Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
standard video convention. Line scan cameras are usually used with moving objects and
hence, new frames are externally triggered and created line by line with a finite, predetermined
length. Thus, there are no VSYNC signals within the video signal, so the frame
grabber must be able to process the various external trigger inputs from the camera or
control electronics.
6.2 DIGITAL FRAME GRABBERS
As the title implies, a digital frame grabber does not need to sample the analogue signal
from a camera, as the input is already digitised. Consequently, the design of such frame
grabbers is simpler. All digital cameras are progressive or line scan cameras, so each line is
stored sequentially with the frame start and line start signals set on individual data lines.
7 MAKING MEASUREMENTS
Now that we have a properly illuminated object and the ability to image it, we are in a
position to begin the process of making measurements using the image processing
software. The first action to be taken is to calibrate the camera to convert the pixel spacing
into units of length. Therefore, a traceable calibration standard or object with known
dimensions is required. Before choosing this, it is important to remember that the ideal
calibration standard for any application is one with similar dimensions, appearance and
texture to the artefacts to be measured.
Sections 7.1-5 describe the processes used to calibrate a Leitz Ergolux AMC microscope,
fitted with a 1024x1024 Pulnix digital camera and a Leica QM550 CW image analyser.
Whilst there may be some variations in manufacturer terminology and facilities, the
calibration and measurement processes described in these sections are essentially
applicable to all microscopy and macroscopy applications using vision systems.
7.1 CHOOSING THE RIGHT TYPE OF MEASUREMENTS
Before making any measurements, it is first worth considering some of the different types
of dimensional measurements that can be made. When using a binary image (see section
7.4) it is likely that most measurements will be point to point. For example, the diameter of
a circular spot may be given by the longest line of detected pixels in the object. This is
known as a feret diameter and is akin to a calliper measurement. These types of
measurement are easily obtainable from the vision system, and for most applications have
an acceptable accuracy of ±2 pixels. If higher accuracy is required, then the diameter of the
spot may be derived from area measurements. Sub-pixel accuracy is readily achievable by
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