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Technological Foundations of the RP Development

But several technological components had to be developed before a three dimensional configuration of a model could be achieved that derived from a digital model:

1.1.3a Creation of 3D CAD software

1.1.3b Numerically Controlled Machines

1.1.3c Machine and Material Interaction

1.1.3a 3D CAD Software development

The visionary application of computers for the creation of technical drawings starts in the early 1950 with the science fiction novel “The door into summer” by Robert Heinlein[10]. Heinlein’s character Daniel Boone Davis developed the idea of a machine named “Drafting Dan” that was able to replace the manual drafting table with a display driven by a keyboard.

One of the first applications of graphical information display can be seen at the project SAGE that was developed by MITRE[11] from 1953 on. The laboratories started a first major real-time, computer-based command and control system that processed network information of arriving enemy aircraft and displayed the aircraft tracks and identification on an interactive monitor build from a cathode ray tube. Light guns that were related to the monitors were used to pinpoint flight tracks. Sage was fully deployed in 1963 and ended in 1983. The required hardware for such an operation was provided by a then little known company called IBM who won the contract to design and build the Whirlwind II computer, otherwise known as AN/FSQ-7, for this proposed new air defense system. When complete, the AN/FSQ-7 weighed 250 tons, and required a 3,000kW power supply and over 49,000 vacuum tubes. When SAGE became fully operational, it relied on 24 AN/FSQ-7s; they remained in service until the Air Force ended the SAGE program in 1983[12].

Sage Interface with Cathode Ray Tube

In 1963 Dr. Ivan Edward Sutherland’s software “Sketchpad” delivered a prototype for future CAD applications. This first graphical software was based on a two dimensional constraint drawing area. It enabled the user to create drawings that included a digital pen based input device for a cathode ray tube monitor, referenced objects and constraint relationships. Its drawing elements were limited to straight and lines and circular arcs. Sketchpad was able to store explicit information about the topology of a drawing. If the user decided to moves one vertex of a polygon, both adjacent sides would be moved in a constraint relationship. If the user moved a symbol, all lines attached to that symbol would automatically move to stay attached to it. The topological connections of the drawing are automatically indicated by the user as he sketches. The Tx-2 computer was connected to a plotter that enabled the physical printout of the created digital drawings.

Due to its very limited distribution “its influence has been via the ideas that it introduced rather than in its execution.”[13] The expensive hardware required for the operation of these early drawing devices reserved their application mainly to major companies in the automotive and defense sector. [14]

In the late 1960ies digital drawing was limited to two dimensional descriptions of objects, but in 1967 SynthaVision by MAGI, the Mathematical Applications Group, Inc. was presented and is considered the first solid modeling system. The software developed out of a nuclear radiation transport analysis that combined early Ray tracing algorithms and their dependant interactions with volumetric entities. The software was a constructive solid geometry (CSG) system that was limited in its ability to create complex objects[15]. With the use of combinatorial geometry[16] as Boolean operations, whole voluminous shapes such as boxes, spheres and cones were added or subtracted to form more complex shapes. The application of SynthaVision can be seen in the first half of the movie TRON[17] in the Game Grid area, where the software was used to create such three dimensional vehicles as the Lightcycles, Recognizers and Tanks.

Tron Lightcycles

The development of an interactive graphics interface and the uprising solid modeling software laid the foundation for a computer aided design (CAD) practice that was changing the workflows of the affected industries.

The next years saw an implementation of the uprising modeling technology with manufacturing methods, that was well received and financed by the different industries from automotive, defense and aeronautic sectors. These financially potent users where the only target group that was able to finance the still expensive hardware requirements needed to operate CAD/CAM processes. At the end of 70s a typical CAD system was a 16-bit minicomputer with maximum of 512 Kb memory and 20 to 300 Mb disk storage at a price of 125,000 USD[18].

Several milestones were important for the integration of this new work pipeline that bundled design and production efforts. The innovations in the usability and mathematical surface description were combined with the advancements in computer calculation power and display qualities. The sinking prices of hard and software led to a broader acceptance that expanded from the initial financially powerful corporations to the contemporary common

A list of key innovations in the areas software, hardware development and data handling standards is listed below:

Software

In 1972 Patrick J. Hanratty wrote ADAM which is considered to be the first commercially available integrated, interactive graphics design, drafting, and manufacturing system[19]. Hanratty is considered the father of CAD/CAM technology with a background in the development of PRONTO, a numerical-control programming system and DAC, Design Automated by Computer, a production interactive graphics manufacturing system developed for automotive fabrication[20]. This kernel of ADAM was written in FORTRAN and Hanratty designed it to run on virtually any machine. Even today, it is estimated that around 70% of all major CAD/CAM companies can trace their software back to code written by Hanratty for Adam, AD-2000, and its follow-on Anvil-4000. Adam, for example, was licensed by Computervision Corp. for Cadds, by Gerber Scientific Inc. for IDS 3, and McDonnell Douglas for Unigraphics[21].

The first stress analysis packages were programmed by Jack Lemon’s Company Structural Dynamics Research Corporation (SDRC), their profile contained applications as Superb FEA, a finite element analysis program, and SuperTAB, the first commercial modeling package that ran on DEC (Digital Equipment Corporation) workstations. At that time, both Ford Motor Company and General Motors started using SDRC software for pre-and post process analysis.

In 1975 Avions Marcel Dassault (AMD), now Dassault Systemes purchased CADAM (Computer-Augmented Drafting and Manufacturing) software equipment licenses from Lockheed and become one of the very first CADAM customers. CADAM was a two dimensional drawing tools and by 1977 AMD assigned its engineering team the goal of creating a three-dimensional, interactive program, the forerunner of CATIA (Computer-Aided Three-Dimensional Interactive Application). Its major advance over CADAM was the 3rd dimension. In 1984 drafting capabilities were added to CATIA, enabling it to function independently of CADAM. By 1985 CATIA Version 2 contained fully integrated drafting, solid and robotics functions, making it the aeronautical applications leader. By 1988 CATIA Version 3 contained AEC functionality and was ported to IBM’s UNIX-based RISC System/6000 workstations. CATIA thus became the automotive applications leader as well[22].

Alias/ Wavefront presented the first commercially available NURBS real-time surface handling and interaction software in 1989 that was running on Sgi’s Irix machines. In 1993 a unified effort between CAS and CAA in Collaboration with the Technical University in Berlin was resulting in the presentation of the first interactive Nurbs Modeler for PCs named NöRBS.

Autodesk was founded in 1982 by John Walker, who created AutoCAD. The software was based on a CAD program written in 1981 by Mike Riddle called MicroCAD, changed later to Interact. It was shown at the COMDEX trade show 1982 in Las Vegas as the first CAD program in the world to run on a PC.

Hardware

Computervision was created in 1969 to produce systems for production. In 1978, Computervision introduced the first CAD terminal using raster display technology. [23]

For a large scale graphical output of the digital information California Computer Products, Inc. (Cal Comp) released in 1959 with its Model 565 the worlds first drum plotter. Calcom later produced the legendary Roll Plotter 1040 in 1984

Conventions

In 1979 Boeing, General Electric and NIST developed a neutral file format as a contract from Air Space called IGES (Initial Graphic Exchange Standard).

Autodesk’s AutoCAD defined the file formats DWG and DXF as the standard exchange format for all CAD software.

1.1.3b Numerically Controlled Machines

The following paragraphs describe a selection of two innovations that were necessary for the mechanical, material and computational developments of Numerically Controlled Machines. These descriptions explain the connection between a numerical input and a mechanical movement in the context of industrial production.

The text therefore doesn’t strive to delineate a complete a timeline of the developments of computer science but rather describe the innovation environment that is prerequisite for the mechanical Rapid Prototyping technology.

The interaction between a mechanical operation and numerical information that stored programmatic information can be traced back to early 18th century. The advancements in this field of matrix based mechanical engineering were founded on Innovations developed during the era of the industrialization.

Jacques de Vaucanson (1709-1782), a French Inventor, engineer and Flight pioneer succeeded in automating the loom by means of perforated cards that guided hooks connected to the warp yarns. Power was to be supplied by falling water or by animals.

“…The [loom] was improved in 1728 by increasing the number of needles and using a rectangular perforated card for each individual shedding motion, the cards being strung together in an endless chain.”[24]

Yet these first automated weaving machines were still restricted to a plain or very simple weaving pattern. In 1801 Joseph Marie Jacquard improved the loom to produce far more complex patterns. Jacquard controlled the action of the weaving process by devising an interface for the behavior of the loom for a specific pattern to be reproduced. Jacquard arranged for the pattern to be depicted as a group of holes `punched’ into a sequence of pasteboard card. “Each card contained the same number of rows and columns, the presence or absence of a hole was detected mechanically and used to determine the actions of the loom. By combining a `tape’ of cards together the Jacquard loom was able to weave (and reproduce) patterns of great complexity, e.g. a surviving example is a black and white silk portrait of Jacquard woven under the control of a 10,000 card `program’.”[25]

These innovations that were driven by an automatization of repeatable actions lay foundation for the upcoming advancements of computer sciences that rendered a more complex goal in the performance criteria expected from the machine.

Analytical Engine by Charles Babbage

The English inventors Charles Babbage and Ada Lovelace developed a calculation machine that was remarkably close to the contemporary computer. His earlier developed Difference Engine was followed up by the Analytic Engine that was planned to perform algebraic operations upon given letters. The engine consisted of several isolated parts, a Mill, or the calculating unit; the Store, which consisted of columns of figure wheels; input devices (the card readers), and output devices (printer or card punches).

Ada Lovelace documented the design and logic and wrote first programs.

“… here are two sets of cards, the first to direct the nature of the operations to be performed - these are called operation cards; the other to direct the particular variables on which these cards are required to operate - these latter are called variable cards.“[26] These variable cards were completed with a set of cards that would describe fixed numerical values as π.

…The Analytic Engine is therefore a machine of the most general nature. Whatever formula [algorithm] it is required to develop, the law of its development must be communicated to it by two sets of cards. When these have been placed, the engine is special for that particular formula. The numerical value of its constants must then be put on the columns of wheels below them, and on setting the Engine in motion it will calculate and print the numerical results of that formula.” [27] The Analytical Engine was planned to contain `looped’ functions and was capable of conditional branching (IF… THEN… statements) i.e. automatically take alternative courses of action depending on the result of a calculation.[28]

Beside the budgeting problems a general lack of tooling precision that was needed for the accomplishment of the mechanical parts prevented Babbage and Ada Lovelace from finalizing the machine.[29]

But the remarkable idea of the Analytical Engine was its ability to perform with a limited number of input information a broad range of mechanical operations that would lead to information output. The systematic employment of punch cards for the configuration was steering differentiated mechanical actions. These early innovations contained complex mechanical processes that would be steered by an elaborate program to put forth an either physical object as a fabric, or analytical information by the Analytical machine.

The further development of industrial manufacturing in the 20th century can be seen in another key innovation, the “Motor controlled apparatus for positioning machine tool”. This tool was innovated by the American John T. Parsons and was patented in 1952. Parsons interests lay in solving complex problems of structural and aerodynamic blade design. He describes the purpose and reason for the invention in the first chapter of his patent:

“This invention relates to a method of and means for shaping and modifying work pieces and more particularly to a method of and means for automatically controlling
machine tools, such as milling machines and the like, from media containing stored information such as cards or tape punched or otherwise modified for this purpose.
In the production and manufacture of machined surfaces, such as airfoil shapes, it is customary to use models or templates either as a guide for the machine tools or as
a checking device for measuring the accuracy of the work as the machining operation progresses. This required the expensive practice of first constructing models or templates,
or both, having an accuracy better than that required for the finished work. Such procedures are long and tedious, and the accuracy has been dependent upon the skill and care of the workmen…”[30]

Parsons developed this automated milling machine for the production of geometrically complex airfoil wings in a controllable and reproducible quality independent of patterns. Parsons defined an airfoil template through 17 points that were given between the radii on the upper and lower surfaces. 200 points were given along the radius of each surface. He interpolated these curves from a defined cloud of points to describe the helicopter blades for Numerical Control machining. The machine used IBM punch-card accounting machines to speed up all the design calculations and to eliminate human errors in the manufacturing. Additional storage media were Mylar or Polyester tapes. Parsons steered the milling head with servos. This development was supervised by the MIT’s Servomechanisms laboratory in the 1940ies and 50ies. The research institute engineered a servo-control system for advanced radar used on U.S. Navy ships. The lab’s war time developments that were centered on fire control and gun-positioning instruments were embodied in the numerical control apparatus of Parson’s airfoil wing milling machine.[31]

These early machines contained hard wiring and fixed operating parameters. They required a technology developed by Edmund U. Cohler and Joseph E. Monafaan for the translation of digital information to analog action via a binary switch. “To activate external devices requiring a voltage whose magnitude is proportional to a number contained in a computer register. In electronic computers this register may consist of a series of “flip-flops”[switches] with one voltage level of each representing the “zero” and another voltage level the “one” of a binary mathematical system.“[32] The need for paper based storage devices, as the punched tapes ceased through the developments in the fields of data storage and computational power. The calculation and storage of the geometric information of the manufacturing pattern was more and more generated and saved within the computer. The control of the NC machine was achieved through a regulation of the engines and machine parts that would interpolate the set point with the actual value by position, angle and status sensors. The controlled milling head received its path information from a chart that described the x, y and z-axis coordinates [33]. The MIT´s Servomechanisms Laboratory Computer Application Group, led by Douglas T. Ross, also developed the Automatically Programmed Tool Language (APT), an easy-to-use, special purpose programming language. Eventually, APT became the world standard for programming computer-controlled machine tools.[34]

The machine’s action information is programmed by G-codes[35] that inform the tool about coordinate system, tool position, units etc. The following example G01 is a straight-line feed move in a combination of X, Y or Z axis. It’s used specifically for the linear removal of material from a work piece.[36]

Example G01 Action

% (PROGRAM START FLAG)

:1002 (PROGRAM #1002)

N5 G90 G20 (BLOCK #5, ABSOLUTE IN INCHES)

N10 M06 T3 (TOOL CHANGE TO TOOL #3)

N15 M03 S1250 (SPINDLE ON CW AT 1250RPM)

N20 G00 X1 Y1 (RAPID OVER TO X1,Y1)

N25 Z0.1 (RAPID DOWN TO Z0.1)

N30 G01 Z-0.125 F5 (FEED DOWN TO Z-0.125 AT 5IPM)

N35 X3 Y2 F10 (FEED DIAGONALLY TO X3, Y2 AT 10IPM)

N40 G00 Z1 (RAPID UP TO Z1)

N45 X0 Y0 (RAPID OVER TO X0, Y0)

N50 M05 (SPINDLE OFF)

N55 M30 (PROGRAM END)

1.1.3c Machine and Material Interaction

The first commercially available rapid prototyping machine was developed 1988 by 3D Systems (Rock Hill, SC)[37] [38]. The company invented the Stereo Lithography apparatus (SLA), a method that uses a liquid photopolymer resin that is solidified by a UV laser to generate parts. An SLA machine consists of the following parts: a build platform, resin vat, recoating blade, ultraviolet laser and a scanning device. The 3D computer based component design is divided into 0.02 to 0.1mm thick layers. An Nd: YVO4 solid state laser is then scanned onto the surface of the monomer, drawing the cross section of the component. The build platform, which translates up and down, is suspended in the vat of resin. The build platform is placed slightly under the surface of the resin. The laser beam hardens the resin when it makes surface contact. Contemporary materials that are used for the SLA printing are photocurable liquid resins, mostly Epoxy based or reinforced ceramic, optically clear resin with ABS like properties for Mold Making and flexible rubber-like materials.

3D systems implemented a build material whose properties had been known for a while yet the technological precision for a manufacturing required for a functioning machine hadn’t been achieved before the closing years of 1980ies. Nevertheless did the idea of a layered assembly for the description of a digital model existed long before the first machine was engineered. [39] Wynn Kelly Swainson described in 1971 a manufacturing system that contained the key elements of the SLA technology. He described a system were “Method, apparatus and product in which a three-dimensional figure is formed in situ in a medium having two active components by causing two radiation beams to intersect in the media. The dissimilar components are selected to respond to the simultaneous presence of the beam and to either react or to produce reactants which render the intersection of the beams physically sensible or distinguishable. The beams trace surface elements of the figure to be produced.”

UV hardening of photosensitive Polymers had been patented already in January 1945. The technique was invented for the coating of circuit boards. Hereby a very thin layer of liquid resin was placed on the source object and hardened through the UV light emitted by “a sunlight, a mercury vapor lamp, a tungsten bulb, or some similar arrangement to give light either in the range of the ultraviolet rays plus visible light, or in the range of visible light alone.”[40] After the initial hardening the object was then baked several hours at about 75° C to insure complete polymerization.

However the resolution of the light source that was needed for the UV curing in the circuit boards was too low and not adaptable for a rapid prototyping method. The creation of a precise materialization required a point based light source that would be directed around the outlines of the sectional layers of the material.

Theodore Maiman

In 1961 Theodore H. Matman, Los Angeles, Calif., assigned to Hughes Aircraft Company, Culver City, Calif. patented a device capable of generating and amplifying coherent light with the name of “Ruby Laser Systems”[41] because of dependency on a ruby crystal grown. The Lasers allows the emission of light with a narrow wavelength spectrum (”monochromatic” light). This light of a specific wavelength that passes through the gain medium is then amplified. The surrounding mirrors of the gain ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light passes through the partially transparent mirror and escapes as a beam of light. The fine laser tip and the controllable UV spectrum rendered the technology precise enough to track the detailed object outlines in a photo curing process of the liquid polymers. Commonly a solid state laser is in the rapid prototyping technology. In the first stereolithographic machine an electronic synchronization of laser beam, servo and mechanical performance was applied that recycled an electronic control system for a linearly slanted print head[42].

The way for the first generation of rapid prototyping machines was paved by seven factors:

o The ability to translate geometric information that was numerically interpolated into a machine path with consideration of possible material- and tooltolerances.

o The arrival of 3D solid modeling software as in 1967 SynthaVision by MAGI.

o Advanced servomechanisms for the steering of tool activity.

o The application of UV curable photopolymers as a build material

o An electronic control systems for the coordination of the ultra violet solid state Nd: YVO4 Laser[43] Laser

o A programming language that would translate geometric information into machine paths.



[10] Heinlein, Robert A.. The Door into Summer. New York: Del Rey, 1997.

[11] The MITRE Corporation was formed out of the Computer System Division of the Massachusetts Institute of Technology (MIT) Lincoln Laboratories” in MITRE - About Us - MITRE History - Semi-Automatic Ground Environment (SAGE).” MITRE–Applying Systems Engineering and Advanced Technology to Critical . 25 May 2005. 20 Aug. 2008 <http://www.mitre.org/about/sage.htm

[12] MITRE–Applying Systems Engineering and Advanced Technology to Critical . 25 May 2005. 20 Aug. 2008 <http://www.mitre.org/about/sage.htm

[13] Sutherland, Ivan Edward . Sketchpad: A man-machine graphical. New preface by Alan Blackwell and

Kerry Rodden. Cambridge CB3 0FD: University of Cambridge, 2003. p. 2

[14] Bozdoc, M. (2006b). Introducing CAD. Retrieved from iMB: Resources and information for professional designers website (http://mbinfo.mbdesign.net)20.08.2008: http://mbinfo.mbdesign.net/CAD-Intro.htm.

[15] “Mathematics Application Group, Inc. (MAGI) Synthavision.” Magi. 20 Aug. 2008 <http://design.osu.edu/carlson/history/tree/magi.html>.

[16] “Combinatorial geometry is a blending of principles from the areas of combinatorics and geometry. It deals with combinations and arrangements of geometric objects and with discrete properties of these objects. It is concerned with such topics as packing, covering, coloring, folding, symmetry, tiling, partitioning, decomposition, and illumination problems. Combinatorial geometry includes aspects of topology, graph theory, number theory, and other disciplines.” in Goodman, Len and Weisstein, Eric W. “Combinatorial Geometry.” From MathWorld–A Wolfram Web Resource. http://mathworld.wolfram.com/CombinatorialGeometry.html

[17] Tron. Dir. Steven Lisberger. Perf. Jeff Bridges, Bruce Boxleitner, David Warner. DVD. Walt Disney Video, 1982.

[18] “CAD Chronology: The 70’s.” i-MB Resources and Information for professional designers. 21 Aug. 2008 <http://mbinfo.mbdesign.net/CAD1970.htm>.

[19] “MCS: Company History.” MCS: ANVIL-EXPRESS - 3D CAD/CAM/CAE Software. 21 Aug. 2008 <http://www.mcsaz.com/about/history.htm>.

[20] “MCS Founder.” ANVIl Express. 20 Aug. 2008 <http://www.mcsaz.com/about/founder.htm>.

[21] “The CAD/CAM Hall of Fame.” The CAD/CAM Hall of Fame. 11 Jan. 1998. 21 Aug. 2008 <www.americanmachinist.com/304/Issue/Article

[22] Section 10: CAD/CAM/CAE/CADD. Retrieved August 21, 2008, from http://design.osu.edu/carlson/history/lesson10.html.

[23] for a detailed description of the historical CAD/ CAM development see the “A Critical History of Computer Graphics and Animation” course presented at the Ohio State UniversityWayne, C. (n.d.). Section 10: CAD/CAM/CAE/CADD. Retrieved August 21, 2008, from http://design.osu.edu/carlson/history/lesson10.html.

[24] “punched card.” Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 22 Aug. 2008 <http://www.britannica.com/EBchecked/topic/483451/punched-card>.

[25] Dunne, Paul E. . “History of Computation - Babbage, Boole, Hollerith.” Department of Computer Science - University of Liverpool. 22 Aug. 2008 <http://www.csc.liv.ac.uk/~ped/teachadmin/histsci/htmlform/lect4.html>

[26] Dunne, Paul E. . “History of Computation - Babbage, Boole, Hollerith.” Department of Computer Science - University of Liverpool. 22 Aug. 2008 <http://www.csc.liv.ac.uk/~ped/teachadmin/histsci/htmlform/lect4.html

[27] Dunne, Paul E. . “History of Computation - Babbage, Boole, Hollerith.” Department of Computer Science - University of Liverpool. 22 Aug. 2008 <http://www.csc.liv.ac.uk/~ped/teachadmin/histsci/htmlform/lect4.html

[28] ” Science Museum - Online Stuff - Babbage.” Science Museum . 22 Aug. 2008 http://www.sciencemuseum.org.uk/onlinestuff/stories/babbage.aspx?page=5

[29] Dunne, Paul E. . “History of Computation - Babbage, Boole, Hollerith.” Department of Computer Science - University of Liverpool. 22 Aug. 2008 <http://www.csc.liv.ac.uk/~ped/teachadmin/histsci/htmlform/lect4.html

[30] *Patent Parsons, John T., Stulen, Frank L. 1958 Motor controlled apparatus for positioning machine tool, United States, PARSONS CORP, 2820187, http://www.freepatentsonline.com/2820187.html

[31] “History: MIT Servomechanisms Laboratory: Institute Archives & Special Collections: MIT.” MIT Libraries, MIT Library. 27 Aug. 2008 < http://libraries.mit.edu/archives/mithistory/histories-offices/servo.html>

[32] Cohler, Edmund U., Monahan, Joseph E. 1960 Digital to analog converter United States SYLVANIA ELECTRIC PROD 2956272 http://www.freepatentsonline.com/2956272.html

[33] “John Parsons–A Master of Manufacturing.” Institute of Precision Engineering, Hong Kong . 25 Aug. 2008 <www.ipe.cuhk.edu.hk/download.ht

[34] Andrews, Elizabeth , Judith Janec, Denis Meadows, and Joshua Schneider. “MASSACHUSETTS INSTITUTE OF TECHNOLOGY..” Records, 1940-1959. 26 Aug. 2008 <libraries.mit.edu/archives/research/collections/collection

[35] fixed in the DIN 66025

[36] “CNC G Codes Definitions Examples Programs Programming Learning Training.” CNC Verification Simulation CNC Mill CNC Lathe G Codes CNC Machines CNC Tools CNC Learning Training Fanuc Fagor Fadal Okuma . 25 Aug. 2008 <http://www.cncezpro.com/gcodes.cfm>.

[37] First outline of the technique was patented by Hull, Charles. “Apparatus for production of three-dimensional objects by stereolithography.” Google. 11 Mar. 1986. 26 Aug. 2008 <http://www.google.com/patents?id=y

[38] Revised version of the initial patent with a optimized resin surface behavior see Hull, Charles. “METHOD OF AND APPARATUS FOR
P
RODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY WITH
REDUCED CURL.” Google. 17 Apr. 1989. 26 Aug. 2008 <http://www.google.com/patents?id=KGsiAAAA

[39] Swainson, Wyn Kelly . “Method, mediumMethod, medium and apparatus for producing three-dimensional figure product.” Google Patents. 9 Aug. 1977. 26 Aug. 2008 <http://www.google.com/patents?id=Uv0

[40] PROCESS OF PHOTOPOLYMERIZATION. ” Patent number: 2367660. 17 Apr. 1989. 26 Aug. 2008 <http://www.google.com/patents?id=UttaAAA

[41] Matman, Theodore H. . “Patent number: 3353115.” RUBY LASER SYSTEMS. 13 Apr. 1961. 27 Aug. 2008 <http://www.google.com/patents?id=b-lU

[42] Peer, Thomas . “US Patent number: 4567570.” Electronic control system for a linearly slanted print head. 16 Feb. 1983. 27 Aug. 2008 <http://www.google.com/patents?id=ncI

[43] HIGHLY EFFICIENT DEVICES USING CENTROSYMMETRIC PEROVSKITE CRYSTALS BIASED TO SEVERAL TT PHASE RETARDATIONS .” Google Patents. 17 Apr. 1989. 26 Aug. 2008 <http://www.google.com/patents?id=L6p