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    <p>There should be no major issues with Code 39 readability. When used in applications where reliable scanning is important Code 39 is normally deployed with protection against misreads in the form a modulo 43 check digit that the scanner is configured to verify prior to passing the code on to the system. Any half-decent barcode generator or barcode reader will support Code 39 check digits.</p> <p>As I haven't seen the print quality of the Code 39 barcodes that your are scanning it is impossible to be certain however I would certainly suggest that you are using reader software that has very poor quality Code 39 scanning.</p> <p>Your barcode library is probably confused for the following reason, but it is impossible to be sure without extensive debugging of the device...</p> <p>Below I have aligned two Code 39 images that were created using the <a href="http://www.terryburton.co.uk/barcodewriter/generator/" rel="nofollow noreferrer">online barcode generator</a> based on <a href="http://www.terryburton.co.uk/barcodewriter/" rel="nofollow noreferrer">Barcode Writer in Pure PostScript</a>. On top is a horizontally flipped image containing "U" and beneath is an image containing "13415566".</p> <p><img src="https://i.stack.imgur.com/NCXRJ.png" alt="Two code 39 barcodes aligned"></p> <p>Reading the top image from right-to-left you can see that there is a degree of similarity with some portion of the bottom image.</p> <p>A scanner might be forgiven for the misread of this unprotected Code 39 except that it has following against it:</p> <ul> <li>It should be expecting a quite zone (whitespace) before the leading start bars sequence.</li> <li>It should be expecting a quite zone after the trailing stop bars sequence.</li> <li>The bar pattern for "U" is not entirely correct.</li> <li>The assumed stop bar sequence is not entirely correct.</li> </ul>
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    1. COI don't think one should necessarily blame the scanner. One can construct code 39 sequences such that, if printed as barcodes with white space above and below, a diagonal line which enters the top and leaves the bottom or vice versa would "see" a perfectly well formed Code 39 barcode, including quiet zones, which represented bogus data. Most combinations of characters would not lend themselves to such misreading, but the combinations which could be misread aren't particularly less likely to occur than any others.
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