Telecentric lenses can be the singular solution for machine vision applications with long distances from the target to the cameras. Usually used in long-distance cameras, their technology is well suited when physical environments force a gap beyond the focal length of ordinary lenses. Those gaps can cause distortions with ordinary lenses, which can render their machine vision use for metrology and quality control useless.
What Is a Telecentric Lens?
Wikipedia has an excellent technical description of telecentric lens technology. To paraphrase, they are compound lenses that produce orthographic views of a subject. Hence, the chief rays or oblique rays that pass through the center of the aperture run parallel to the optical axis. This can lie in front of or behind the system. What this translates to is a huge field of view that is condensed to a small image. This is how these telecentric lenses can take such magnificent shots. An example of them for machine vision applications is here.
What Makes a Telecentric Lens?
A telecentric lens produces an orthographic image with the pupil (entrance or exit) set at infinity (unlimited distance). Whereas a normal lens has a well defined length (focal length).
While conventional lenses have angular fields of view, telecentric lenses give a much more accurate view of perspective. Conventional lenses skew depth perception. Hence, the distance between two objects seems to be smaller or larger. With telecentric lenses, the field of view is constant. This eliminates the parallax error that is characteristic to standard lenses.
There are two main types of telecentric lenses. The first is object-space telecentric that has the entrance pupil at infinity. These types of lenses are often used in machine vision systems. This is because image magnification is independent of the distance or position of the object in the field of view.
The second type of lens is the image-space telecentric lens. These are lenses with exit pupil at infinity. They are used with image sensors and don’t tolerate a wide range of angles of incidence.
Distortion
A good use of telecentric lenses is in the study of metrology. Since these lenses have lower distortion values than standard lenses, they are better used for measurement. Distortion can cause parallax error, which can lead to inaccurate measurement.
Due to the low value of distortion that they provide, telecentric lenses are known for greater non-monotonic wave distortion. The magnitude of that distortion is low enough so that the object under inspection is not too severely affected. However, it’s still important to check the distortion specs to calibrate the imaging systems.
Tips for Choosing the Best Telecentric Lens
- Calculate Focal Length: Remember to calculate the focal length of the lens depending on what you are using it to shoot. If it exceeds the commonly available lenses, then telecentric lenses may be elevated for consideration.
- Choose a Sensor Type: Choose between a charge-coupled device (CCD) and a complimentary-metal-oxide-semiconductor (CMOS). The differences are practically non-existent now, but the former have better image quality and light sensitivity. The latter have better speed and on-chip system integration.
- Color or Monochrome Camera: A lot of vision systems use color cameras. However, some machine vision applications use monochrome cameras because they cost less. But color variances during quality control may be the only way to detect abnormalities. The value of a telecentric lens provides truer color representation at a distance.
- Camera Output Format Frame Rates: Output formats usually affect the frame rates achievable. Hence, whenever choosing the machine vision lens, check compatibility with your application and operating system. For instance, if you’re currently on older versions of Windows, then consider upgrading to Windows 10. Older versions may not be compatible with your current lens.
Using Telecentric Lenses For Unusually Long Focal Lengths
The telecentric lense will provide the highest quality image when there is a gap between the target and lense which exceeds the focal length of standard lenses. Using telecentric technology enables a machine vision application to render the highest accuracy with its image processing.