Q: At my job at the Central Sterilization/Distribution Center at a university hospital, we use bar codes as a means a keeping track of all the sterile instruments and products we distribute. As part of the tracking process, the bar codes of the used products are sent back to us later in the day, so that we can re-scan them in order to keep tally on production and ordering. But, sometimes these bar codes come back to us partially worn or scratched away during transport or usage. When this happens, the hand-held scanner can't read it, so we often have to register the bar code's number manually via one of several computers in our department. But, we've learned that we can very often simply draw in the missing lines on the bar code with an ink pen - and it works!! So, assuming that this barcode and scanning system must be pretty complicated, my question is how can simply drawing in the missing lines work? Thanks, Leprichan for your great answer! However, I'd like to add that we know that it works, because there's a monitor at each scanning station, so that we can immediately see that correct product was read and processed. Weird, isn't it?
A: Actually, the bar code and scanning system isn't that complicated. You may have noticed that your scanner is using a very thin line of light to read the bar code. What it does is send a laser light to the target, and read the reflected light. The software knows that there should be a certain amount of light reflected and reads that. When you fill in the missing or worn lines, you are correcting the view the scanner gets. As long as the pen you use has the correct density, and doesn't reflect light it should be absorbing, you will get good reading. You say that very often, redrawing the lines works, I assume that means sometimes, you can't get a correct reading simply by trying to fill in worn or missing lines. The amount of light reflected by the bar code is critical, and trying to duplicate a complete bar code by hand is nearly impossible.