How do Barcode Scanners work?
To understand how a barcode scanner works, we have to explore the different parts of the device. Basically, there are 3 functional parts to the barcode scanner itself, the illumination system, the sensor / converter, and the decoder.
The simple explanation...
Barcode scanners begin by illuminating the code with red light. The sensor of the barcode scanner detects the reflected light from the illumination system and generates an analog signal with varying voltage that represent the intensity (or lack of intensity) of the reflection. The converter changes the analog signal to a digital signal which is fed to the decoder. The decoder interprets the digital signal, does that math required to confirm and validate that the barcode is decipherable, converts it into ASCII text, formats the text and sends it to the computer the scanner is attached to.
Let's look at each functional part of a barcode scanner in more detail:
Illumination Systems - The illumination system is the method by which the bars and spaces on the barcode are illuminated. There are a variety of illumination systems commonly used in barcode scanners:
Single Point LED - This technology is exclusive to the barcode wand reader and the barcode slot reader. The illumination of the barcode comes from either a single or pair of LED's and is focused through a single ball-type opening. This technology requires the ball to physically touch the barcode being scanned.
Since the illumination is on a single point, the operator has to provide motion to the barcode past the light source. In the case of a barcode wand, the operator drags the illumination ball across the barcode. For swipe or slot readers, the barcode is typically printed on a credit-card like media. The operator pulls the card through a fixed slot, past the illuminating head.
Slot and wand readers are inexpensive, and can accommodate any length of barcode. There are several disadvantages of the single point illumination method. Slot and wand readers require the operator to control the speed at which the barcode passes in front of the illumination head. Because barcodes must be in contact with the illumination head to read, the barcode can easily be damaged by abrasion of the head on the media that hosts the printed barcode. Although the illumination head is hardened, it will wear out and must be replaced regularly.
Linear Multiple LED - Expanding on the single-point illumination system, placing multiple LED's in a line give the ability to light the entire width of the barcode. This type of illumination is used in CCD scanners and Linear Imagers.
When used in CCD scanners, the LED's are paired with a line of photocells to detect the reflected light from the barcode Since the LED's are relatively low in power, and the photocells are low in sensitivity, the range of CCD barcode scanners is generally limited from being in contact with the barcode to 1" away.
Laser - This type of illumination method uses a single point red laser diode similar to a laser pointer. The point of light is expanded into a line by oscillating the laser into a stationary mirror, or projecting the point into an oscillating mirror. This illumination method is very popular because of the working distances typically achieved are superior to the point illumination or linear LED illumination methods. Typical working distances are from 1" to 18". By increasing the power of the laser and decreasing the angle of oscillation, ranges of over 20 feet can be obtained.
LED Imager - The linear and full imager is very similar to the CCD device, with some important changes. In linear imagers, the amount of illumination is increased by using high light LED's, and the sensing photocells are more sensitive. Linear imaging technology mimics both the range and focus of laser scanners.
In full imagers, high-intensity LED's illuminate a square scanning "target". The light sensors in full imagers are very similar to the light sensors in monochrome cameras. The sensors search the scanning square target for a valid barcode. By pairing the target square with sensors that search the target square for a valid barcode, LED full imagers are omni directional - you don't have to line up the barcode in any way in order for it to be decoded. The target / snapshot method give LED imagers the ability to read 2-dimensional barcodes as well.
Regardless of the method used to illuminate the barcode, the illumination method is causes reflected light to return to the scanner head and be seen by the sensor.
Sensor and Converter - A photo detector senses the reflected light and generates an analog signal with varying voltage. The voltage fluctuates based on whether the sensor sees the reflected light from the white spaces because the black bars absorb the red light.
The technology used in the sensor can vary depending on the illumination method. The output is always the same - a voltage wave form with peaks for the white spaces, and troughs for the black spaces in the barcode.
In an imaging barcode scanner, the sensor covers the entire scan target and generates a 2-dimensional wave form. In both cases, this analog signal is sent to the converter. The converter changes the analog signal to a digital signal. This signal is the digital representation of what the sensor detected from the reflected light. Now that the barcode scanner has a digital signal, the signal is transferred to the barcode scanner decoder .