Utilization of COTS (commercial off-the-shelf) products is now almost a bylaw of government and military design projects and is becoming of increasing interest in commercial designs, as well. Optical systems are no exception; the use of stock optics can provide tremendous advantages in terms of reduced cost and development effort. The key is finding the most appropriate way of employing stock optics in a custom design.
The obvious benefit of using COTS products is reduced cost. Volume production yields reduced unit cost, especially in optical components. Using a stock product avoids setup fees and tooling costs that can make a custom optic as much as ten times more expensive in small quantities. Further, that differential only begins declining when production volume reaches 1000 pieces, a quantity seldom reached in high-end optical system manufacturing.
Stock optics can also help reduce design and development time because they are readily available. Rather than waiting months for a custom optic to arrive before assembling a prototype to begin testing, designers can obtain stock optics overnight. This high availability also provides benefits during final production by eliminating the need to maintain an expensive in-house inventory. Because vendors seldom discontinue or alter their stock optics, supply issues seldom arise.
Custom vs. Stock
There are drawbacks to using stock optics in certain situations, however. Stock optics work best when the optical system is relatively slow and paraxial in its design requirements. An optical speed of f/8 or higher with a field angle of 15° or less are easy to obtain using only stock components. When seeking lower f/#, though, the ability to customize the exact curvature, composition, and coatings of lenses may prove more efficient than restricting a design to using only stock optical components. Despite the vast range of stock components available, there may not be a precise match to the design’s needs. Any mismatches will compound the need for making design iterations in order to optimize performance. When performance needs are extremely demanding, then, custom optics may provide the fastest route to final design despite the wait for initial delivery.
Custom optics may also be necessary when the optical system must meet tight size and weight constraints. While a design composed of stock optics may be able to achieve the same performance as one using custom components, the stock design may involve multiple lenses and a longer optical path to achieve the equivalent performance of a single custom lens. When size and weight are critical, then, custom optics may be essential.
Often a system that will employ custom optics in the final design, however, will want a stock component solution for the early stages of development where availability and low cost are critical. A development team might, for instance, need to create a demonstration or proof-of-principle prototype in order to obtain funding for full product development. A prototype optical element may also be needed in order to support the development and evaluation of other system elements before establishing definitive final optical performance requirements. Custom optics can quickly become prohibitively expensive when there are many design iterations involved.
Stock Empowers Prototypes
In many cases, too, the constraints that might require a custom final solution may not apply to the prototype. An imaging system for airborne reconnaissance, for instance, may tolerate longer light pathways and more lenses (hence more weight) in the system’s optical components during test and evaluation of image sensors and processing algorithms than the final deployed system will have. This tolerance allows developers the option of creating a stock component solution for system test purposes while developing final specifications or awaiting delivery of custom optics.
Even in applications where such stock solutions are not optimal, developers can use them in combination with other stock items or custom lenses. An application requiring a high-quality imaging lens with an odd magnification, such as 17.5x, can be formed by combining stock items in a hybrid configuration. In this example a 20x infinite conjugate objective designed for use with a 200mm focal length tube lens can be made into a 17.5x system by replacing the tube lens with a 175mm off-the-shelf doublet. Because the tube lens runs slow and has small field angles, this substitution will not introduce significant aberration and so will still provide the desired imaging quality.
Sometimes the only thing wrong about a stock optical element in a specific application is that it is not quite the right size (Figure 1). Vendors with their own production facilities can often address such slight mismatches by modifying an off-the-shelf lens, such as by grinding the edges of a lens that is too large. Vendors can also re-process stock lenses by applying custom coatings to meet application needs.
Developers can also find clever ways of customizing stock optical components to address their design needs. There are not as many stock options for mirrors, for instance, as there are for lenses, but developers can still modify a stock part to get the mirror they need. Simply select a lens that has the right surface curvature and have it mirror coated with aluminum, silver, or gold as needed to gain access to mirrors in a huge range of sizes and focal lengths.
Maximizing Stock Opportunities
To maximize the opportunity of using stock optics in a system design it is important to consider the optical subsystem as early in the system design effort as possible. Early planning helps optical developers secure an adequate space and weight budget within the system design to allow for a solution based on multiple stock lenses rather than a single custom lens.
A convenient way to create stock solutions is to break the optical design into subsections based on focus. Collimators and collectors are easy to find as stock products because many achromat and PCX lenses are already optimized for those applications (Figure 2). In addition, many stock lens assemblies are optimized for infinite conjugate on one side coming to a focus on the other side or as a 1:1 relay (4F system). This allows partitioning of optical designs into a series of infinite conjugate systems, each partition consisting of a stock solution.
A developer might, for example, need a small image lens with 2.5x magnification for a small CCD camera. The lens can be implemented using two achromats with effective focal lengths in a 5-to-2 ratio, such as a 6.25mm dia. x 20mm focal length lens and a 6.25mm dia. x 50mm focal length lens. Placing the 20mm lens with its close conjugate side toward the object will collimate the object image. Placing the 50mm lens with the close conjugate side toward the camera will focus the collimated image onto the sensor. The result is a 2.5x image on the CCD.
Finding the right stock element for a design does not need to involve pouring over large catalogs from multiple vendors and distributors. Lens design software such as ZEMAX and CODE V allows developers to quickly search a collection of catalog listings for individual lenses based on parameters such as size and focal length. Also, most vendors have a knowledgeable support staff that can answer questions and quickly identify candidate items as well as suggest design alternatives to achieve the desired ends. The more a developer works with the vendor at defining the design’s needs, though, the better the chances of a vendor locating an optimum set of choices.
Creating Hybrid Designs
Mitchell C. Ruda, President of Ruda Cardinal, Inc., states that developers can start with a stock assembly such as a microscope objective, imaging lens, or eyepiece as their design’s basis, and then add lenses to correct residual aberrations and adjust performance to match system requirements (Figure 3). This hybrid approach leverages the optimized performance and relatively low cost of stock assemblies for their given function while still achieving the required end-toend performance.
To follow this approach, however, developers need detailed information on the stock assembly’s design and optical characteristics so they can model it in their optical design tools to determine the corrections they will need. Most companies are reluctant to give out this kind of detail on their proprietary designs, but there are alternatives available. Companies such as Edmund Optics will create Black Box descriptions of their assemblies for design tools such as those from ZEMAX (Figure 3). These descriptions allow developers to model the performance of the stock assembly in their overall system without compromising the vendor’s design secrets, simplifying the creation of appropriate corrections and adjustments in the overall optical design.
Stock optics, then, can offer system developers significant advantages in terms of cost and design effort under many circumstances. Depending on the system constraints, developers can create designs entirely from stock components, blend stock and custom components, or modify stock components to avoid the delays and costs inherent in creating custom optical elements. Even if only used for prototyping and proof-of-principle development, with custom optics an inevitable part of the final design, leveraging stock optics in optical system designs can yield a substantial dividend.