Evaluation Engineering - December 2008 - (Page 40)

MACHINE VISION Non-Telecentric Region Object Telecentric Region Object Image on Monitor a. Conventional Lens Comparison of Conventional and Telecentric Lenses Courtesy of Edmund Optics Object Image on Monitor a. Conventional Lens Object Software drivers that bypass the host operating system improve performance by directly accessing memory. Also, some special hardware incorporates frame grabber functionality and reduces host overhead, especially for multiple camera systems. Once an image has been transferred to the host, it needs to be analyzed. Vision software toolboxes from several manufacturers allow you to address a wide range of applications without being a programming expert. Often, you can download a demo version for evaluation. A particularly interesting approach is offered by SILICONSOFTWARE, in which users string together VisualApplets® to deﬁne a series of image operations. The VisualApplets actually are compiled modules based on lowerlevel operators from the company’s image processing library. You use the company’s synthesis program together with Xilinx place-and-route software to generate an FPGA layout for your custom machine vision system. Noteworthy Online Help Technical Notes The Viewbits Division of Computer Modules lists several cameras and states that CMOS sensors now are at least as good as CCD sensors. The site references an article on CMOS vs. CCD written by Rockwell Scientiﬁc. In it, the author explains that the SNR of CCD sensors has an inherent 3-dB/octave decrease as the readout rate is increased. In a CCD, charge is transferred from well to well during the readout process so each well must have a much higher bandwidth than is required to capture the image. According to the paper, the output from high-definition sensors with a high frame rate often is followed by a 30-MHz to 33-MHz ﬁlter to reduce noise. The very wide bandwidth of the CCD output ampliﬁer is largely responsible for excessive thermal noise. In contrast, especially with submicron technology, CMOS sensors can separately amplify each pixel and have a much lower bandwidth. “The noise bandwidth [is reduced] from tens of megahertz for CCDs to tens of kilohertz for CMOS [sensors].”1 This means that the SNR associated with CMOS sensors can be much higher than that associated with CCD sensors used at high output data rates. CCD sensors are a mature technology so their production will not stop abruptly. Nevertheless, for many applications and especially where a high pixel rate is needed, CMOS sensors are performing better. Most importantly, CCD sensors require special silicon processing that’s not compatible with usual CMOS fabrication. Nevertheless, large CCD arrays used in lower-speed applications such as astronomy have excellent performance with SNR of approximately 120 dB. On the Melles Griot website, you can ﬁnd the Machine Vision Lens Fundamentals technical note. It covers important aspects of lens operation and terminology. This is a good reference in general and especially useful for anyone just starting to work with machine vision. After reading this paper, at a minimum, you will know which questions must be answered before you can choose a lens. Because a large number of lens properties are discussed together with their effects, some of the trade-offs within your application should become more apparent. Edmund Optics® provides a great deal of lens-related information on the company’s site. Often, you can learn the basics from a well-illustrated paper written in plain English. For example, at the end of the white paper Will a 4 0 • E E • December 2008 www.e v al u a ti o n e n g i n e e r i n g . c o m http://www.evaluationengineering.com

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Evaluation Engineering - December 2008 Contents Editorial Product Briefing Test Software C-V Measurements Nanoelectronics Test Product Guide Company Guide Machine Vision EMC Test Index of Advertisers

Evaluation Engineering - December 2008

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