BS ISO 17850:2015 pdf free download.Photography一Digital cameras – Geometric distortion (GD) measurements.
4 Measurement methods
4.1 General
As defined in 3J. geometric distortion basically derives from the variation of magnification in the image field. If this phenomenon occurs in an image. it means that a regular structure in an object does not appear to be regular in the image taken with the camera. There are two ways defined in this International Standard to quantify the amount of geometric distortion in an image. Both have their pros and cons.
4.2 Local geometric distortion
Geometric distortion can be measured on a white chart containing black dots at the position of a regular grid or on a grid chart formed by straight lines. The local geometric distortion method analyses the grid formed by the test chart in the centre of the image and calculates the ideal positions of the structure based on the measured distances. After that, it analyses the rest of the image and locates all actual positions of the grid. The distance between the ideal position and the actual position is the geometric distortion at that location in the image.
The distance between the two positions can be plotted as a function over the distance to the image centre. This curve indicates the variation of image magnification versus the actual image height, which is an expression of the geometric distortion called local geometric distortion. In order to limit the result to a single value that might get reported with the cameras specifications, the maximum (peak to peak) value shall be reported.
The manufacturing tolerances, such as lens tilt or off-centring, can result in a non-rotationally symmetric GD behaviour, lithe system is not rotationally symmetric, it can lead to increased distortion levels in the image corners. In this case, the measured geometric distortion is correct for the camera under test but might not represent a standard camera of the tested model.
4.3 Line geometric distortion
The principle of line geometric distortion is to measure the bending of a straight horizontal or vertical line at defined distances from the image centre and to report the maximum of the measured bending. This bending is preferably measured on a chart with a regular line grid.BS ISO 17850 pdf download.

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