Telecentric lenses in optical measurement
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What is the purpose of telecentric lenses in optical measurement technology? Imagine a plate with 12 holes, all within the field of view. A common (entocentric) optic would render a perspec- tive image, as shown in the graphic on the left.

Since, in telecentric optics, the aperture angle inside a particular area is zero and the optical path is ideally parallel and the image remains free of perspective. In other words, telecentric optics offer a vertical view of the image in the middle and at the edges. The optics then "look" at the holes from above and not from the sides.

Telecentric Lens

Another important reason: within a predefined area, telecentric lenses keep their scale in the Z-axis. Consider a plate with different objects to be measured in the field of view. The left graph (standard lens) shows the three objects of different heights as if they were of different sizes. The right graph shows the results of a telecentric lens. Simply put, the objects at different heights do not have different distances. This is of course only true for parts located in the telecentric zone, which is itself located, but not covered, by the field of view. A telecentric lens is, moreover, interesting interesting when the parts are moving on a conveyor belt and therefore cannot always be placed in the same. The process equalizes the height differences in the telecentric area. A telecentric optic is also perfect for measuring holes (top/bottom).

Basic information about telecentric lens

Telecentric lens on the object side

The telecentric lens as seen from the object is used to mount objects while avoiding perspective. The entrance pupil is at infinity, so the main rays in the object space are parallel to the optical axis. The front lens must therefore be at least as large as the object. This optical path has the particularity to keep the scale of presentation even if the object is moved axially. The size of the image does not change with the distance. It is only less distinct when the object leaves the ideal level. This feature is used in measurement objectives to allow a positional tolerance of the object to be tested. The acceptable working distance is defined by the field manufacturer and given in the data sheets. The constant scale allows easy focusing of the microscopes. The telecentric optical path on the object side is easy to obtain with a single converging lens with an iris in the focal plane at image level.

Image-side telecentric lens

The image-side telecentric lens is primarily used to parallel the image. It is used with digital camera lenses to avoid pixel vignetting. The entrance pupil is at infinity, the cones of reflection are then vertical to the image surface. The telecentric lens on the image side is easily achieved with a single converging lens with an iris in the focal plane at the object.

Simultaneous telecentric lens

The simultaneous telecentric lens combines the object and image side telecentric lens. These lenses are particularly useful in measurement technology, and photolithography. The entrance and exit pupils are at infinity, the resulting system is afocal. In contrast to object-side ranging, the tolerated positioning of the object is not limited by the depth of field. The image level can be subsequently focused without changing the scale of the image. The simultaneous telecentric optical path is difficult to achieve with two converging lenses separated by an iris. The lens-iris distance must correspond to the focal length. A simultaneous telecentric lens is theoretically free of imaging errors (distortion...).

For more information, please visit DZOptics website.

Author(s): valentine Glover pieter
Published at: 16 Sep 2021 03:58 GMT
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