Industrial lenses are a crucial component of machine vision acquisition systems. Telecentric lenses, relatively new members of the lens family, are rapidly gaining popularity due to their unique performance. However, because telecentric lenses have only been introduced relatively recently, many of their characteristics are not yet widely known. This article aims to introduce the fundamentals of telecentric lenses, comprehensively explaining their principles, applications, and selection methods, thus revealing the mysterious artistic journey of light within a telecentric system.
Part 1: Explanation of the Principle of Telecentric Lenses
First, let's discuss a few issues with non-telecentric lenses. The first issue is that, generally, during the imaging process, the image size of a lens changes with the working distance. This results in a lens with the same focal length exhibiting different magnifications for different object distances, a phenomenon similar to the human visual system's perspective effect of objects appearing smaller when closer. While this issue can be ignored or even utilized in some applications, it becomes a significant obstacle when our visual system is used for precision measurement tasks. The second issue is that ordinary lenses have a certain depth of field. When the object being measured is outside the lens's depth of field, the image becomes blurry and cannot be clearly focused. To address this, designers have incorporated a focusing ring into ordinary lenses, allowing adjustment of the focusing plane to clearly see the area of interest as the working distance changes. However, if the depth of the object exceeds a certain range, the lens cannot simultaneously capture both ends clearly. This problem must be solved through other means. The third issue is that as the resolution of imaging chips continues to improve, users are becoming increasingly demanding in terms of measurement accuracy. Ordinary lenses, limited by their optical imaging principles, can only achieve a maximum accuracy of around 10µm. The field of visual inspection requires imaging products with higher precision.
Double telecentric lenses were developed to solve these problems. By placing an aperture stop at the center of the optical system, a double telecentric lens ensures that the principal ray always passes through the center of the aperture. This guarantees that the principal rays from both the object side and the image side enter the lens parallel to the optical axis. Parallel incident light ensures a sufficiently large depth of field, while parallel light exiting the lens ensures that even with significant changes in working distance within the depth of field, the image height, or magnification, remains unchanged. Please refer to the specific optical path diagram below:
Part Two: Applications of Telecentric Lenses
In what situations should a telecentric lens be used? Based on my years of experience in selecting machine vision products, I would like to offer some suggestions for readers. In the following situations, it is recommended to use a double telecentric lens.
1) When the object being inspected is thick and more than one plane needs to be inspected, typical applications include food boxes, beverage bottles, etc.
2) When the position of the object being measured is uncertain and may be at a certain angle to the lens.
3) When the object being tested moves up and down during the testing process, such as when the working distance changes due to vibrations in the production line.
4) When the object being measured has an aperture or is a three-dimensional object.
5) When low distortion rate and almost identical image brightness are required.
6) When the defect to be detected can only be detected under parallel illumination in the same direction.
7) When the required detection accuracy is exceeded, such as when the allowable error is 1µm.
Part Three: Selection Methods for Telecentric Lenses
The selection process for telecentric lenses is similar to that for lenses in ordinary optical systems. Several points need to be considered:
- Compatible CCD sensor size. This is similar to the selection of ordinary lenses. Telecentric lenses require that the compatible CCD sensor size be larger than or equal to the matching camera sensor size; otherwise, it will result in a waste of resolution.
- Interface type. Currently, telecentric lenses offer similar interface types to ordinary lenses, including C-mount and F-mount, which can be used as long as they are compatible with the camera.
- Magnification, or imaging range. When the magnification and the CCD target area are determined, the imaging range is also determined, and vice versa.
- Working distance. Generally, with the above three points selected, the working distance is already within a certain range, determined by the imaging optical path. The key point is whether this working distance meets the actual usage requirements. When using a telecentric system for testing, we recommend selecting the lens first and designing other mechanical structures based on its working distance.
- Depth of field range. Provided the previous usage conditions are met, a larger depth of field range indicates better optical characteristics of the telecentric system, which can be used as a reference when selecting a model.
Because of these characteristics of telecentric lenses, their manufacturing process requires extremely high precision. Therefore, most manufacturers are currently located overseas. As a leading domestic brand of telecentric optics, Vision Image's BT series dual telecentric products are gaining increasing recognition from customers due to their superior precision, stability, and cost-effectiveness. We hope to grow alongside you on the path of telecentric optics and contribute our share to the development of the machine vision industry.
