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Spatial resolution

Spatial resolution refers to the number of active pixels on the sensor. When designing a system, it is important to calculate the optimum camera resolution in order to be able to solve the inspection task in a robust and cost-effective way.

To achieve this, it is necessary to know details about the features that need to be detected. The smallest feature in combination with the required field of view, defines the smallest possible spatial resolution. In addition care has to be taken concerning po tential limitations due to the optical system (lens) or the software used.

For example, consider a 1 x 1 mm feature that needs to be inspected on an object. The feature can occur anywhere on the object, which measures 100 x 100 mm. The software requires that the feature occupies 3 x 3 pixels to achieve a reliable detection. It will therefore be necessary to have 3 pixels per mm, multiplied by the object size, which equates to a spatial resolution of 300 x 300 pixels.

This will hold true for mono chrome cameras, however, special consideration needs to be taken when using colour cameras that use a Bayer pattern. Here the colour of each pixel is interpolated using colour information from adjacent pixels. As a guide the minimum resolution should be doubled when using Bayer sensors.