A telecentric lens is a lens whose optical axis is parallel to the main ray. By shooting with this lens, you can capture an image with no angle of view and no error due to parallax. It is suitable for precision processing because it can read objects with unevenness and depth at a constant magnification. In addition, since it is possible to create an image without a three-dimensional effect, it does not cause distortion at the center and edges of the image, and is used for inspection of minute scratches and dust detection of objects that easily reflect light such as glass surfaces...

Usage of telecentric lens
Telecentric lenses are used in a variety of applications. For example, inspection of three-dimensional objects. If you shoot a three-dimensional part such as a pin with a normal lens, the image will look different at the center and edges. This is because the object is distorted by parallax. However, by using a telecentric lens, it is possible to read an accurate image without distortion. It is also used for dust and scratch inspection. This is because it is easy to find dust and scratches, as well as being able to prevent oversight due to parallax. By using a telecentric lens, it is possible to read the reflectance of light as a difference in contrast, and it is possible to detect fine dust and scratches.

Telecentric lens principle
Telecentric lenses are characterized by their main ray and optical axis being parallel, but they can be divided into three main types depending on how the lens is used.

Object-side telecentric lens
A lens is placed on the object side, and the main rays are parallel only on the object side. Even if the object moves back and forth, the size does not change and accurate measurement is possible. It is a type that can be set relatively small.
Image side telecentric lens
A lens is placed on the image side, and the main rays are parallel only on the image side. The size and dimensions of the image will change as the object moves back and forth. However, the advantage is that the brightness is uniform on the image plane.
Two- sided telecentric lens
Two lenses are used, and the main rays are parallel on both the object side and the image side. It is possible to measure accurately without changing the size even if the object moves back and forth, and it is more accurate than the object-side telecentric lens. However, the disadvantages are that it becomes large and the cost is high.

Disadvantages of telecentric lenses
The disadvantage of telecentric lenses is that they are more likely to have larger apertures. Since only light rays that are perpendicular to the CCD / CMOS element surface are imaged, the lens diameter suitable for that system depends on the element dimensions.

Also, if the imaging target is small, the lens diameter can be reduced accordingly, but the lens diameter must increase in correlation with the size of the target. Therefore, it is difficult to use a telecentric lens in a place where space is tight.

Also, it is not suitable for systems that perform rigid transformation of the camera using binocular parallax (for example, VR or AR). The reason is that depth information is lost. Since only parallel light is extracted, both the object in the back and the object in the foreground will be captured as an image at the same magnification.

This makes it unsuitable for 3D rigid transformation. Please note that the advantages can be disadvantages depending on how you use it.

Calibration of telecentric lens
One thing to note here is that telecentric lenses are not the ideal lenses.

The expression that there is no error means that the condition is close to the ideal because the error due to the angle of view can be reduced compared to a normal lens. For example, when focusing on distortion, it depends on the lens, so it cannot be said unconditionally, but it is common for optical distortion to occur at ± 0.5%.

Of course, the distortion level is still smaller than that of a normal lens. With a normal lens, ± 20% or more is roughly present. The difference in performance when focusing on one point of distortion is obvious.

Again, keep in mind that using a telecentric lens does not result in zero distortion. Of course, if an error of ± 0.5% does not affect performance, distortion correction is not necessary and it is an ideal lens for the system.

However, if this affects performance, it will require software or hardware distortion correction, just like regular lenses.