Six tips to teach you how to select telecentric lenses

The matching selection principle of telecentric lenses and cameras is the same as that of ordinary industrial lenses, as long as the specifications of the target surface are larger than or equal to the target surface of the camera. Please note during use that the area vertically below the objective lens of the telecentric lens is telecentric imaging, and the area beyond this range is not telecentric imaging in the strict sense. This must be noted in actual use. Be careful, otherwise unnecessary bias will occur.

When customers choose a telecentric lens, they should first understand when to choose a telecentric lens. According to the principle characteristics and unique advantages of the telecentric lens, it is best to use a telecentric lens when the inspection object encounters the following 6 situations:

1) When it is necessary to detect objects with thickness (thickness>1/10 FOV diameter);

2) When it is necessary to detect objects that are not on the same plane;

3) When it is not clear what the distance between the object and the lens is;

4) When it is necessary to detect apertured and three-dimensional objects;

5) When low distortion is required and the brightness of the image effect is almost exactly the same;

6) When defects can only be detected under parallel illumination in the same direction.

When choosing a telecentric lens, you should first understand the usage conditions corresponding to the relevant indicators of the telecentric lens:

1) Object size------shooting range.

2) Image square size------the size of the target surface of the CCD used.

3) Working distance------The distance between the front surface of the object lens and the subject.

4) Resolution---------The CCD pixel size used.

5) Depth of field------------The range within which the lens can form a clear image. The greater the image/object magnification, the smaller the depth of field.

6) Interface------------Camera interface, mostly C, T and other interfaces.

According to the usage situation (object size and required resolution), select an object lens and CCD or CMOS camera with appropriate object size. At the same time, get the image size, you can calculate the magnification, and then select the appropriate image lens according to the product list . During the selection process, you should also pay attention to the influence of the depth of field index, because the greater the image/object magnification, the smaller the depth of field. In order to obtain a suitable depth of field, you may need to reselect the lens.

The most important things to consider when selecting a telecentric lens are the following three points:

1. Target surface matching

There is a very important parameter in industrial cameras: chip specifications (chip physical size), generally 1/3”, 1/2”, 1/2.3”, 1/2.5”, 2/3”, 1”, 3 /4", 35mm full frame, 4K, 12K, etc. These are the diagonal size specifications of the camera chip, which defines the diameter of the circumscribed circle of the camera chip (rectangle). We often have to do this when choosing industrial lenses One-to-one correspondence means that the imaging size of the industrial lens should be greater than or equal to the chip size of the camera. Generally, it is best when the two are equal. If the size of the lens is larger than that of the camera, it can also be used, but it will cause a certain amount of cost waste. . If the size of the lens is smaller than that of the camera, the future imaging will produce a black corner phenomenon. If the deviation is large, the image will become a circle. This will not ensure the field of view of the lens and will affect the imaging resolution of the camera. cause losses.

2. Interface matching

After the imaging target surface is matched, the interface between the lens and the camera must also be considered. Because cameras from different manufacturers and different target surfaces will have different interfaces. For example, 2K, 4K, 6K, 8K, 12K, etc. in line scan cameras will have different interface specifications such as F-mount, C-mount, M58*0.72, M72*0.75, etc. These interface specifications have different threads or bayonet outer diameters. , the distance from the flange end face to the chip will also be different. Therefore, when selecting a model, you must consider the corresponding interface issues. The interfaces of the camera and lens must be consistent, otherwise they cannot be installed. Like the bi-telecentric lenses provided by LANO, each lens will correspond to different interfaces according to the customer's specific practicalities to ensure reasonable installation and use.

3. Precision matching

General vision projects will have accuracy requirements. This accuracy is generally the comprehensive accuracy of hardware and software. When we select cameras and lenses in the early stage, we usually focus on the accuracy that the hardware can achieve. Generally, we define the basic accuracy of the hardware in terms of the field of view size corresponding to the unit pixel. The calculation method is: side length of camera pixel/magnification of lens = hardware accuracy

All lenses of telecentric optics have magnification indicators in the product parameter column, which can be directly used for accuracy calculations. If you choose a fixed-focus industrial lens, you must first calculate the magnification when the lens is working, and then calculate the accuracy. Because fixed-focus lenses have different magnifications at different working distances, our formula for calculating fixed-focus lenses is: focal length f/object distance WD = magnification. When doing high-precision vision projects, the accuracy requirements are often too high and cannot be guaranteed by hardware alone. At this time, the software algorithm needs to be corrected and improved through sub-pixel and other calculation methods.