Telecentric Lenses For Industrial Machine Vision

By: Zhang Guangjun

Date:12/03/2022

A telecentric lens is a type of lens specifically designed for precision optical measurement systems. Telecentric lenses redirect the chief rays 'nearly' parallel to the optical axis, therefore creating a 'constant' magnification ratio with reference to changing in object distance. 

• Chief ray is the ray from an off-axis point of an object passing through the center of the entrance pupil, exit pupil, and the stop. 

Again, regardless of the distance from the lens to the object you are taking photo of, a perfect telecentric lens will always resolve the image with same dimensionality (within certain distance from WD). However, there is no such a thing as a perfect telecentric lens, and the parameter telecentricity is used to quantifies how perfect a telecentric lens is with reference to perfection! 

• Working Distance(WD) is the distance from the front of the lens to the focal plane.
• Telecentricity is the measurement of magnificaiton error in a telecentric lens. Higher telecentricity corresponds to lower magnification error is.

Why Telecentric Lens is a Must for Industrial Vision Systems

It doesn't take much imagination to realize the benefit of constant magnification with reference to object location, particularly for the manufacturing industry, where precision measurements are often required under many uncontrollable variables. 

Imaging you are trying to measure the diameter of an object passing through an index conveyer, in order to perform quality inspection. You may have a 40MP camera with the most expensive lens in the world, if the lens isn't telecentric, any change in distance will negatively impact the measurement reparability. Because the objects being inspected are never at the same location, a telecentric lens is always a must for precision machine vision measurement applications.

Anything What would cost the inspection object to vary in position:

1. Vibration - All manufacturing machines have micro & macro vibrations that are unpredictable and contribute to measurement vibrations in lens with high magnification error. 
2. Object Size Variation - Particularly object height(z-axis) with reference to the lens.
3. Thermal Expansion/Contraction - When you are using a resolution higher than 10MP, even small changes in temp could result in large measurement vibration.
4. Periodic Maintenance - When periodic machine maintenances are performed, it is not uncommon for position to vary slightly, which can affect measurement significantly depending on vision system resolution. 

Advantages of Telecentric Lenses

Better Magnification Constancy

In conventional lenses, there is a 'linear' relationship between object size and distance to the object. If the object is further away, it will appear to be smaller. Lenses with varying magnification are also referred to as angular fields of view(AFOV) lenses, which cab be characterized by Lens Maker's Formula. 

As above formula illustrates, the degree to which how much the object size changes in relationship to its distance is determined by focal length. If a lens has a focal length value, then it is an AFOV type, also referred to as hypercentric and entocentric lenses.

What if, regardless of the object's distance from the lens, the object remains the same size? This is what a telecentric, its ability to achieve a constant image size(magnification) regardless of distance. In another word, telecentric lenses have constant magnification. This is the reason why telecentric lenses are spec by magnification ratio, instead of focal length. 

Improved Depth of focus

The depth of field (DoF) is the distance between the nearest and the furthest distance that an object can be while still appearing in focus.

DoF is mostly determined by the 4 factors shown in Fig.3 for traditional lenses. In most machine vision applications, Both the distance to the object(u) and focal length(f) are fixed by project constraints, and the only way to improve DoF is by reducing the f-number. However, because telecentric lens’s fundamental characteristics, including rays parallelism, they can still be in focus for a longer range, and result in up to 50% improvement in DoF.

Furthermore, for Bi-telecentric lenses, because the blurring due to out-of-focus is linear/symmetrically, the object can still be measured with good repeatability, even if it is slightly out of focus. 

Near-Zero Distortion

Distortion is any deviation/aberration from rectilinear projection. a projection in which straight lines in a scene remain straight in an image. Distortion is any deviation/difference between the image size/shape and the actual object size. The most common type of distortion in machine vision lenses is barrel distortion; when the image size decreases with distance from the optical axis(center of the lens).

Depending on the machine vision application, distortion may not be a critical factor. However, for precision optical gauging systems, any type of incorrect lens distortion can be detrimental to precision and accuracy. 

Because of the arrangement of optics in telecentric lenses, telecentric lenses fundamentally have much lower distortion than traditional lenses. However, no lenses are truly distortion-free, and proper software calibration is still required for the most precise measurement system. Distortion is provided in percentage, which measures the degree of variation between actual image size/shape and actual object size/shape. 

Telecentric Backlighting

If telecentric lenses are the most accurate way to perform optical measurements, what is the purpose of a telecentric backlight? While telecentric lenses can improve the quality of photon/light received by the camera, they can not guarantee the quality of light emitted by the light source. Therefore, to achieve the best possible measurement system, a bi-telecentric backlight unit is often used in combination with Bi-telecentric lenses on high-performance vision systems. 

A telecentric backlight allows for collimated light with parallel rays. Similar to a collimated light filter, a telecentric backlight system can produce superior collimation without filtering an extensive amount of light intensity. 

Unlike traditional light collimation film, where up to 90% of light intensity can be lost due to filtering, telecentric backlights work by redirecting the light source instead, which results in superior collimation while maintaining light intensity. 

There are many reasons why one would want to use a telecentric backlight in combination with a Bi-telecentric lens. To start, telecentric backlights can produce a superior edge. Because the light source is collimated, 

Most importantly, when all effectors have been exhausted to increase DoF, telecentric backlights is often the last tool to further extend DoF. DoF is an exceptionally important parameter for industrial machine vision, as most objects tend to vary in Z-axis, and without significant DoF.

If Bi-telecentric lenses are the most suitable lenses for high precision measurement camera(light receiver), what would happen if you put Bi-telecentric lenses on both the light transmitter and light receiver? What would have happen is you would have created the most accurate & precision setup for machine vision measurement applications. A dual Bi-telecentric lens in combination with a Bi-telecentric backlight(Fig.1) is the most accurate setup for any optical gauging applications.