Paul C. Chevrette

Pixel-less infrared imaging based on the integration of an n-type quantum-well infrared photodetector with a light-emitting diode

This letter presents the recent developments of large-area focal plane “pseudo” arrays for infrared (IR) imaging. The devices are based on the epitaxial integration of an n-type mid-IR (8–10 μm) GaAs/AlGaAs quantum-well detector with a light-emitting diode. The increase of spontaneous emission by the midinfrared-induced photocurrent is detected with a charge-coupled device camera in the reflection configuration. The mid-IR image of a blackbody object is up-converted to a near-IR transformed image with very small distortion.

Evaluation of the microscanning process

Focal plane arrays allowed tremendous improvement in the robustness and compactness of thermal imagers reducing both mechanical and optical requirements. However, these will always be limited by the pixel size, the fill factor, and by the sampling theorem. As compared to older one-detector scanning systems, focal plane arrays can only reproduce half the frequencies scanning systems do for a given instantaneous field of view. To overcome this limitation, microscanning seems to be a winning approach. Microscanning can be seen as an oversampling process. A series of images representing the same scene are taken while displacing each time the image over the array by a fraction of the detector pitch. The oversampled image is built by interlacing all the pixels from all the images in both directions. It can be shown that microscanning can bring the resolution to the same level it is with standard scanning system. Furthermore, by characterizing the process, one can compensate for it and bring the resolution to the level of a microdisplacement. This article describes work that has been undertaken at the Defense Research Establishment Valcartier to evaluate the requirements for the microscanning process and to determine gains that can be obtained by using that technique in a surveillance application.

Realization of a fast microscanning device for infrared focal plane arrays

Microscanning is a technique that allows to double the resolution of a given staring array imager. It consists in taking multiple images of the same scene while displacing each time the image over the detector plane by a distance equal to a fraction of the detector pitch. The technique is limited by the time required to shift the image from one point to the other and by the precision of the movements. This article describes work that was done under contract for the Defense Research Establishment Valcartier as part of the Wide Area Coverage Infrared Search System (WACISS) project to develop a fast microscanning imaging device. The system includes three main sections: the microscanning head, the controller and the power amplifier. THe microscanning head is made of a lens and a two-axis microtranslation table driven by two piezoelectric translators. The controller drives a high voltage power amplifier which in turn drives the translator. The controller allows four operation modes: fixed position, 2 X 2, 3 X 3, and 4 X 4 microscan. It works in open as well as in closed loop for precise displacements. The systems will be integrated to the WACISS project and will serve as an aid for the identification of detected objects.

Step-stare technique for airborne high-resolution infrared imaging

The Infrared Eye project was developed at DRDC Valcartier to improve the efficiency of airborne search and rescue operations. A high performance opto-mechanical pointing system was developed to allow fast positioning of a narrow field of view with high resolution, used for search and detection, over a wide field of view of lower resolution that optimizes area coverage. This system also enables the use of a step-stare technique, which rapidly builds a large area coverage image mosaic by step-staring a narrow field camera and properly tiling the resulting images. The resulting image mosaic covers the wide field of the current Infrared Eye, but with the high resolution of the narrow field. For the desired application, the camera will be fixed to an airborne platform using a stabilized mount and image positioning in the mosaic will be calculated using flight data provided by an altimeter, a GPS and an inertial unit. This paper presents a model of the complete system, a dynamic step-stare strategy that generates the image mosaic, a flight image taking simulator for strategy testing and some results obtained with this simulator.

Infrared Eye: an operational prototype

A new concept of surveillance system called Wide Area Coverage Infrared Surveillance System (WACISS), based on the human vision, was developed and a first laboratory prototype was demonstrated recently. A second prototype, more operational, is named the Infrared Eye is being built and will be tested in cooperation with the NRCC Flight Research Laboratory. The Infrared Eye will use the new pixel-less quantum well infrared photodetector sensors, coupled to light emitting diodes (QWIP/LED), currently being developed at NRCC Institute for Microstructural Science under DREV sponsorship. The multiple advantages of the pixel-less QWIP/LED over conventional sensors will considerably simplify the design of the system. As the WACISS, the IR Eye will integrate two cameras: the first, with a wide field-of- view, will be used for detection while the second camera, with a narrower field with higher resolution for identification, will be mobile within the WFOV and slaved to the operators line-of-sight by means of an eye-tracking system. The images from both cameras will be fused and shown simultaneously on a standard high resolution CRT display unit, interfaced with the eye-tracking unit. The basic concepts pertaining to the project and the design constraints of this second prototype are presented.

Wide-area-coverage infrared surveillance system

Current infrared imaging systems used for surveillance and search and rescue operations possess two fields of view which may be alternately selected by the operator: a wide field of the order of 20 degrees is used for the search and detection of targets, and a narrower field of a few degrees is selected for the recognition tasks. However, the degraded sensitivity and resolution of the wider field prevents it from fulfilling its function adequately. A new concept based on the focal plane array detector technology is intended to correct this drawback and to improve future infrared surveillance system for search and rescue operations. Simulating the properties of the human eye, the concept allows the simultaneous surveillance and image acquisition in two fields of view. A wide peripheral field of view (60 degrees) with increased sensitivity but lower resolution is dedicated to search and detection. A narrower field (6 degrees), which can be steered within the wider field, allows the recognition of detected objects with an improved resolution obtained by the use of microscanning techniques. THe high resolution required for the simultaneous display of both fields of view has led to the development of a new type of display, based on optical projection and superposition, better adapted to the human eye and hence optimizing the human interface. The constraints on the opto-mechanical and electronic design imposed by the mobility of the narrower field within the larger one, the microscanning mechanism and the calibration requirements of the focal plane array are discussed, and the selected solutions are presented. The limitations of the system in its present state of development are exposed and the plans for future improvements are elaborated.

Enhancement of point-source targets in an IR staring FPA sensor

This paper describes the results of experiments that were conducted in order to characterize the types of noise limiting the performance of an amber InSb charge injection device focal plane array (3-5 microns) of 256 by 256 pixels. This is part of the work done at the Defense Research Establishment Valcartier to develop a wide-area-coverage infrared surveillance system. The emphasis is put on the analysis of the postcorrection spatial noise that reduces the array sensitivity to weak point-source targets. This residual noise limits the improvement provided by an increased array integration time. Furthermore, the results show that a temporal low frequency noise component has a more severe effect than detector nonlinearities. However, this problem can be partly resolved with a periodic offset compensation obtained by reference image subtraction. The reference image is acquired when the blade of a flat black chopper wheel completely blocks the aperture of the camera. The chopper wheel is synchronized on the acquisition process. Results show that this compensation method can efficiently reduce the low frequency noise level and enhance point-source target detection.

Field calibration software for thermal imagers and validation experiments

A software developped in C language for a quick field calibration of thermal imagers is presented. The software allows to take into account the atmospheric transmission between the target and the imager, and to evaluate the corrected blackbody radiation temperature of any resolved area on the target at a known range. The software and the field procedure for calibration are described. Some validation tests carried out during the NATO RSG-17 trial in Meppen Germany, and more extensive tests performed at DREV after the trial are also presented. The error budget in calibrating thermal imagers and evaluating both extended and point target signatures from thermal imagery is analysed and discussed.

Point source detection with pixelless QWIP: comparison study to optimize the spectral band

Recent developments on QWIP-LED detectors have led to Pixelless Imaging Devices. These detectors convert a thermal IR image into a near-IR image giving the possibility to image an IR scene at a higher resolution on the same detector area. Their use into a surveillance system is of great interest. The aim of this theoretical study is to compare the Signal-to-Noise ratio obtained with different spectral bands of these new pixelless sensors.

Automatic MRTD objective measurement for IR systems

An objective methodology that can be used to perform automatic MRTD tests on infrared imaging systems is presented. It is based on the assumption that a unique threshold function should exist between the signal-to-noise ratio measured by a computer that perform a spatio-temporal filtering on digitized images and the MRTD.