Line-scan cameras build images a single pixel row at a time using a sensor which passes in a linear motion over an object, or more often when the object passes under the sensor. As a conveyor or web moves, a 2D image can be reconstructed in software line-by-line for inspection. This makes it ideal for high-speed continuous applications such as document scanning, print inspection, web inspection of paper, aluminum, steel or glass, and in detailed inspections requiring measurements be done in microns. Line-scan can also inspect round bodies without perspective distortion.
Because line-scan cameras use a single row of pixels, they can build continuous images not limited to a specific vertical resolution, allowing for much higher resolutions than area-scan cameras in both 2D and 3D. Chromasens has been at the forefront of line scan technology for more than a decade. It recently introduced its 3DPIXA HR 2 µm 3D line scan camera that covers a 16mm field of view and achieves 0.5µm height resolution or twice the resolution of previous versions. As a result of this precision, manufacturers of semiconductors and other electronics can improve productivity by preventing defects, reducing waste, and increasing yield while achieving compliance with traceability requirements.
In contrast, area-scan cameras are more general purpose. Used in the majority of machine vision systems, they contain a large matrix of pixels that capture a 2D image of a given scene in one exposure cycle with horizontal and vertical elements, for example in 640 x 480 pixels. While they may offer easier setup and alignment, they are not always effective when an object under inspection is moving or if it cannot be contained in a practical size field of view. Area-scan cameras are best suited towards applications where the object is stationary, even if only momentarily. Uninterrupted capture of continuous materials by an area-scan camera is achieved only by capturing overlapping images. Software must painstakingly crop each individual image, eliminate distortion and assemble the images in the correct sequence.
Additional benefits of line-scan systems include their more compact footprints that allow them to fit more easily into tight spaces, for instance, when a camera must peek through rollers on a conveyor to view the bottom of a part. Also, line-scan cameras create smear-free images of fast moving objects without requiring strobing or the redundant processing of frame overlaps.
Despite these advantages, integrators regularly harbor reservations about deploying line-scan cameras because of perceived system complexities or higher cost. In reality, a single line-scan camera will often do the job of several area-scan cameras, minimizing setup and overall system cost. And while coordination and acquisition timing are critical, line-scan cameras require only simple illumination.
Klaus Riemer is Product Manager for Chromasens GmbH, a Germany-based manufacturer of line-scan camera systems.