A technique is introduced to extend traditional polarized-light microscopy to obtain quantitative c-axis orien- tation images of reflective non-cubic crystal grains such as a titanium. The technique is based on multiple generalized illumination and detection states in a laser polarimeter and a physical model mapping resulting im- age irradiance to crystal orientation, and is demonstrated by comparing relative-orientation images with EBSD orientation maps of a Ti-6Al-4V sample. The new technique is shown to be somewhat more tolerant than EBSD to mechanically-induced surface roughness and deformation, although grain contrast for a Ti-7Al sample was be only weakly correlated with roughness as measured by an AFM.
A technique employing a 3D morphological image-registration algorithm is demonstrated for stitching together high-resolution surface im- ages obtained with a commercial atomic-force microscope (AFM), producing 3D surface images up to 1mm long with lateral resolution ~ 100nm: These images can be applied to reflectance modeling by extracting surface parameters to be used as inputs for reflectance models, for instance the previously-published Coherence Model [BG. Hoover and VL. Gamiz, J. Opt. Soc. Am. A 23, 314 (2006)], which utilizes the surface roughness and autocorrelation derivatives in the large-roughness approximation. Surface moments estimated from extended-range AFM images demonstrate lower uncertainty at all frequencies and substantial reduction of sampling artifacts at low frequencies, enabling improved estimates of surface parameters. The autocorrelation of a nearly monoscale diffuse-gold surface is measured out to 800μm separation, and the autocorrelation of a multiscale tin surface provides parameters that verify the Coherence Model t to the measured quasimonostatic BRDF.
Active (Mueller matrix) remote sensing is an under-utilized technique for material discrimination and classication.
A full Mueller matrix instrument returns more information than a passive (Stokes) polarimeter; Mueller
polarimeters measure depolarization and other linear transformations that materials impart on incident Stokes
vectors, which passive polarimeters cannot measure. This increase in information therefore allows for better
classication of materials (in general). Ideally, material classication over the entire polarized BRDF is desired,
but sets of Mueller matrices for dierent materials are generally not separable by a linear classier over elevation
and azimuthal target angles. We apply non-linear support vector machines (SVM) to classify materials over
BRDF (all relevant angles) and show variations in receiver operator characteristic curves with scene composition
and number of Mueller matrix channels in the observation.
When using a MMP for a detection or identification task, a user considers certain elements of
the Mueller matrix. The usual way of performing this task is to measure the full Mueller matrix
and discard the unused elements. For polarimeter designs with speed, miniaturization, or other
constraints it may be desirable to have a system with reduced dimensionality that measures
only the important elements of the Mueller matrix as efficiently as possible. In this paper,
we develop a framework that allows partial MMPs to be analyzed. Quantitative metrics are
developed by considering geometrical relationships between the space spanned by a particular
MMP and the space occupied by the scene components. The method is generalized to allow
the effects of noise to come into the equation when noise performance is important as well.
Surface scattering can be formulated in terms of coherence functions averaged over surface realizations. The
resulting integrals for the average scattered intensity are superficially similar to those derived in conventional
formulations like the Kirchhoff, Beckmann, and physical-optics models, but the coherence function is subject
to some essential conditions, which are extensions of previously-derived conditions on the radiometric parame-
ters of primary, partially-coherent sources and their propagated fields, that significantly influence the resulting
scattered-intensity or BRDF solutions. The field approximation that leads to conventional radiance-like models
is compared to a field approximation that leads to a particular coherence model of surface scattering, which is
reviewed and verified against radiometric and atomic-force microscope (AFM) data due to a standard diffuse-gold
reflector, representing apparently the first verified inverse reflectance solution for a non-contrived diffuse rough
Non-imaging monostatic laser polarimetry has been used in a number of scenarios to probe characteristics of
both surfaces and intervening media. While the measurement technology required for laser polarimetry has
matured, sophisticated data-processing algorithms have been relatively slow to develop; hence laser-polarimeter
data has been typically under-utilized. This paper presents systematic applications of components analysis
to laser-polarimeter data that distinguish among electromagnetic-wave scattering characteristics of materials
and enable the development of adaptive discrimination and monitoring algorithms that are invariant to selected
variables in a scene. Both principal-components analysis (PCA) and non-linear components analysis are used to
derive orientation- or pose-invariant channels from Mueller matrices measured over all probe angles. Invariant
channels trained by using data due to isotropic scatterers are then used to conduct blind monitoring, i. e.,
predicting the presence of the target in a scene of arbitrary orientation, with the resulting cluster diagrams
presented with photos of the illuminated scene components. Training of a monitor invariant over dual variables
is demonstrated using data due to anisotropic scatterers.
Ion irradiation of polymer films is a promising process technology for photonics applications that require flexible, lightweight devices resistant to selected environmental variables. Crossed phase gratings that may serve as laser-beam array generators are fabricated using the dry process of irradiation of acrylic (PMMA) films with various doses of high-energy alpha particles through a stencil mask. The gratings are examined with the aid of AFM and SEM images, and Raman-Nath diffraction analysis is applied to estimate the generated refractive-index modulation as a function of the dose. SEM images of a stained grating cross-section suggest a mechanism of unsaturated bond formation and accompanying contraction of the irradiated polymer. Post-irradiation baking is shown to increase the contraction or generated surface relief by around an order of magnitude. Since the index modulation and surface relief due to irradiation tend to cancel, the overall diffraction effciencies of unbaked gratings do not surpass 67%, although baked gratings can provide higher diffraction effciencies.
The diffractive bidirectional reflectance distribution function (BRDF) of a surface with one-dimensional roughness is derived from coherence theory using an expansion valid for large effective roughness. Expansion of the surface autocorrelation function in the limit leads to representative Cauchy and Gaussian BRDFs and an intermediate closed-form general solution, which is fit to specular-plane BRDF data from an aluminum satellite sample at the wavelengths 1.06 microns and 10.6 microns.
Remote-sensing technology for weathering assessment should be sensitive to both textural and compositional changes in a surface. The capabilities of active polarimetry in weathering assessment are investigated through measurements of the Mueller matrices of bare and painted metal and dielectric surfaces at visible laser wavelengths in the quasi-monostatic geometry. Weathering mechanisms investigated include particulate erosion and solar irradiation, which are found to alter certain off-diagonal Mueller elements by around 5% in measurements over illumination angle.
Off-diagonal Mueller elements indicate polarization transformations as occur in polarizers and retarders. Target scattering may also generate off-diagonal elements, which then provide information unavailable from passive polarimetry or active depolarization measurements. The target and observation parameters required in active, monostatic systems for the detection of off-diagonal Mueller elements due to target scattering are investigated. The dependences of off-diagonal elements on incident angle, surface roughness, material composition, and target symmetry are investigated through analysis and measurements from two polarimeters. Multiple scattering and anisotropic roughness, which may result either from innate surface anisotropy or oblique incidence, are found to generate off-diagonal elements in the monostatic geometry. Results from a polarized microfacet scattering model corroborated with polarimeter data reveal a particular application of off-diagonal Mueller elements in the discrimination of dielectric from metal targets of similar roughness.
Full order Stokes polarimeters are often composed of an analyzer consisting of a rotating quarter wave plate in front of a horizontal polarizer. A number of measurements are then made with the wave-plate oriented at different angle. The four-element Stokes vector is then computed from a linear combination of these measurements. A disadvantage of this device is that only a limited range of analyzer states can be generated. As a result a large number of measurements may be required to reduce the noise gain in the Stokes vector reconstructor. In this paper we describe a polarimeter based on a linear polarizer and two variable wave plates. It can be shown that such a device can produce an arbitrary polarization state. An active polarimeter consists of a generator stage, which transmits a laser illuminator with different polarization states and a receiver with a polarization analyzer stage. In our system both generator and analyzer stages consist of a horizontal polarizer and two variable wave-plates. A sixteen element Mueller matrix of resolved images is then formed for target characterization.
We describe a number of methods for imaging into and through highly scattering media, all based in optical imaging processing methods. We describe methods that describe image formation in scattering media in new ways that complement transport theory and other traditional ways.
3D images have been formed through a pair of single mode fibers, using monochromatic light of reduced spatial coherence. Two fibers are required: one carries the object beam, the other the reference beam. Light comes out of the exiting end of the two fibers, interferes to form a hologram, which then forms a fully 3D image. Also, we have pursued the problem of imaging through highly scattering media, such as biological tissue. We recover both the amplitude and the phase of the uncorrupted wavefield from the scattered light. This is accomplished with considerable effort; we record up to 8000 electronic holograms and read them all into a computer; the resulting computer processing is quite intensive, requiring many hours of computing. With the phase thus recovered, we can get significantly improved image resolution.