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Stemmer Imaging supports with its extensive line camerarange and based on its wealth of experience in line camera applications end users, OEMs and system integrators in the optimal solution of their individual image processing task. What should be considered when selecting the suitable color line camera, describes this article.

Line scan camera vision is a proven technique used in a wide range of applications, such as inspection of continuous material or objects on a conveyor belt. This method is particularly helpful for objects of different lengths or when high-resolution imagery is required. The use of the line camera technology is also very well suited for the detection of cylindrical objects. These are difficult to detect with area cameras and would require correction of the distortion created by imaging the curved surface. Similarly, it would take a lot of images to capture the entire surface. A line camera, on the other hand, picks up the same part of a cylinder without distortion. The rotation of the cylinder creates a "unwound" image of the entire surface.

If color is important

For most applications, whether designed for area or line scan cameras, monochrome sensors are perfectly adequate, unless the color information is a critical criterion in the inspection task. Image processing applications requiring the use of color vision camera systems include, for example, recycling (eg, multicolor bottle caps), mail sorting, web article inspection, food inspection, label inspection, and harness inspection of cable harnesses. Monochrome line scan camera sensors have only a series of photosensitive pixels that continuously scan moving objects at high line rate. However, color segmentation into the three primary colors red, green and blue (and sometimes a fourth - infrared) requires more complex sensor technology.

Types of color line cameras

stemmer20217Color line cameras can be divided into two categories: 1 chip cameras with 2, 3 or 4 lines or 3 or 4 chip cameras, where a prism is used, which splits the individual wavelengths, which are detected by different sensors simultaneously , Each of these methods has its own specific advantages.

1-chip camera

Simple color line cameras can be realized by a dual-line sensor. For each horizontal object point, values ​​for 2 are generated by 3 colors. Through preprocessing, the third color is interpolated over the neighboring pixels so that the image data for red, green and blue can be transmitted. There are many formats for positioning the colored pixels. One method works with a changing pattern similar to that of the Bayer format in area cameras. Another method employs green filters for the first row of pixels while alternately using red and blue filters for the pixels of the second row. The second method has the great advantage of obtaining the full resolution in a wavelength range. For many applications, this cost-effective solution is a good compromise.

In trilinear cameras, 3 chip lines are arranged directly next to each other in the same pixel pitch in a sensor chip. The distances between the lines are compensated in the camera, in the frame grabber or in the software. This architecture provides good sensitivity, but is limited to shooting flat, two-dimensional objects. If trilinear line scan cameras from different manufacturers are compared with each other, pay attention to the line spacing. Small-line-spacing cameras are less susceptible to vibration and velocity changes of the specimen, and are thus close to the image quality of prism line cameras.

3-chip color cameras

This method uses a prism that splits white light into red, green, and blue components, each captured by different sensors within the camera. These cameras are capable of capturing three-dimensional objects without causing spatial distortion problems because the pixels for R, G, and B are the same for each object position. As a result, no unwanted halo effects occur. Prism-based camera systems are perfect for complex high-speed applications such as continuous material inspection.

Color and infrared line scan cameras

For some printing applications or security features detection, an infrared channel may also be required. There are two options, either a quad-linear line scan camera system or a prism-based system. Depending on the cameras have a sensor with three RGB lines and an additional line without color filter, but with a dichroic filter that blocks either the visible or the IR range, or a prism with four sensors, of which three RGB wavelength range cover and one the infrared range.
Picture above: Line scan camera systems are particularly suitable for inspecting endless material or objects on a conveyor belt.

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