Why barcode scanner can not read

We believe that many users will encounter such problems when using barcode scanner. If we can't accurately grasp the distance between barcode scanner and barcode, it will easily appear that "can't read barcodes" or "we need to scan several times to scan." At this time, we believe that many users will wonder if there is any problem with the scanner, or because the scan performance of the scanner is not good. In fact, this is a wrong understanding.
 What is the reason? It involves a term for a scanner, called the optimal working distance. The Barcode scanner have an optimal working distance for the highest resolution performance, the so-called focal length. The resolution of the bar code scanner is adjusted according to the focal length. It can also be said that the bar code scanner can decode the narrowest strip width. For this reason, if the distance between the bar code scanner and the bar code changes, the narrow bar and blank area of the bar code may not be read correctly, so there is a phenomenon that the bar code scanner cannot scan the bar code.
The following will be the difference between the ccd bar code scanner and the laser bar code scanner.
1) ccd barcode scanner:

In the camera, a bar code image on the red image sensor is captured through the lens based on the ccd bar code scanner. Therefore, if the captured image is out of focus or the bar code is blurred, it is more difficult to identify the bar code.
2) Laser barcode scanner:

The spot diameter is the smallest in focus. At a position larger or smaller than the focal length, the spot diameter becomes large, so that the barcode/space area cannot be recognized. The resolution is highest at the focal length position. Therefore, bar code scanners may also treat uneven print density, dust and scratches as part of the bar code. So when reading bar codes with low printing accuracy or fibers (such as cardboard) on the background, we need to pay attention to match the focal length and the narrow bar width.