Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
5.4 DIGITAL CAMERAS
A digital camera is ideal for dimensional measurement using vision systems. Its operation
is completely different to an analogue camera, in that the analogue to digital (A/D)
conversion of the image takes place within the camera. This has significant benefits, as all
the unwelcome effects of analogue signal transmission such as noise, interference and line
loss are removed. The main drawback to these cameras is that they are more expensive
than analogue types. Digital cameras built before the Camera Link standard was adopted
also suffer a second drawback, that is, the complex cabling required for their operation. In
its simplest form, an analogue camera requires only one coaxial cable to send an image.
The digital signal, however, is sent byte by byte, with additional signal lines reserved for
synchronisation, pixel clock and handshaking. The result is a camera that requires a bulky,
inflexible cable with multiple wires, which often has to be custom made, increasing the
cost still further. The industry wide adoption of the Camera Link standard will signal the
end of complex cabling and significantly reduce the cost of cables for digital cameras.
5.5 LINESCAN CAMERAS
The active area of this camera consists of a single row of pixels. Linescan cameras are
mostly used for imaging large, or moving objects. Complete 2D images can be built up by
physically scanning the object or camera, with the start controlled by a trigger pulse and
the acquisition stopped after a pre-determined number of lines have been scanned. The
processing of linescan images does not follow the same rules as those for area scan
cameras therefore specific frame grabbers may be required.
6 FRAME GRABBERS
The frame grabber is the interface between the camera and the control PC. Its task is to
assemble a digital image, which can be processed by the computer. There are many
commercial frame grabbers available, but their basic operation is as follows:
6.1 ANALOGUE FRAME GRABBERS
If the video signal is analogue, then the frame grabber must digitise it, using an A/D
converter, usually with 8-bit resolution. The sampling interval of the A/D converter,
called the pixel clock, can be internally generated or transmitted by the camera. The
digitised values are stored in the frame grabber’s memory area until a complete image is
assembled. The following sections describe the two modes of operation of frame grabbers
and how they are used with various camera types.
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