The choice of a vision system lens is very important

The vision system expands the capabilities of automatic control of technological processes, allowing you to determine the size, position and orientation of assemblies on the assembly line. However, in order to realize the full potential of such systems, the camera lens used is of great importance.

The role of vision systems in process control continues to grow, especially in the fields of robotics, object recognition and quality control. Modern vision systems have gone beyond the standard process of recognizing a single element in a group of objects and determining its location and now help to perform more complex operations, such as moving objects.

For robotic systems used to assemble or control a large number of items, for example in the automotive industry and on monitoring lines, the conveyor belt is the reference point. On it, robots perform two functions: identification and movement.

For most vision systems, lighting is one of the most important characteristics. The system must provide high data repeatability, therefore, vibration must be minimized to obtain a clear image.

In large batch inspection lines, for example, in pharmacology, the vision system must identify damaged packaging, unreadable labels and lack of medication, quickly recognize and measure square, round and rectangular objects with high accuracy. Maintaining a consistent appearance and color of packaging can improve system accuracy. In food control systems, size, color, density and shape are determined by multi-element examination of the item. Multi-element vision systems can have color and monochrome cameras and use structural lighting to determine the profile and internal composition of the product.

While the camera, imaging software, and lighting are critical parts of a vision system, perhaps the most critical component is the imaging lens. In order for the system to realize all its capabilities, the optical lens must correspond to the task at hand. When choosing a lens, vision integrators must consider four critical factors:

• The type of the observed object and its properties;
• Depth of field or focus
• Location of the system and monitored objects;
• Operating conditions.

By analyzing these four characteristics, you can select the ideal lens for your application.

Properties of the observed object

Before choosing a lens for a vision system, the system integrator must know which object will be observed and what it will be surrounded by. This visible area means the field of view of the optical system, measured both diagonally and horizontally. Typically, the ratio between the horizontal and vertical dimensions of the optical system's field of view is 4: 3. This ratio depends on the area of ​​the active part of the camera matrix. The size of the matrix is ​​very important in determining the value of the magnification of the objective, on which the area of ​​the field of view of the optical system depends. The magnification of the sensor is equal to the ratio of the matrix size to the field of the optical system and represents the "work" performed by the lens, and this value must be taken into account when choosing it.

Lens magnification is of great importance when attaching a lens to cameras with different sensor sizes; however, lens magnification should not be confused with microscope magnification, which is determined by the length of the optical tube and the focal length of the lens, which does not take into account the size of the camera sensor.

Lens magnification - the ratio of the image obtained by the matrix to the actual size of the observed object.

OOS is the product of the lens magnification by the ratio of the monitor size to the matrix size. This is the total magnification, from the subject to the image on the monitor, the "work" performed by the entire system. The magnification of the optical system is important. Because it is necessary to evaluate the image of the observed object on the monitor

Item stats are also very important. The ability of a lens to recognize individual details of an object depends on the contrast of those details. One way to determine the resolution of a system is to use a Ronchi grating on a target during US Air Force testing. This lattice has one pair of lines - white and black of the same width.

The ability of a lens to distinguish between a pair of lines or dots of a certain width in a given lighting condition characterizes the lens's resolution. Resolution is often represented graphically by a modulation transfer function (MTF).

The graph displays the relative contrast at different frequencies of the line pair. Curvature, chromatic aberration, and other wavefront distortions result in a greater slope of the graph than in the ideal situation. Lens specifications sometimes indicate resolution in pairs of lines per millimeter (l / mm). Dividing this value by 1000 will give you an approximate micron resolution.

To determine the topography of a surface, several cameras and lenses are usually used. And it is necessary to know the degree of image distortion by the lens. Distortion (aberration) - an optical error due to the lens, causing deterioration in image quality at various points. Topographic applications often use laser beams or other light source to ensure accurate measurements. Some imaging software can help compensate for a range of lens distortions.

The large format and field of view of the optical lens is excellent for use in control systems. That's due to its high resolution, low distortion and limited chromatic aberration. The wide field of view of the optical system and compatibility with large format sensors make these lenses suitable for inspecting tissues, LCD monitors, food and beverages.