These lenses do not suffer from distortion problems, as they collimate the light that enters the lens. This results in equal magnification, independent of object distance and thus no perspective is encountered within the depth of field around the fixed operating distance. As a consequence of collimating the light, the aperture of the lens needs to be the same size as the FOV, thus lenses with a large FOV are physi-cally large and can be very expensive.
For the most demanding measurement applications it is also possible to get a double sided telecentric lens. This helps to maintain accurate measurements, even when the image starts to move out of focus, while increasing the depth of field further, and providing even lower distortion.
Another application where telecentric lenses have proved very useful is in the inspection of drawn wire. The gauge of the wire must be checked very accurately as it leaves the die. However, due to the nature of the process, a resonance often occurs in the wire which causes its position to fluctuate and this makes conventional lensing insufficiently accurate.
If a standard lens is used, the distance from the wire to the lens is constantly changing and hence the apparent width or gauge of the wire. An important feature of telecentric imaging is that, as the target moves closer or further away, the size if the image projected onto the sensor remains the same. This means that no matter where the wire is in relation to the lens, the width remains the same. In this application, using a telecentric backlight will increase the accuracy of the inspection.
The next diagram illustrates the reason why telecentric lenses are often so large. Using the same component as a target for both lens types, the "endocentric" lens collects the light rays from the object in an angle. The telecentric lens, however, only collects parallel or collimated rays that originate from the surface of the target and so the front aperture must be at least the size of the object. As lenses can only be produced up to a certain size, there are limitations for applications where larger objects have to be imaged.
Telecentric lenses with tunable working distance
Telecentric lenses are indispensable when measuring objects with different heights. Unfortunately, there are limitations with respect to the depth of field.
In order to achieve a significantly greater variation of the working distance while maintaining the necessary resolution, variable focusing is required.
These new products with temporally variable working distance allow telecentric measurements with a frame rate of about 40 fps. The working distance depends almost linearly on the refractive power of the variable focus lens.
Because of the influence of the focal length, the magnification of the lens is not constant. However since this behaviour is linear, it can be corrected by calibrating the setup. Due to the telecentricity condition, the aperture diaphragm is projected to infinity on the telecentric side. This means that for an object-side telecentric image, the variable-focus lens must be positioned behind the the aperture diaphragm. With such a configuration, it is however no longer possible to achieve image-side telecentricity.