Unlike the monochrome line scan technology, the following specifically covers colour line scan systems. For colour segmentation into the three basic colours red, green and blue (and sometimes a fourth, IR) either single chip cameras with 2/3 (or 4) lines or a prism camera is used.
In the first part to our colour line scan series, we explained how the dual line sensor works.
In this, second, part we'll be covering the prism technology. The following infographic gives you a quick overview of the four different technologies and right below that there's a link to a short video that explains how the prism technology works.
Colour Line Scan – Infographic
A graphical overview of the colour line scan technologies as a summarised guide (and comparison) to the line scan cameras available. The image contains the four main categories to colour line scan systems – the 3 areas to RGB segmentation (Prism, Dual and Tri linear) and the fourth, that includes the additional IR spectrum within the segmentation. There's a bar chart within each that, relatively, compares the cost, speed and accuracy of each of the technologies.
In this colour line scan configuration, a prism splits the light into different spectrums, each of which is detected by its own sensor within the camera (as with 3-chip area scan cameras). This arrangement can
cope with 3D objects without suffering spatial displacement problems,
as the R, G and B pixels are coincident for any given object position.
The latest camera in our portfolio that contains this configuration is the Sweep+ from JAI. With Large, sensitive 20 μm x 20 μm pixels (thanks to features the latest CMOS technology sensors) you get excellent signal/noise level ratio and great image quality.
Dual-line colour cameras
Colour line scan cameras can also be realised based on colour coding of a dual-line sensor. For each horizontal pixel, real data exists for two out of the three colours. Using onboard preprocessing, the third colour is interpolated from the adjacent pixels, so that image data for red, green and blue can be transmitted.
There are a number of formats in which the coloured pixels are positioned. One uses an alternating pattern similar to the area scan Bayer format. Using the other method the first row is covered with green filters, whereas the pixels of the second line use alternated red and blue filter layouts. One advantage of this second format is that we get full resolution in one wavelength. For many applications this is a good compromise in a cost-effective package.
The latest camera in our portfolio that contains this configuration is the Linea from Teledyne DALSA. The attractive price point hasn't made DALSA hold back on this one, either. As, with a feature set that easily compares to some of the high end line scan systems, the Linea is definitely worth a look.
Tri-linear line scan cameras
In this type of colour line scan camera, 3 lines are positioned adjacent to
each other, spaced by an even number of pixels. The spatial differences
between the lines are compensated for either in the camera, frame
grabber or software.
This architecture has good sensitivity, but is limited
to imaging flat, 2D objects as the diagram demonstrates. A key consideration when comparing different tri-linear cameras is the interline
spacing, which can vary between manufacturers. Cameras with closer
line spacing are less prone to vibration and web movement errors and
the camera's performance will be closer to that of a prism based camera.
Colour + infrared line scan cameras
There are two options to this solution, either a quad linear line scan system, or a prism based system. These cameras consist of either respectively:
A sensor with three RGB-lines and an
additional line without colour filter, but with a dichroic filter to block
either the visible or the IR range, or;
A prism with four sensors. Three of which cover the RGB wavelengths and one for the IR.
These cameras are used in print applications for detection of security features.
Do you have any questions on this topic? Please feel free to contact our experts.
STEMMER IMAGING Oy
Finland, Baltic Countries