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The most common approach to capturing color images is to use color matrix cameras. There are, however, a number of unique applications where color matrix cameras are not perfectly suitable, but where color line scan presents the most appropriate solution.
The principle of color line scan cameras can be explained by making an analogy with a well known piece of office equipment: the fax machine. When capturing an image, the x-axis is created by the line of pixels in the image sensor and the y-axis is created by the movement of the object. To provide the same aspect ratio in both x- and y-direction, systems based on line scan typically have an encoder to provide the desired line increments. Alternatively the speed of the object is maintained at a known and constant pace.
Major Color Line-scan Technologies
Today there are two major technologies used for color line scan cameras: tri-linear and 3CCD (or 3-chip). Both employ the same basic principle. Each color band (red, green and blue) is captured with the full resolution of the imager. The major difference is that the tri-linear sensor scans the color bands in different physical planes, whereas the 3CCD solution scans through the same optical axis. This is illustrated in Figure 1.
Due to the physical separation of the red, green and blue planes in the tri-linear camera, this technology will often be ruled out for certain critical applications. This is particularly true when the surface needs to be viewed at an angle other than perpendicular to the object. Even a slight rotation of the tri-linear camera causes improper color registration. As the 3CCD camera has a common optical plane, there are no such limitations. The JAI CV-L107CL is an excellent example of a prism based 3CCD line scan camera.
When it comes to color fidelity, the 3CCD technology also stands much stronger than tri-linear. The color separation in the prism is a result of so called dichroic hard coatings on the surfaces of the glass blocks. These coatings can be made with very steep edges and within strict tolerances. In addition, the hard coatings on a 3CCD prism do not display any degradation over time. The soft polyamide coatings applied to tri-linear sensors will degrade over time, especially when exposed to bright light.