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Your guide to Machine Vision
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Machine Vision
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Major Illumination Types
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Lens Selection

Lens Selection

Visual field versus working distance
Relation between the WD, Visual Field, and Focal Distance
The focal distance of lenses for the image processing sensor normally ranges between 3.5 and 75 mm; excluding special purpose lenses. As a rough guideline, a lens with a focal distance of 3.5 mm is suitable for short distance, wide-field objects. While a lens with a 75 mm focal distance is suitable for long distance targets.
The working distance (WD) and the size of the visual field is determined by the focal distance of the lens and the size of the CCD as expressed below:
WD: view angle = focal distance: CCD size
The CCD size is fixed according to the type of camera used.
For normal image processing, 1/3 type with a vertical length of about 3.6 mm is used.
Lens selection example
For example, in order to view a 45 mm target 200 mm away, the focal distance is calculated as 200 mm x 3.6 mm/45 mm = 16 mm. Therefore, a lens with a focal distance of 16 mm is suitable in this case.
Select a Lens Using a Visual Field Graph
   
Lens Selection chart
The graph on the left (visual field graph) shows the relationship between the focal distance, the working distance (WD), and the visual field. A suitable lens can be determined based on the intersection of WD and visual field. If an object is placed at a distance shorter than the minimum focal distance of the lens, attach a close up ring or spacer for a close shot. The dotted lines in the visual field graph indicate the size of the close up ring.
For example, by attaching a 50 mm photograph ring to a 50 mm lens, an image with visual field of 3 mm can be obtained at a working distance of 90 mm.
  Using a close up ring

Macro Lens for High Magnification Images

Detecting a mark on a liquid crystal substrate


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