Vision & Sensors

Raising the Bar

January 5, 2009
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Consider several factors in bar code reading using machine vision.

Data matrix code is shown here. Source: HTE Inc.


Determining what type of reading system to use to decode a bar code is dependent on a number of application specific requirements as well as the type of bar code.

Variables that affect the decision include, but are not limited to, size and density of code, read rate, distance from code to reader, life of unit, contrast of code from background and control of bar code placement in front of the reader.

Code type is a good place to start. The familiar linear, or one-dimensional, codes used in retail have now been joined by 2-D codes.

Linear codes are usually printed in dark ink on a white label to create a high contrast between the lines and the background. All types of readers including laser, CCD, CMOS and imaging systems can understand these codes. Laser scanners create a line of light that must pass over the entire code from left to right to decode. Rotating mirrors are used to project the laser in multiple directions.

Imaging systems take a picture of the code and surrounding area, locate the bar code in the image and then decode the content. Laser scanners dominate this market; however, orientation of the code in relation to the laser beam is of key importance.

Two-dimensional codes come in two types: stacked linear codes and data matrix codes. Two-dimensional bar codes contain more information than conventional one-dimensional linear bar codes. Conventional bar codes get wider as more data is encoded, while 2-D bar codes make use of the vertical dimension to pack in more data. Each 2-D code must be read using imaging technology.

A simple stacked linear code, as its name implies, is really no more than one set of linear code located upon another. The stacking is not obvious until the operator understands the simple stacked linear code.

The final type of 2-D code is called the data matrix code. Two-dimensional bar codes contain more information than conventional one-dimensional linear bar codes, which get wider as more data is encoded. Two-dimensional bar codes make use of the vertical dimension to pack in more data. Data matrix, or 2-D, codes have become increasingly popular because of their small size, large data content and error correction capabilities. Originally developed for marking directly on the part as opposed to marking a label and then placing the label on the part, they are now moving into the retail, pharmaceutical and document paper label markets. Two-dimensional bar codes have become possible as auto scanning charge-coupled device (CCD) and laser scanners have replaced the original “light pen” type of scanner.



This figure is a linear image. Source: HTE Inc.

Alignment of the Reader

The laser scanner is really a line scan camera with a coaxial or angular laser beam that is aligned so that the reflected light strikes the line scan elements and the bars can be read.

If the bar code is not aligned in such a way that the laser beam can pass through all of its bars and spaces, the scanner cannot read it. Orientation must be even more precise when a laser scanner is used to read a PDF417 stacked code. For image-based bar code readers, orientation is significantly less of an issue, as they capture the entire image in the field of view.

This figure shows a simple stacked linear code. Note that, as its name implies, it is really no more than one set of linear code located upon another. Source: HTE Inc.

Distance from code to camera

Laser scanners are capable of a large range in distance in the Z-axis. A typical focal point can range from inches to feet with the depth of focus sometime greater than a foot. This means the part can get closer or father away from the scanner and still be read. This allows for the label or package to move around and still be readable. An imager-based system can also decode at a large range in distance of the Z-axis, but the depth of focus is usually ±2 inches maximum. This requires the label or product to be accurately placed in the Z-axis. Because the entire field of view is captured with an imager, rotation of the mark can occur in software, eliminating the alignment issues of a laser scanner.

Size of code

Laser scanners are fit to the application by determining the physical size of the code as well as distance from the label to the reader. Density, or number of lines per inch, also is a consideration when choosing a laser scanner. Different models have wider angles of view as well as variation in depth of field.

Imaging systems are susceptible to both size of code and depth of field. Large linear bar codes are not easily decoded by imaging-based systems and depth of field is usually ±1 inch. These systems work well on small linear and data matrix codes. Read rates are very good on both types of readers.

Imagers generally do not require as high a decode rate as laser scanners because they capture in two dimensions. In dynamic applications, the decode rate is actually more important than the scan or image-capture rate because this determines how fast the scanner will be able to process the encoded data and send it to the host. For imagers, shutter rate and trigger timing are the most important considerations for speed. Decode rate comes into play only when throughput is discussed. Miniature megapixel im-agers are not designed for high-speed ap-plications, but are better suited for slow or stationary applications.

While imagers do not have the same high-scan rates, they are able to simultaneously read multiple stationary speci-mens such as a complete tray of specimens.

This figure is also a stacked linear code. Its stacking is not as obvious until you have understood the simple stacked linear code. Sorce: HTE Inc.

Contrast

Laser scanners require high contrast from the code and its background. This is why linear codes typically put black lines on a white background (the label) creating very high contrast. If a code has low contrast from code to background, an imager using LED illumination is a better choice. The data matrix code is a good choice when marking directly onto the part creating a low-contrast situation.

Only imaging-based readers can decode a data matrix. The algorithms originated in full machine vision systems with data matrix specific readers coming on the scene in the past 10 years. Imagers are generally more expensive and require lighting to illuminate the part or label. This is an advantage in low-contrast applications, but results in a more complex reader. Imagers are a solid-state technology; they consist mainly of semiconducting materials, components and related devices, and include no moving parts. Laser scanners do have moving parts, including a motor that rotates or moves a mirror to sweep a laser across an object and then direct the returning energy. In comparison, imagers illuminate an object by means of an array of LEDs and use a lens to focus the target image onto a complementary metal-oxide-semiconductor (CMOS) or CCD image sensor.

An LED, in itself, has a longer life expectancy than a laser diode. While a laser diode has a failure rate of 120 in 109 hours, the LEDs in an imager fail at a rate of three in 109 hours. Also, the failure mode of LEDs tends to be gradual, whereas laser diodes generally fail without warning.

Imagers

Imager-based bar code readers, as an alternative to current laser bar code scanners, are emerging as the more robust code-reading technology. Until recently, however, a 2-D-capable bar coding system was neither easy nor cost-effective to use. Most imagers had been too large and expensive. The average list price of an array imager currently is around $3,000, compared with $1,000 for a high-performance laser scanner.

Laser scanning, however, is the most widely used method for decoding-that is, reading-linear bar codes. As the laser spot, which moves so fast that it appears to the eye as a line, passes over the dark and light elements of the code, a photodetector in the scanner measures the amount of light reflected back by each element. The photodetector con-verts the reflected light energy into electrical energy, which is then transformed through signal processing from an analog to a digital signal. The digi-tal signal is decoded according to the formulas of the symbology. V&S



David R. Wyatt is staff engineer at Applied Manufacturing Technologies (Orion, MI). For more information, call (248) 409-2000, e-mail dwyatt@appliedmfg.com or visit www.appliedmfg.com. Dan Reed is president of HTE Inc. (Auburn Hills, MI). He can be reached at (248) 371-1918, dreed@hte.net or www.hte.net.

Tech Tips

- Laser scanners are fit to the application by determining the physical size of the code as well as distance from the label to the reader.

- Density, or number of lines per inch, also is a consideration as to which model of laser scanner to use.

- Different models have wider angles of view as well as variation in depth of field.

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