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Optical Design & Engineering

Optomechanical Design in Five Easy Lessons

The development of practical optical systems requires careful optomechanical design.

From oemagazine February 2004
31 February 2004, SPIE Newsroom. DOI: 10.1117/2.5200402.0006

Optomechanical design is the sub-discipline of optical engineering in which optics such as lenses, mirrors, and prisms are integrated into mechanical structures (cells, housings, trusses, etc.) so as to form an optical instrument. The design for a typical optical instrument results primarily from the cooperative efforts of a team of lens designers, optical engineers, and mechanical engineers. They seek and apply input from experts in fabrication, assembly, alignment, and testing as well as from specialists on light sources, film, detectors, focal plane arrays, electronics, signal processing, and so on that might be used in the instrument.

Instrument design starts with the definition of a need for a particular device and progresses through conceptual, preliminary, and final design phases with analyses, experimentation, reviews, and documentation accomplished as appropriate. Along the way, the team must consider how the operational and survival environments (such as temperature, humidity, contamination, vibration, shock, etc.) will affect the design and which materials would be best for each component.

The engineers also should be aware of the projected cost of fabricating the device and keeping it operating. The total life cycle cost of an instrument is driven largely by performance requirements because they usually determine design complexity, but it can also be affected strongly by choices in instrument configuration, materials, and dimensional tolerances. In general, it is best to emphasize simplicity of design and use the most lenient tolerances that allow the instrument to meet performance requirements.

Making the right detailed choices in optomechanical design demands the logical application of intuition—and experience—with unknowns verified through analysis and testing. Each facet of the design is well known to those active in their specific area, but not necessarily to those working in other disciplines. Team members must make decisions in five basic design categories: materials, structural design, lens-to-mount interfaces, mountings for prisms and mirrors, and assembly and alignment. We offer here some guidelines that may help workers in or outside of the design team to achieve better optomechanical designs and understand why certain choices are made. Note that these guidelines are not absolute rules. The only absolute rule of engineering is that exceptions apply in special cases.

Material Decisions

The choice of materials is critical to the performance and cost of a system. Choose the wrong materials and your system won't meet spec. Choose overly exotic materials and your system may meet spec but it won't matter because it may be cost prohibitive. In general, designers should choose optical glasses designated by the manufacturer as preferred types. These are usually easier (and cheaper) to process in the optical shop and are most likely to be available for future production.

When a variety of materials with otherwise acceptable properties is available, the choice may well be made on the basis of density because that tends to reduce total weight of the instrument. Insofar as possible, match the coefficients of thermal expansion (CTEs) of materials used in connected mechanical and optical parts to minimize differential expansion or contraction in the event of temperature changes. Low-expansion materials such as ULE (Corning Inc.; Corning, NY) or Zerodur (Schott Glass; Duryea, PA) may prove the best choice for mirror substrates. Aluminum is the most frequently used metal for structures, but in some cases stainless steels, titanium, or Invar may provide more advantageous CTEs.

Specify the heat treatment of critical metal parts after rough machining to maximize their dimensional stability; recommended procedures must be followed. Adhesives and sealants (elastomers) with low outgassing properties are the best choices for vacuum applications; usually, sealants are not used as adhesives. Choose elastomers with minimal CTE and low shrinkage during curing.

Structural Design

An optical system functions properly only as long as the optics remain within allowed tolerances of their nominal locations and orientations, and structural deflections caused by gravity and other external forces or by temperature changes do not excessively distort the optical surfaces. Structural designs must be stable enough to control these effects throughout the operating temperature range. The structure must constrain the optics in such a manner that they are not damaged or irreversibly moved when exposed to extreme environmental conditions. Temporary deflections of optics and mechanical parts are acceptable during vibration, shock, or temperature changes beyond the operating ranges so long as the parts survive and come back to their nominal positions after exposure.

Figure 1. Threaded retainers (top) or ring flanges (bottom) can provide axial preload for lenses that will suffer extreme acceleration loads.

To minimize the effects of temperature changes, designers can engineer the structure to be passively athermal, i.e., insensitive to these changes over the full operating temperature range. Remember to consider changes in both optical and mechanical parts.

To maximize performance without excessively tight tolerances on dimensions, design a carefully optimized number of mechanical adjustments into the instrument. Establish tolerance budgets after analyzing the sensitivities of aberrations to component positional errors. Design structures for maximum possible stiffness within weight and packaging constraints because that tends to reduce deflections from external forces such as gravity; in addition, isolate the supported optical components from mechanical resonance effects under vibration conditions.

Lens-to-Mount Interfaces

Lens mounting and positioning is critical to the performance of a refractive optical system. For best results, design metal reference surfaces to interface with polished surfaces on lenses rather than with ground rims or bevels; you will then use the most accurately made surfaces for lens positioning. Avoid glass-to-glass edge contacts between lenses whenever appreciable acceleration forces are expected. It is better to use separate lenses with spacers or shoulders machined into the mounts.

Figure 2. Preferred types of glass-to-metal axial constraints include conical metal surfaces for convex lenses (top), convex toroids for concave lenses (middle), and flat metal surfaces for flat bevels on lenses (bottom).

Lenses should be preloaded axially for the maximum expected acceleration loads at extreme anticipated temperatures. The applied force should equal lens weight times the acceleration factor. Changes in temperature can significantly affect preload, so remember to consider them. Threaded retainers provide one means to apply preload (see top view of figure 1). The thread fit should be specified as Class 1 or 2 per ASME Publication B1.1 so that the ring aligns itself to the lens surface for maximal symmetry of force distribution. A good rule of thumb is that the delivered axial force equals five times the torque applied to the ring divided by the pitch diameter of the thread; remember, this is only an approximation. Ring-flange-type axial constraints provide good alternatives to threaded retainers for constraining lenses whenever tighter control of axial force is needed (see bottom view of figure 1).

Lens-to-mount interfaces should be designed for low axial contact stress. To do so, use conical metal surfaces to touch convex lens surfaces tangentially; use convex toroidal (donut-shaped) interfaces to touch concave lens surfaces, and use flat metal surfaces to interface with flat bevels on lenses (see figure 2). In the case of the toroid-to-concave-lens interface, make the toroid radius at least half the radius of the lens surface. Flat bevels should be accurately perpendicular to the optical axis to facilitate alignment.

For systems expected to experience severe radial accelerations, constrain lens rims radially by specifying a close fit to the inside diameter of the mount. Customized spacers can be used to fill measured gaps, or the mount diameters can be machined at assembly to closely match the measured diameters of the lenses.

Elastomeric mountings are frequently used to support lenses, windows, and small mirrors. The thickness of the annular ring of elastomer around the optic can, in most cases, be chosen to make the design insensitive to temperature changes.

Analytical techniques can aid in estimating mounting stresses in optics and mechanical parts. Finite element analysis methods can then be used to confirm these estimates, if necessary.

Mounting Prisms and Mirrors

Reflective components such as mirrors, gratings, and some prisms have their own sets of mounting issues because they are more sensitive to surface distortions than refracting optics. Support prisms and small mirrors semikinematically, if possible. In such an interface, all six degrees of freedom (three tilts and three translations) are uniquely constrained against reference surfaces by forces delivered through small areas. If the mount incorporates more than six constraints or if the contacts are too large, moments transferred through the interfaces may distort the optical surfaces.

Most prisms and small mirrors can be bonded to their thermally compatible supports using multiple small adhesive areas with the total bond area given by

Abond = w aG fs /J

where w is the weight of the system, aG is maximum acceleration loading, fs is a safety factor (better than four), and J is the strength of the bond joint. Prisms and small mirrors also can be clamped in place on their mounts with multiple springs. Forces from the springs should be normal to the contacted surface and oriented such that the force vectors pass directly through the optic to reference pads on the other sides of the optic. Flat pads touching flat surfaces on the optic must be lapped coplanar prior to assembly.

Support larger mirrors at multiple points around their rims and on their backs to minimize gravitational distortions at all elevation angles of the line of sight. Typical mount configurations include Hindle-type mounts using multiple levers and arrays of pneumatic/hydraulic actuators. Multiple-point supports deliver forces as needed to support the localized portion of the mirror's weight at the support points.

Assembly and Alignment

The best optical system will be useless without accurate assembly and alignment. Proper engineering means designing for success at this step. The guidelines are not always intuitive, however, such as minimizing the number of adjustments for fine alignment. Too many adjustments are as bad as too few.

In the actual assembly and alignment process, start by cleaning all parts thoroughly. Carry out the actual assembly process of optical instruments in a clean, dry environment such as a clean room or under a laminar flow hood. Use only approved lubricants and apply them carefully to avoid contamination.

For best performance of multiple-lens assemblies, rotate the lenses differentially around their axes to "phase" residual wedges so they counteract each other. Adjustments should be locked after the optics are aligned; techniques include mechanical clamping, epoxy pinning, laser welding, and soldering. Seal optical instruments during assembly to protect optics from moisture and particulates. They can be purged with dry N2 or He and pressurized as appropriate.

Optomechanical design is key to optical system performance. Good communication between team members and awareness of the issues touched on above will help ensure that the optical system actually fielded is practical, robust, and performs to specification. oe

Further Reading

1. D. O'Shea, Selected Papers on Optomechanical Design, SPIE Milestone Series vol. 770, SPIE Press, Bellingham, WA (1988).

2. P. Yoder Jr., Opto-mechanical Systems Design, 2nd Edition, Marcel Dekker Inc., New York, NY (1992).

3. D. Vukobratovich, "Optomechanical System Design," Ch. 3 in The Infrared & Electro-Optical Systems Handbook, M. Dudzik, ed., ERIM, Ann Arbor, MI, and SPIE Press, Bellingham, WA (1993).

4. P. Yoder Jr., "Mounting Optical Components," Ch. 37 in OSA Handbook of Optics, 2nd edition, vol. I, M. Bass, ed., McGraw-Hill Inc., New York, NY (1995).

5. A. Ahmad, ed., Handbook of Optomechanical Engineering, CRC Press, Boca Raton, FL (1997).

6. P. Yoder Jr., ed., Optomechanical Design, SPIE Selected Papers on CD-ROM, vol. 5 (1999).

7. P. Yoder Jr., Mounting Optics in Optical Instruments, SPIE Press, Bellingham, WA (2002).

Mentored to Mentor, Yoder Keeps the Faith

 Paul Yoder's remarkable career was nurtured early by his physicist father and other important mentors. Today he continues the cycle by sharing his knowledge in many different settings. Yoder's father was head of the physics department at Juniata College (Huntingdon, PA). Yoder followed suit, earning a BS in physics from Juniata in 1947 before going on to an MS in physics at Penn State University (University Park, PA).

At Penn State, David Rank, head of the university's spectroscopy lab, introduced the young physicist to the emerging field of military precision optics as then spearheaded at the Frankford Arsenal (Philadelphia, PA), where Rank had been a consultant. After graduation, Yoder spent 10 years at Frankford, designing and manufacturing optical instruments.

After Frankford, Yoder's career began to be best expressed as a sum that was greater than its constituent parts. Yoder spent 25 years with Perkin-Elmer Corp. (Norwalk, CT), rising to assistant to the director of research for the multinational company. After immersing himself in military and aerospace applications, Yoder turned his talents to serving the medical community by co-founding Taunton Technologies, which pioneered vision correction by laser corneal recontouring and later became VISX Corp. (Santa Clara, CA).

Beyond his business success, Yoder has spent much of his career ensuring that the optical community that gave him his start would continue to thrive. Yoder is a fellow of both SPIE and OSA, a member of Sigma Xi, and received the Director's Award from SPIE in 1996, the Engineering Excellence Award from OSA in 1997, and the George W. Goddard Award from SPIE in 1999. Yoder has also completed the cycle of mentored-to-mentor by organizing symposia at SPIE and OSA on optical engineering and optomechanical design, teaching graduate optics courses at the University of Connecticut (Storrs, CT), and conducting dozens of short courses and guest lectures around the world. He has also fostered the ongoing development of his craft by authoring more than 60 papers and some of the most prominent texts used today on optomechanical design. —Winn Hardin

Paul Yoder Jr.
Paul Yoder Jr. is a consultant in optical engineering, Norwalk, CT.