Contact image sensors (CIS)
Contact image sensors use the same concept as used in fax machines and desktop scanners. They include a sensor, lens and optional light source with large pixels being mapped 1:1 with the object. The combination of the sensor being as long as the object itself and the 1:1 lens provides a telecentric view of large objects with a minimum of optical distortions.
The sensor head is made-up of a lens array using graded rod lenses. The refracting index of each rod has its maximum on axis and decreases proportional to the square of the distance. Each individual lens captures an image of a very small region of the target, and thanks to the small overlap in the captured images, a clear, sharp and continuous image is produced along the narrow line of the sensor head. As detectors single line monochrome sensors or even triple line sensors with filters for vibrant colour acquisition are used. Contact image sensors can be stacked or shifted to offer extended lengths and offer basically the same features as standard line scan cameras as far as dark current, peak response non-uniformity or dynamic range are concerned, but without the common trade-offs concerning spatial resolution and light efficiency. Contact image sensors are optimised for a wide range telecentric acquisition of flat objects with limited space for installation.