Last Word - Mark Williamson

Contact image sensors (CIS) have been around for many years. Used extensively in copiers and scanners as well as currency verifiers and card and ticket reading systems, CIS form an image line by line, so there is a natural comparison with the line scan cameras traditionally used in industrial web inspection applications.

Until recently, the classic limitations of CIS, such as very close working distances and low depth of field, lack of speed and the
absence of an industrial interface standard, have meant that their use in industrial applications has been very restricted. However, the recent introduction of the new KD series of contact image sensors for machine vision applications by Mitsubishi Electric Europe means that this technology is now a viable and cost-effective alternative. Using a proprietary tri-line CMOS colour sensor and integrated white LED illumination, web inspection can be carried out in colour at a resolution of 600dpi.

CIS technology

Image formation in CIS is achieved via a double-row lens array, using graded rod-lenses, matched to a colour tri-line CMOS sensor. Each individual rod-lens captures an image of a very small region of the target. Thanks to a small overlap in the individual images, a clear, sharp, combined image is produced along the narrow line of the sensor with a resolution of 600dpi. The selected rod-lenses provide an optimised working distance of 12mm.

CIS combines high data rates, high sensitivity, excellent colour rendition and perfect resolution without any additional effort in set-up, camera alignment or calibration. Image data output is provided via standard industrial CameraLink interfaces. Built-in timing control includes encoder inputs and light control, and is configured by RS232 via CameraLink.

The images generated by a contact image sensor show virtually no distortion as the long sensor line maps the object on a one-to-one scale.

The CIS products are available in three different lengths, offering scan widths of 309, 617 and 926mm. The two sets of integral white LED lighting can be varied in intensity and controlled independently, increasing the range of applications. The lighting is homogenous, with no shading along the length of the sensor, so defects can be detected even at the edge of the scan.

Best of both technologies

Contact image sensors 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.

Even the best line scan 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.

Although this can lead to higher costs for cameras with the highest resolution (line scan cameras with 2k pixels or more require large format lenses), it also gives more flexibility in the system. 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 line scan 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' line scan cameras can be used. These utilise multiple line scan stages, which results in increased integration times and dramatic sensitivity improvements over standard line scan.

Independent advice

With monochrome and colour versions of both contact image sensors and line scan cameras available, and different performance characteristics between the two technologies, it is clear that the CIS technology has 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 contact image sensors, the company offers cameras from the world leader in line scan technology, and so is able to provide clear independent advice as to what is the best solution for any particular web inspection application.

The author's views expressed on this page may not be shared by this publication.