TELECENTRIC OPTICS: INCREASING THE ACCURACY OF OPTICAL MEASUREMENTS

machine visions

The growth of processing power of processors, the development of software for image processing, as well as an increase in the resolution and sensitivity of digital cameras contribute to the widespread use of machine vision systems for industrial and research applications, and often they require not only visual control of the presence of certain objects, but also measurements. their sizes. This article is devoted to how telecentric optics can help in solving this problem.

In some cases, the object of control of machine vision systems has a rather complex volumetric shape, which, due to the well-known optical laws, diffraction and interference, can distort the resulting image. To a large extent, such distortions can be minimized using telecentric optics and properly selected lighting. Let’s try to understand what telecentric optics is and where it is used, using the example of the line of telecentric lenses and illuminators of the Italian company Opto Engineering.

TELECENTRIC LENSES

There are two main sources of distortion of images of volumetric objects: edge effects of refraction on the object under study and distortion and refraction in the optical system. In both cases, the boundaries are “blurred” and the shape of the object is distorted due to the multidirectionality of the light beams. And if the defects arising in the lens and usually called distortion can be reduced by more careful processing of the lenses and compensated by a mathematical apparatus, then edge effects on the object can be minimized only by organizing the parallel path of light rays in the control zone. It is for this purpose that telecentric lenses are used, which ensures the parallelism of the light beam to the optical axis.

In addition to minimizing optical distortion, the use of telecentric optics provides the following advantages:
• no perspective distortion;
• increasing the depth of field;
• minimization of “extraneous” illumination.

DESIGN FOR PRECISION

Unlike conventional lenses, telecentric lenses rarely have a diaphragm and usually lack focusing capabilities, since focusing and moving lenses introduce additional distortions that are difficult to account for. Therefore, the relative position of telecentric optics and the controlled object is clearly defined in advance. The most accurate measurements are taken at the center of the telecentric zone. For the same reasons, the repeatability of measurements is increased and the use of an adjustable aperture is limited. The depth of field and resolution of the lens depend on the aperture size (diffraction limit), and the shape of the adjustable aperture (blades) is usually far from circular, which also serves as an additional source of distortion. Therefore, telecentric optics usually have a fixed aperture value.

Due to the physical principles used, that is, the parallelism of the beams, the size of the input lens for telecentric optics is always larger than the observed object. For example, OptoEngineering TC 23 036, a popular lens for cameras with a 2/3 ”sensor, for a 34.7 mm × 29 mm (∅36 mm) control zone, has a body diameter of ∅61 mm with a length of almost 165 mm. For monitoring larger objects, the size of the entrance lens can be tens of centimeters or even several meters. Obviously, the manufacture of such optics requires adherence to very high production quality standards.

COLLIMATED LIGHTING

The maximum effect can be achieved when using telecentric optics both for obtaining an image on the camera and for organizing a parallel, collimated beam of light. An example of a telecentric pair for controlling the geometry of OptoEngineering TCBench objects is shown in Fig. 3. An additional advantage of the ready-made assembly is that there is no need to ensure the alignment of the light source and the lens, as well as an optional convenient kit for calibrating the optical pair.

CORE COMPACT TELECENTRIC OPTICS AND LIGHTING SERIES

A breakthrough in minimizing lens size is the OptoEngineering CORE compact telecentric lens series. Thanks to special solutions, the Italian company has managed to more than halve the size of its lenses. The rather large dimensions of traditional telecentric optics did not allow to place it on a robot arm. But at the exhibition held in May this year, cameras with a small pixel size of up to 1.7 microns to capture the image of the smallest details.

EXPOSURE DURATION

In the course of industrial production, the control object is often in motion, and this introduces its own error in the measurement accuracy. When the subject moves even at a relatively low speed of 0.2 m per second when shooting with an exposure of 0.01 s, the “blur” of the image in the direction of movement will be 2 mm. To obtain a more contrasting image, it is necessary to minimize the exposure time, while, preferably, providing sufficient illumination. Fortunately, advances in LED light sources are opening up a wide range of possibilities. For example, they are embodied in the OptoEngineering series of LED illuminators. LTCL collimated light sources (based on telecentric lenses) have the ability to set the pulse duration in a wide range (from 5 μs), which makes it possible to provide the luminous flux not only in the desired direction. Check for more here.

APPLICATIONS

Telecentric optics are indispensable for measuring geometry or shape control, especially when it comes to volumetric objects that are difficult to study using conventional optics (for example, bolts, nuts, rubber seals and gaskets, plastic covers and containers, electronic components, various parts automobile and other engines, transmissions, holes, studs ranging in size from fractions of millimeters to tens of centimeters). In addition, a large depth of field and the absence of perspective distortions make it possible to increase the reliability of control of printed circuit boards and electronics when the controlled surface has a relief.

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By Cary Grant

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