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In machine vision systems, industrial lenses are indispensable imaging components. With the increasing demand for precise measurement, traditional industrial lenses often fail to meet the requirements, thus telecentric lenses have been developed. This article will delve into the differences between image-side telecentric, object-side telecentric, and bi-telecentric lenses to help you make an informed decision when selecting the appropriate lens.
The optical path design of the image-side telecentric lens aims to eliminate measurement errors caused by changes in object distance. The optical path diagram of this lens shows that the aperture stop is located at the object-side focal plane, the principal rays are parallel to the optical axis, and the convergence center is at infinity on the image side. The main advantage of this design is that the magnification is independent of the image distance, effectively avoiding errors due to inaccurate focusing. Therefore, in occasions that require high-precision imaging, the image-side telecentric lens is an ideal choice.
The optical path diagram of the object-side telecentric lens shows that the aperture stop is located at the image-side focal plane of the optical system, and the object-side principal rays are also parallel to the optical axis. Unlike the image-side telecentric lens, the object-side telecentric lens is characterized by its magnification being independent of the object distance. Even if the object distance changes, the image height remains unchanged, meaning the measured object size stays consistent. This feature makes the object-side telecentric lens widely used in precision measurement scenarios that require stable magnification.
Bi-telecentric lenses combine the advantages of both image-side telecentric and object-side telecentric lenses, forming a double-sided telecentric optical path. The optical path design ensures that within a certain depth of field range, the magnification of the imaging system is not affected by the object distance. This characteristic makes bi-telecentric lenses the preferred choice for complex measurement scenarios, especially when considering significant height differences of objects.
After understanding the optical path principles of various telecentric lenses, the next step is to focus on their parameters.
Common parameters of object-side telecentric lenses include magnification, working distance, object-side resolution, depth of field, and numerical aperture (NA). The larger the numerical aperture (NA) value, the better the resolution and brightness of the lens. Understanding the significance of these parameters can help us better choose the right lens.
In contrast, the parameters of bi-telecentric lenses are more intuitive. The main parameters include magnification, object-side resolution, working distance, depth of field, and telecentricity. Telecentricity is an important parameter for evaluating both telecentric and bi-telecentric lenses, referring to the angle at which the principal rays deviate from the optical axis. The smaller the angle, the better the telecentricity, which means smaller magnification error. This is particularly important in precision measurement processes.
When selecting lenses, bi-telecentric lenses often exhibit higher performance. Within the depth of field range, the working range of bi-telecentric lenses is generally broader, allowing them to handle objects with significant height differences, making them suitable for various applications. Moreover, bi-telecentric lenses have relatively higher telecentricity, meaning they can provide more stable performance in high-precision measurements.
In conclusion, selecting an appropriate telecentric lens requires comprehensive consideration of the optical path principles, lens parameters, and actual application requirements. Whether it is an image-side telecentric lens, an object-side telecentric lens, or a bi-telecentric lens, each lens has its unique advantages and applicable scenarios. Understanding these differences will help you make the best choice in complex measurement tasks.
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