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Micro Video Lenses Grow Up

Photonics Spectra
Nov 2009
Oliver Barz, Edmund Optics

A few years ago, micro video lenses were used in machine vision almost exclusively for limited-space and low-resolution applications. But the ongoing reductions in size and cost of optical sensors, along with increases in pixel count, are allowing the creation of higher-resolution compact video systems. At the same time, new optical designs are yielding micro video lenses with increased resolution to match today’s sensors, rendering the micro video lens ready to tackle new applications.

The technological progress in semiconductors has resulted in increased resolution and lower cost for sensors. The following digitization of machine vision has moved many system designs from analog cameras and frame grabbers to vision sensors and intelligent cameras. While the electronics have changed a lot, the optics have remained basically the same. Most of the video lenses developed for machine vision use a C-mount and are designed for a sensor size of 1/2 to 2/3 in. covering an image circle of 8 or 11 mm, respectively. With high-resolution CCD and CMOS sensors now available in sizes around 1/3 in. or less, an opportunity has opened for optical manufacturers such as Edmund Optics to reduce the size of the lenses accordingly, creating “micro” video lenses.

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Micro video lenses provide all the performance of traditional C-mount lenses in a highly compact form.

The typical micro video lens is approximately 15 to 25 mm long and has an outer diameter of 14 to 18 mm (shown at top of page). The mount is usually an M12 × 0.5 thread, more commonly known as an S-mount. This compact size provides an advantage over traditional C-mount lens and camera systems not only for smaller assembly dimensions, but also for reduced costs.

Because micro video lenses are frequently used in large quantities, cost is an important concern. However, to maintain high quality, the lenses should be manufactured with an all-glass-and-metal design – no plastics. Cost reduction must come from elsewhere.

Naturally, the decreased lens dimensions result in lower glass costs. Micro video lens designs reduce costs further by abandoning expensive mechanical parts. For instance, micro video lenses can avoid use of a helical mount for focusing and instead use the mounting thread to adjust position. Another cost reduction design step is to make the aperture not of iris leaves but with an aperture spacer of the proper diameter located between the lens elements.

Lens selection

The cost reductions have allowed manufacturers to offer a variety of micro video lens design styles from stock to address various resolution and working distance requirements. For instance, basic infinite conjugate imaging lenses, with roots in security applications, are suitable for standard-resolution cameras with a working distance of 100 mm to infinity. Such lenses are available to cover focal lengths from 1.7 to 50 mm, enabling angular horizontal fields of view between 6.8° and 134°. For more demanding requirements, high-resolution infinite conjugate micro video lenses also are available for megapixel sensors.

For vision applications that require close-in operation, finite conjugate imaging lenses are available with a typical working distance of 150 to 250 mm. These lenses can be used also with aperture modification if the depth of field demand does not match the aperture at around f/2. The highest performing among these finite conjugate lenses cover sensor resolutions of up to 200 line pairs per millimeter and are used for a wide range of close-focus applications that require accurate images. Focal lengths between 5 and 25 mm are available, and the lenses act as a base for a variety of modifications, including aperture adjustments and filter changes.

If the working distance of an application is shorter than the listed working distance for a stock lens with the desired focal length, all is not lost. Developers have found that they can use a micro video lens at a working distance of only a few centimeters with little loss of performance.

Micro video lenses are also available in a sealed optical assembly for use in harsh environments beyond typical lab and factory floor installations. Two coating options make these lenses suitable for either visible or near-infrared imaging applications.

When choosing a micro video lens, a good point to start from is the sensor size and resolution. For a sensor with more than one megapixel, a high-resolution lens is best. If the sensor is bigger than the specified 1/3 in., a short focal length will result in some vignetting. However, some users have reported a good experience with using an 8-mm lens with up to a 1/2-in. sensor.

The next parameter to investigate is the (angular) field of view. In many vision systems, this is a more useful specification than focal length because it takes into account areas of the image that may be distorted, whereas focal length determinations deal only with the undistorted image center. Most of the images taken by vision sensors are never displayed, so distortion is not as important as it is for human-viewed images. Making a selection based on field of view, easily calculated from working distance, sensor size and object size, thus leads to a more suitable choice.

Creating custom lenses

If no stock lenses are available for a given application, the optical manufacturer may be able to help with a modification of a standard product or with a custom design. For instance, a sewer inspection system needed a small camera with a short working distance. To avoid the expensive option of refocusing a stock camera, the customer asked Edmund Optics to increase the lenses’ depth of focus. This was easily done by changing the aperture spacer to f/5. The method of increasing depth of focus by decreasing aperture size is used also for applications where the Z-position of objects might change. Checks of optical performance using design software have shown that versions down to f/16 are practical.

Another typical modification that developers require is the addition of a filter. This can be handled in two ways. One is to insert a small glass filter into the housing. The other is to coat one of the lens elements with a filter layer. Typical filter requests include infrared-cut or daylight-cut filters, which confine the camera to using the visible or near-infrared spectrum only. Including this filter in the lens system can result in cost savings, as no separate filter is needed inside the camera.

Barrel changes are another modification option. If a standard housing does not fit a given camera, the manufacturer can readily modify the barrel to incorporate a different threading. Such modifications can even become standard. This month at the Vision 2009 conference in Stuttgart, Germany, for instance, Edmund Optics and Baumer Optronic GmbH announced a new camera/lens interface standard without threading. Reduced mounting tolerances help enhance the lens’s optical performance, and integrated focusing capability will enable the lenses to be incorporated into preadjusted cameras. In the first use of this new interface standard, the lens mount of Baumer’s new GigE camera will mate to the new barrel design from Edmund Optics (Figure 1).

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Figure 1. A new GigE camera from Baumer utilizes a novel barrel designed together with Edmund Optics. Courtesy of Baumer Optronic GmbH.

For most modifications, however, the changes will be proprietary to the customer. The process begins with the manufacturer and customer first discussing the desired specifications. A typical custom design will consist of three to six lens elements and will depend on the target price, the optical performance (modulation transfer function, distortion, relative illumination) and mechanical constraints such as mounting style, sensor size and working distance. The manufacturer’s optical designers are often able to make helpful design suggestions to optimize cost and performance. They can also develop virtual performance tests to evaluate the design options.

Once the modifications are specified, prototypes must be made to prove the concept. The manufacturing series can then start with just 50 to 100 pieces. Most modifications can be very cost-effective, and the volume price of modified micro video lenses can be comparable to the standard product.

Some customers like to develop the micro video lens themselves to simulate the whole device. In that case, the manufacturer double-checks the design for optical quality and producibility and discusses options to optimize the cost and performance. For example, in recent years Leuze electronic GmbH has developed several designs for its products. Two of the latest are used in the company’s LSIS 412 vision sensor (Figure 2).

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Figure 2. Custom micro video lenses are used in the Leuze LSIS 412 vision sensor. Courtesy of Leuze electronic GmbH + Co. KG.

Applications abound

Custom or stock micro video lenses represent a new wave of opportunities for imaging system designs, allowing them to serve in applications where the classic camera/objective lens combination is too expensive and bulky. Typically, these new applications call for a highly compact configuration that uses a micro video lens together with a more-or-less specialized vision sensor. In many cases, the vision sensor even has the lens built in, as with Leuze’s LSIS 412. The result is an imaging system that can fit almost anywhere.

With a wide-angle (>90° horizontal) micro video lens, for example, it is possible to place a vision sensor in one corner of a rectangular space and monitor whether something passes through that space. This can be used to replace safety light curtains in workshops, with the system shutting down the machinery if the danger zone is entered.

The OpticScore electronic shot evaluation system from Knestel Elektronik GmbH uses two high-speed sensors with micro video lenses to capture images of a bullet in flight (Figure 3). Operating at 40,000 fps, the system can capture several images of the bullet as it passes through the light curtain. This yields an accurate and reliable measurement of shot value and position – an improvement over existing electronic scoring systems.

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Figure 3. This electronic shot-scoring system from Knestel Elektronik uses two compact imaging systems to capture images of a bullet in flight for accurate evaluation of score value. Courtesy of Knestel Elektronik GmbH.

The BMOS 5000 keyboard from Desko GmbH uses a custom micro video lens as part of a built-in multidocument reader (Figure 4). This keyboard is used in security and passenger service stations at airports and incorporates an optoelectronic reader module no larger than a matchbox. The module handles magnetic cards and provides optical reading of travel documents such as ID cards and passports that conform to the International Civil Aviation Organization.

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Figure 4. The Desko BMOS 5000 keyboard incorporates an optical document reader that a micro video lens helps keep smaller than a matchbox. Courtesy of Desko GmbH.

Other applications for micro video lenses are in produce-detecting scales in supermarkets, passport readers at customs stations, lottery scanners and automated contour measurement devices in manufacturing. Some 3-D measurement systems calculate height from the deformation of a laser line – a good opportunity to use a micro video lens to keep the housing compact. In nearly all these cases, the key to success is the availability of compact imaging solutions for large-quantity production at a favorable price.

With the advent of micro video lenses, these imaging systems are now available. Offering compact size, high resolution and low cost, micro video lenses are making imaging systems more affordable and easier to use in tight quarters. And, with the help of the lens manufacturers, even unique system requirements can be met without sacrificing cost or performance.

Meet the author

Oliver Barz is a member of the sales team at Edmund Optics in Karlsruhe, Germany; e-mail: obarz@edmundoptics.de.


GLOSSARY
depth of field
The distance, on either side of the object plane focused on, through which satisfactory image definition can be obtained. For the special case of an imaging system with lens axis perpindicular to the image plane, focused at a range of 25 ft, and with definition acceptable for objects from 20 to 40 ft; the depth of field extends from 5 feet in front of, to 15 feet behind, the object plane focused on.
resolution
1. In optics, the ability of a lens system to reproduce the points, lines and surfaces in an object as separate entities in the image. 2. The minimum adjustment increment effectively achievable by a positioning mechanism. 3. In image processing, the accuracy with which brightness, spatial parameters and frame rate are divided into discrete levels.
working distance
In microscopy, the clear distance between the specimen being viewed and the first optical element of the objective lens.
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