Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
with the scale oriented on the Y-axis, to check the squareness of the calibration, as the
camera may not have square pixels. Failure to account for this may introduce significant
errors into the measurements. Most systems can produce a software - generated grid. If
this grid is superimposed on the image of the graticule grid, any large-scale image
distortions will become evident.
7.4 SETTING THE CORRECT DETECTION THRESHOLD
After calibrating the system, detecting objects will require setting a threshold. A vision
system performs measurements by detecting changes in intensity and hence, the contrast
of a feature e.g. a black spot against a white background. As each pixel has an 8-bit grey
scale intensity value range, 0 being black and 255 white, the user can define a threshold on
the grey scale so that regions with intensity above or below that value can be detected. In
this way, a binary image is created, whereby pixels with a greyscale intensity value below
the threshold are assigned a binary 0, and those above, a binary 1. Depending on whether
bright or dark features are to be measured, adjacent detected pixels are grouped together
to form a detected feature. Modern vision systems may have the facility to perform grey
image processing of features, but for the purposes of this guide, only binary image
processing will be covered. Figure 28 shows a typical intensity profile of a dark feature on
a bright background. The maximum intensity range of the camera is 0-255 greyscale, but
the actual dynamic range of the image may well be less, especially if there is low contrast.
The correct threshold level should be calculated from the image grey scale range. This is
called normalising the intensity threshold. For most vision systems, where the optical
magnification is less than 20x, the 50% normalised threshold should be used. The effects of
coherence from the illumination on the true edge position of a feature can be neglected at
this scale, as it is well below the measurement uncertainty achievable. The formula for
calculating the 50% threshold is given in equation 14:
t = b + (a – b)/2
…..(14)
Where ;
t = Grey scale intensity threshold
b = Mean dark level intensity
a = Mean bright level intensity
50