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The normalisation filter is an non-linear filter useful to detect dark narrow bands with a background of varying intensity. In such cases it can be difficult to set a threshold, especially if you search for threshold passing, but also if you search for intensity jumps. In both cases the varying background is a problem. The normalisation filter flattens the background, thus the dark bands turns out with their relative strength compared to the background. If they have similar relative strength compared with the background, a common detection threshold will work.

The general background intensity in a neighbourhood of all points on the intensity profile is calculated. Then the point’s intensity is divided by the background intensity and multiplied with 100. You get an intensity profile consisting of percent values close to 100. Only where dark bands it will be below 100. The bands can know be measured as threshold passing if setting a threshold less than 100, e.g. 80.

To parameters manages the normalisation. The normalisation width decides the width of the point neighbourhood. The number sets how many points to each side you calculate. A normalization width of 5 means for example that the intensity is measured for a total of 11 points. All points in the neighbourhood are sorted by increasing intensity. The normalisation selector decides which of the sorted points to use as background value. If we use the example with normalisation width set to 5, the normalisation selector must be in the –5..5 area. –5 picks the lowest of the sorted numbers, 0 picks the median and 5 picks the largest number. Other values pick a number in between. If you have a large normalisation width (most of the neighbourhood points having a background intensity), 0 will e a robust and good normalisation selector. If you have a narrower width and only a few of the points having any background intensity, you must use a higher selector value since you want to pick one of the higher values. Avoid however the maximum value since it often is noisy.

The normalisation filter can likewise be used for light bands on varying background. For example to find weaknesses in a transparent material with varying background light.

Parameters guiding filtering

Filter parameters


Smooth base

Sets the filter width, or how large the smoothing neighbourhood is. Increasing this value does not considerably increase the time of analyses.

Smooth counter

Sets the number of smoothing iterations. In praxis the filter turns wider. The time of the analyses linearly increases with this parameter.


Sets the differentiation order.
The values are  

0.   No differentiation
1.   First derived (gradient)
2.   Second derived (curve)

The time of the analyses linearly increases with the order.

Normalisation width

Sets the width of the neighbourhood calculating the background intensity. The time of the analyses linearly increases with the width.

Normalisation selector

Selects the background intensity from a sorted list of neighbourhood values decided by the normalisation width k. Valid values are then –k..k. This parameter does not affect the time of analyses.



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