Chapter 10 Interfacing and Vision Systems 249
Image Acquisition
Image acquisition involves illuminating a workpiece and digitally scan-
ning its image. Fluorescent lamps, incandescent bulbs, strobe lights, or arc
lamps usually provide the illumination. The type and amount of light needed
depends on the application. Front lighting is used to enhance surface features,
such as bar codes or labels. Side lighting is used when three-dimensional (3D)
images are desired. Back lighting provides a silhouette of the object.
Scanning an image is done with a video camera. The solid-state digital
cameras used with vision systems have either a charge-coupled device (CCD)
or a charge-injection device (CID). At the heart of the camera is a silicon chip
used with an array of photosensitive elements. The light reflected into the
camera lens from the workpiece falls onto this photosensitive array and is
converted into an analog electrical signal. The image is broken down into
dots of light called picture elements, or pixels. Pixels are the means used to
form the image on a TV screen. A typical digital video camera can produce
an image that measures 1024 by 768 pixels.
Image Preprocessing
During image preprocessing, an analog-to-digital (AD) converter
changes the analog signal into an equivalent digital signal, Figure 10-10.
The digital signal represents light intensity values over the entire image.
These values are stored in the memory, which allows the digital image to
be analyzed and interpreted.
Keyboard
Typical Vision System Components
Digital
camera unit
Digital video
processing
Ouput control
interface unit
Computer
Video monitor
To robotic
workcell
Keyboard
Part
Light source
Image
Acquisition
Image Preprocessing, Analysis,
and Interpretation
Interface to Robotic
Workcell Equipment
Light source
Figure 10-9. These digital electronic subsystems are found in typical machine vision systems.