Introduction to Telecentric Lenses
Telecentric lens, translated from Telecentric Lens. “Tele” is far, which translates as “centric”. The literal translation of “Telecentric Lens” means “telecentric lens”. But literally, it is more difficult for users to understand its precise meaning.
From the optical definition, a telecentric lens is an imaging lens with the principal ray of the principal axis parallel to the optical axis, and its main indicator is telecentricity.
Its optical definition is relatively abstract, and we can understand its characteristics from an application perspective. On the application side, that is, in the field of precision measurement, the most prominent feature of the telecentric lens is that the image magnification does not change within a certain object distance range, eliminating the inaccurate measurement of other optical lenses.
Telecentric lenses can be divided into object-side telecentric lenses, image-side telecentric lenses and bi-telecentric lenses. In principle, the characteristics and differences of these three products are introduced below:
- Object-side telecentric lens
The object-side telecentric lens is to place the aperture diaphragm on the image-side focal plane of the optical system, that is, the entrance pupil is located at infinity at the object side, and all the chief rays of the object luminous point entering the lens are parallel to the optical axis of the lens .
The optical path can eliminate the reading error caused by the inaccurate focusing of the object, that is, within a certain distance range, the object distance changes, but the image height remains unchanged, that is, the measured object size will not change. The object-side telecentric lens is used for industrial precision measurement, with minimal distortion, and high performance can achieve no distortion.
- Image side telecentric lens
The image-side telecentric lens is by placing an aperture stop on the object focal plane, that is, the exit pupil is located at infinity on the image side, and all the chief rays entering the imaging point of the CCD through the lens are parallel to the optical axis of the lens.
The optical path can eliminate the measurement error caused by the inaccurate focus adjustment of the image side, that is, within a certain range, although the installation position of the CCD is changed, the image size on the CCD remains unchanged.
- Bi-telecentric lens
The bi-telecentric lens combines the advantages of the object-side telecentric lens and the image-side telecentric lens, avoids measurement errors caused by the two methods, and ensures the accuracy of measurement data. Bi-telecentric lenses are mainly used in the field of machine vision inspection.
Technical indicators of telecentric lenses
Due to its unique design principle, the telecentric lens has optical characteristics such as high resolution, large depth of field, and ultra-low distortion. When choosing a telecentric lens, you usually need to pay attention to technical indicators such as magnification, depth of field, telecentricity, distortion, and resolution.
- Magnification
The magnification of the telecentric lens, β=photoreceptor diameter/field of view diameter, the size of the photoreceptor are shown in Figure 5. When selecting a telecentric lens, the target surface of the sensor compatible with the telecentric lens is required to be greater than or equal to the target surface of the matching camera. Otherwise, it will cause a waste of resolution.
- Telecentricity
Telecentricity describes the angle at which the chief ray of a telecentric lens deviates from the optical axis. The smaller the angle, the better the telecentricity, the smaller the imaging magnification error, and the more accurate the measurement.
- Depth of field
Depth of field refers to the distance between the closest object point and the farthest object point where the lens can form a clear image. Due to the unique design principle of the telecentric lens, it has a much larger depth of field than ordinary industrial lenses.
- Distortion
Distortion is the general term for the inherent perspective distortion of optical lenses. Telecentric lenses have very small distortion. The distortion of the telecentric lens products of United Optics has achieved below 0.06%.
- Resolution
Resolution refers to the minimum distance between two points that can be distinguished by a telecentric lens or the number of line pairs that can distinguish black and white stripes within 1mm. It can also be expressed by the highest spatial frequency that the sensor can resolve. The resolution of a telecentric lens = 1/2 x pixel size.
For example, the sensor pixel size is 3um, and the resolution of the telecentric lens is 166lp/mm. According to the requirement of high resolution, the MTF of the lens needs to be greater than 0.3 at 166lp/mm.
When choosing a telecentric lens, in addition to the above main technical indicators, the interface type, lighting method, F-number, and other technical indicators of the telecentric lens also need to meet the requirements.
Telecentric lens selection
Because the telecentric lens has unique technical advantages, it is currently used in high-precision inspections such as mechanical parts measurement, plastic parts measurement, glass products and medical parts measurement, and electronic component measurement. The telecentric lens, the light source, and the camera together constitute an image acquisition system. Therefore, the selection of the telecentric lens is restricted by the requirements of the entire system. Comprehensive judgments must be made when selecting:
1. The field of view of the lens ≥ the size of the object to be measured; also consider the lens magnification and the size of the compatible sensor;
2. The depth of field of the lens ≥ the height of the surface undulation of the measured object;
3. The resolution, distortion, and telecentricity of the lens match the detection accuracy of the detection system;
4. The working distance and overall dimensions of the lens match the size and space of installation;
5. The interface of the lens matches the interface of the camera;
6. It is also necessary to consider the lighting method of the system. There are two types of non-coaxial and coaxial telecentric lenses to choose from. At the same time, the F-number of the lens and the size of the lighting interface must be considered.