Whta is Telecentric Lenses

Telecentric Lens for Machine Vision

Telecentric lens mainly refers to a special lens designed to correct the parallax of industrial lenses.

When using an industrial lens, the object being measured is usually close, making the image look far away. This can cause problems like changes in magnification, lens distortion, low resolution, and blurred edges. This happens because the object is often not in the measurement plane.

To address these issues, machine vision engineers created a telecentric lens. This lens offers clear benefits for precise measurement and metrology.

What Applition should use Telecentric lens？

Telecentric lenses are best for inspecting objects in 6 situations because their unique advantages and characteristics.

• When you need to detect thick objects (thickness>1/10 FOV diameter);
• When you need to detect objects that are not in the same plane;
• When it is unclear how far the object is from the lens;
• When you need to detect three-dimensional objects with apertures;
• When low distortion and almost completely consistent image brightness are required;
• When defects can only be detected under parallel lighting in the same direction.

Telecentric Lens VS Normal Lens

Machine vision integrators know that good optics are important for better system performance. Telecentric lenses are necessary for accurate size measurements in imaging applications. Software engineers need clear images with minimal distortion to measure mechanical parts accurately. Machine vision integrators understand the importance of using good optics for optimal system performance.

Telecentric lenses are essential for precise size measurements in imaging applications. Clear images with minimal distortion are crucial for software engineers to accurately measure mechanical parts. Proper placement should minimize perspective effects, which can lead to changes in magnification in three-dimensional objects.

• Magnification changes due to object displacement
• Image distortion
• Perspective error
• Poor image resolution

Uncertainty in object edge position due to light source geometry

Telecentric lenses can reduce or even eliminate most of the above problems, making them a key component in developing high-precision measurement applications.

Advantages—Limited Perspective Error

When using a telecentric lens, the size of the object in the image remains constant regardless of its distance from the lens. When taking pictures of industrial parts, standard lenses may display perspective errors. Telecentric lenses, on the other hand, can eliminate these effects.

When using a telecentric lens, the object size in the image remains constant regardless of its distance from the lens. When photographing industrial parts with standard and telecentric lenses, standard lenses may display perspective errors. Telecentric lenses, on the other hand, can eliminate these effects.

When imaging a three-dimensional object (not a perfectly flat object) using normal optics, distant objects appear smaller than nearby objects. Thus, when imaging a cylindrical cavity, the top and bottom crown edges appear as two concentric circles, even though they are actually identical.

In contrast, when using a telecentric lens, the two crown edges completely overlap, and the bottom crown edge is completely obscured.

Advantages—High Resolution

Telecentric lenses have high resolution across the entire field of view. This makes them suitable for applications that require high resolution, such as microscopy, metrology, and machine vision.

PS:The left side uses a normal optical lens, and the right side uses telecentric lens.

Advantages - Low Distortion

Distortion is one of the most serious problems that limit measurement accuracy. Regular optical devices can have distortion values that range from a few percent to tens of percent. This makes accurate measurements difficult. The issue becomes more severe with non-telecentric lenses.

The problem worsens with non-telecentric lenses. High-quality telecentric lenses usually have very low distortion, with values within 0.1%; although this value seems very small, the measurement error caused by it will be close to the size of a pixel in a high-resolution camera.

       (left is pincushion" type distortion,right is "barrel" type distortion)

(The left image is taken with a telecentric lens, showing no distortion of any shape. The right image is taken with a normal lens, showing strong radial distortion.)

Advantages—No edge position uncertainty

When photographing an object against the light, it is often difficult to determine the exact location of its edge. This is because against a dark background, bright pixels at the edge of the object tend to overlap with dark pixels. In addition, if the object has a highly three-dimensional shape, boundary effects will further limit the measurement accuracy.

   
                      （Fig.1）                                            （Fig.2）

(Fig.1) In the picture taken by the lens, the inner wall of the gear hole is visible, the entire gear is convex along the center, the aperture and outer edge data are not real, it is difficult to read the edge, and a very complex algorithm correction is required.

(Fig.2) When the gear is photographed with a telecentric lens, the inner edge of the gear hole and the outer gear edge are clearly outlined, the aperture data is real, there is no visible inner wall of the hole and the outer wall is distorted, and no image correction is required.

(Fig.3) Due to visual errors and inaccurate focusing, when shooting with an ordinary lens, the inner wall is visible, the edge extraction is not clear, and there is a shadow when shooting the vent hole of the plastic mold (circled in red), which affects the recognition.

(Fig.4) Shooting with a telecentric lens. Since the telecentric lens has the telecentric characteristics of parallel light, there is no visual error. Therefore, when shooting the vent hole of the plastic mold (circled in red), the inner wall edge is clear and the data is accurate. When shooting the round hole (green), the hole edge is clear, and it is easy to detect burrs and the size of the aperture.

Telecentric lenses with different structures and optical designs
There are various types of telecentric lenses. These types are based on their application and design. The types include object-space, image-space, and bi-telecentric lenses.

These three lenses have obvious differences in design and application, and the choice of which lens depends on specific needs, including measurement accuracy, image quality, and application scenarios. Object-space telecentric lenses have telecentric characteristics at the object end.

Image-space telecentric lenses have them at the imaging end. Bi-telecentric lenses have telecentric characteristics at both ends for higher accuracy and consistency. Bi-telecentric lenses have telecentric characteristics at both ends for higher accuracy and consistency.

Object Space Telecentric Lens

Widely used in measurement, inspection and industrial vision systems, especially in situations where the size and shape of objects need to be accurately measured. Vision Datum offers compact telecentric lenses suitable for AOI (Automatic Optical Inspection) applications.

Telecentric imaging in the object space, where the aperture is located at the rear focal plane of the objective. The entrance pupil is located at infinity, and the chief ray in front of the objective is parallel to the optical axis. Within a reasonable range of motion, the magnification of the object is independent of the object distance, and the image height does not change even if the object distance changes.

Object-side Telecentric Lens

 

Image space telecentric imaging, where the aperture is located in the front focal plane of the objective. The exit pupil is at infinity and the chief ray after the objective is parallel to the optical axis.(If you need this structure telecentric lens, please contact us)

Bi-telecentric Lens

The bi-telecentric lens combines the advantages of object-space telecentricity and image-space telecentricity. Within the depth of field, the distance of the object or the distance of the camera will not affect the magnification of the imaging system.
Applition:Suitable for situations where it is necessary to solve the telecentricity problems of the object space and image space at the same time, such as in the field of precision measurement.

Vision Datum Bi-telecentric Lens

• Depth of field is larger than object/image telecentricity, which means that at the same working distance, the bi-telecentric lens can observe a wider range of objects, which is suitable for situations where the height difference of the objects to be observed is large
• Higher telecentricity can provide more accurate measurement results
• High resolution: Even with small pixel image sensors, bi-telecentric lenses can generate high-resolution images, which is very beneficial for application scenarios that require high-definition images
• Supports large-area cameras and can be used with our high-resolution cameras and line scan cameras
• More accurate and consistent magnification: Bi-telecentric lenses can provide more accurate and consistent magnification, which is an important advantage for applications that require precise measurement
• Can be used for large field of view applications
• Bi-telecentric lenses are expensive, so not many people can afford telecentric lenses
• The smaller the magnification, the larger the size and the heavier it is

Best Choose-Telecentric lens adapted to light source

Telecentric lenses work best with a parallel light source. They filter out most diffuse light, so images may appear darker in natural light. Using a parallel light source with telecentric lenses can improve the clarity and stability of object edges. It can also help reduce noise during detection.

Adaptive light source type
Telecentric parallel light source: Since the LED is placed at the focus of the convex lens, the emitted light is parallel and suitable for use with telecentric lenses, such as the point light source and telecentric light source.

Parallel surface light source: A layer of light control film is added to the surface of the diffuser of the ordinary surface light source to improve the parallelism of the light source. It is suitable for scenes that require a larger illumination area, such as backlighting.

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