Electro-optic staring sensors, which sample a scene with pixels of finite size, generate images that are affected by aliasing and blurring caused by the sampling process. One potential method to reduce the effects of sampling is microscanning. In the microscan process, multiple images of the scene are generated. Between each successive image, the location of the image on the detector array is moved a fraction of a pixel. The set of images produced in the microscan process are then combined to form a single image. We present an analytical model of the microscan process. The model shows that the microscan process can significantly reduce aliasing in the reconstructed image, and that the process does not blur the image beyond the blur caused by the finite pixel aperture. The model also shows that factors such as the blur produced by the imaging optics and the fill factor of the detector array affect the reduction in aliasing produced by microscanning. We present a quantitative description of the effect of microscanning for selected cases of fill factor, optics blur, and number of microscan steps. We also present images produced by computer simulation which qualitatively verify the reduction in aliasing associated with the microscan process.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Aliasing and blurring in microscanned imagery

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.

Evaluation of the microscanning process

The development of a unique discrete modulation transfer function (DMTF) [1] has solved the previous problem of multivalued modulation transfer function associated with the non-isoplanatic nature of discrete systems. This paper proposes two methods for the measurement of the DMTF and shows that the use of the DMTF makes the performance analysis and measurement of discrete systems easier.

Measurement of the Discrete Modulation Transfer Function

A discrete modulation transfer function for focal plane arrays

There is widespread requirement for low cost lightweight thermal imaging sensors for both military and civilian applications. In Europe, these requirements are now being met by systems using large uncooled ferroelectric detector arrays offering performance levels which, until recently, could only be achieved by expensive cryogenically cooled systems. The uncooled technology is the result of collaboration between the UK Defence Research Agency (DRA) and GEC-Marconi Electro Optics (GMEO) under a 'dual use technology program' (DUTP). This has resulted in the design of an uncooled module suite which constitute the core elements of the European uncooled thermal imaging sensors. This module suite has been adopted by UK MoD to meet the requirements of the UK Stairs A demonstrator program producing a lightweight thermal surveillance and weapon sight. A European consortium including Delft Instruments and Signaal Usfa (DISU) in Holland have also adopted the modules to meet the requirements of the lightweight infrared observation nightsight (LION). This product is being marketed in collaboration with Thomson CSF Optronics, of France. This paper discusses the current status of the underlying detector and processing technology including enhancements already in development. In addition, the designs and benefits offered by the Stairs A and LION products are addressed.

European uncooled thermal imaging technology

Focal plane arrays (FPA) of CdHgTe detectors have been incorporated successfully into infrared imagers with sensitivities in either the 3-5 µm or 8-14 µm bands. Whilst it has been possible to obtain good imagery with minimal fixed pattern noise using digital non-uniformity correction electronics, these imagers suffer from problems caused by the sampling of the scene by the FPA; in particular, aliasing results in the obscuration of high frequency detail in the scene and its appearance at lower frequencies below the Nyquist frequency. It is demonstrated how microscanning may be used to reduce these sampling effects and in addition, ways of improving the ease of viewing of the final image by reducing the degree of pixelation are discussed.

D. J. Bradley

Papers

Sampling effects in CdHgTe focal plane arrays