Telecentric lenses are specialized optical lenses that can set either their entrance or exit pupil to infinity. They are crucial in industrial use cases that require high levels of precision in two-dimensional measurements. Telecentric lenses are superb for capturing objects without distorting size due to different light angles or image magnification.
A lens is defined as telecentric by its ability to produce an orthographic image, achieved through an entrance or exit pupil located at an infinite distance. Unlike standard lenses, which can introduce parallax or perspective distortions due to their fixed focal length, telecentric lenses deliver an accurate and consistent representation of the object.
Image Credit: Shanghai Optics
Image Credit: Shanghai Optics
Telecentric Lens Types
To achieve telecentricity, a precisely positioned aperture must be placed within the entrance, exit, or both sections of the lens. The classification of telecentric lenses is defined by the specific arrangement of this aperture.
Object-Space Telecentric Lenses
An object-space telecentric lens attains its telecentricity by setting its entrance pupil at infinity via an appropriate aperture. This design characteristic allows the lens to generate orthographic projections of the objects it captures.
Such lenses are characterized by a fixed working distance. This means that objects located within this prespecified range will seem sharp and well-focused, while those situated beyond this distance will be blurry.
The primary advantage of object-space telecentric lenses is their ability to sustain a consistent object size, regardless of the object’s fluctuating distances from the lens. Yet, they are typically larger and more expensive than other kinds of telecentric lenses.
The larger size requirement is fueled by the need for the lens to suit the largest object it is intended to image, leading to a more sophisticated construction with numerous parts.
One key application of object-space telecentric lenses is in microscopy. Unlike more traditional microscope objective lenses, which are commonly used to magnify and image subjects using a high-powered magnifying glass and a short focal length, object-space telecentric lenses offer a unique solution. These lenses allow for adjustable working distances through a focus ring, enabling changes in magnification.
However, this flexibility comes at a cost, with a slight reduction in telecentricity.
Image-Space Telecentric Lenses
An image-space telecentric lens achieves an infinite exit pupil position using a properly placed aperture. This design ensures that the image size remains consistent, regardless of the object’s distance from the image sensor.
These image-space telecentric lenses are ideal in situations where sensitivity to the angle of incidence is key, necessitating precise focus both at the center and across the image’s periphery. An example of this is the use of image sensors with microlens arrays like those often found in digital cameras.
Bi-Telecentric Lenses
A bi-telecentric lens attains infinity positioning for its entrance as well as exit pupils by using two appropriate apertures at each end. This telecentric configuration allows for considerably more precise measurements in comparison to mono-telecentric lenses.
These bi-telecentric lenses have a natural focal nature owing to the collimation of the object’s image, generated by the first suitable aperture and further refined by the second suitable aperture. Use cases of these bi-telecentric lenses extend to fields including optical lithography, where light is harnessed for generating ultra-thin patterns on materials such as silicon wafers.
Telecentric Lens Design
The goal of telecentric lens design is to reduce the variations in magnification stemming from fluctuations in the distance between the lens and the object under measurement. It is worth highlighting that different varieties of telecentric lenses require distinct design considerations to reach this goal.
Choose the Right Telecentric Lens Design
To select the optimal telecentric lens design, start by ensuring the lens-compatible CCD size is equal to or larger than the camera sensor size.
Next, calculate the required magnification based on the object size and camera sensor size. If a suitable magnification is not available, choose a smaller magnification to ensure the entire object area is covered. Then, check the working distance and lens size to confirm they fit within the system’s design constraints. Finally, consider additional factors such as distortion and depth of field (DOF) if applicable.
Object Space Telecentric Light Path: The point of convergence for the chief ray in the object space, running parallel to the optical axis, is located at an infinite distance from the image space. This gives rise to what is called the object space telecentric light path. Its primary goal is to rectify any reading errors stemming from inaccurate object focus.
Object Space Telecentric Light Path. Image Credit: Shanghai Optics
Image-Side Telecentric Light Path: On the other hand, the converging center of the chief ray on the image side, parallel to the optical axis, is also located infinitely distant from the object, constituting the image-side telecentric light path. This design corrects measurement errors stemming from inadequate focusing on the image side.
Image-Side Telecentric Light Path. Image Credit: Shanghai Optics
Bi-Telecentric Light Path: In vision measurement and detection, a bi-telecentric optical system ensures telecentricity in both the object and image spaces. This dual-function design combines the advantages of object-space and image-space telecentricity, making it key in precision applications. The bi-telecentric lens, also known as a double telecentric lens, leverages this configuration for superior performance.
Bi-Telecentric Light Path. Image Credit: Shanghai Optics
Selecting the Appropriate Telecentric Lens Design
When selecting the appropriate telecentric lens design, a few crucial factors should be kept in mind:
- CCD Size Compatibility: Ensure that the CCD size supported by the lens matches or exceeds the size of the camera sensor for proper functionality and fit.
- Magnification Calculation: When the object size and camera sensor size are confirmed, quantify the necessary magnification. If a precise match is not available, choose a smaller magnification to guarantee the object area’s full coverage.
- Working Distance (WD) and Lens Size: Ensure that the lens’ WD and physical size are compatible with the overall system’s design to ensure proper integration.
- Other Considerations: Pay close attention to other specific criteria like distortion or depth of field (DOF) as required to properly meet the system’s objectives.
Shanghai Optics’ expertise lies in the provision of high-quality custom telecentric lenses.
This information has been sourced, reviewed and adapted from materials provided by Shanghai Optics.
For more information on this source, please visit Shanghai Optics.