Usually devices are pre-calibrated from the factory and does not need calibration during its lifetime.
However, at some point in time, calibration algorithm may be changed in new SDK versions and after SDK update you may be asked to perform device calibration. To perform calibration you need to have special calibration objects. They are included in accessories package that comes together with the device in its box when purchased.
In case you have lost them, you can print them from the PDF files. These files can be found either in SDK installation folder
C:\Program Files (x86)\Regula\Document Reader SDK\Doc\test object #1.pdf
C:\Program Files (x86)\Regula\Document Reader SDK\Doc\test object #2.pdf
Once printed, please follow onscreen instructions to perform calibration.