The main issue is that the image view is too out of focus in the range 14-18 cm with mono-camera node. And I have set the IR floodlight to 0 mA, because even zero amount of floodlight gives me enough brightness to see object with barcode in dark environment (where focus is not in oder). Even if I set 10 mA of IR floodlight, the view is more brighter where barcode is hard to see.
Now I got clear that I can not adjust the focus manually with the MonoCamera node, but with the ColorCamera node. I may have to mount LED separately with the Camera to work in dark environment.