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Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

Choosing High-Speed Surface Inspection Sensors

Comparing area CMOS to linear CCD for speed, cost and flexibility
LMI high-speed color web/print inspection sensor
© LMI Technologies
An LMI high-speed color web/print inspection sensor built using its FireSync™ component toolbox.
color imaging system
© LMI Technologies
The system is capable of color imaging at 90 KHz effective line rate, and was shown imaging monochrome and color reference surfaces moving at 30 m/second when exhibited at Vision Stuttgart in November.
lumber on a conveyor
© LMI Technologies
Twenty CMOS chroma+scan 3300 sensors are used to inspect six-meter long pieces of lumber on a conveyor, resulting in 100 percent inspection with full color and 3D wireframe models.
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By Leonard Metcalfe

Traditional high-speed web or surface inspection has been reserved for linear CCD imagers, but today we also need to consider area CCD and area CMOS imagers. Each technology has different features that are best suited for a specific application. This article will discuss the differences between area CMOS and linear CCD for high-speed surface inspection.

Over the years LMI has implemented many types of surface inspection sensors. In my experience, the CMOS imager has a speed and flexibility advantage due to the onboard circuitry. Traditionally, CCDs are recognized by the industry for achieving high speeds, but at a higher cost and complexity. This article will address the advantages of CMOS area imagers for implementing extremely fast, scalable, and compact surface inspection systems.

A typical statement requirement for a high-speed monochrome web/surface inspection application is below. The specifications required for this example include:

  • Resolution 0.5mm—mono-
    chrome
  • Inspect 1000mm-wide web surface
  • Minimum 2000 pixels for 0.5mm resolution across the web
  • Speed: 25 meters per second (needing 50 KHz effective line rate)
  • Exposure time is 20 µs for 0.5mm resolution at speed
  • Deliver image to a PC for image processing

In most cases, this type of requirement can use a linear CCD or area CMOS. In this example, we will first explore a solution using a linear CCD implementation, and then compare with area CMOS operations.

Linear CCD Implementation

This solution requires one 50 KHz 2048 pixel camera, likely with a camera link interface, a large, high-resolution lens and a camera link frame grabber. It has a 100 percent duty cycle and a high-intensity light source (250mm standoff). The camera standoff is 1,000 mm for 1,000 mm FOV.

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