Infrared image system for microelectronic devices: Auto focus and image correlation techniques

An infrared (IR) microscope camera system is used to measure the temperature distribution of power devices. In general, a microscope camera system contains a fixed lens; this means the whole camera has to be moved to detect focus images. During the heating up or cooling down process the thermo-mechanical expansion influences the measurement results. For the calibration, a pixel-by-pixel registration of individual images is required. The issues concerning finding the focus image and guarantee a pixel-by-pixel overlap in the image sequence are solved by the proposed auto focus and the image correlation algorithm. To detect the focus position and corresponding focus image, passive focusing is used, where a focus curve is recorded. The gradient operator is used to compute the focus value. Image registration is applied to compute the distortion between the images and guarantee a pixel-by-pixel overlap. In our case, the most significant parameter is the displacement, hence a simple image correlation algorithm is implemented.