Keeping a tight focus on optics

July 10, 2002
Up-front planning can often help avoid the need for custom retrofits.

By Wallace Latimer
Director of Custom
Product Group
Edmund Industrial Optics
Barrington, N.J.
[email protected]

Edited by Leland Teschler

Optics suppliers generally have software that helps in designing systems. Nevertheless, such tools do not excuse developers from understanding basic optical design parameters.

An understanding of design elements will influence the decision about whether to design for custom-made or off-the-shelf optics, as depicted here. The best time to make such decisions is early in the design cycle.

Recently a semiconductor capital-equipment maker was designing a new wire-bonding machine that included a vision system. Engineers there knew the demands on the vision system were not particularly strenuous, so they concentrated most of their efforts on the electronics and motion control. They did, however, leave some space for the camera and optics.

That's when my company got involved. The OEM asked us to design a vision system with certain magnifications that could be changed in the field and which, of course, would fit in the space available.

Problem: The engineers hadn't left enough space for the optics. Too bad, because with better planning, we could have provided the required magnifications with ease using optics that were both inexpensive and off the shelf. As it was, the only alternative was a custom system that came at some cost. Moreover, some of the specs had to be relaxed simply because of the mechanical constraints.

Though the engineers involved had the right idea, they hadn't considered that the working distance for lower magnifications tends to be longer than that for higher magnifications. And lack of sufficient room complicated the focusing and compensation methods.

The irony was that optical specifications were not, in general, unreasonable. They would have been easily met had we been able to place the lenses at our discretion.

The situation these engineers ran into is not at all uncommon in semiconductor manufacturing equipment. Few manufacturers in this area have more than a single engineer in charge of optical systems. This despite the fact that it may be unusually difficult to design these systems because they must fit into the design after (and around) other systems.

Often, the integration of a vision system means snaking the optical system through the equipment without interfering with the primary process. This is particularly true for operations that include wirebonding, die packaging, aligning wafers, and lining up registration marks before lithography or metrology.

With basic parameters nailed down, the next step is to work out a combination of focal lengths and object/image distances. The bad news is that this usually involves calculating thin-lens equations, which you probably saw last in a college physics textbook.

The good news is that you don't have to do it yourself. Most modern optical companies have optical-design software that can quickly and easily provide a preliminary solution, unless the problem is extremely complex.

This is the point — before the design is finalized — at which to consider whether you need lenses custom made or if off-the-shelf optics will do. Custom lenses are almost always used to correct aberrations or package requirements. But sometimes correction is not important or a combination of off-the-shelf lenses will work as well.

Off the shelf or custom
Economy-of-scale is everything when it comes to the price of optics, because of how lenses are made. Low volume will always favor off-the-shelf elements. But the advantages diminish as volume rises and other factors take over. As a general rule of thumb, the custom approach makes economic sense only when one needs thousands of lenses. (As with any rule of thumb, there are always exceptions). But if a custom approach is absolutely necessary, it can be done at a reasonable cost for 100 or more pieces.

Off-the-shelf optics are made in quantity, in continuing production, and kept in stock by suppliers. These stock lenses are typically designed into standard matrixes in a wide variety of sizes and focal lengths.

Prototype using off-the-shelf components: It is fast, inexpensive, and lets you confirm image quality requirements. Moreover, custom lenses are expensive in the small quantities normally used for prototyping. Custom lenses made by traditional methods may require long lead times. If one uses lenses made with deterministic grinding and polishing, the lead time is less but the cost will be high. Of course, if prototyping shows the design must change, any one-time expenses associated with custom lenses must be considered as part of the development cost and are not typically recoverable.

Both custom and off-the-shelf elements are viable options for between 1,000 and 100,000 pieces. No one stocks off-the-shelf lenses at such a large volume without a forecast from a specific customer. But it's easier for the vendor to handle increases in volume with an off-theshelf option, because there is less risk in overstocking these lenses than in overstocking a custom lens. And if a custom lens can reduce the number of elements in the design, this approach becomes even more cost-effective at such volumes.

Custom lenses are almost always the choice above 10,000 pieces. At these volumes, the benefits of eliminating elements become more pronounced. There are real economies of scale. And the 10,000-piece level is a significant breakpoint.

Other factors
Additional factors that govern the choice between stock and custom lenses include whether or not designs need minimal weight, small size, tight tolerances, or strenuous specifications for the focal length or aberration correction.

If the design is already complete and assumes custom lenses, changes to incorporate off-the-shelf lenses may be expensive. Changing the lens inevitably means different mounting to accommodate any changes in focus. Even lenses with identical focal lengths can mount differently because a change in radius alters where the lens must sit. All in all, the cost for engineering these changes may outweigh the savings of an off-theshelf lens.

Though custom lenses can reduce the number of elements in a design, this may or may not cut costs. Additional elements, however, inevitably add weight to the system. Most offthe-shelf lenses are larger then if designed for a specific need, because the components are meant to cover a broad base of applications. In a tight space, a custom lens can be a real advantage.

Another consideration is tolerancing. Tolerance stack-up can accompany the use of numerous elements (rather than custom optics) to correct aberrations. A marked decrease in performance result. In addition, some designs require a certain element have a specific tolerance that may not be standard for off-the-shelf versions.

There are also situations that demand a very specific focal length and there is just no easy way to get around it. Some designs may need a specific form of optic, such as a meniscus lens, to correct aberrations; these types of lenses are often unavailable off the shelf.

Finally, there are many ways of customizing off-the-shelf elements in lieu of going full custom: Some of the most widely used include edging down a component, cutting it to a specific size, or custom coating it.

The easiest customization is changing the diameter of a stock element. It is easy to edge down or cut an element even in small volume. This is often important for mounting in an existing housing or in cramped quarters. "Edge downs" can be quick and inexpensive.

Special coatings are frequently a motivation for a custom lens. Sometimes designs require low reflectance at a specific wavelength or an antireflection coating in the UV or near-IR range. Lens suppliers are accustomed to fielding requests for special coatings on batches of uncoated lenses. As with edge downs, the costs are reasonable depending on the lot size and desired turnaround time.

System requirements

Important parameters for specifying an industrial vision system are easily understood when presented diagrammatically. They include sensor size, working distance, field of view, depth of view, and resolution.

An understanding of basic optical requirements is a first step in designing an imaging system that is economical. The primary purpose of any imaging system is to get enough image quality to extract necessary information. There is no single number that determines image quality. Instead, the approach is to consider the fundamental parameters of an imaging system and the values needed for the application at hand.

Field of view (FOV) is the viewable area of the object under inspection. In other words, this is the portion of the object that fills the camera sensor. Working Distance is the distance from the front of the lens to the object under inspection.

Resolution is the minimum feature size of the object under inspection.

Depth of field (DOF) is the maximum object depth that can be maintained entirely in focus. The DOF is also the amount of object movement (in and out of focus) allowable while maintaining an acceptable focus. Sensor size measures a camera sensor's active area, and typically specifies the horizontal dimension. This parameter is important in determining the right lens magnification to get a desired field of view.

Besides resolution and depth-of-field, three other factors affect image quality: image contrast, perspective errors, and distortion. One can use these factors to determine the minimum acceptable image quality. Image contrast is the only item which the user can control by selecting the appropriate illumination type and configuration. The others will be inherent in the optical design and controlled by the optical designer. The tightly packed optical systems that characterize semiconductor manufacturing generally sacrifice image quality to accommodate mechanical constraints. This is where a good understanding of image quality requirements can help save both time and money.

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