Barcodes

There are currently four different types of bar code readers available. Each uses a slightly different technology for reading and decoding a bar code. There are pen type readers (e.g. bar code wands), laser scanners, CCD readers and camera-based readers. Pen type readers consist of a light source and a photo diode that are placed next to each other in the tip of a pen or wand. To read a bar code, you drag the tip of the pen across all the bars in a steady even motion. The photo diode measures the intensity of the light reflected back from the light source and generates a waveform that is used to measure the widths of the bars and spaces in the bar code. Dark bars in the bar code absorb light and white spaces reflect light so that the voltage waveform generated by the photo diode is an exact duplicate of the bar and space pattern in the bar code. This waveform is decoded by the scanner in a manner similar to the way Morse code dots and dashes are decoded.

Laser scanners work the same way as pen type readers except that they use a laser beam as the light source and typically employ either a reciprocating mirror or a rotating prism to scan the laser beam back and forth across the bar code. Just the same as with the pen type reader, a photo diode is used to measure the intensity of the light reflected back from the bar code. In both pen readers and laser scanners, the light emitted by the reader is tuned to a specific frequency and the photo diode is designed to detect only this same frequency light.

Pen type readers and laser scanners can be purchased with different resolutions to enable them to read bar codes of different sizes. The scanner resolution is measured by the size of the dot of light emitted by the reader. The dot of light should be equal to or slightly smaller than the narrowest element width ("X" dimension). If the dot is wider than the width of the narrowest bar or space, then the dot will overlap two or more bars at a time thereby causing the scanner to not be able to distinguish clear transitions between bars and spaces. If the dot is too small, then any spots or voids in the bars can be misinterpreted as light areas also making a bar code unreadable. The most commonly used X dimension is 13 mils (roughly 4 printer dots on a 300 DPI printer). Because this X dimension is so small, it is extremely important that the bar code is created with a program that creates high resolution graphics (like B-Coder). For a good description of the different graphic file formats that are commonly used to create bar codes see: Raster vs. Vector Graphics CCD (Charge Coupled Device) readers use an array of hundreds of tiny light sensors lined up in a row in the head of the reader. Each sensor can be thought of as a single photo diode that measures the intensity of the light immediately in front of it. Each individual light sensor in the CCD reader is extremely small and because there are hundreds of sensors lined up in a row, a voltage pattern identical to the pattern in a bar code is generated in the reader by sequentially measuring the voltages across each sensor in the row. The important difference between a CCD reader and a pen or laser scanner is that the CCD reader is measuring emitted ambient light from the bar code whereas pen or laser scanners are measuring reflected light of a specific frequency originating from the scanner itself. The fourth and newest type of bar code reader currently available are camera-based readers that use a small video camera to capture an image of a bar code. The reader then uses sophisticated digital image processing techniques to decode the bar code. Video cameras use the same CCD technology as in a CCD bar code reader except that instead of having a single row of sensors, a video camera has hundreds of rows of sensors arranged in a two dimensional array so that they can generate an image. The factors that make a bar code readable are: an adequate print contrast between the light and dark bars and having all bar and space dimensions within the tolerances for the symbology. It is also helpful to have sharp bar edges, few or no spots or voids, a smooth surface and clear margins or "quiet zones" at either end of the printed symbol. All application programs support bar code reading as long as you have the right equipment. Bar code readers are available with two types of output - either "keyboard wedge" output or RS232 output. The bar code readers with keyboard wedge output plug directly into the keyboard port on your PC and they also provide a pigtail connector so that you can plug in your keyboard at the same time. When you scan a bar code with the keyboard wedge bar code reader, the data goes into the computer just as if it were typed in on the keyboard. This makes it extremely easy to interface the bar code reader to any application that is written to accept keyboard data.

The keyboard wedge interface is extremely simple however it has a few drawbacks. If you swipe a bar code, the cursor has to be in the correct input field in the correct application otherwise you end up reading bar code data into whatever application has the focus. This can cause all sorts of potential problems as you can imagine. The keyboard output also is limited in that you cannot modify the data in any way before sending it into the program that is to receive the data. For example, if you needed to parse a bar code message into multiple pieces or remove some of a bar code message or add in a date or time stamp you would not be able to with a normal keyboard wedge reader.

The other possible output option is to get a bar code reader with an RS232 or "Serial" interface. With these types of bar code readers, you connect the reader to an available serial port on the back of your PC. You would then need a program called a "Software Wedge" to take the data from the bar code reader and feed it to the application where you want the data to go. The disadvantage to this approach is that it is a little more complex however you gain much more control over how and where your data ends up when you read a bar code.

Bar codes aren't generally used to hold prices. Prices are held in the computer of the store where the items are carried. Scanning a bar code allows the computer to identify the item, and its price is called up from computer memory and charged to the customer. Prices are changed electronically as a function of data processing, and this allows the market to change prices (like for sales) with key strokes or mouse clicks. Some markets use another bar code for some sale items, but as stated, that bar code isn't generally for holding price information.

There is a newer information tag that is actually a 2-dimensional bar code, and it is more information dense than the standard bar code. It's called a quick response (QR) code. Like the original bar code, it can be scanned and the item identified. Appropriate pricing resides in the computer. We also see QR codes used to identify things or provide information other than just that of products on a store shelf.

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When a barcode is scanned at the checkout, the scanner reads the barcode's unique pattern of lines and spaces, converting it into a numerical code. This code corresponds to a specific product in the store's inventory database. The system retrieves information about the product, such as its price and description, and updates the transaction total accordingly. Finally, the scanned data can also help track inventory levels in real-time.

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1D has vertical bars that are read horizontally, 2D has small squares that are read simultaneously both horizontally and vertically.

2D barcodes can store much more information than 1D barcodes can.

The graphic structure of a bar code is very simple; it comprises only dark areas and light areas which humans see as the familiar "bars". These bars form a binary system of representation, in other words the barcode symbol has only two states -- "light" and "dark" --- there is no third state. The graphical simplicity means that it is very easy to place a barcode onto an object. Barcodes can cheaply and easily be printed onto many different materials. Because of this graphical simplicity it is also quite easy to create an electrical gadget that can "see" the barcode. The electronics of the gadget needs to be able to distinguish between "dark" and "light" and that is not technically difficult. Within the software of the gadget the barcode can be represented by a binary numbering system "1" and "0". Consequently it is very easy to interpret that number and convert it to the letters and digits that we humans usually read. Additionally a barcode can suffer quite a lot of damage before it becomes unreadable. The machine reading the barcode only needs to find a straight line (any straight line) between one end of the bar code and the other. That line can be diagonal. If it is possible to draw a line that crosses every bar and does not cross any smudges or gaps then the bar code can, theoretically, be read. There are many different bar code formats in use. Some of them can only represent numbers and some can also represent letters. Some barcode formats also contain extra information that can be used to check that the barcode has been read correctly. This means that if part of the barcode label is missing so that some bars are missing, the barcode read will know that what it is seeing is not a valid barcode. The machine that is reading the barcode can know immediately if it has seen a valid barcode or not. If it cannot see a valid barcode then it can immediately alert the operator of the machine so that they can correct the problem in some other way. These characteristics mean that in many situations a barcode is a very reliable way to transfer information because it is quite easy to avoid false reads and often even damaged barcode labels can still be read very easily. It is hypothetically possible to create a barcode of any length but, in practice, most barcode readers can only read quite short barcodes. Consequently barcodes are used where the information to be transferred is very simple. For example, most of the barcodes used on things that are bought in shops contain only eight, twelve or thirteen digits.

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Barcodes are read by optical devices.