Ingmar Renhorn

Wavelength band selection method for multispectral target detection

A framework is proposed for the selection of wavelength bands for multispectral sensors by use of hyperspectral reference data. Using the results from the detection theory we derive a cost function that is minimized by a set of spectral bands optimal in terms of detection performance for discrimination between a class of small rare targets and clutter with known spectral distribution. The method may be used, e.g., in the design of multispectral infrared search and track and electro-optical missile warning sensors, where a low false-alarm rate and a high-detection probability for detection of small targets against a clutter background are of critical importance, but the required high frame rate prevents the use of hyperspectral sensors.

Analytical fitting model for rough-surface BRDF.

A physics-based model is developed for rough surface BRDF, taking into account angles of incidence and scattering, effective index, surface autocovariance, and correlation length. Shadowing is introduced on surface correlation length and reflectance. Separate terms are included for surface scatter, bulk scatter and retroreflection. Using the FindFit function in Mathematica, the functional form is fitted to BRDF measurements over a wide range of incident angles. The model has fourteen fitting parameters; once these are fixed, the model accurately describes scattering data over two orders of magnitude in BRDF without further adjustment. The resulting analytical model is convenient for numerical computations.

Nonuniformity correction of infrared focal plane arrays

The non-linearity of the responsivity of infrared (IR) focal plane arrays (FPA) is a major concern when IR images are evaluated. This will affect both the detection and classification ability of the IR sensor. The nonuniformity correction (NUC) method proposed here is based on polynomial fitting to pixel responsivities. It is applied to infrared data obtained from two cooled multi-band sensors in the MWIR region. The method is shown to give both suitable data restoration and a simple and efficient computer implementation. Bad pixels were identified by evaluating the coefficents obtained from the multi degree polynomial. Bad pixels were also replaced by using a nearest neighbour-hood algorithm. The NUC method, including the bad pixel identification and replacement, was implemented in a user friendly GUI (graphic user interface) for calibration of registered IR data.

Imaging Q-switched CO2 laser radar with heterodyne detection: design and evaluation

A Q-switched CO2 laser radar with heterodyne detection has been designed and evaluated. A simplified theory has been used to optimize the Q-switched laser for high-resolution ranging. The return signal statistics from diffuse, glint, and topographical targets have been investigated, and statistical distributions have been fitted to the experimental data. Detection of specific targets in laser radar images using range gating has also been studied.

Measured signal amplitude distributions for a coherent FM-cw CO2 laser radar.

Measurements of signal amplitude distributions with a FM-cw CO2 laser radar have been made against various targets in both imaging and staring modes. Data show good agreement with theoretical distributions. From the measurements conclusions are drawn about the atmospheric- as well as target-induced effects. Beam wandering effects are shown to be of importance in the staring mode.

Noise sources in laser radar systems.

To understand the fundamental limit of performance with a given laser radar system, the phase noise of a testbed laser radar has been investigated. Apart from the phase noise in the transmitter laser and the local oscillator laser, additional phase noise was introduced by vibrations caused by fans in power supplies and cooling systems. The stability of the mechanical structure of the platform was also found to be of great importance. Furthermore, a model for the signal variations from diffuse targets has been developed. This model takes into account the stray light, the speckle decorrelation, and Doppler shift due to moving targets.

Coherent laser radar for vibrometry: robust design and adaptive signal processing

A coherent laser radar system based on semiconductor laser technology has been designed and built. The compact design and the absence of adjustments makes the system mechanically robust and easy to use. The present system has an output power of 50 mW and a line width of 280 kHz (HWHM). The laser radar system has been used in vibrometry measurements. For vibrometry of moving objects, adaptive signal processing is required in order to obtain the vibration signature. Especially for unresolved objects, interference between different vibrating parts will complicate the analysis. Model based estimation techniques are used to obtain the parameters which determine the dynamics of the reflecting object.

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.

Terrain segmentation using laser radar range data.

A novel approach to segmentation of laser radar range images is presented. The approach is based on modeling horizontal and vertical scans of the terrain as piecewise-constant or piecewise-linear functions. The approach uses adaptive estimation based on Kalman filtering techniques. The performance of the segmentation algorithm is evaluated by application to laser range measurements. We also discuss how the output from the segmentation algorithm can be used for, e.g., object detection.