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Telecentric lenses aid machine vision accuracy

23 Nov 2007

Finding the right set of lenses for your machine vision system can be a daunting task. Wiebke Marzahn of Sill Optics addresses some common concerns by breaking the puzzle down into a number of key questions to ask yourself before starting your search.

Despite machine vision being one of the fastest-growing fields in optics, users often struggle when choosing the right components for their system. Electronic parts, cameras and software are expensive items, and the optic, which is responsible for the final visual reproduction of the object, cannot be neglected simply due to a lack of expertise. In this article, we hope to dispel some of the typical concerns that users have when selecting optics for machine vision.

Where do I start?
If you are looking for a quick solution, a good option is to contact one of the known suppliers of complete machine vision systems. You can find good package deals on the market for standard applications such as sorting, inspecting and measuring. However, if you have special requirements, this approach will soon reach its limits.

The next option is to study the large variety of optical components that are available in catalogues. If you do not find a suitable solution, your only option is to contact a supplier that offers specially manufactured solutions as well as new designs. Do not be deterred by this option. Low volumes are usually available at affordable prices and it is worth achieving your ideal result with specifications suited exactly to your needs instead of making costly and unsatisfactory compromises.

Endocentric or telecentric lenses?
Before approaching a supplier, ask yourself what result you would like your system to achieve. Is the task to gather measurements and evaluate them using a computer, or a simple observation task conducted via a monitor? In addition, you need to know if you require a standard charge-coupled device (CCD) lens (known as an endocentric lens) or a telecentric lens.

An endocentric lens reproduces the object perspectively, in the same way as a standard photographic lens, whereas a tele-centric lens dissolves the perspective. As telecentric lenses produce a parallel path of rays, it is possible to measure, for example, the diameter of the object independent of its location.

The object that you are studying will dictate the type of lens that is required. If you require a simple overview of an object, then a standard lens will most likely be sufficient. This is also the case for simple measuring and monitoring tasks. However, if you want to measure drill holes exactly, or if the parts to be measured are on different levels, it is better to use a telecentric lens. The same is true if your measurement requires a high level of accuracy.

The distortion of a telecentric lens is far lower than that of a standard CCD lens. In addition, the telecentric range is usually greater than the depth of focus, so the position of the object is not critical. The object can be evaluated despite slight blurring.

It is vital that you choose a telecentric lens that can illuminate your camera's sensor. Most lenses are suitable for use with smaller sensors such as 1/4- or 1/3-inch formats. Today, many suppliers produce lenses for larger sensors, but in some cases it may be necessary to have a new optic developed.

Telecentric options
Telecentric lenses are available for nearly all reproduction scales between 0.04 and 20x. It is important to note that the characteristics of the optics will vary greatly depending on the magnification. A lens with a reproduction scale of 0.04x will have a large external diameter, a long working distance and is usually only available for smaller sensors. A magnifying lens on the other hand can have a very small external diameter, a relatively short working distance and will be compatible with larger sensors. The external diameter is determined by the object size.

Standard telecentric lenses are said to be object-side telecentric. This means that the parallel beams originating from the object are reproduced on the sensor. Image-side telecentric lenses are also available and work like standard CCD lenses. These lenses have an aperture angle on the object side and reproduce in a telecentric manner on the sensor side. This is advantageous when using larger sensors or when the sensor is equipped with a microlens array. Image-sided telecentricity also helps to prevent non-homogeneous illumination of the sensor.

The last category is double-sided telecentricity, which combines the advantages of both systems described above. There is virtually no distortion with double-sided telecentricity. If the highest possible measuring accuracy is your goal, then a double-sided telecentric lens is the best option.

Illumination for machine vision
When selecting your lens, it is important to consider your illumination source. Standard ring or area lights can be used with telecentric lenses, but if you want to exploit the characteristics of the lens fully, telecentric illumination should be used, especially if high accuracy is important.

Telecentric illumination is available in two variations: as transmitted light illumination with a telecentric condenser, and as surface illumination integrated into the lens. In the second case, the reproduction optic is used as the illumination optic.

When you are imaging non-transparent objects, you illuminate the object from the camera side with surface illumination. If you illuminate with ring lights, area lights or fibre bundles, shadows can appear on the object and complicate the evaluation. In comparison, telecentric surface illumination (which is integrated into the lens) results in uniform illumination of the object. Polarization effects can be used to continuously adjust the contrast between diffusely and directly reflected light with a quarter-wave plate.

Transmitted light illumination is more suitable for measuring an outline, monitoring semi-transparent objects and objects with drill holes. So-called condensers are equipped with LEDs and can be obtained as telecentric illumination or partial diffuse illumination. When using the latter, the alignment effort is reduced as these do not have to be placed exactly on the optical axis of the object. Standard light tables are sufficient to achieve diffuse background illumination. However, since unwanted reflections may occur on the edges of the object that can lead to an inaccurate evaluation, telecentric illumination is an alternative to achieve maximum accuracy.

Critical factors to consider
Once you have decided to use a telecentric lens, there are several other key quantities that you should have to hand before approaching any supplier. These are the size of your image sensor, the size of your object field (including some tolerance for adjusting) and the working distance that you require. The camera connection as well as the flange distance should also be known.

The maximum overall length and maximum diameter of the optic should be clear if the lens is to be incorporated into a tight space or a machine. We recommend that you consider gathering vibration information to help decide whether the lens casing needs to be glued or secured. If you plan to translate or scan the lens, you should determine the maximum lens weight that your system can cope with. Finally, you must know the required resolution if a critical measuring task is concerned. For this, however, it is sufficient to know the pixel size of your sensor and the size of the structures to be measured.

Outlook
The object fields that are being tested with telecentric lenses are continually increasing. As the front lens has to be at least as big as the object, telecentric lenses are in turn getting larger, a trend that has become more noticeable over the last few years.

The steady increase in the size of image sensors is also pushing the size of the lenses to even higher levels to ensure that the entire sensor is illuminated. The numerical aperture of the lens is also steadily increasing. This is important as pixels get smaller and the camera's resolution improves. However, it does not make sense to simply increase the resolution of the camera, since the image will still be blurred if the lens does not offer the appropriate resolution.

• This article originally appeared in the November 2007 issue of Optics & Laser Europe magazine.

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