Industrial lens distortion, also known as aberration, refers to the distortion where a straight line outside the principal axis of the subject plane becomes a curve after being imaged by an optical system. The imaging error of this optical system is called distortion. Lens distortion generally includes three types: pincushion distortion, barrel distortion, and linear distortion.
When using optical dimension measurement, any type of distortion will severely affect the measurement results. This typically manifests as a significant data deviation between the detected values in the center region and the edge region of the image.
To address the image distortion problem, we employed two solutions.
Software correction resolves lens distortion
The XAVIS R&D team has developed a distortion correction algorithm that can minimize measurement deviations caused by lens distortion in any area of the entire image through software correction. The measured values change very little when the product is placed in any area of the image.
We call this calibration method "multi-point calibration." The method involves first taking a visual photograph of a calibration board. The software can automatically analyze the coordinates of the center of the black dots on the calibration board. By simply inputting the true coordinates of two adjacent black dots, the entire image can be effectively calibrated. Because the black dots on the calibration board are distributed throughout the entire camera's field of view, the problem of discrepancies between the detection data in the image's center region and the detection data in the edge region is eliminated.
Distortion correction software
Hardware correction resolves lens distortion
Telecentric lenses and double telecentric lenses can correct the distortion problems of traditional industrial lenses. Within a certain object distance range, a telecentric lens ensures that the magnification of the acquired image does not change with the object distance, which is crucial for applications where the objects being measured are not on the same plane.
In general industrial lenses, the closer the target object is to the lens (the shorter the working distance), the larger the image formed. When using a general lens for dimensional measurement, the closer the target object is to the lens, the larger the image formed.
The following problems exist:
1. The difference in magnification occurs because the objects being measured are not on the same measuring plane;
2. Large lens distortion;
3. Parallax, also known as the magnification of an object, changes as the object distance increases.
4. The lens resolution is not high;
5. Uncertainty in the position of image edges due to the geometric characteristics of visual light sources.
Telecentric lenses offer excellent solutions to the aforementioned problems. Regardless of location or working distance, they provide the same magnification after refocusing because the maximum field of view of a telecentric lens is directly related to the proximity of the lens's aperture. The larger the lens size, the larger the required field of view. Therefore, they can be used in high-precision measurement and metrology.
Telecentric lenses offer superior image quality in measurement applications, exhibiting less distortion than traditional fixed-focus lenses. This optical design results in a more symmetrical image plane. Due to their unique optical structure, the image remains unaffected by changes in the distance between the object and the lens, and its distortion is generally less than 0.1%, with a distortion coefficient only 1/20th that of ordinary lenses. This significantly improves the accuracy and stability of inspections. Therefore, they have become the preferred accessory for high-precision industrial inspection and measurement.
