Reworking existing lenses is often seen as an attractive alternative to the manufacture of a brand new lens. We are often asked if
an existing lens can be refurbished, rebuilt, or modified to either restore it to its original operating performance, or change it to operate for a different function altogether. Among those most interested in seeking such an alternative have been members of the machine vision, defense, astronomy, and industrial processing industries. Indeed the most recent, and probably the most famous, refurbishing job was the correcting of the short focal length of NASA's Hubble telescope after it was already in space.
The decision to rebuild, refurbish, or modify is based on feasibility, complexity, tooling, spectral performance, uniqueness, available machinery, and cost. However, in
A successful example of repairing and refitting an existing lens is the repair of the Hubble Space Telescope shown here during its construction.
the final analysis, these variables reduce to two basic issues: is the proposed work cost effective and will significant time be saved by the repair?
Feasibility The first order or determining if an optic can be refurbished is to establish what the customer wants the lens or optical system to do. While it may seem elemental, too often a customer wants the performance of the lens restored to that which it never achieved in the first place. A well-defined set of operating specification is needed to determine the cost feasibility where dollars match a customer's wish list.
The next step is to determine the physical feasibility of reworking a lens. This requires a detailed analysis of the internal components, which includes the optical elements themselves, together with all the mechanical ones such as spacers, apertures, irises, and barrels. For complex optical systems it is desirable to have a step process of increasing complexity, in which one first performs a first order analysis to determine the scope of the rework, which in turn is used to prepare a cost estimate for the customer. Please note, in this article the words repair and refurbishment are used interchangeably, as are lens, element, and optic.
Refurbishment is found in defense systems, particularly those involving large or expensive vehicles, such as aircraft and tanks. Here the basic structure, such as the tank hull and turret, are massive units that house a collection of systems: propulsion, fire control, ammunition, crew, etc. Since the cost of the hull and turret is high, it makes economic sense to refurbish the internal systems to improve the tank's performance and survivability in warfare. The same concept applies to aircraft in which the navigation, fire control, and ordnance systems are refurbished and upgraded, as for example by adding IR night vision capability to an existing visual navigation system. Companies that we have worked directly with in this area include Boeing, General Dynamics, Kollsman Instruments, DBA, Sanders Associates, Texas Instruments, Magnavox, and McDonnell Douglas. In the commercial reproduction market of copying documents, Xerox, 3M, and Ricoh, to name a few, have upgraded their systems by replacement/rework of their lenses with additional optics to increase the magnification range, resolution, and light gathering capability.
Evaluations Since an inordinate amount of engineering can be spent reviewing an optical design, the first step is a limited evaluation. Determine what is desired, then move in logical steps. For instance, if a lens simply has a scratched front or rear element, then determine if the lens can be disassembled without damaging the barrel or element. Next, evaluate the elements for surface and/or coating defects. As part of this individual element evaluation, data should be simultaneously collected on the radii and diameters to determine if tooling, test gages, etc., exist to remove the coating and resurface the elements. The spectral band for the coating must also be determined.
If the performance of the lens is an unknown, a preliminary design evaluation is required. This evaluation is accomplished with computer based modeling of the lens. Both proprietary and commercial computer programs (i.e., Zemax, ACOS, Oslo, and Code V) are available for this purpose. The real key is gaining access to the individual element and spacing data. This design analysis also allows one to determine if restoring the lens to its original operating condition will achieve the required performance, or if the lens can be modified to meet that performance.
Spectral performance When the goal is to change the operating wavelength of the lens, the issue becomes one of design change, plus material, plus coating costs. Most good, wide-band, optical designs are achromatized for their spectral operating region. When the spectral band needs to be changed, then the materials from which the elements have been made need to be evaluated to see if they transmit in the desired spectral band. It is almost always the case that the optical coatings must be changed, because optical coatings have a defined and usually limited bandpass. Most older lenses were coated with a single layer of magnesium fluoride, and most reflective optics with simple metallics. With multi-layer optical coating machines, coatings can be significantly improved and higher transmissions or reflectivities achieved at reasonable cost.
Performance criteria Refurbishing is more complicated when the optical design has to be modified to meet new performance criteria. If the design evaluation indicates that a limited number of elements need to be changed and these changes involve reworking the surfaces to new radii, then it may be feasible to modify the lens. The more elements to be changed, the lower the feasibility. For some changes, the most cost-effective solution is to remove an element and replace it with a new doublet, or to replace part of a viewing system with a new beamsplitter and focusing elements. In either case, the existing optical system is fairly expensive, so the cost of this complexity is less than designing and manufacturing a new system.
Aspherization Aspherization is the new buzz word in refurbishing. Aspherized elements can greatly improve certain systems. This is particularly the case for both the mid- and far-IR where many of the components can be diamond turned to complex aspheric curves. Understand that the replacement of a spherical element with an aspheric element always requires an optical redesign.
One feature of aspherization is that, with clever optical redesign, it is often possible to both improve the performance of and reduce the number of optical elements in the system, since the correction of secondary spectrum and higher order Seidel factors can be achieved without the requirement of additional surfaces.
In the visible, while some machinery is now available for aspheric polishing, most aspherics are still made con-
40 Optics & Photonics News/January 1999
ventionally and are labor intensive. Therefore, aspherizing an existing system is more feasible for expensive systems or those with confined access for which conventional optics will not fit.
Complexity Complexity divides into two basic areas: optical and mechanical. First, is the repair/refurbishment simple? Second, how easily can the lens be taken apart?
The complete disassembly of a lens is not a trivial exercise. In fact, the method of original assembly is one determinant in a refurbishment decision. If the lens was originally assembled so that a small amount of explosive will not dent the barrel, then forget refurbishing. In this case, the barrel and the internal spacers will, at best, have to be replaced and, at worst, the elements will be damaged beyond use. For the less destructive case, if the lens barrel is swaged together or the elements are installed in a plastic barrel that has been staked or glued together, then the barrel will probably be destroyed or at best deformed, and will have to be replaced. Here we enter into a gray area where the cost of manufacturing a one-off lens barrel and/or internal spacers combined with the mechanical design labor will determine if the lens can be refurbished.
Let us assume, however, that the lens was assembled with threaded retaining rings and can be taken apart with limited damage to the elements. The next issue to address is how the individual elements were mounted. For production lenses, elements are mounted into metal or plastic cells (rings) that have a step and a thin lip around one edge. The element is centered in the cell, and the thin edge peened over the circumference of the element to securely hold it in place. Elements that need to be refurbished almost always have to be removed from their cell. While it is not difficult to cut the lip and free the optic, care has to be taken not to chip the lens.
Some lenses are actually designed to be refurbished. For example, the Angenieux brand of zoom lenses have threaded retaining rings; replaceable cams, galleys and pinions in the zoom mechanics; and a repairable iris assembly, including individual iris blades. By contrast, many Asian zoom lenses are riveted and peened together. Disassembly in this case means that the rivets must be drilled out, distorting the rivet holes and making direct reassembly not feasible.
Tooling and machinery The physical procedure of manufacturing, coating, and testing optics requires a range of tooling to hold and work the elements, and of proper sized machines to regenerate and polish their surfaces. While measurements of the curves can be accomplished interferometrically, in a working optical shop a test gauge or test plate is used by the optician to measure, the figure while the lens is on the lap. Having the right set of test plates as well as grinders and polishers is a requirement for efficient production. This tooling is expensive. At our shop we have a test plate library of over 1,000 gauges, and every refurbishment job seems to require additional test plates.
Additional tooling complications are the assembly
and test fixtures. Most systems require special fixture tooling to hold the optics in position during assembly. While most optical assembly shops will have tooling to assemble various sizes of doublets, assembling the optical barrels with the elements and spacers often requires dedicated tools and fixtures to insert and hold the elements, spacers, reticules, and prisms.
The testing of the refurbished systems also is a fertile ground for fixture multiplication. These include special alignment and resolution targets. Some systems also need calibrated IR targets, as well as a means of determining boresight, and MTF and OTF measurements.
Cost and delivery For the vast majority of customers, the cost of the element is paramount and the delivery is second. The one exception to this rule is for optics that are unique. These include lenses or optical systems installed in restricted spaces and for which there are no other solutions. (When one wants to refurbish an original third-order Fresnel lighthouse lens made by Le Master himself, cost is a secondary issue to craftsmanship.)
As indicated above, the refurbishing production schedule is a function of a number of factors. These include some or all of the following
redesign disassembly modification or replacement of optical elements modification and/or replacement of mechanical components reassembly alignment testing.
Delivery time is shortest for the rework of complex multielement optical systems when basic mechanical assemblies are preserved or reworked, and changes to the optical components are minimal. Obviously substantial time can be saved in the production schedule when changes are limited. What is not so apparent is that ancillary operations are also reduced. These include
Optics & Photonics News/January 1999 41
Computer Optics Inc. f/1.7 40 x (25 to 1000 mm) Zoom Lens refurbished to operate in the near-IR from the visible and also redesigned to change the image format.
painting or anodizing, engraving, and the shipping time for components such as new O rings.
To refurbish or not The concept of modifying an existing lens or optical system for use in a new project is appealing. The process is highly individualized, with a variable cost envelope, and must be evaluated on a case-by-case basis. A driving feature of rework is that one can reuse elements that can be expensive to replace, have overall unique properties, and save a significant amount of time in the process. But, the rework procedure must be approached with attention to detail, due to the wide variety of redesign, refurbishment/rework, and other modifications that may be necessary to ensure that the refurbished lens will perform as redesigned.
The key is the initial evaluation of both the lens and its new performance. It definitely pays to perform an initial evaluation to get a rough idea of what is required. Once rework budget limits are established, a decision can be made as to what modifications are feasible, including aspherization of selected elements, modification of spherical curves, or insertion of additional elements. In addition, significant effort has to be allocated to the mechanics, which include changes to the housings, spacers, apertures, irises, and mounts. From this in-depth analysis, a performance specification can be established and the cost and delivery determined.
While the variety of refurbishment requests vary in general, as a general rule a lens costing under $500 is not economical to repair. From $500-5,000 the refurbishment decision will be determined by the amount of disassembly and the number of elements to be repaired or replaced. Assuming a non-unique optic, the system has to be over $5,000 for a redesign to be cost effective.
The development of modern computer programs has significantly transformed optics particularly in design evaluation. The ability to rapidly trace thousands of rays
makes it possible to quickly model a variety of design changes and innovations, and to determine the feasibility of optical refurbishment of an existing system. The most significant issue in the application of computerized evaluation is having a trained optical designer who understands optical manufacturing and tolerances. To actually be able to build a redesigned system, the optical designer must have direct optical manufacturing experience, so that the redesign can be manufactured within the cost envelope.
Manufacture of optical elements has also improved with the addition of a number of machines and the testing necessary to confirm the performance of the optical element. Included in this category are new aspheric generators and polishers, lapping and high-speed polishing machines, and precision interferometric measurement instruments for the shop. For the mechanics, CNC machining centers routinely can be programmed to make the mechanical parts to the very fine tolerances that are required for precision optics.
Since these machines are expensive, one can predict that the refurbishment of optical systems will be most feasible when the optics are expensive, or when there are a number of systems to be refurbished. In these cases, the development and setup costs are distributed over a large number of units. For example, this cost envelope probably limits the refurbishment work on photographic lenses to repair damaged coatings and exterior optical surfaces, while large telescopes, military instruments, and some industrial systems are good candidates for refurbishment.
Thus, while there is a place for refurbishment of older existing optics, each case is unique and has to be evaluated on cost and complexity.
Gordon Kane has been president and chief engineer of Computer Optics Inc. (COI) In Hudson, NH, since 1985. COI is an opto-mechanical company specializing in optical design, manufacture, and rework of lenses and optics for the machine vision, defense, and Industrial processing Industries.
Lightcraft propulsion Continued from page 25 The eventual cost of launching objects to LEO using laser propulsion could be as low as $200/kg.
There are many problems to overcome before a low-cost, reliable laser-propelled launch system becomes a reality. These challenges include development of active on-board flight control to compensate for atmospheric variances (such as wind and dust), advanced high-temperature materials for the propulsive surface of the cowl, and miniaturized on-board propellant management for cooling and delivery of a gaseous working fluid for flight above the atmosphere. The current program will address these issues over the next few years.
Moderate advances in high-power lasers and high-temperature light materials could enable the Lightcraft technology project to launch a 1-kg nanosatellite into LEO in the next 5-7 years. The further development of pulsed lasers with power exceeding about 10 MW would enable the routine launch of nanosatellites at a reasonable cost. The use of laser propulsion to launch
heavy payloads and people into space is improbable, needing gigawatt-class lasers and highly advanced materials technology.
For more information on, and photographs of, these Lightcraft experiments go to www.ple.af.mil and other links therein.
References 1. A. Kantrowitz, "Propulsion to orbit by ground-based
lasers," Astronaut. Aeronaut. 10, 74-76 (1972). 2. L.N. Myrabo et al., "Ground and flight tests of a laser pro
3. F.B. Mead, Jr. et al., "Flight and ground tests of a laser-boosted vehicle," 34th A I A A / A S M E / A S E / A S E E Joint Propulsion Conference, paper AIAA 98-3735, July 13-15, 1998.
Patrick Carrick is chief of the Propellants Branch, and Franklin Mead Jr. is senior scientist and co-program manager of the LTD project, in the Propulsion Sciences and Advanced Concepts Division, Propulsion Directorate, at the Air Force Research Laboratory (AFRL/PRSP), Edwards Air Force Base, CA. Leik Myrabo is associate professor of Aeronautical Engineering at Rensselaer Polytechnic Institute, Troy, NY, and co-program manager and affiliated research scientist with AFRL/PRSP.
