Machine vision lens selection

Machine vision adds a new dimension to industrial control systems that provide the size, position and orientation of parts on the assembly line. And the right lens choice is very important for machine vision to play its due role.
Optical control is very important in most machine vision applications. Robot vision systems also require extremely high repeatability, so reducing jitter provides a clear image.
On large-scale unit inspection lines like pharmaceutical factories, vision systems must be able to identify defective packages, unreadable labels, and missing products. The vision system must be able to quickly identify and measure square, circular and elliptical objects with extremely high accuracy. Improving the precision of the machine vision system can help maintain a uniform package surface and color.

Although cameras, analysis software, and lighting are important to machine vision systems, perhaps the most critical component is the industrial camera lens. If the system wants to fully utilize its functions, the lens must be able to meet the requirements. When choosing a lens for a control system, machine vision integrators should consider four main factors:

â–  Can detect object categories and characteristics;
â–  depth of field or focal length;
â–  Loading and detecting distances;
â–  Operating environment.

Physical characteristics

The ability of the lens to resolve an object's features depends on whether the contrast of the features is strong. A method of determining the resolution of the system, or the minimum and more analytical features of the object, may use a resolution method such as the Ronchi reticle method. The ability of the lens to recognize the line width or pitch of a particular width under specified lighting conditions determines its resolution. The resolution is usually displayed graphically by the module conversion function (MTF).

Distance constraint

The so-called working distance refers to the distance between the object and the front end of the camera lens when the image is in the focal length range. It limits the space required for the vision system and the devices that work with the vision system. In the limit range, the working distance can be changed by refocusing the lens. The infinite conjugate lens has a focusing distance from the minimum working distance to infinity, and the finite conjugate lens has a specific working distance range.

Depth of field

Deep effect (DOF) is the blur caused by the movement of an object. The DOF is the largest object depth in the focal length range. It is also the amount of movement allowed by the object to maintain the ideal focus (moving from the best focal length back and forth). When the object is placed closer to or farther than the working distance, it is out of focus, so the resolution and contrast will be adversely affected. For this reason, the DOF matches the specified resolution and contrast. When the depth of field is constant, the DOF can be made larger by reducing the lens aperture (that is, increasing the F/# value), and also requires light enhancement.

The importance of the environment

Environmental factors in machine vision systems include object reflection coefficients, light, temperature, vibration, and contaminants. Reflections from objects can cause highlights and can also blur features. In high temperature environments, problems may occur due to thermal expansion of the optics in the lens. Not all lenses can adapt to temperature changes. When detecting hot objects, it is best to use a lens with a long working distance.