Flexible Imaging Lens Designs
Through clever engineering and adaptable manufacturing, vision system lenses can be quickly and cost-effectively modified for optimum performance.
The needs of the imaging world are continuously evolving, requiring lenses to be less expensive, higher resolution and more versatile than ever. As a result, lens manufacturers and designers must develop more adaptable, more efficient and more economical lenses.
Before a lens is produced or designed, the versatility and manufacturability of the lens must be considered. Clever features and proprietary techniques can transform a single-use design into a multipurpose one. Such adaptability increases volume production, lowers costs for end-users, and makes projects more economically viable when considering long-term forecasts.
On the other hand, it can also mean a lengthy and complex design process. With more variables, more can go wrong in the initial prototyping phase.
Ultimately, the end-result is worth the extra design work up front: production lenses that are advanced pieces of technology, superior quality and more suited to the ever-changing needs of machine vision integrators, OEMs and laboratories.
For instance, when designing a lens, it is critical to first define its overall scope. Should the new high-resolution, fixed-focal-length lens design maximize a 0.67-inch, 5-megapixel sensor with 3.45-micron pixels? If the answer is yes, this quickly leads to additional questions. What working distance is needed? What magnification is needed? Is performance at the edge of the field equally as critical as performance at the center?
Many lens manufacturers have produced 5-megapixel lenses based on the first question without considering the importance of the second set of questions. Many 5-megapixel lenses will maximize the sensor, but only in the center of the image, and only over a small working distance range that cannot be changed, greatly reducing the usability and predictability of the lens.
Lenses with true 5-megapixel performance across the entire image field and over a broad range of working distance options are available. These lenses have several advantages, including the ability to resolve high resolution with strong contrast at any point of the image, and the ability to achieve that anywhere from close up to infinity.
A clever design makes this possible. The design may add a small amount of manufacturing complexity, but it increases the useable range of high-resolution lenses. By substituting subtly different front or rear optical elements into these multielement designs, the lens manufacturer can shift the optimized region of the lens. Instead of producing a lens that has been optimized at one working distance and provides inferior performance everywhere else, this design technique yields lenses with multiple optimized conjugates (working distance ranges) for the customer to choose from.
Typically, two or three versions will cover the full range of options, from 200 millimeters to infinity, allowing engineers to select the most suitable lens for their application.
Sometimes, all that is necessary to increase the functionality of a lens is to modify the metal that surrounds it. For instance, a compact micro video lens must forego many of the features in a traditional fixed focal length lens, such as a focus ring, an adjustable iris and filter threading. However, this does not mean that many of the same benefits cannot be achieved with a bit of effort from the lens manufacturer.
Focusability is the most straightforward feature to achieve. It is accomplished simply by putting an extended amount of threading onto the lens. This allows the operator to screw the lens in and out to achieve the best focus. The focus can be fixed in place with locking nuts.
An iris, although not cost-feasible in most micro video lenses, allows the operator to vary the f-stop of the lens, resulting in increased control over the balance between resolution, light collection and depth of field. To mimic this feature, the aperture at the stop of the lens can be mechanically designed so that different sizes can be substituted by the lens manufacturer. This allows an 8-millimeter lens with a minimum aperture of f/2.5 to be quickly changed to f/4, f/8 or whatever is needed for an application, without having to redesign the lens.
Such adaptability provides options and significant value to customers. A good lens manufacturer or system integrator can work directly with customers to determine the specific f-stop requirements of their systems, and can quickly deliver an off-the-shelf lens or a slightly modified custom lens, tailor-made to the customer’s needs.
The most complex feat is trying to gain some of the benefits of filter threading, particularly the ability to mount an infrared cut-off filter. Within a micro video lens, this is predominantly achieved using one of two methods.
The first method involves leaving all of the metal components the same and coating one of the lens surfaces with an infrared cut-off coating. The downside to this route is that filter coatings can be highly angle-dependent, and putting one on the surface of a lens, particularly one that still requires high visible transmission, can be costly and problematic.
The second option is to insert a thin, flat infrared cut-off filter into the lens path, allowing for higher levels of transmission and light control. However, this cannot always be accommodated with the existing metal design. A new housing may then be needed. Both methods are used by lens manufacturers, showing that a simple lens design does not have to decrease flexibility.
The last example of a clever optical design is the most complex, but also the most rewarding to customers. The lens can be designed so that the spacing between optical elements can be changed, providing a magnification range of 0.33X to 1X. In addition, by allowing the entire assembly to be flipped, or rotated, 180 degrees, the lens can provide a magnification range of 1X to 3X. Thus, with a minimal number of mechanical changes, a single lens design can be adapted to many magnifications, while maintaining high-resolution and fast f-stops.
This allows rapid customization of magnification for any standard magnifications offered. For example, if the standard lens has a 2X option, it can be customized into a 2.4X part with just a few internal part changes at the factory during assembly.
Art and Science
Lens design is as much an art as it is a science; it requires vast technical know-how as well as creative imagination and the courage to try something unconventional.
Designing lenses today is more difficult than ever, thanks to increasing camera sensor resolutions and decreasing pixel sizes. Through clever engineering, adaptable manufacturing and forethought, superior lenses can be produced. Whether the end result is an off-the-shelf solution that fully meets the customer’s needs, or a lens that is quickly and cost-effectively modified for optimum performance, the customer’s application, budget and time to market are all beneficiaries.
For more information, call Edmund Optics at 800-363-1992, ext. 6855, or visit www.edmundoptics.com.
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