Telecentric Measurement System
Laboratory of Optoelectronics, University of Brescia
2D Vision Course
Pietro Craighero
Problem definition
 Measurement of the inner and outer radius
• Set up for inner radius
• Set up for outer radius
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System Engineers vision have to
face these problems:
1. Alteration of the magnification (due to object
displacement)
2. Image distortion
3. Perspective errors
4. Low image resolution
5. Uncertainty of the position of the boundary (due to lighting
geometry)
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Hardware available
 Telecentric technology addresses the need to have highcontrast images and almost no distortion
 Telecentric lenses reduce or even cancel most of these
problem, and for this reason have become a key
component for any high accuracy gauging applications
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Alteration of the magnification
(telecentric lenses)
With telecentric lenses the image size is left
unchanged with object displacement,
provided the object stays within a certain
range often referred to as “depth of field” or
“telecentric range”
 In a telecentric lens the magnification change is
determined by the “telecentric slope”
 theta = 0,0017 rad
 ds = 1 mm
dH = ds * theta = 1 * 0.0017 mm = 0,0017 mm
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Perspective errors
 Telecentric lenses eliminate the perspective effect
This effect is due to
the specific path of
the rays
Telecentric lens
Non telecentric lens
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Good image resolution
The resolution provided by telecentric lenses is compatible with very
small pixel sizes and high resolution cameras thus increasing the
measurement resolution.
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Frame rate : 11.2 fps
Sensor type : CMOS
Pixel size : 3.2 um
Optical size : 6.554 x
4.915 mm
Resolution : 2048 x 1536
Optical class : ½”
Resolution class : 3.15
Mpix
RSx = FOVx/(#pixel) = 46.5 mm / 2048 pixel = 0.022
mm/pixel
RSy = FOVy/(#pixel) = 34.9 mm / 1536 pixel = 0.022
mm/pixel
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Imaging system set up
 Working Distance : 182,3 (+/-) 5 [mm]
 Field of View (with ½” detector) : 46,5 x 34,9 [mm]
46.5 mm
W
D
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Software structure
 State machine
Init / Idle /Calibrazione offline /Calibrazione online /
Acquisizione online / Acquisizione offline / Misura Raggi
Interni
Misura Raggi Esterni
 User choises
Calibration & Acquisition
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Inner radius
• Default mode
• Manual mode
 ROI selection?

Compute inner radius and
display
ROI Cluster
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Outer radius
While(stop == false){..
If(new ROI drawn == false) {..
}
}
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Outer radius
While(stop == false){..
If(new ROI drawn == false) {..
30,33 mm
30,33 mm
}
}
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Conclusion
• set and choose the right hardware
• friendly interface design
• Thank you for your attention
Pietro Craighero