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| The raw data is first cleaned to remove erroneous noise and a background correction is applied to remove any nonuniformity of the illumination conditions. A threshold value, which is a % greyscale value is then applied. This value is user definable depending upon the sample but is usually set at around 90% of the background modal value. Every pixel darker than this threshold value is then defined as particle and everything lighter than this is defined as background. |
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| The particle perimeter is then traced using a technique known as chain code which assigns a value to each pixel reflecting its relationship with its immediate neighbours. For example – a pixel in a straight line with two of its neighbours (pixel ‘e’ in the diagram) will have a different value than a pixel on a corner (pixel ‘d’). All these chain code values are then summed together for the entire particle giving a more accurate perimeter value than if it was calculated by just counting the pixels and giving them all an equal weighting. |
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