Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
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Grey scale value
0
Average
Single
0 100 200 300 400 500
Pixels
Average profile 50%
threshold
Single profile 50%
threshold
Figure 29: Comparison of average intensity profile with a single pixel line intensity profile.
7.5 QUANTIFYING THE EFFECTS OF DISTORTION AND
MAGNIFICATION ERRORS
Now that the camera has been calibrated and the correct threshold determined, the
useable measurement area must be identified. Localised magnification errors are quite
common and knowledge of where and by how much the scaling breaks down, allows the
determination of the usable measuring area of the image. This is done by imaging an array
of calibrated spots, or other suitable standard, over the entire image, as shown in Figure
30. The diameters of the spots are then measured and any deviation in the sizes of the
spots can be seen by comparison with the calibrated values. Measurements must be made
rather than direct visual observations because screen distortions may affect judgment.
Errors due to distortion are usually found at the edge of the field of view. These may be
quantified by measuring the diameters, or centre-to-centre pitches, of the array of spots
and comparing them to the calibrated values. It is also possible to perform the pixel
calibration using this method and this can provide a more accurate and reliable calibration
value for the system, as the centroids of the spots are determined with sub-pixel accuracy.
Both axes may be calibrated at the same time and an error map produced very quickly for
the entire image area. Once the useable area of the screen has been determined, the pixel
calibration should be repeated within that area to minimise errors.
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