QMT Features: June 2015
Specialist lenses for machine vision
New types of lenses can help solve challenging measurement problems says Mark Williams of Stemmer Imaging


All machine vision systems rely on the production of an image of sufficient quality to allow the required measurements to be made. Clearly the lens or lens combinations used in the vision system play a crucial role in determining the quality of the image produced as well as its magnification. This article looks at some novel lens types that have become available and others that help solve otherwise challenging measurement applications.
Polymer tuneable lenses


Polymer tuneable lenses (Figure 1) are an interesting recent development. These are shape-changing lenses, based on a combination of optical fluids and a polymer membrane, which basically copy the principle of the human eye. A circular ring pushes onto the centre of the membrane and the shape of the lens (and hence its focus) can be changed by moving the ring, or exerting a pressure on the outer part of the membrane, or by pumping liquid into or out of the membrane. A change in lens radius of several microns can have the same optical effect as moving the entire lens several centimetres. The liquid lenses do not replace conventional optics in a machine vision system. However, when being used with the appropriate optics they allow a much faster change of the focus than would be possible by moving the conventional lenses using mechanical actuators. Users therefore benefit from smaller, cheaper and faster optical systems. They are particular useful where objects of different height need to be measured. Lens response times of the order of just milliseconds means that they can accommodate high throughput inspections.

Telecentric imaging and illumination
The use of telecentric lenses is well-established in machine vision applications. Telecentric lenses collimate the light entering the lens across the whole field of view meaning that all objects in the image have the same magnification irrespective of their distance from the lens, removing perspective distortion and making it easier to make measurements. Applications involving the measurement and inspection of flat surfaces and their defects place greater demands on the illumination system. For such applications, which include recognition of silicon wafer patterns and inspection of LCD displays, polished metal surfaces, plastic and glass panels among others, telecentric lenses are available with integrated coaxial light sources to homogeneously illuminate uneven surfaces and detect small surface defects such as scratches or grooves. An integral built-in LED source can provide excellent illumination stability and homogeneity with a reduction in the back-reflections often associated with conventional coaxial illumination systems. They are especially suitable for use when imaging highly reflective flat surfaces with reflectance > 30%. For applications where objects need to be back illuminated, such as high accuracy measurement of round or cylindrical parts, there are high-performance telecentric illuminators specifically designed for use with telecentric lenses. These offer higher edge contrast when compared to diffused back light illuminators and therefore allow higher measurement accuracy.
360° view optics

Imaging of complex image shapes frequently requires the use of more than one camera. However some novel lens designs that go under an umbrella description of 360° view optics have been introduced to allow viewing of every surface of an object with as few cameras as possible. Here the top and side views of an object or the bottom and inside views of an object with a hole in it, or multiple side views of an object can be captured in a single image. Applications come from a variety of market areas, such as the beverage, pharmaceutical and automotive industries.

Cylindrical objects such as bottles, jars, batteries, bolts etc can be imaged using special pericentric lenses. The specific path of the light rays through the lens means that light from the part of the object closest to the lens is imaged in the centre of the camera sensor while light from further away is imaged further out on the detector. This means that a single image can show detail from the top and the sides of the object simultaneously. Common applications in the beverage and pharmaceutical industries include bottle neck thread inspection, and datamatrix code reading – the code will always be correctly imaged no matter which direction it is facing.
The smaller the object diameter, the larger the object height which can be inspected, while thin objects can be inspected over a larger diameter. For small objects (down to 7.5 mm diameter) catadioptric lenses are available. This lens design results in the sides of the object being inspected being observed over a wide viewing angle, approaching 45° at its maximum, which makes it possible to inspect complex object geometries under a convenient perspective.

A special range of hole inspection optics have been designed for viewing of objects containing holes, cavities and containers to allow imaging of both the bottom of a hole and its vertical walls. With a large viewing angle (>82°) and innovative optical design, these lenses are compatible with a wide range of object diameters and thicknesses. Hole inspection optics can be used to inspect a variety of different object shapes such as cylinders, cones, holes, bottles or threaded objects.

Another novel optical configuration is offered by polyview lenses. These provide eight different views of the side and top surfaces of an object.?The wide perspective angle (45°) enables the inspection of the side features of an object (for example the threads of a screw or a nut) which would otherwise be impossible to acquire with a single camera. Both the external walls of an object and its top can be imaged at the same time, while internal surfaces of holed objects can be completely inspected from the outside. A combined view of the internal and external surfaces is possible and an image displaying both the inner walls and the bottom of a cavity can be obtained.

Choosing the best lens for the application
This article highlights just some of the multitude of optical configurations available to provide inspection solutions using machine vision. Choosing the most appropriate lens and indeed all of the other components needed for a machine vision system such as illumination, cameras and frame grabbers, right up to the software requires detailed understanding of the application requirements and technology capabilities.

As Europe’s largest independent supplier of vision technology and services, Stemmer Imaging is constantly validating these and other new techniques so it can offer its customers vision technology that’s right for the job.
www.stemmer-imaging.co.uk
  
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Rob Tremain Photographer
www.4exposure.co.uk
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