Why you need calibration target?
High accuracy alignment or measurement systems require the calibration as described above to provide results with a high accuracy. You will find a distortion in any lens (no matter if it is telecentric or non-telecentric). Beside the normal ˇ°barrel distortionˇ± and ˇ°cushion distortionˇ± the camera installation might not be exactly parallel to the scene, or the non-coincidence between the camera and lens may result in a so called ˇ°keystoneˇ± distortion. However, the so called intrinsic and extrinsic transformations can be calculated by using a calibration target.
For example: If the optical distortion noted in a telecentric lens data sheet provided by the manufacture is 0.1% and if we take a picture of 100mm field of view, we can calculate an error of 0.1mm in the fringing field. An error of 0.1mm may be acceptable for many systems, but if it is not acceptable we can use a calibration target to correct the measurement to eliminate the error. That makes the difference between a low accuracy and a high accuracy system.