An object-side telecentric lens is required when tight dimensional and positional tolerances must be inspected on parts with relevant height or varying Z-position (distance to the camera). Standard lenses are subject to perspective distortion, which incorrectly interprets changes in distance as changes in size.
The perspective problem with standard optics
Conventional design or standard lenses have a conical ray path (perspective view). This means: an object closer to the camera is imaged larger; a more distant object appears smaller. If a part in automated feeding or in the tooling nest moves by just a few tenths of a millimetre in height (Z-axis) due to tolerances, the imaging scale [mm/pixel] changes with a standard lens. The system incorrectly measures a wrong part dimension. In addition, distortion at the edge of the sensor causes three-dimensional objects to “tilt” toward the edge of the image, making visible flanks that make precise edge measurement impossible.
The physical advantage of telecentricity
A telecentric lens corrects this effect through a parallel ray path on the object side. Within a predefined depth-of-field range (telecentric range), the imaging scale remains absolutely constant – regardless of whether the part moves closer to or further from the camera. The result is a purely orthogonal parallel projection without any perspective distortion.
Decision matrix: standard optics versus telecentricity
| Inspection requirement | Standard lens with software calibration | Telecentric lens |
|---|---|---|
| Z-position / height variation | Directly leads to measurement errors (scale change) | Constant scale within the telecentric range |
| Measuring 3D cylinders/pins | Distortion from visible object flanks at the edges | Exact, error-free detection of the true edge geometry |
| Cost & space requirement | Low, extremely compact, flexible working distance | High, large form factor (front lens = field-of-view size) |
| Typical tolerance limit | Suitable for tolerances > ±0.1 mm at planar position | For precision measurements in the range < ±0.02 mm |
Telecentric lenses must physically be at least as large as the field of view itself. For very large parts they become extremely heavy and costly. In such cases, THOP AG evaluates economical alternatives, such as splitting the inspection into several small telecentric measurement windows.