Web inspection technology

Contact Image Sensors (CIS) (Fig 1) offer an interesting alternative approach to conventional linescan cameras for web inspection applications. They record images of moving objects in a very similar way to flatbed scanners and photocopiers. With line rates up to 75kHz, scanning widths up to 4m (at 25-600dpi) or 1m at 1,200-3,600 dpi, pixel resolution from 1mm to 7 micron and integral light sources, the CIS offers benefits in terms of space required, ease of alignment, scalability and costs, says Mr Williamson.

The Contact Image Sensor Head is made up of a lens array using graded rod lenses. Because they are graded, these lenses do not suffer from any alteration in their refractive index. Each individual lens captures an image of a very small region of the target object and, thanks to the small overlap in the captured images, a clear, sharp image is produced along the narrow line of the sensor ‘head’. Because the rod lenses do not magnify or reduce the image, a 1:1 image is created without the distortion commonly found when using conventional lens/sensor (camera) systems.

The CIS is made up of silicon photodiodes using either CCD or CMOS technology. The diodes are combined with support circuitry to allow the formation of a long linear array up to 4m in length. Signal processing allows for high data rates independent of the scanning width. Interface to the image processing system is via industry standard CameraLink or Gigabit Ethernet. The light source is long linear source, typically a fluorescent tube driven from a high frequency inverter, preferably with daylight characteristics, or a long LED line light. LEDs are preferred for their long lifetimes and the option of pulsed operation to reduce stray light effects. A built-in timer control includes encoder inputs for triggering and light control.

The best of both technologies

CIS and traditional linescan cameras each have a number of advantages and disadvantages, and so it is important to understand the needs of the application before deciding on the system that is most suitable. CIS benefit from a flat construction, so that they fit into narrow spaces. They come as a system complete with optics, illumination and encoder logic and can be readily stacked together according to the width of the web with no reduction of line rate with width, giving a constant viewing angle, sharpness and brightness across the width.

Since CIS operate with a fixed working distance (typically around 10mm) and a depth of field of around 2-3mm, they are best suited for inspecting very flat objects where there is no vertical movement of the object at the point of movement. Even the best linescan cameras must be combined with the appropriate lenses and illumination sources to create the best system. Optimum interaction between the individual components from the illumination unit through the lenses and the camera up to the image capture hardware is decisive for the quality of the overall image.

The magnification and area of inspection can easily be changed by adjusting the working distance or even the lens. In addition, a much wider range of incident illumination is available for linescan cameras, together with a choice of illumination and viewing angles, allowing inspection for particular types of surface anomalies. For applications that require higher sensitivities, advanced ‘time delay and integration’ linescan cameras can be used. These utilise multiple linescan stages which results in increased integration times and dramatic sensitivity improvements over standard linescan.

Independent advice

With monochrome and colour versions of both CIS and linescan cameras available, and different performance characteristics between the two technologies, it is clear that CIS have an important role to play in web inspection applications. It is also important to get independent advice to make the best choice. Stemmer Imaging is Europe’s largest independent supplier of vision technology and services to industrial and scientific OEMs, system integrators and reseller markets. In addition to a range of CIS, the company also offers cameras from the world leader in linescan technology.