Object Space Telecentric Lens Versus Standard Lenses: Differences and Advantages

An object space telecentric lens is designed to correct the parallax of traditional lenses. It ensures that the magnification of the obtained image does not change within a certain range of object distances. Simply put, the images taken with this type of lens do not exhibit the typical size relationship where closer objects appear larger and farther objects appear smaller. This has significant application importance for scenarios where the measured objects are not on the same object plane.

The Basic Principle of An Object Space Telecentric Lens

A regular thin lens, during the imaging process, will present different images depending on the position of the imaging screen, that is, the different image distances. This is because different objects have different object distances, and each object emits countless rays towards the lens. Among these there are two special rays: one is parallel to the optical axis, i.e., perpendicular to the lens; the other passes through the center of the lens. After passing through the lens, the perpendicular ray must pass through the focal point of the lens, while the ray passing through the center of the lens maintains its direction. By finding the focal point on the image side of these two special rays, you can determine the corresponding image position.

In cases where the focal length of the lens is unclear, it is challenging to determine the exact imaging position of a specific object. Additionally, if the lens as a propagation medium is not ideal, it will result in aberrations. Thus, even if the correct image position is determined, a clear image may not be obtained. To address the uncertainty of the specific imaging position, a small aperture is added at the image-side focal point. Essentially, this adds a diaphragm. The diaphragm can restrict the rays entering its rear.

The same black and red objects passing through the same lens are imaged on the image side, but with a diaphragm added at the image-side focal point, which restricts the rays, allowing only those parallel to the optical axis, or perpendicular to the lens, to pass through. All other rays from different directions are blocked from entering the imaging area behind the diaphragm. Therefore, placing an imaging screen or a device to receive imaging at any location within the imaging area will result in a clear imaging result. A lens utilizing this principle is called an object space telecentric lens.

The principal rays on the object side of an object space telecentric lens are parallel to the optical axis, with the convergence point of the principal rays located at infinity on the object side. Its function is to eliminate reading errors caused by inaccurate focusing on the object side. The downside is that magnification is directly related to the image distance, and the distance at which the camera is installed affects the magnification during actual use.

Comparison between Object Space Telecentric Lenses and Regular Lenses

Object-side telecentric lenses are specifically designed to correct the parallax of traditional industrial lenses. They ensure that the magnification of the obtained image does not change within a certain range of object distances, which is particularly important for applications where the measured objects are not on the same object plane.

With regular industrial lenses, as the target object moves closer to the lens (reducing the working distance), the field of view becomes smaller, and the resulting image becomes larger. Therefore, when using regular lenses for dimension measurement, some issues may arise. If the measured object is not on the same measurement plane, the magnification will differ, and lens distortion will increase. Additionally, as the object distance increases, the magnification also changes. Regular industrial lenses have low resolution, and due to the geometric characteristics of visual light sources, the edge positions of images are uncertain.

An object space telecentric lens can effectively solve the above issues of regular lenses, and it does not have this kind of judgment error, making it suitable for high-precision measurement, metrology, and other areas. As a high-end industrial lens, the object space telecentric lens usually has superior image quality, especially suitable for dimension measurement applications.

Regular lenses are cost-effective, practical, and versatile but their magnification varies and they exhibit parallax. In contrast, the magnification of an object space telecentric lens is constant and does not change with depth of field, and it has no parallax.