Translation and Interchange forMechanical CAD and Lens Design Data
G. Groot Gregory, Edward R. Freniere, Richard A. HasslerLambda Research Corporation, 80 Taylor Street, Littleton, MA 01460
ABSTRACT
Computer Aided Design (CAD) is often associated with design of mechanical systems. Today optical systems are becomingarbitrarily complex and are often designed via CAD and are always constrained by the mechanical system. Although lensdesigners were one of the early adopters of computer processing, merging lens design information into CAD programs isproblematic. In addition, moving design data from one CAD program to another is neither easy nor straightforward.CAD programs are based on a modeling engine. These engines may be proprietary or open. Moving data from one engine toanother may be achieved using the STEP or IGES interchange formats but will suffer from loss of some modeling data. Themathematical precision of the engine also affects the reliability of the exchanged data when moving from a low precision to ahigh precision engine.
It is also desired to move lens design data into the CAD model. This requires converting the surface representation of theprescription into a usable format for the modeling engine. Optical properties and special component issues must beinterpreted and resolved.
This paper will outline some of the issues encountered when moving data between optical design and mechanical CADprograms.
Keywords: Translation, IGES, STEP, Data Interchange
1. INTRODUCTION
Optical analysis has benefited by the advances in computing and computational power since the first rays were traced in the
early 1960s1. Now in the midst of the age of information, the detail of the computer models has improved due in part to theexplosive growth in the availability of data. LensView™ from Optical Data Solutions provides databases of opticalprescriptions from patent data and many optical component manufacturers have made their lens data available, bothproviding a ready source for assembling models of optical systems. Up-to-date optical glass data is available via the Internet.The Solexis database by Stellar Optics Research International Corporation includes surface proprieties and BSDF curves onCD-ROM via subscription. While merging the lens deign output into a CAD (Computer Aided Design) program has beenpossible for several years, tools are now available to combine all this diverse data into a single design environment
Optical and mechanical information is frequently combined for both imaging and non-imaging applications. The problemcomes in determining in what format the desired data is available and how to convert the data into a format that may be usedby the analysis program at hand. Over the years a number of mechanical design formats have been proposed and adopted butnone provides a perfect solution. No standard optical prescription interchange format has arisen but data exchange ispossible. In order to utilize this data, an understanding of the different types of data representations is necessary. A workingknowledge of the capabilities and shortfalls of the data formats aids in determining how to get the most use from a particulardata set.
2. GEOMETRIC DATA REPRESENTATIONS
In order to begin the data assembly for a comprehensive optical analysis, the geometric data for both optical and mechanicalcomponents must be combined. Several data representations may be used including surface and solid based models. In allcases the model must provide sufficient information to bound the individual components and position them in space. Surfacebased models provide detailed information about curved surfaces2 and the boundary that defines the extent of the surface.Solid models provide additional detail through the relationship of adjacent surfaces that in combination form an enclosed
Copyright © 1999 Lambda Research Corporation
Published in Optical Design and Analysis Software, Proceedings of SPIE, Volume 3780, Denver, 1999
volume. A wireframe representation may also be encountered and provides information about some of the curved edges,which lie on the curved surface, but no information about the surface itself. Wireframe data may be used to build surfaces.Optical prescriptions contain geometric and optical information that can, with care, be converted into any of the aboverepresentations.
Solid models use a detailed topology to define the interrelations between geometric entities within the model. The topologicalelements do not define the geometry but will have geometrical data elements attached to various objects. Figure 1 illustratesthe topology used by the ACIS®3 solid modeling kernel.
BodyLumpsShellsSubshellsFacesLoopsCoedgesEdgesVerticesFigure 1. ACIS Topology
The highest level of model object is a body, and is composed of lumps. Lumps are 1D, 2D, or 3D sets of points in space thatare disjoint with all other lumps. Shells bound lumps with a set of connected faces and wires, and can bound the outside of asolid or an internal void (hollow). Subshells form a further decomposition of shells for internal efficiency purposes. A faceconnects a portion of a surface and is bounded by a set of loops. Loops connect a series of coedges. Generally, loops areclosed, having no actual start or end point. Wires connect a series of coedges that are not attached to a face. Coedgesrepresent the use of an edge by a face and two adjacent edges will share a coedge. A coedge may also represent the use of anedge by a wire. Edges are curves bounded by vertices. The interrelation of the topological elements assures that the modelingengine uses complete and valid geometric entities when moving around the model and interrogating the data.
The geometric data contained in a lens prescription is similar to a surface based representation in that each surface is definedseparately without any topological control. Curvature parameters, an aperture, and additional orientation data such as a tiltand a decentration typically define a lens surface. The distance between each surface is also provided and may be relative to apreceding surface or referenced to an absolute coordinate system. Since lens design raytracing algorithms have historicallybeen sequential, surfaces may fold over and intersect each other without any serious effect to the program’s analysis.
Wires3. MECHANICAL INTERCHANGE FORMATS
Sharing data between various programs is not a new idea. The mechanical CAD industry has launched several interchangemethods from published formats to standards bodies to open architectures.
Individual vendors have developed and published interchange formats to allow other applications to share data. Autodesk®provides DXF (Drawing eXchange Format). DXF allows a neutral interchange format for drawing data without exposing thedetails of AutoCAD® core data format. 3D Systems, Inc. developed STL (StereoLithography Interface Specification) forstereolithography and rapid prototyping but STL is used for other data exchange as well. This format is comprised of a set ofsmall triangular facets that bound the underlying geometry.
Copyright © 1999 Lambda Research Corporation
Published in Optical Design and Analysis Software, Proceedings of SPIE, Volume 3780, Denver, 1999
The most widely used standard format is IGES (Initial Graphics Exchange Specification). IGES dates back to the 1970’s. TheIGES Specification is overseen by the IGES/PDES Organization (IPO). The National Institute for Standards and Technology(NIST) has officially recognized IPO as the official organization responsible for the content of the IGES Specification. IPO isalso responsible for the United States input to the content of the PDES (Product Design Exchange using STEP) standard.STEP (Standard for the Exchange of Product Model Data) is a new and evolving broad-based format, which is beinggenerally adopted by application developers. Other standards include VDAFS (German Automotive Industry Standard) andSET (National French Standard).
Some companies are developing modeling toolkits to provide application developers a common platform and loss-less datatransfer between similarly based programs. ACIS® by Spatial Technology provides modeling functionality and a commondata format for applications. ACIS is the most widely adopted modeling kernel boasting 180 applications and 1.4 millioninstalled seats4. Parasolid® by Unigraphics Solutions® is another popular kernel with some 80+ applications5. Othermodeling engines and toolkits are available.
Figures 2-4 are examples of the STL, IGES and ACIS formats of a thin sheet object. The sheet is a square with height of 2mm oriented in the XY plane with center coordinates of (0,0,1). Note that it is generally not necessary to work directly withthese formats.
solid sheetfacet normal 0 0 1outer loopvertex -1 1 1vertex 1 1 1vertex 1 -1 1endloopendfacet
facet normal 0 0 1outer loopvertex -1 1 1vertex 1 -1 1vertex -1 -1 1endloopendfacet
endsolid sheet
Figure 2 - STL Sheet
File created by the ITI ACIS/IGES 4.0 translator. S 1,,,11Higs_stp.sat,24HITI ACIS/IGES TRANSLATOR,3H4.0,32,38,6,308,15,, G 11.0D0,10,2HCM,1,0.0D0,13H990617.095819,.001D0,0.0D0,,,9,0,13H990617.0958G 219; G 3 143 1 0 0 0 0 0 000000000D 1 143 0 0 1 0 0D 2 128 2 0 0 0 0 0 000010000D 3 128 0 0 4 1 0D 4 141 6 0 0 0 0 0 000010000D 5 141 0 0 1 0 0D 6 110 7 0 0 0 0 0 000010000D 7 110 0 0 1 0 0D 8 110 8 0 0 0 0 0 000010000D 9 110 0 0 1 0 0D 10 110 9 0 0 0 0 0 000010000D 11 110 0 0 1 0 0D 12 110 10 0 0 0 0 0 000010000D 13 110 0 0 1 0 0D 14143,0,3,1,5; 1P 1128,1,1,1,1,0,0,1,0,0,-1.2D0,-1.2D0,1.2D0,1.2D0,-1.2D0,-1.2D0, 3P 21.2D0,1.2D0,1.0D0,1.0D0,1.0D0,1.0D0,-1.2D0,-1.2D0,1.0D0,1.2D0, 3P 3-1.2D0,1.0D0,-1.2D0,1.2D0,1.0D0,1.2D0,1.2D0,1.0D0,-1.2D0,1.2D0, 3P 4-1.2D0,1.2D0; 3P 5141,0,1,3,4,7,1,0,9,1,0,11,1,0,13,1,0; 5P 6110,1.0D0,1.0D0,1.0D0,-1.0D0,1.0D0,1.0D0; 7P 7110,-1.0D0,1.0D0,1.0D0,-1.0D0,-1.0D0,1.0D0; 9P 8110,-1.0D0,-1.0D0,1.0D0,1.0D0,-1.0D0,1.0D0; 11P 9110,1.0D0,-1.0D0,1.0D0,1.0D0,1.0D0,1.0D0; 13P 10S 1G 3D 14P 10 T 1
Figure 3 - IGES Sheet
Copyright © 1999 Lambda Research Corporation
Published in Optical Design and Analysis Software, Proceedings of SPIE, Volume 3780, Denver, 1999
501 0 1 014 TracePro - 2.0 11 ACIS 4.0 NT 24 Thu Jun 17 09:58:11 19991 9.9999999999999995e-007 1e-010body $1 $2 $-1 $-1 #
display_attribute-st-attrib $-1 $3 $-1 $0 1 #lump $-1 $-1 $4 $0 #
id_attribute-st-attrib $-1 $-1 $1 $0 2 #shell $-1 $-1 $-1 $5 $-1 $2 #
face $-1 $-1 $6 $4 $-1 $7 reversed double out #loop $-1 $-1 $8 $5 #
plane-surface $-1 0 0 1 0 0 1 1 0 0 forward_v I I I I #coedge $-1 $9 $10 $-1 $11 forward $6 $-1 #coedge $-1 $12 $8 $-1 $13 forward $6 $-1 #coedge $-1 $8 $12 $-1 $14 forward $6 $-1 #edge $-1 $15 $16 $8 $17 forward #
coedge $-1 $10 $9 $-1 $18 forward $6 $-1 #edge $-1 $16 $19 $9 $20 forward #edge $-1 $21 $15 $10 $22 forward #vertex $-1 $11 $23 #vertex $-1 $11 $24 #
straight-curve $-1 1 1 1 0 -1 0 I I #edge $-1 $19 $21 $12 $25 forward #vertex $-1 $13 $26 #
straight-curve $-1 1 -1 1 -1 0 0 I I #vertex $-1 $18 $27 #
straight-curve $-1 -1 1 1 1 0 0 I I #point $-1 1 1 1 #point $-1 1 -1 1 #
straight-curve $-1 -1 -1 1 0 1 0 I I #point $-1 -1 -1 1 #point $-1 -1 1 1 #End-of-ACIS-data
Figure 4 - ACIS Sheet
The three examples show how differently these formats can be written. The STL and ACIS formats are human readable and itis easy to see the topological references in the ACIS file. IGES defines entities and relations by id codes in the first part of thefile and provide dimensional information in the second part. No relational detail is provided in the STL file.
Each of the examples uses an ASCII (Text) format but binary versions are also available. Binary files takes less space but areharder to troubleshoot if a problem occurs when moving the data between computers or via email. Also binary file transfer ismore difficult when moving data between computers using different operating systems.
4. INTERCHANGE PITFALLS
The success of any data interchange will depend on a variety of conditions. Common failures occur when insufficient data isavailable in the file for the receiving modeling environment. Tolerance and precision difference can also interfere with asuccessful interchange. And while the physical representation has been transferred, the history and constructional details areoften lost. Moving data from a solid representation to a surface representation is easier than moving in the opposite directionbut steps can be made to repair and update the data once the conversion has taken place.
Different systems use different tolerances. The more significant digits in the data, the longer it takes to visualize the modeland perform operations. Problems arise for example when two adjacent edge bounding adjacent faces are out of specificationdue to an increase in precision, so that positions of bounding geometry and reference points are no longer within tolerance.Solving this may require generating a new data file from the originating application with increased precision. In some cases itmay be necessary to use the imported data as a template, and recreate the model. And depending on the type of analysis thedata may be close enough and not affect the results.
Importing solid data into a surface representation works well because the extra topological data may be ignored. Surface datamay have to be converted into a solid when moving data into a solids-based program. Techniques for this include joining andstitching. These operations will add the required topological relationships to the surfaces. Again precision issues play a rolewhen stitching two surfaces which may have adjacent edges slightly askew. Leaks and holes over all bounding surfacesrequire extra surfaces to be defined and appropriately attached to fill the gaps.
Copyright © 1999 Lambda Research Corporation
Published in Optical Design and Analysis Software, Proceedings of SPIE, Volume 3780, Denver, 1999
It is possible to generate surfaces or solids from two-dimensional data and wireframes. A wireframe is a series of connectededges defining the outer boundary of a surface or volume. Two-dimensional data may also be thought of as wireframewithout depth. Covering the wire boundary creates surfaces. These surfaces do not have to be planar but they must intersecteach point of the bounding wire. For instance, if the boundary is circular any surface with cylindrical symmetry could coverthe wire. A centered lens would fit this criterion. Facet data from a STL file defines triangular boundaries that can be coveredwith planar surfaces. Any closed series of edges in the same plane may be likewise covered.
Depth can be added to two-dimensional sheets through sweeping operations. Figure 5 illustrates a boundary consisting of fiveedges covered to form a planar sheet. Sweeping can be used to make extrusions or surfaces of revolution from the sheet. Ineither case the geometry may be made complete for surface or solid data representations.
2D coveredprofile
Extrusion
Figure 5 - Sweeping a 2D profile
Revolution
Reliable and efficient data translation is market driven and constantly improving. Healing is a relatively new technique thatprovides the ability to simplify surfaces, stitch geometry and rebuild the model to the appropriate topology and precision.Surface simplification attempts to replace spline and NURBS surfaces with planes and other analytic surface types. This cancause problems when importing weak aspherics when the simplification tolerance is greater than the surface departure thefrom lens base curvature.
Unfortunately, data representation is not the whole story behind the geometry. Each CAD program has its own method ofbuilding the objects and this information is lost by any intermediate exchange format. Any parameterizations used to definethe part will be left out and the history of modeling operations cannot be translated. These data are limited to the generatingapplications. This is not a problem if the data does not have to return for further modification but is a consideration during theactive design phase of the system. Often times when sharing CAD data with other analysis tools it is best to restrict all designchanges to a single tool and use the current model data for the analysis.
5. CONVERTING LENS DATA INTO GEOMETRY
In principle converting optical prescriptions to surface or solid models is straightforward. A simple bi-convex lens can beconstructed into a solid by defining the boundary of its enclosed volume. Using a mechanical CAD program two spheres canbe created and positioned such that the edges of the spheres along the optical axis intersect the vertex positions from the lensprescription. A Boolean intersection will leave a lens element, which has an oversized aperture. Defining a cylinder with adiameter equal to the clear aperture of the desired lens provides a tool for a second Boolean intersection resulting in thecomplete lens element. Figure 6 illustrates these steps. A Boolean operation can be thought of as the interaction of a piece ofmaterial and a tool. During a subtraction operation the result is the material after removing the portion occupied by the tool.Experience in a machine shop or with mathematical set operations makes thinking in Boolean quite easy.
Copyright © 1999 Lambda Research Corporation
Published in Optical Design and Analysis Software, Proceedings of SPIE, Volume 3780, Denver, 1999
The approach used in Figure 6 is called CSG (Constructive Solid Geometry) but the data is stored in a BoundaryRepresentation (B-REP) with all of the topological elements described above. It is easier to envision how to construct anobject using CSG so it is left to the modeling kernel to convert the user operations into the appropriate data representation.
Lenselement
Build spheres from lens
curvatures
Intersect spheres anddefine cylinder for
aperture
Figure 6 - Constructing a lens from primitives
In reality, optical systems tend to have much tighter dimensional performance specifications than those found in mechanicalsystems. This can cause problems when trying to analyze a system with 1/10th wave surfaces and tight positionalrequirements when the analysis tool has a limiting resolution of .001 model units with model units set to inches. Standardoptical surface polynomial forms are not standard in the mechanical world. Building an aspheric surface may requireapproximating the surface using splines or NURBS (Non-Uniform Rational B-splines).
Over the years lens designers and lens design programs have discovered means to enter more complex data than sequentialsurface records of lens elements. Non sequential groups, prisms, axicons and other data can be a nightmare when trying toconvert the data to geometry. Figure 7 is an excerpt of a lens prescription that includes a Dove prism from ACCOS V™ byOptikos Corporation. The lens prescription defines three surfaces: an input, an internal reflection and an output. A real prismor its solid representation requires six surfaces. By including the clear aperture data (CLAP) and understanding the intent onecan build the surfaces of a solid representation, but with the general nature with which these surface types may be defined, anautomatic or cookbook approach is difficult to design.
DEC -4.1E-01 TILT -45 CLAP 1.415 GLASS GERM C SURFACE 3 TILT -45 REFL DEC 2.0 TILT 135 CLAP 1.415 TH 1.5 AIR DEC 6.502242E-01 RTILT 45 CC -1.0 ASI CV -5.0E-01 CLAP 3.20E-01 TH -1.0 REFLInputOutputInternal ReflectionFigure 7 - Dove PrismCopyright © 1999 Lambda Research Corporation
Published in Optical Design and Analysis Software, Proceedings of SPIE, Volume 3780, Denver, 1999
6. CONCLUSIONS
A wide array of data is available for use with today’s design tools to assist the engineer in building more accurate andcomplete system models. Merging data from various data formats is becoming commonplace and does not require the user ofdata to be an expert in the use of the tool with which it was generated. However, an understanding of the methods,capabilities and limitations associated with the tools and the formats will save much effort when trying to figure out whythings just don’t work. Mechanical interchange formats are well established although diverse. These are market driventechnologies with a great deal of competition and will no doubt improve rapidly. Additional techniques are becomingavailable to improve the interchange after conversion.
Incorporating optical prescriptions into CAD based analysis programs is and will be challenging. Optical tolerances oftenexceed those used in mechanical devices and can easily exceed capabilities of the CAD tools. Apsheric surfaces may requirealternative surface forms and there are no geometric equivalents for Diffractive or Holographic optical elements. And lensdata does not have to exist past the aperture or for any surfaces not in the path of a ray.
The differences in capabilities between CAD based optical analysis programs and lens design software is narrowing. Morecomplex analytic and parametric surface types are being added to geometric modeling engines. These surface forms allowmore accurate modeling of aspheric optical surfaces. Lens design programs are improving their non-sequential raytracecapabilities, permitting more complicated geometry to be designed within the program. Both trends will make datainterchange easier and more reliable. On one hand the CAD based programs will be able to capture more of the opticalprescription and surface data, while on the other the level of detail required by the lens design program will be closer to thatrequired by CAD based data set.
7. ACKNOWLEDGEMENTS
Trademarks and registered trademarks are the property of their respective owners.
8. REFERENCES
1
E. R. Freniere and J. Tourtellot, Lens Design, Illumination, and Optomechanical Modeling, Volume 3130, 170-178, SPIE,San Diego, 19972
M. Mantyla, An Introduction to Solid Modeling, Computer Science Press, Rockville, MD, 19883
ACIS Save File Format Manual, Chapters 3-4, Spatial Technology, Inc., Boulder, 19964
Spatial Technology, http://www.spatial.com/5
Unigraphics Solutions, http://www.unigraphics.com/
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