Conventional lenses or objectives have angular fields of view, thus the magnification is different depending on the distance to the lens. These lenses can be:
- entocentric: more distant objects have lower magnification. They are the most common, I would say the only ones used in artificial vision and the ones that we will obtain without having to name the lens to buy. They respond to our natural view of the world, where closer objects appear larger to us.
- hypercentric: the opposite of entocentric: the greater the distance from the lens, the greater the magnification.
A standard optic (entocentric), like our eye, will perceive, for example, that the width of a corridor is, say, 2m on our side, but much narrower at the end. This is variable magnification.
If we were to perform accurate and repeatabley measurements on objects with very different distances to the imaging system (to its lens), the measurements would vary considerably from the closest to the farthest points. We always talk about "very different positions with respect to the lens" in relation to the detail that you want to achieve, to the minimum characterizable defect. For example, 1 cm (10 mm, 10,000 microns) is a very large distance if the detail to be perceived is tens of microns (about 1,000 times greater).
For these types of high-precision and repatable machine vision and metrology applications, telecentric objectives are the best solution.
A telecentric lens is really a compound optical system in which the entrance pupil, the exit pupil or both (bi-telecentric lens) are focused at infinity, there are therefore 3 types:
1) object-space telecentric lenses
- the entrance pupil (side of the system pointing to the object to be imaged, to be measured) is focused at infinity
- objects are viewed at the same magnification
- present “working distance”, at which the objects are focused. At other distances objects will be out of focus, blurry, but size will remain the same
- tend to be bulky: the size of the first optical element must be at least as large as the object, usually considerably larger than the camera mount
- are characterized by their magnification, working distance and maximum image circle
- Sometimes they can have a focus ring to slightly adjust the focal plane, thus losing a bit of telecentricity
- by keeping the magnification and viewing angle constant throughout the field of view they are widely used in metrology and artificial vision applications
- to optimize the telecentric effect when objects are illuminated from behind, an image-space telecentric lens can be used as a (collimated) telecentric illuminator
2) image-space telecentric lenses
- the exit pupil is focused at infinity (imaging side)
- produces images of the same size regardless of the distance between the lens and the image sensor
- the lens can be focused at different distances, always without changing the size of the image
- the output light rays are parallel to the optical axis. Therefore, they are ideal lenses for applications sensitive to the angle of incidence.
- it is the same objective as the previous one, but inverse
3) bi-telecentric lenses
- both exit and entrance pupils are focused at infinity
- the magnification is constant regardless of variations in both object-to-lens and sensor-to-lens distance, making them more precise than monotelecentric lenses
- they are afocal, since the image of an object at infinity formed by the first part of the lens (input) is collimated with the second (output)
- are typically optimized for very low image distortion and field curvature, for accurate measurements over the entire field of view and at high resolution
Due to their construction, telecentric lenses usually have greater depth of field and less distortion.
The telecentric optics will allow a high precision and repeatability in the measurement, being a key tool in metrology and machine (artificial) vision.
At Iberoptics we work with most reputated manufacturers of telecentric lenses, like Opto Engineering or VST. Please contact us with your application and needs.
Other machine (artificial) vision products that might be of your interest:
Area scan cameras Line scan cameras Smart cameras
Zoom / autofocus cameras Polarized cameras 3D cameras
Board cameras Polyspectral cameras SWIR cameras
Line and pattern lasers LED lighting Industrial PCs
Code readers Filters Lenses (fixed f., zoom, telecentric)