About the Book

Machine vision lenses

The choice of the appropriate lens for a machine vision application depends on the sensor used. The lens must be able to illuminate the entire sensor surface to avoid shading and vignetting. The lens must also be able to resolve the pixel size of the camera sensor used. The ratio of image to object size describes the magnification of a lens and is directly related to both the focal length of the lens and the working distance (distance of the object from the lens). As mentioned earlier, fixed focal length lenses are used for the vast majority of industrial applications where the object size and working distance remain fixed. In order to ensure accurate capture of object details and their contours, these details must be reproduced on approximately 4 pixels of the sensor. Thus, the magnification required depends on the resolution of the object and the size of the sensor pixels.

For fixed focal length lenses, the available adjustments are usually only the focus and/or aperture size (iris). Reducing the aperture size can minimize some aberrations because it reduces the amount of incoming light and thus increases the depth of field. As the depth of field increases, the image will be sharper over a wider range of object distances.

However, with a fixed focal length and a fixed position of the lens relative to the sensor, this range of distances produces perspective effects where objects of the same size have different magnifications depending on their distance from the lens. This can significantly compromise the accuracy of the measurement. The size of the aperture is specified by the f-number. But care must be taken here: the diffraction effects described above limit the sharpness of the image when the aperture is smaller than F8.

Macro lenses

Macro lenses are specially designed for small fields of view, almost the same size as the camera sensor. They are classified according to their reproduction rate in relation to the size of the sensor and are optimized for close focusing. Thanks to their very good MTF properties and negligible distortion, they can be adapted to many vision applications. Their only shortcoming would be a lack of flexibility, however, as it is not possible to change their magnification ratio or working distance. Some standard or high resolution lenses can be used as macro lenses by adding special inverted rings.

The most common magnification for these objectives is between 0.5x and 2x; higher magnification is already in the range of microscopic vision.

Telecentric vision and lighting

Telecentric lenses are generally used in measurement applications where perspective distortions and incorrect reproduction scale can cause problems. They are particularly suited for 3D image capture where scaling can often be misinterpreted. Telecentric lenses collimate the light entering the lens over the entire field of view (image left). Thus, all objects have the same magnification in the image regardless of their distance from the lens, which eliminates perspective distortion and makes measurements easier.

Image (a) shows an electrical assembly to be checked for defects. One of the pins is bent and the vision system must locate this defect. The use of a standard endocentric lens distorts the perspective of the image, which complicates considerably the search for defects (image b): the pins seem to "spread out" from the central axis and go to the sides. In this image, the defective pin does not differ much from the others, representing a major challenge for the image processing software.

Image (c) shows an image of the same component produced with a telecentric lens: all the pins now appear perpendicular to the lens, without perspective distortion, except for the bent pin. The defective component is thus clearly visible on the image, making the recognition work much easier.

Another example is the precise control of the thickness of drawn wires as they exit the mold. The challenge here is that as the wire exits the mold, it can vibrate and change position, complicating measurements if a universal lens is used (image 4). Indeed, the distance between the wire and the lens is constantly changing, so with a standard lens, the thickness of the wire will appear to vary accordingly (images 5a and 5b).

A telecentric lens will be able to provide an accurate image with uniform magnification and focus on the wire regardless of its position (image 5c).

Because of the parallel beam path, the aperture of the telecentric lens must necessarily be the same size as the field of view. Thus, lenses for large fields of view are large and can be very expensive.

For particularly demanding measurement applications, there are double-sided telecentric lenses. They allow accurate measurements even when the image is blurred. They increase the depth of field and provide low distortion.

Telecentric applications for the measurement and inspection of flat surfaces and their defects place high demands on the illumination system. For applications such as pattern detection of silicon wafers and inspection of LCD screens, polished metal surfaces, plastic or glass panes, telecentric lenses with integrated coaxial light sources allow even illumination of uneven surfaces and detection of small surface defects such as scratches or grooves.

An integrated LED source provides excellent light stability and homogeneity, and reduces counter-reflection effects, common in conventional coaxial lighting systems. This type of lighting is particularly well suited to the inspection of highly reflective flat surfaces with a reflection rate of over 30%.

For applications where objects require backlighting (e.g. precision measurements of round or cylindrical parts), powerful telecentric illumination systems have been specially developed to work with telecentric lenses. Compared to diffused transmitted light illumination, they offer a higher contrast at the edges of the image and thus allow for greater measuring accuracy. Even transparent objects get a clear and sharp outline.

What is the right lens?

With so many criteria to consider when choosing a machine vision lens, the help of a specialist is not superfluous. He or she will be able to match the requirements of the application with the technological capabilities of the different types of lenses. This know-how is one of the strengths of DZOptics. The company holds a place among the leaders in the machine vision market, with an extremely broad product portfolio, expertise, customer service and technical support of the highest order.

About the Author

kaki