February, 2013
www.us-tech.com
Image-Based Barcode Readers Can Help the Whole System
By Michael C. Moed, Ph.D., Director of Engineering, Cognex Corporation, Natick, MA L
aser scanners use a moving pinpoint of light to illuminate the barcode. A single photocell receives the reflected light and converts the
barcode into an electrical signal as it moves across a barcode. The scanner then measures the relative widths of the bars and spaces, translates the dif- ferent patterns into the barcode data, and sends them on to a computer or portable terminal. Laser scanners are able to read one-dimen-
sional (1-D) barcodes in traditional high contrast applications, providing excellent performance for high-speed unidirectional scanning, offering large depth-of-field and a simple setup. Stitching sever- al lasers together in an “X” or star pattern allows for omnidirectional scanning. However, less than ideal conditions — poor contrast codes, poor code quality, and perspec- tive (angled) reading — as well as the need to read the increasingly popular Data Matrix and other more complex two-dimensional (2-D) matrix codes — require a more robust approach. Although laser-based systems are easy to set up, it can be a challenge to maximize read rates because users cannot see the images that the scan- ner is attempting to interpret. In addition, it is often difficult to deter- mine the reason for an unsuccessful read, so optimizing the process proves next to impossible. The other choice: conventional
image-based readers generally con- nect commercially available compo-
In the past, manufacturers have had two main cate- gories to choose from when solving ID challenges: laser scanners and conventional image-based readers.
nents such as CCD or CMOS imagers and digital signal processors (DSPs) on a conventional printed circuit board (PCB). The reader captures an image, passes that image to the DSP to locate and decode any barcodes present, and then captures the next image in a pipeline fashion. A produc- tion environment with well-formed high contrast 1-D barcodes can achieve reading speeds up to 45 decodes per second — suitable for many high speed applications that can benefit from the more robust reading capabilities of image-based systems.
Advantages of Imagers Image-based readers use solid-
state camera technology to capture full-frame images in a single snap- shot, avoiding the need to cross a bar- code with scan lines from end-to-end the way lasers do. When an imager captures a frame containing a bar- code, it can locate that code anywhere within the frame. Thus, in effect, each image contains thousands of scan lines in all directions making finding and reading barcodes more robust. Image analysis software algorithms can interpret a higher percentage of the captured codes than laser scan- ners can, and they can also manage the complexities of 2-D codes. In addi- tion, the images themselves can be stored for training, historical analy- sis, or documentation. Conventional image-based read-
ers are typically designed for a single focal plane and offer only limited depth of field. System setup requires
Most laser scanners sweep the laser beam horizontally using an electronically controlled oscillating mirror.
fixing variables such as the focus of the lens, aper- ture size, and target brightness. Changes in dis- tance after setup cause images to be out of focus, overexposed, or otherwise misaligned. When a process provides consistent focus and lighting, how- ever, conventional image-based readers offer clear benefits. ID readers that incorporate liquid lens tech-
nology with autofocus, allow users to change focal distances easily during line changeovers, thus reducing laser scanners’ depth of field advantage. Also, software tools make optimizing a setup easier with features such as automatic target brightness and robust image analysis algorithms that are
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