Tomas Hallberg

Scattering and polarization properties of the scarab beetle Cyphochilus insulanus cuticle

Optical properties of natural photonic structures can inspire material developments in diversified areas, such as the spectral design of surfaces for camouflage. Here, reflectance, scattering, and polarization properties of the cuticle of the scarab beetle Cyphochilus insulanus are studied with spectral directional hemispherical reflectance, bidirectional reflection distribution function (BRDF) measurements, and Mueller-matrix spectroscopic ellipsometry (MMSE). At normal incidence, a reflectance (0.6-0.75) is found in the spectral range of 400-1600 nm and a weaker reflectance <0.2 in the UV range as well as for wavelengths >1600u2009u2009nm. A whiteness of W=42 is observed for mainly the elytra of the beetle. Chitin is a major constituent of the insect cuticle which is verified by the close similarity of the measured IR spectrum to that of α-chitin. The BRDF signal shows close-to-Lambertian properties of the beetle for visible light at small angles of incidence. From the MMSE measurement it is found that the beetles appear as dielectric reflectors reflecting linearly polarized light at oblique incidence with low gloss and a low degree of polarization. The measured beetle properties are properties that can be beneficial in a camouflage material.

Four-parameter model for polarization-resolved rough-surface BRDF

A modeling procedure is demonstrated, which allows representation of polarization-resolved BRDF data using only four parameters: the real and imaginary parts of an effective refractive index with an added parameter taking grazing incidence absorption into account and an angular-scattering parameter determined from the BRDF measurement of a chosen angle of incidence, preferably close to normal incidence. These parameters allow accurate predictions of s- and p-polarized BRDF for a painted rough surface, over three decades of variation in BRDF magnitude. To characterize any particular surface of interest, the measurements required to determine these four parameters are the directional hemispherical reflectance (DHR) for s- and p-polarized input radiation and the BRDF at a selected angle of incidence. The DHR data describes the angular and polarization dependence, as well as providing the overall normalization constraint. The resulting model conserves energy and fulfills the reciprocity criteria.

Efficient polarimetric BRDF model

The purpose of the present manuscript is to present a polarimetric bidirectional reflectance distribution function (BRDF) model suitable for hyperspectral and polarimetric signature modelling. The model is based on a further development of a previously published four-parameter model that has been generalized in order to account for different types of surface structures (generalized Gaussian distribution). A generalization of the Lambertian diffuse model is presented. The pBRDF-functions are normalized using numerical integration. Using directional-hemispherical reflectance (DHR) measurements, three of the four basic parameters can be determined for any wavelength. This simplifies considerably the development of multispectral polarimetric BRDF applications. The scattering parameter has to be determined from at least one BRDF measurement. The model deals with linear polarized radiation; and in similarity with e.g. the facet model depolarization is not included. The model is very general and can inherently model extreme surfaces such as mirrors and Lambertian surfaces. The complex mixture of sources is described by the sum of two basic models, a generalized Gaussian/Fresnel model and a generalized Lambertian model. Although the physics inspired model has some ad hoc features, the predictive power of the model is impressive over a wide range of angles and scattering magnitudes. The model has been applied successfully to painted surfaces, both dull and glossy and also on metallic bead blasted surfaces. The simple and efficient model should be attractive for polarimetric simulations and polarimetric remote sensing.

Noise properties of a corner-cube Michelson interferometer LWIR hyperspectral imager

Interferometric hyperspectral imagers using infrared focal plane array (FPA) sensors have received increasing interest within the field of security and defence. Setups are commonly based upon either the Sagnac or the Michelson configuration, where the former is usually preferred due to its mechanical robustness. However, the Michelson configuration shows advantages in larger FOV due to better vignetting performance and improved signal-to-noise ratio and cost reduction due to relaxation of beamsplitter specifications. Recently, a laboratory prototype of a more robust and easy-to-align corner-cube Michelson hyperspectral imager has been demonstrated. The prototype is based upon an uncooled bolometric FPA in the LWIR (8-14 μm) spectral band and in this paper the noise properties of this hyperspectral imager are discussed.

Calibrated sensitive polarization measurement methods in the regions 3-5 μm and 8-12 μm,corrected for contributions to the detector signal from the polarizer

Exploitation of polarization increases the contrast in imaging sensor and makes reconnaissance and surveillance sensors more efficient. This paper reports about two methods of polarization measurements. The used equipments are two IR Thermo vision 900 cameras, one in the wavelength region 3-5 micrometers and the other in the region 8-12 micrometers . The cameras have polarizing filters in front of the sensors. A calibration method is applied. In that way it is possible to correct for the signal contribution from the polarizing filters. The contribution can be separated into two parts, one consists of contributions to the detector signal due to the temperature of the filters and the other consists of reflections at the filter surfaces. The measurement methods are illustrated by laboratory measurements of surfaces of different surface roughness. The objectives of these measurements are to understand the physical properties of different surfaces for camouflage work. Also, calibrated sensitive polarization measurements of backgrounds are reported. The methods make it possible for the sensors to increase their ability for detection, recognition and classification.

Simplifying BRDF input data for optical signature modeling

Scene simulations of optical signature properties using signature codes normally requires input of various parameterized measurement data of surfaces and coatings in order to achieve realistic scene object features. Some of the most important parameters are used in the model of the Bidirectional Reflectance Distribution Function (BRDF) and are normally determined by surface reflectance and scattering measurements. Reflectance measurements of the spectral Directional Hemispherical Reflectance (DHR) at various incident angles can normally be performed in most spectroscopy labs, while measuring the BRDF is more complicated or may not be available at all in many optical labs. We will present a method in order to achieve the necessary BRDF data directly from DHR measurements for modeling software using the Sandford-Robertson BRDF model. The accuracy of the method is tested by modeling a test surface by comparing results from using estimated and measured BRDF data as input to the model. These results show that using this method gives no significant loss in modeling accuracy.

Optical polarization and the dependence of angle of incidence for different surfaces: comparison between different wavelengths from UV to IR

As the sensor technology for polarimetric imaging is advancing into more robust commercial systems such sensors could soon be expected for, e.g., military surveillance and reconnaissance applications in addition to more conventional sensor systems. Thus, there might be an upcoming need to understand limitations on present camouflage systems to meet this new sensor threat. Some of the reasons why polarimetric imaging has drawn attention is the ability to achieve a higher contrast for artificial surfaces against natural backgrounds, by analyzing the degree of linear polarization, which in this work has been analyzed for different types of surfaces as a function of wavelength. We also compare with the polarimetric vision of horse-flies and other aquatic insects via the polarization properties of different colors of horse coat hair in order to give some further insight into polarimetric vision techniques developed by nature. In this work we have used different measurement techniques, such as angle dependent polarimetric spectral directional hemispherical reflectance and polarimetric imaging.

Infrared absorption bands measured with an uncooled interferometric LWIR hyperspectral camera

This paper presents stand-off measurements with an interferometric LWIR hyperspectral camera, which is based on a corner-cube Michelson interferometer and an uncooled micro-bolometer. Spectrally resolved image data may provide a more robust discrimination and, moreover, the ability to also classify and identify materials. This is however dependent on three parameters: instrument, source intensity and vibrational bands. Strong vibrational bands in LWIR can be found in minerals (solid, crystalline substances), textile fibers (polymers) and hazardous substances like toxic chemicals and explosives (smaller molecules). Spectral bands measured e.g. in textile polymers may be used in applications such as discrimination and identification of camouflage. Experimental setups and measured vibrational bands are presented, along with interpretations of vibrational frequencies in LWIR.

Design, calibration and characterization of a low-cost spatial Fourier transform LWIR hyperspectral imaging camera with spatial and temporal scanning modes

Hyperspectral imaging in the longwave infrared region (LWIR) offers a unique day and night sensor capability currently not available in other spectral ranges. Current proliferation of the technology is however often limited by the size, weight, power and cost (SWaP-C) requirements of the available instruments. This paper presents a low-cost spatial Fourier Transform LWIR hyperspectral imaging camera, based on a corner cube Michelson interferometer and an uncooled microbolometer. In addition to conventional spatial pushbroom scanning, e.g. provided by a moving platform, the current interferometric setup can use temporal scanning of a stationary field-of-view due to a spatially offset stepper motor controlled corner cube retroreflector. The design and calibration and a characterization of the instrument are presented. Applications and future modifications are also discussed.

Optical polarization: background and camouflage

Polarimetric imaging sensors in the electro-optical region, already military and commercially available in both the visual and infrared, show enhanced capabilities for advanced target detection and recognition. The capabilities arise due to the ability to discriminate between man-made and natural background surfaces using the polarization information of light. In the development of materials for signature management in the visible and infrared wavelength regions, different criteria need to be met to fulfil the requirements for a good camouflage against modern sensors. In conventional camouflage design, the aimed design of the surface properties of an object is to spectrally match or adapt it to a background and thereby minimizing the contrast given by a specific threat sensor. Examples will be shown from measurements of some relevant materials and how they in different ways affect the polarimetric signature. Dimensioning properties relevant in an optical camouflage from a polarimetric perspective, such as degree of polarization, the viewing or incident angle, and amount of diffuse reflection, mainly in the infrared region, will be discussed.