Introduction To Line Scan Vision Technology

There are two types of acquisition methods to acquire an image of an object—area scan and line scan. The most common 2D machine vision systems use area scan cameras, which require a complete matrix of pixels to be exposed at the moment of acquisition. By contrast, line scan cameras contain a single row of pixels, building the final 2D image pixel line by pixel line.

Building a line scan image requires movement between the camera and object, usually along a conveyor belt or rotating shaft. As the objects passes in front of the camera, a new pixel line is acquired. Software on a vision processor or frame grabber stores each line, then reconstructs pixel data into the final 2D image. This unique image acquisition process excels at capturing fast-moving discrete parts on a conveyor, inspecting all sides of cylindrical objects, and building images of very large objects. Commercial devices like document scanners, photocopiers, and fax machines which scan documents into memory use line scan technology, as do production and distribution lines in manufacturing and logistics, which rely on this special technology to acquire high resolution images quickly for detailed part inspections.

Area scan cameras illuminate an object and capture all exposed pixels to form an image while line scan cameras illuminate and capture pixels line by line.

With line scan imagers available from 500 to 8000 pixels per line—and some even able to match high speed line acquisition rates of 67,000 lines per second—the latest generation of line scan cameras generate all the power necessary for applications that formerly required a cluster of high resolution vision imagers. These attributes make line scan cameras an ideal choice for capturing target objects in continuous or discrete “web” surface inspections, such as for plastic, textiles, metal, or paper.

Line scan cameras can also “unwrap” cylindrical objects to capture their entire surface area. Large objects that require high-resolution imaging for precise measurement and defect detection–such as flat panel displays, solar cells, and car parts—are also well-suited to this technology. And because line scan systems only need to view a small portion of the target object for each line acquisition, they did not require a large, unobstructed view of the target object. For this reason, they work well in installations with restricted fields of view or mounting space.

Why Choose Line Scan?

Regardless of industry, line scan camera technology helps manufacturers acquire high-resolution images for fast-moving, large, and cylindrical objects that challenge area scan cameras.

Unwrap cylindrical objects for inspection: Inspecting round or cylindrical parts can require multiple area scan cameras to cover the entire part surface. A line scan camera can produce an “unwrapped” 2-D image of a cylindrical object as it spins on an axis. This avoids special fixturing and complex algorithms to stitch together several images at varying coordinate spaces. This feature is useful for inspecting the size and uniformity of objects, fill levels, labels, and safety seals.

Add vision to space-constrained environments: Since a line scan camera builds an image a single pixel line at a time, it only needs to see a sliver of the object as it moves past. This attribute makes line scan cameras ideal for applications with restricted fields of view or space, such as in the electronics and pharmaceutical industries, or when the camera needs to peek through rollers on a conveyor to view the bottom of a part.

Acquire high-resolution images at low cost: Line scan systems can also generally provide much higher resolution than traditional area scan cameras. High resolution inspection is critical in most tech-oriented industries. A line scan camera with the appropriate imager can produce very large images at significantly lower cost than an area scan camera. For this reason, a line scan camera in motion is a practical replacement for a high-resolution area scan camera.

Inspect web surfaces: Since line scan systems require parts in motion to build an image, they are often well-suited for applications with products in motion, such as on high-speed production lines and conveyors. This makes it easy to perform long surface (or “web”) inspections and to verify codes and text. Using an area scan camera would require extra processing to reconcile frame overlaps, complicated image stitching, and hardware synchronization. Some line scan models can obtain a stream of continuous (i.e. gapless) images between frames, while others can do so with minimal gaps of a few lines. Models that perform discrete continuous inspections can take pictures of randomly-sized parts without large blank spaces between images.

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