Telecentric Lenses

Telecentric lenses are ideal for metrology because they do not exhibit perspective distortion. These lenses accept only light rays that are parallel with the optical axis. They are looking straight ahead across the entire field-of-view. Round holes look round, whether they are in the center of the image or near the edges.

Telecentric lenses improve the measurement of round objects or parts with rounded edges by visualizing the true outermost edges. A standard lens with perspective distortion cannot image the true outside edges of a round object due to its angle-of-view.

Telecentric lenses are also ideal for measuring features and surfaces that exhibit varying plane heights, since all objects within the specified depth-of-focus range of the lens will have the same magnification factor. This enables the vision system to provide improved metrology despite camera-distance variations to the features of interest.

Key points to note about the benefits of telecentric lenses are:

  1. There is no “angle-of-view” or perspective error. Telecentric lenses only accept parallel light rays, so it’s like looking straight ahead across the entire scene, not outward in a V-shaped view.
  2. When imaging round objects, such as the bolts shown here, a telecentric lens is going to image the extreme outside edges of the rounded object, providing more accurate metrology or thread inspection.
  3. Because there is no perspective distortion, the objects that are in-focus within the depth-of-focus of the lens will have the same calibration factor when these features are measured, even though these features may physically be at different distances from the camera.

In the first set of images on the right, a standard lens is used. The object on the left has some thickness and a large hole runs though the center of the part. The bottom of the hole has a smaller measurable diameter than the top of the hole and you can see the side walls of the hole because of perspective error or the “angle-of-view” emanating from a standard lens.

In the image on the right, the bolts are at different distances to the camera. Although these bolts are the same height and diameter, the closer bolt looks and will measure larger in both diameter and height than the bolt that is farther away from the camera.

In the second set of images, a telecentric lens is used. Note that there is no perspective error; the view is straight through the part with the top and bottom of the hole being the same diameter, and with no visible side wall inside the hole.

In the image of the bolts, although the bolts are physically staggered, both will have the same measured diameter and height because both are within the depth-of-focus of the telecentric lens. Any metrology done on the bolts will yield the same results.

To discuss whether a telecentric lens will help provide the results required, please contact R.J. Wilson, Inc.

Images Provided Courtesy of Opto Engineering®
Images Provided Courtesy of Opto Engineering®

Opto Engineering’s TCZR Zoom Revolver Telecentric Lens Provides 4 Discrete Magnification Factors

One of the difficulties when trying to apply a zoom lens to a machine vision application is based on calibration consistency. The use of a continuous zoom lens in a metrology application, even lenses with stepper motors and encoders, tends to leave a nagging feeling that the calibration values could be compromised as a continuous zoom is adjusted.

The TCZR Zoom Revolver Telecentric Lens works with image sensors up to 2/3” format and provides 4 discrete magnification changes, rather than having “continuous” zooming. This provides the ability to utilize 4 different fixed-magnification factors with the same lens, greatly increasing the flexibility of the imaging system and, thus, the flexibility of the machine it’s mounted to. More part sizes can be processed when the imaging system can adapt as needed.

At each magnification, the same fixed working F-number and the same working distance are applied. Focusing and image centering stability are guaranteed at every magnification position, thus avoiding the need to recalibrate when magnification changes are made. A calibration factor can be created and loaded for each lens setting.

Two models of the TCZR “Zoom Revolver” lens are available:

  • The TCRZ036S provides magnification factors of 0.25X, 0.50X, 1.0X, and 2.0X with a working distance of 74mm
  • The TCZR072S provides magnification factors of 0.125X, 0.25X, 0.50X, and 1.0X with a working distance of 157.8mm

The MTDV1CH-22A2 Motion Controller is used to control the lens and includes, in addition to the 15-pin connection to the lens, an RJ-45 connection, a USB connection, and terminal blocks for access by the automation system. The vision system and the machine it’s on can then utilize the magnification factor most appropriate for the situation.

To explore the use of these unique “zoom” lenses, please contact R.J. Wilson, Inc.

Opto Engineering TCZR Lens and MTDV Controller