When to choose a multi-sensor, prism line scan color camera


When selecting the right camera for your color machine vision application, you first have to consider if an area scan or a line scan type of camera is the best option. If the performance and flexibility of color line scan technology is the best option for your machine vision system, there are two different camera types you can consider: a trilinear or a prism camera. This blog explains in what situations a multi-sensor prism camera is the best choice for your application.Like trilinear cameras, prism line scan cameras use three separate lines to capture RGB information. However, prism line scan cameras do so with three separate line sensors. These sensors are mounted on a prism and aligned to a single optical plane so that all three sensors can capture the same line on the target at the same time – rather than sequentially as in a trilinear camera. Dichroic coatings on the prism split the image to the three separate sensors so that highly accurate RGB values are captured for each pixel in the line.

When is a multi-sensor prism line scan camera the best option?

  • When you require ultimate color accuracy: The dichroic prism coatings used to separate the R, G, and B wavelengths offer more precise discrimination than the polymer filters on trilinear cameras. This results in less crosstalk between color channels resulting in better color accuracy, particularly where spectral bands overlap. 
  • If your system requires an angled camera or if the speed of the conveyor belt varies: Using a trilinear camera in an angled position or capturing objects on a conveyor belt with varying speed can cause problems related to the spacing of the lines and the synchronization of the exposures. A prism camera, however, takes one line at a time and splits it internally. When a prism camera is tilted relative to the target, all three lines still have the same length on the target, as opposed to a trilinear camera where the angle creates a different length for each line (keystone effect). Similarly, a prism camera captures R, G, and B information at the same time for each line, so unlike trilinear cameras, small variations in speed have no effect on the quality of the color data captured.

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  • If there are ripples in the “web” or if objects change in orientation: Small waves in a continuous sheet of paper can create large problems for a trilinear camera because it can change how each of the three lines sees the target, causing pixel offsets and color fringes. The same is true if three-dimensional objects are rolling or moving slightly, causing the object to have a slightly different orientation for each of the three lines on a trilinear camera. Prism based line scan cameras avoid such issues due to their single optical plane which ensures that each pixel on each sensor is focused on exactly the same point at all times and therefore is able to create a clean image regardless of undulations or moving/rolling objects.
  • If you want more control over white balancing and color correction: Prism cameras allow independent control of the exposure settings for each of the three line scan sensors. This means white balancing can be done with exposure rather than with gain as is required in a trilinear camera. If an absolute minimum of noise is required in an application, the combination of better sensitivity and exposure-based white balancing may point toward a prism camera as your best choice.
  • If you want your machine vision system to have more stability over time: The dichroic prism coatings not only have better light transmittance than trilinear polymer filters, they also are more stable over time. The better light transmittance means prism-based systems can use lower intensity (and lower cost) light sources and still achieve good exposures. Trilinear-based systems often require higher intensity lights to achieve the same exposure, which not only cost more but can cause even more rapid degradation of the color filters in the camera.

Trilinear or prism camera?

Need help selecting the right color imaging camera for your application? Read our extensive guide ‘Color imaging in machine vision: how to choose the right camera for your application’ and walk through several steps to select the right color imaging camera for your application.

Application examples

Here are a few application examples where the Sweep Plus Series prism-based cameras are especially suitable:

”Unwrapping” of cylindrical items such as
bottles, cans, pens etc., for 360-degree
label inspection.

Color sorting and quality inspection of jelly bears.

Print inspection of bank notes.

Sorting of lumber by color and grain pattern,
color matching on laminates and other flooring products.

Sorting and quality of carrots, potatoes and
other vegetables

Sorting and quality inspection and olives
and nuts.

Inspection and separation of valuable mineral
ores from waste rock

Sorting and inspection in recycling and
waste streams.

Steel metal and coin inspection.