In the industrial production, spring has the important application. The quality of the spring is usually related to the precision and stability of the industrial system. Thus the importance of the spring inspections is self-evident. Nowadays, there are more and more kinds of springs, the market demand is getting larger and the requirement of the precision is much higher. Thus primitive artificial inspecting rate and precision could not reach the requirement of the contemporary industrial production. Modern inspecting methods arose at the historic moment . Visual inspection is widely used in inspecting the spring as the most common and effective method belonging to the non-contact measurement. However, traditional optical inspections still have some problems remaining to be settled. These problems are as follows:

1.The edge is unstable while inspecting the size or angle, which leads to the low precision.

Traditional visual inspections use high-resolution CCD camera and industrial camera lens, the principal of the industrial camera lens imaging is as follows:

This picture is cited as the theoretical model, but a little bit depth of field error is permitted when the industrial camera imagines. When the position of S’ is moved forward or backward, the camera can still get a clear image. But when the position of the inspected object S changes, the height of the image that S’ imagines will change too. It means magnification will change along with the working distance changing. In this way, in the process of actual imagination, different degrees of changes would happen in the imagining proportion, the edge is unstable, the results after the standardization would change, which lead to the low precision in inspecting the angle and the size.

Correspondingly, we can cope with these problems using the BT series of Double Telecentric Lens. The principal of the BT series of Double Telecentric Lens is as follows:

the BT series of Double Telecentric Lens transform lights which go in and out into paralleled lights and filtrate the rest diffuse reflected lights by setting diaphragm between filters. In this way, whether the object distance changes or not(of course we should get a clear image), the magnification will not change. This feature can solve not only the stability of the edge of the inspected object, but also the system precision problems caused by uncontrollable shake. It satisfies many online inspecting requirements which require high precision.

2.There is reflect light in the corner while inspecting continuity which leads to the error.

The image processing judges whether there is edges or not via the changes of the grey level. Because the special structure of spring itself, whatever we adopt fall shot light source or backlight, images consistent with the grey level of the inspected object are always not available, especially in the corner where lights are reflected, we often get grey levels contrary to other areas , which leads to the final results to show that there is a fracture in the corner. These results will result in the direct failure of the inspecting works.

However, when we use the paralleled light source and Double Telecentric Lens, the continuity of images is rather good. The defects are clear at a glance and effects are rather good while conducting inspecting sizes. The picture below is the profile image of the spring when we use the paralleled light source and Double Telecentric Lens. The grey level is fairly obvious and easy enough to deal with. We can hardly make a inspecting mistake.

3.The depth of field cannot cover the whole spring.

The depth of field of ordinary industrial camera lens is comparatively smaller, while the spring has certain height. It is common that depth of field cannot cover the whole spring, thus we cannot get a clear image. In this case, measurements and inspections cannot be conducted. Optical inspections cannot cope with this case in this field.

But when we use Double Telecentric Lens, its depth of field can be several times or even several tens of times of the ordinary camera lens due to its paralleled light source. We may solve this problem absolutely.

Among present domestic manufacture factories, we sincerely recommend the products of the Telecentric Optical System Company. Especially in the spring inspecting applications, after testing for many times and long-time applications, the quality of the products is rather reliable and of high stability that satisfy the industrial requirements of inspections completely.

Author：BTOS Editor