuming that the system is photon noise-limited.

Bidirectional reflectance distribution function of gold-plated sandpaper.

Gold-plated sandpaper was investigated for use as a Lambertian standard reference reflector for the IR spectrum. Various grit sizes from 3 to 400 microm and material types (i.e., silicon carbide and aluminum oxide) were studied. The different gold-plated sandpaper grit sizes were measured in the same way using three laser wavelengths (0.6328, 3.39, and 10.6 microm) at five angles of incidence of the source (0, 10, 20, 30, and 60 degrees ). All the scattering measurements were performed in the plane of incidence. The best choices of sandpaper grit sizes were 9-microm A1(2)O(3) for 0.6328- and 3.39-microm radiation and 600 grit by Armak Co. for 10.6-microm radiation. These choices were compared with other commonly used reflectors such as magnesium oxide, halon, sintered bronze, and flowers of sulfur. An attempt was made to correlate surface roughness (size of grit) to the degree of approximation to a good Lambertian reflector, but it was found that grit size is not as important as the filling factor, or density of particles, over a given area. It was found that fairly good approximations to Lambertian behavior result when the angle of incidence is small but not when the angle of incidence is as large as 60 degrees .

Infrared propagation and performance modeling at the Electro-Optical Test Facility.

Fog spectral transmission data generated in the Electro-Optical Test Facility and propagation models usedm by the authors are presented. Preliminary results of a modeling effort are presented that support the following conclusions: (1) atmosphere emission is a major factor in IR system performance despite the fact it is commonly omitted; (2) the 3.5-4.1-microm interval offers superior performance to the 3-5 band at both relatively short and long ranges; and (3) the longer wavelength 9.5-11.5-microm band is superior to either the 3-5 or 3.5-4.1 bands in the presence of fog.

Cryogenic refractive indices and temperature coefficients of cadmium telluride from 6 microm to 22 microm.

The index of refraction and its variation with wavelength and temperature were measured for cadmium telluride made by chemical vapor deposition (CVD). The measurement at 20 K using liquid helium as coolant is reported for the first time. The refractive index at 10 microm and 20 K is 2.6466, and the average temperature coefficient at 10 microm and in the range 20-80 K is 4.9 x 10(-5)/K.

Application of a synthetic aperture optical system to infrared imaging.

This paper discusses the application of a synthetic aperture system (SAS) for the formation of infrared, images. The SAS of primary interest is a hexagonal array of six circular apertures; special emphasis is placed on a one-dimensional model that describes the salient features of the two-dimensional model. In infrared images, in the 8-14-microm region, conventional detectors are too large to sample the image without aliasing the spatial spectrum. The unusual transfer function characteristics of the SAS are shown to obtain larger sampling distances than those required by the Whittaker-Shannon sampling theory. One possible detector array that will correctly sample the image is presented.

A comparison of the theoretical operation of high-impedance and low-impedance detectors.

The purpose of this paper is to compare the theoretical spectral detectivity (Dlambda*) of a low-impedance photoconductive detector with a high-impedance photoconductive detector at low background temperatures. This theoretical prediction assumes that both detectors are limited only by photon noise and Johnson noise. Examples of infrared detectors that have these ultimate limits are mercury cadmium telluride (Hg(x)Cd(1-x)Te) and mercury-doped germanium (Ge:Hg).