Line-Scan Cameras

In Figure 2.14 we showed a one-dimensional CCD array for measuring positions or lengths of objects. In general, we can use the same sensor to acquire two-dimensional images; we only have to scan the second dimension. There are two principal ways to do this:

• The sensor moves relative to the object to be scanned; this principle is used in flat-bed image scanners , whereby two-dimensional images (mostly on paper) can be transferred into digital media.
• The object moves relative to the sensor; at first glance, there seems to be no difference from the first possibility, but some differences do exist:
  • Normally, objects can be moved quickly and easily. If a large number of objects had to be scanned, it would obviously take more time and effort to position the camera rather than to simply position the objects.
  • The object is not limited regarding its length. Figure 2.18 shows an example in which a one-dollar note is being scanned. It is easy to imagine that it would be no problem to add a number of dollar notes to a single image.

Figure 2.18. Principle of a Line-Scan Camera

With this technology, one can easily generate images with a large number of pixels. For example, the sensor of the line-scan camera in Figure 2.18 has 2048 pixels, which means that it is a high-resolution type. Imagine further that the resolution in both directions should be the same, resulting in a total resolution of about 2048 x 4800 9.8 million pixels ”and that with a simple sensor of 2048 CCD cells .

The main application area for line-scan cameras is product inspection; in particular, products transported by a conveyor belt or rotating cylindrical objects are easy to scan. Figure 2.19 shows a line-scan sensor for those or similar applications.

Figure 2.19. Line-Scan Sensor Used in a Flat-Bed Scanner

CMOS Image Sensors