PERFORMANCE SPECIFICATIONS AND DESIGN CONSTRAINTS

Two of the most important inputs to the design process are the performance specification and the definition of imposed constraints. The former sets forth the user’s definition of what the end item must do and how well it must work to be judged acceptable, whereas the latter defines the physical limitations, such as size, weight, configuration, environment, and resource consumption that affect opto-mechanical and electrical interfaces with the surround. In the case of a scientific payload for a space probe, these generally would consist of many separate, complex, and lengthy documents. In the simplest cases, the specification could consist of one short document giving a few general requirements and parameters would be left to the discretion of the optical and mechanical design- ers and engineers. In almost all cases, the preparation of at least one drawing to specify the item’s opto-mechanical interfaces would be appropriate.

A suggested list of items to be considered in the typical performance specification and con- straint definition for an opto-mechanical system may be found in Table 1.1. These items are not nec- essarily in order of importance nor all-inclusive. Careful consideration of these features (and others that may be unique to the design in question) should help the design teams create a satisfactory end item or product. It is advisable also to indicate clearly the intended purpose of the instrument at the beginning of the specification.

Figure 1.4 illustrates an opto-mechanical interface drawing. This drawing defines the required external configuration for a particular 9 in. (22.9 cm) focal length, f/1.5 objective lens assembly with coaxial laser output and image-forming input channels that is discussed in more detail in Section 5.5. The drawing also sets limits on overall package size, defines critical dimensions, states requirements for perpendicularity of the optical axis of the imaging system (datum -A-) and of the image plane to the mounting flange (datum -C-), and establishes tolerances for critical dimensions and angles. The technical performance specification for this lens defines the optical characteristics

Corrector plate no. 1 Corrector plate no. 2 Primary mirror Field lens group Image derotator

Scan mirror Mount and scan mechanism Control mechanism Mount Cell, spacers, and retaining ring Cell, spacers, and retaining ring Housing Lens cone

FIGURE 1.3 Lower level block diagram of the Imaging and scanning optics block shown in Figure 1.1 here

configured as a catadioptric Newtonian-type optical system with an object–space scanning mirror and image derotation system.

(focal length, relative aperture, field of view, image quality, vignetting, transmission, etc.) as well as constructional features needed for the assembly to accomplish its intended function in a specific environment.

TABLE 1.1

Checklist of General Design Features Typically Included in Specifications and Constraint Definitions for Optical Instruments

● _{Performance requirements such as resolution, MTF at specified spatial frequencies, radial energy distribution, encircled}

or ensquared energy at specific wavelengths, or numerical aperture

● _{Focal length, magnification (if system is afocal), magnification and object-to-image track length (if system has finite}

conjugates)

● _{Angular or linear field of view (in specified meridians if anamorphic)} ● _{Entrance and exit pupil sizes and locations}

● _{Spectral transmission requirements} ● _{Image orientation for a given object}

● _{Sensor characteristics such as dimensions, spectral response, element size and spacing, and/or frequency response} ● _{Size, shape, and weight limitations}

● _{Survival and operating environmental conditions} ● _{Interfaces (optical, mechanical, electrical, etc.)} ● _{Thermal stability requirements}

● _{Duty cycle and useful life requirements}

● _{Maintenance and servicing provisions (access, fits, clearances, torquing, etc.)} ● _{Emergency or overload conditions}

● _{Center of gravity (CG) location and lifting provisions}

● _{Human-instrument interface requirements and restrictions (including safety aspects)}

● _{Electrical requirements and restrictions (power consumption, frequency, phase, grounding, etc.)} ● _{Material selection recommendations and limitations}

● _{Finish/color requirements}

● _{Corrosion, fungus, rain, sand/dust, and salt spray erosion protection requirements} ● _{Inspection and test provisions}

● _{Electromagnetic interference restrictions and susceptibility} ● _{Special markings or identifications}

● _{Storage, packaging, and shipping requirements}

−A− −C− −D− CG TBD −B− Input object beam Object beam image Output laser beam (expanded) Input laser beam 9.75 max. 11.50 max. Mounting surface thru CG within 0.50 A .002 TIR A 0.010 DIA. D 0.010 TIR C 0.003 TIR 5.688 6.90 max. 5.18 max. Beam splitter 35° 20° 2.62 35° 0.205±0.003 ±0.5°

Dimensions are inches Angles Decimals

xxx 0.005 xx 0.01

DIA (3) Holes

FIGURE 1.4 Example of an opto-mechanical interface drawing showing the configuration, critical dimen-

One aspect of optical instrument performance specification preparation worthy of special consid- eration here is the quantification of what is really needed from the equipment once it has been designed and built. Smith (1989) advised us that specifications should ask for just enough to accomplish the intended purpose and no more. Technical requirements should be clear and concise; not overburdened with details, yet not so general as to foster confusion on the part of the designers trying to determine what is wanted. For example, although it is easy to say that a new photographic lens is to be “diffrac- tion-limited,” it is not so easy to prove that some lower level of performance would not suffice. It has become a common practice for those wishing a device to be developed to ask first for an analysis of the trade-offs between performance and cost. The time and cost of such analyses, if properly conducted and documented, are usually worthy expenditures. It has often been said that requirements are not absolute and performance is not always the most important attribute of a system. For instance, life cycle cost is sometimes the most vital aspect of new hardware. An affordable system that works adequately may be better in the long run than a more expensive version that offers a small technical advantage, but requires more maintenance. Strict schedule constraints such as having a new space payload ready to meet a spe- cific launch window that will not occur again for years also might lead to acceptable compromises in performance because some scientific information from the mission would be better than no informa- tion at all. Above all, the project team must understand what the user (read customer) really wants — not just what the initial specification reads! In this case, understanding requires communication and willingness on the part of all parties to examine all aspects of the application to see if the “requirements” are realistic.

Price (1985) went a bit further by defining a trade-off as a “balancing of factors or conditions, all of which are not attainable at the same time.” He cited and then discussed three useful view- points, one or more of which is generally applicable to almost any system:

1. The hardware system including all components from the object to the final output (e.g., a video recording or display system comprising object, illumination, atmosphere, lens, camera, detector, electronics, recorder, tape, player, monitor, and observer’s eye). 2. The product-user system including the interaction between the person and the apparatus

(e.g., controls, platforms, handles, switches, eye position, eye-hand coordination require- ments, time delays between actions and reactions, etc.).

3. The manufacturing system including raw materials, materials handling, parts manufac- ture, assembly, quality control, optics-to-product interfaces and tests, and the attendant costs, schedules, processes, and personnel utilization.

Price’s paper concluded with the profound statement: “a well prepared analysis is an essential, but not necessarily sufficient, condition to obtaining acceptance of a proposed system design.”

The extent to which the cost of an optical system can be reduced or the product can be made more attractive to prospective buyers for other reasons is often intimately related to the allowable degrada- tion from “perfect” operation. Customers faced with the predicted cost of buying state-of-the-art aer- ial reconnaissance camera systems built to a given specification have been known to ask for a “shopping list” of alternative designs showing system costs in large quantities as a function of resolu- tion in line pairs per millimeter. Although a reliable relationship between these factors is quite diffi- cult to derive, its serious consideration would surely help all parties understand the importance of compromise. Shannon (1979) illustrated this point by pointing out the magnitude of optical distortion introduced by the curved windshields of modern automobiles that is tolerated for style and cost rea- sons. Walker (1979) dealt at length with the compromises appropriate in the design of visual systems such as telescopes, binoculars, or periscopes. Parameters particularly amenable to trade-off in such instruments are image quality, vignetting, and light transmission. To a lesser degree, one might trade field of view, pupil diameter, or exit pupil distance against system complexity, size, and cost. At the end of his paper, Walker provided his version of the dictionary definition of a specification as follows: “A detailed and exact statement prescribing materials, dimensions, workmanship and performance,

arrived at after careful and cooperative consideration of the system application and the realistic needs of the end user.” This seems to express accurately the viewpoint of many of us active in opto-mechan- ical system design.

For many years, specifications for optical instruments procured for U.S. government use referred to military specifications, standards, and other government publications. These documents defined general requirements and provided guidance for the selection of materials, design, inspection, and testing of a variety of equipment items. In 1994, the U.S. Armed Services issued a directive stating that all future military procurement contracts should refer to national and international commercial standards rather than military specifications. As of this writing, of the many military specifications that relate to optical products, several have been canceled and others have been declared inactive. Inactive specifications can be applied to existing procurement contracts, but not to new ones. Specifications for optical coatings can be replaced by the international standard ISO 9211 “Optics and optical instruments — Optical coatings,” but other U.S. military specifications are being reviewed for relevancy to current manufacturing tech- niques. It is expected that many of these will be rewritten as new voluntary optical standards. Standards applicable to commercial products are usually prepared by voluntary standard bodies and distributed through the standard organizations of the various countries producing and/or procuring those products. Work on international voluntary optical standards began in 1979 under the auspices of the International Organization for Standards (ISO) headquartered in Geneva, Switzerland. This effort is conducted within ISO/TC (Technical Committee) 172, Optics and Optical Instruments. The Deutsches Institut fur Normung (DIN) of Germany functions as secretariat for this Committee. Currently 13 nations are actively participating in this work through their national standards bodies. The latter bodies are listed in Table 1.2. In addition, 27 nations are observers. Another international organization involved in standardization efforts is the International Electrotechnical Commission (IEC). It is the leading global organization that prepares and publishes international standards for all electrical, electronic, and related technologies, including electronics, magnetics and electro-magnet- ics, electro-acoustics, multimedia, telecommunication, and energy production and distribution.

ISO/TC 172 was established to promote standardization of terminology, requirements, inter- faces, and test methods in the field of optics. This includes complete systems, devices, instruments, optical components, auxiliary devices and accessories, as well as materials. Its scope excludes stan- dardization efforts relative to specific items in the field of cinematography (the responsibility of ISO/TC 36), photography (the responsibility of ISO/TC 42), eye protectors (the responsibility of ISO/TC 94), micrographics (the responsibility of ISO/TC 171), fiber optics for telecommunication (the responsibility of IEC/TC 86), and electrical safety of optical elements.

TABLE 1.2

International Organizations Involved in the Development of Voluntary Standards Related to Optics and Optical Instrumentation under ISO TC172

● _{Association française de normalisation (AFNOR) from France} ● _{American National Standards Institute (ANSI) from the United States} ● _{Asociatia de Standardizare din România (ASRO) from Romania} ● _{British Standards Institution (BSI) from the United Kingdom} ● _{Deutsches Institut für Normung (DIN) from Germany (Secretariat)}

● _{State Committee of the Russian Federation for Standardization and Metrology (GOST R) from Russia} ● _{Japanese Industrial Standards Committee (JISC) from Japan}

● _{Korean Agency for Technology and Standards (KATS) from Korea} ● _{Österreichisches Normungsinstitut (ON) from Austria}

● _{State Administration of China for Standardization (SACS) from China} ● _{Standards Australia International Ltd. (SAI) from Australia}

● _{Swiss Association for Standardization (SNV) from Switzerland} ● _{Ente Nazionale Italiano di Unificazione (UNI) from Italy}

To facilitate the development of optical standards and fill the void left by the absence of the U. S. military specifications, a consortium made up of seven professional societies, trade associations, and companies sponsored the incorporation of the Optics and Electro-Optics Standards Council (OEOSC),

which acts as the administrator of national optical standards for the United States.† _{OEOSC has}

received accreditation from the American National Standards Institute (ANSI) for a committee called ASC/OP “Optics and Electro-Optical Instruments.” ASC/OP is now authorized to develop U.S. national standards. OEOSC is also responsible for supporting ISO/TC 172 through a U.S. Technical Advisory Group (TAG). This group is a committee made up of U.S. optical experts whose primary responsibility is to review drafts of proposed international optical standards so that it can formulate U.S. opinions regarding the suitability of those drafts to become international standards, and then to transmit those opinions, through ANSI, to the ISO technical committee. The TAG also is responsible for reviewing U.S. national optical standards to determine which of them should be offered as drafts for new voluntary international standards.

Within ISO/TC 172, seven subcommittees (SC) have been established to address different major topics. Under each active SC, there are several working groups (WG) that do the actual writ- ing. Table 1.3 depicts the organizational structure to WG level as of mid-2002. Draft international standards prepared and adopted by the various ISO technical committees are circulated to the inter- national members of ISO for approval before the ISO Council formally approves them. Approval requires at least 75% acceptance by the member bodies voting.

Most U.S. optical companies and the Department of Defense have long based their engineering drawings for mechanical and optical parts on ANSI Specification Y14.5M-1982, “Dimensioning and Tolerancing,” and ASME/ANSI Specification Y14.18M-1986, “Optical Parts,” respectively. One result of activity by ISO/TC 172 that is particularly germane to this practice is the promulgation of ISO 10110, “Optics and optical instruments — Preparation of drawings for elements and systems,” written by WG 2 of SC 1. This standard is expected eventually to replace ASME/ANSI Y14.18M. One important feature of this standard is that it expresses as many concepts as possible in terms of symbols to minimize the need for notes that would require translation for the drawing to be under- stood in the languages of various countries. Default tolerances are given in the standard for cases in which a specific tolerance is not required. This simplifies the appearance of drawings in those cases.

ISO 10110 has 13 parts as listed in Table 1.4. A few of these parts are worthy of special attention here. The following descriptions are based largely on Parks (1991) and Willey and Parks (1997). The first part deals with the mechanical aspects of optical drawings including lists of items to check for completeness of system layouts, subassemblies, and individual optical element drawings. Only such items as are unique to optics are included. All strictly mechanical aspects of optical drawings are cov-

ered by ISO standards on technical drawings, as contained in ISO Handbooks l2 and 33.‡_{These ISO}

standards are largely compatible with ANSI Y14.5M. In cases where there are differences, wording is included to permit usage of national standards so long as they are called out on the pertinent drawings.

The next three parts of ISO 10110 deal with optical material specifications and are straightfor- ward adaptations of glass catalog specifications for stress birefringence, bubbles and inclusions, and inhomogeneity (including striae).

Part 5 of ISO 10110 deals with optical surface figure errors. Either a visual test plate assess- ment of figures or computer reduction of interferometric fringe or phase data can be employed. Centering tolerances are the subject of Part 6. It shows how to specify centering relative to various datum surfaces. Part 7 covers surface imperfections or cosmetic defects such as those commonly

†_{Information concerning the activities, membership, and progress of this council can be found on the OEOSC web site:}

www.optstd.org.

‡_{It is customary for the ISO to issue groupings of published standards dealing with various aspects of a related subject in the}

form of a handbook. For example, ISO Standards Handbook 33, Applied Metrology—Limits, Fits and Surface Properties was published in 1988 to bring together under one cover 58 standards developed within 7 different TCs; all are related to the sci- ence of measurement. The handbook includes terminology; indication of mechanical tolerances and surface conditions on technical drawings, limits and fits, and properties of surfaces and measuring instruments.

called “scratches and digs.” Either of the two techniques may be used to evaluate these defects. The defect areas can be measured directly or their visibility assessed against an appropriately illumi- nated background. Baker (2002) has described a simple and inexpensive apparatus for quantifying these types of defects. Baker (2004) is a definitive reference on this subject.

Part 8 of ISO 10110 concerns ground and polished surface texture while part 9 tells how to indi- cate that a surface is to be coated. It does not specify what type of coating is to be applied nor what the coating’s characteristics and performance should be. These details are covered in another stan- dard, ISO 9211, Optical Coatings.

Part 10 of ISO 10110 outlines ways to specify simple optical elements in tabular form without preparing a drawing. This is useful, as it allows the opto-mechanical designer to communicate

TABLE 1.3

Listing of Subcommittees and Working Groups under ISO/TC 172, “Optics and Optical Instruments.” Secretariat/Convener for each is Shown in Parentheses (See Table 1.2 for definitions of acronyms)

Subcommittee Title/Working Group

SC1 Fundamental standards (DIN)

WG1 General optical test methods (DIN)

WG2 Preparation of drawings for optical elements and systems (AFNOR) WG3 Environmental test methods (DIN)

WG4 Electronic data transfer (BSI)

SC3 Optical materials and components (AFNOR)

WG1 Raw optical glass (DIN) WG2 Coatings (ANSI)

WG3 Characterization of IR materials (AFNOR)

SC4 Telescopic systems (GOST R)

WG1 Binoculars, monoculars, and spotting scopes (GOST R) WG3 Astronomical telescopes (JISC)

WG4 General test methods (DIN) WG5 Night vision devices (GOST R)

SC5 Microscopes and endoscopes (DIN)

WG3 Terms and definitions (BSI) WG6 Endoscopes (JISC)

WG7 Infinity corrected optics (DIN)

SC6 Geodetic and surveying instruments (SNV) (WG not formalized)

SC7 Ophthalmic optics and instruments (DIN)

WG1 Terminology (BSI) WG2 Spectacle frames (BSI) WG3 Spectacle lenses (AFNOR)

WG6 Ophthalmic instruments and test methods (ANSI) WG7 Ophthalmic implants (SIS)

WG8 Data interchange (ANSI) WG9 Contact lenses (ANSI)

SC9 Electro-optical systems (DIN)

WG1 Terminology and test methods for lasers (DIN) WG2 Interfaces and system specifications for lasers (AFNOR) WG3 Safety (ANSI)

WG4 Laser systems for medical applications (ANSI) WG5 Laser systems for general applications (BSI) WG6 Optical components and their test methods (DIN) WG7 Electro-Optical systems other than lasers (JISC)

manufacturing requirements by computer link. Part 11 of the ISO standard gives a table of default tol- erances applicable to dimensions of manufactured elements for which no tolerances have been given on the drawing. For example, when not otherwise specified, elements from 10 to 30 mm in diameter are expected to have diameters within 0.5 mm of the specified nominal value. If this level of accuracy is adequate for the application, the drawing can be simplified by simply omitting the tolerance.

Part 12 tells us how to specify an aspheric surface in a widely understood and accepted man- ner. Finally, Part 13 describes how to specify a threshold for laser damage to optics. The subject of laser damage is considered in Section 2.2.12.

To assist designers and engineers in the interpretation and application of ISO 10110, the Optical Society of America prepared a Handbook of Optical Standards. This handbook (Kimmel and Parks,