|
|
|
Barcode Readers and Reading Devices |
| Barcode Readers and Reading Devices INTRODUCTION Barcode readers, or scanners, come in a variety of sizes, shapes, and prices. The most familiar bar code scanner is the checkout scanner used in many grocery stores. This scanner uses a helium neon laser. The laser scanner is mounted below a glass plate on the surface of the checkout counter. It reads most of the common barcode symbologies on an item as it is drawn across the plate. The decoded value of the bar code is transmitted to a POS processor. This information is compared to a list of product numbers stored in the POS terminal. When a match is found , the data, such as description, price, and taxability, are transferred to the checkout terminal for processing. There are few varieties of scanning equipments: hand-held wand (light pen), hand-held laser barcode reader, stationary fixed beam barcode reader, stationary moving beam scanners, barcode slot readers, hand-held CCD barcode readers. Barcodes convey information. The information conveyed depends upon a particular arrangement of bars and spaces. The bars and spaces are translated from their printed form to an analog electrical signal that represents the relative width of the bars and spaces in the symbol. This is accomplished by illuminating and using a light detector (Sensor IC) to measure the strength of the reflected light. The measurement from a dark area, or bar, will be considerably less than from an adjacent light area, or space. As the light moves from space to bar, the relative intensity of reflection is converted to an analog pattern by the detector. LIGHT PENS (WAND BARCODE READERS)Light Pen's popularity stems from its portability and low cost. Use of a light pen requires the operator to find the barcode symbol and move the light pen smoothly over it. The operator locates the quiet zone of the symbol, places the tip in the quiet zone, and sweeps the light pen across the code through the trailing quiet zone. No motors or moving parts are employed, since a human performs the scanning function. This makes the light pen a low cost, small, rugged barcode reader. BARCODE SLOT READER AND STATIONARY FIXED BEAM READERS Fixed beam readers are close relative of light pens. They use a stationary light beam to read a moving barcode. Fixed beam readers have no moving parts. They illuminate an area of the symbol with a visible or infrared light source. When a barcode symbol passes through the beam, the reader detects the light reflected from the symbol and reads the barcode. As with light pens, the difference in reflected light is converted to an analog signal, digitized, decoded, and transmitted to the host computer via Keyboard wedge PS/2, RS-232, or USB connections. LASER BARCODE READERSLaser technology is the science that deals with the generation of coherent light in small but powerful beams. Laser light has unique properties. A laser beam is one color, or monochromatic. If the light is one color, then it is also of one wavelength. If the wavelengths are stimulated so that they are in phase, the beam is also coherent. Incoherent light is illustrated by an ordinary light bulb, which emits white light. White light is incoherent because it has all visible colors and therefore all visible wavelengths. It is also transmitted in all directions from the light source and cannot be concentrated or controlled. Laser light can be controlled because the beams are of one wavelength and therefore in step with each other. All barcode readers have a specified depth of filed. The directional and coherent nature of laser light permits an expanded DOF. and the power of the laser can well dictate its optical throw, and in combination yield a greater operating range. Laser further focus the light source while other light sources focus the reflected light. Combining this focused light with the polarized nature of laser light, a polarizing filter can tune the received signal to only that specific wavelength of the laser. This provides for greater immunity to ambient lighting conditions leading to improved signal quality and generally better scanning. Laser reading provides possibly the greatest benefit to the industrial and commercial community where rapid and accurate data acquisition is needed. The coherent, monochromatic laser light exists in a focused spot, generally less than 10 mils in diameter. This spot is then reflected through lens assemblies and onto a mirrored surface. The mirror can be either rotating in nature or can be rotated left and right, as is the case in flipper mirrors and dithers. This oscillated movement then causes the laser barcode reader's light to appear as a single line of light. Moving beam laser barcode readers operate with a spot forming the laser's line of light at speed of 33 to 2K scans per second. Handheld devices generally operate at the lower end of these ranges. laser scanners are complex devices with moving parts, whose components are considerably more fragile than the light pen or CCD scanner counter parts. Further, laser scanners are significantly more expensive than the CCD barcode readers. Often costs 3 to 10 times as much (except the low cost Metrologic's Eclipse Laser Scanner which costs (and performs) as much as a low cost short/middle range CCD Barcode Reader) Some laser barcode readers are capable to make the hands-free operation by installing a tiny infrared auto-sensor on the laser engine itself, so that the laser light can automatically be triggered by it once there is any moving object is in front of the infrared sensor. CCD BARCODE READERSEach of the preceding barcode reader uses a light source and a single photo sensor. Relative movement between the scanner and symbol results from operator (Wands), symbol (Slot reader), or light source (Laser scanner) motion. The photodector produces a signal representing the bars and spaces of the symbol. CCD barcode readers operate similarly to photographic cameras. The symbol is illuminated by a light source generated by the reader's LED components. The CCD Barcode Reader focus the reflected symbol image onto a photosensitive semiconductor device called a linear photodiode array. These array typically have 1024, 2048, or 4096 tiny photodiodes. The photodiodes are sampled by a microprocessor to produce an analog of the symbol being read. The analog signal is then conditioned and decoded. CCD barcode reader offer additional benefit over light pen and laser scanners since they are able to sample an entire symbol without movement of the reader, and it generates soft visible red light instead of laser so that the end-users usually has less worries about using it. This capabilities produces less operator frustration than light pens when scanning marginally printed symbols. Like laser scanners, most of CCD barcode readers nowadays are providing hands-free scanning functions by output the LED light on and off continuously or keep simply it on all the time. Other
Related information
|
|
| Copyright 2004 Fametech Inc.(Tysso) Home | Products | Contact |
