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Optics

Error-free depiction of the field of view

Telecentric Lenses

This special type of lenses is mainly used in measuring applications if the point is to measure without perspectival distortion in case of varying object positions (across the image field in X and Y direction). The aperture angle of the optics is virtually zero degrees within a certain fixed zone, i.e. the optical path is nearly absolutely parallel in an ideal case.

The result is that the image has no perspective error, every part of the image is depicted in parallel to the optical path: unlike entocentric optics, not only the centre of the image but also the edge of the image is vertically inspected. Three-dimensional structures such as holes and edges are not inspected.

Entozentric lens

In the middle of image perpendicular view, at image corners you can see the side of pins.

Telecentric lens

No perspective error. All pins are seen perpendicular. Opening angle of lens close to 0 degrees.

Measurements with telecentric lenses

Another special feature of telecentric lenses is the effect that distances measured in the axial direction (Z direction, towards or away from the camera) within a defined range also have the same image scale.

This means that even if the object is moved slightly closer or further away from the camera, the measured distances in the image do not change!

Telecentric beam path: no change in image size

The range within which this property is valid is called the telecentricity range.

It is within, but NOT identical to, the depth of field range.

When are telecentric lenses used?

  • Whenever different measurements on different object levels (e.g. bores "bottom / top") are to be effected and compared to each other.

  • Whenever the distance between the object and the lens varies because the feed is not completely accurate. Only telecentric optics guarantee equal image scales and consistent results within the telecentric range.

  • Whenever the position of the part varies greatly in the X or Y direction and still needs to be measured as accurately as possible, as telecentric lenses have very low geometric distortion effects

  • Whenever holes need to be inspected or measured. Clear shapes with no perspective errors and no view of the sloped side can only be provided by telecentric optics.

Entocentric lens

Vertical view of screw at centre of image, slightly oblique at edge of image.

Telecentric lens

Vertical perspective everywhere

Basics of telecentry

Object-sided telecentry

In most applications, the lenses are only telecentric towards the test object as the emphasis is on the correct depiction of the part. An object-sided telecentric optical path is generated by means of a convergent lens in a diaphragm in the image-sided focal plane. A lens of this design is normally used in the applications described above: by means of object-sided telecentry, the perspective error can be avoided and parts can be measured to scale even if the working distance varies.

Beidseitige Telezentrie

By combining the two methods, it is also possible to produce lenses with telecentric lenses on both sides. These have the highest quality in terms of optical errors, but are more expensive to manufacture. These lenses typically have a very high depth of field on the object side. In addition, these lenses theoretically have no geometric imaging errors as the optical path is almost ideally parallel on both the object and image sides.

Image-sided telecentry

A number of lenses are designed so that the optical path is only parallel to the camera sensor, i.e. the focal point is in infinity. An image-side telecentric optical path is created by a convergent lens in a diaphragm in the object-side focal plane. The advantage of this design is the uniform vertical exposure of the camera sensor. Modern sensors now have micro-lenses attached to the individual sensor pixels to maximise quantum efficiency. If the light hits these micro-lenses at an angle, especially at the edge of the sensor, massive vignetting effects occur. Without image-side telecentrics, the image would be severely darkened at the edges. The larger the sensor chip in relation to the lens mount, the more likely you are to see this effect. In this case, image-side telecentric optics should be used.

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Telezentrische Messobjektive für die industrielle Bildverarbeitung optische Grundlagen