Final Research

‫جامعة الملك فيصل‬ ‫كلية العلوم الدارية والتخطيط‬ ‫الدمام‬

‫‪Optical Scanning‬‬ ‫مقدم من الطالبة‬ ‫أروى سعيد الغامدي‬ ‫شعبة ‪453 /‬‬ ‫الرقم الكاديمي‪252300152 :‬‬

‫للحصول على باقي درجات مادة تدريب العملي ومقدارها ستون علمة‬ ‫‪ 1430_ 1429‬هــ‬

Table of Contents

Introduction..............................................................................................................................III 1. Optical Scanning Holography...............................................................................................V VI........................................................................................Principle of Optical Scanning 1.1 2. Optical Scanning Technologies............................................................................................XI XI................................................................................................................Galvanometric 2.1 XIII....................................................................New Method for Precision Scanning 2.1.1 XX................................................................................................................Summary 2.1.2 XXI.....................................................................................................................Polygonal 2.2 XXII.................................................................................Types of Scanning Mirrors 2.2.1 3. Optical Scanning Holography Applications.................................................................XXVII XXVII......................................................................Principle of Digital Holographic 3.1.1 ......Numerical Evaluation of Digital Holograms by Non-Diffractive Reconstruction 3.1.2 XXIX XXX....................................................................................................3D TV and Display 3.2 XXX..........................................................................................................Description 3.2.1 XXXI........................................................................................Types of 3D Displays 3.2.2 XXXII..........................................................................................................Problems 3.2.3 XXXII.............................................................................................Existing Displays 3.2.4 4. Types of Optical Scan Systems ..................................................................................XXXIV XXXIV..............................................................................................Mark Sense Systems 4.1 XXXV.....................................................................................Security and Concerns 4.1.1 XXXVI....................................................Benefits of Optical Scan Voting Machines 4.1.2 XXXVI.......................................................................................Electronic Ballot Marker 4.2 XXXVIII................................................................................Digital Pen Voting Systems 4.3 5. Optical Scanning Errors......................................................................................................XL XL.....................................................................................................Optic-Related Errors 5.1 XL.................................................................................................................Polygons 5.2.1 XL..............................................................................................................Monogons 5.2.2 6. Error Correction.................................................................................................................XLI XLI......................................................................................................................Polygons 6.1 XLI....................................................................................................................Monogons 6.2 Index.....................................................................................................................................XLII Glossary................................................................................................................................XLII Resources.............................................................................................................................XLV

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Introduction Mark sense systems employ a ballot card on which candidates and issue choices are preprinted next to an empty rectangle, circle, oval, or an incomplete arrow. Voters record their choices by filling in the rectangle, circle or oval, or by completing the arrow. After voting, the voters either place the ballot in a sealed box or feed it into a computer tabulating device at the precinct. The tabulating device reads the votes using "dark mark logic," whereby the computer selects the darkest mark within a given set as the correct choice or vote. Mark sense technology has existed for decades and been used extensively in such areas as standardized testing and statewide lotteries. Although Mark sense systems are often referred to as "optical scan," Mark sense technology is only one of several methods for recognizing marks on paper through optical reading techniques. Mark sense systems were used by 24.6% of registered voters in the United States for the 1996 Presidential election, and their use is on the rise. Also Optical Programming area provides scoring of exams, data collection, and programming services for the campus. The Optical Scanning area uses a National Computer Systems OPSCAN 10 optical mark reader (See Figure 1.1) capable of scanning up to 5500 sheets per hour. Faculty members giving multiple choice/True-False exams can have them scored and receive the results usually on the same day. When scanning an exam, the number of correct responses a student has is printed on the individual test sheet. In addition, a report is provided. Exam files are kept for a semester. A copy of the scanned data may be obtained on a diskette or sent to the user id at user request.

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Figure 1.1 Shows OPScan 10 optical mark reader Machine

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1. Optical Scanning Holography As three-dimensional (3D) imaging and fluorescence techniques become standard in optical microscopy, novel approaches to 3D fluorescence microscopy are emerging. One such approach is based on the incoherent holography technique called optical scanning holography (OSH). Optical scanning holography (OSH) is a form of electronic (or digital) holography. It is a unique, real-time technique where holographic information of a three-dimensional (3-D) object can be acquired by using a single 2-D optical scan. OSH was first implicated by Poon and Korpel when they investigated bipolar incoherent image processing on their acoustooptic heterodyning image processor [1979]. The original idea is later formulated and becomes known as scanning holography [Poon (1985)]. The first experimental results were then demonstrated and the technique was eventually called optical scanning holography in order to emphasize the novel fact that holographic recording can be achieved by active optical scanning [Duncan and Poon (1992)]. Thus far, applications of OSH include scanning holographic microscopy [(Poon, Doh, Schilling, Wu, Shinoda, and Suzuki (1995)], 3-D image recognition [(Poon and Kim (1999)], 3-D optical remote sensing [Kim and Poon (1999)], 3-D TV and display [Poon (2002a)], and 3-D cryptography [Poon, Kim, and Doh (2003)]. Scanning holographic microscopy is, by far, the most developed technique that utilizes OSH. Unlike any other holographic microscopes, scanning holographic microscope has a unique property that allows it to take the holographic information of fluorescent specimens in three dimensions. Recently, scientists have been able to achieve better than one-micron resolution in holographic fluorescence microscopy [Indebetouw and Zhong (2006)]. Here we will discuss the basic principles of OSH. Then we will discuss some of the previously mentioned applications of OSH in detail.

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1.1 Principle of Optical Scanning Optical scanning holography involves active optical scanning and optical heterodyning. We will discuss the basics of optical scanning. An optical scanner or optical processor scans out a transparency, i.e., information, with an optical beam by moving either the beam or the transparency. A photodetector accepts all light and gives an electrical output that can either be stored or displayed by some means or another. Hence, optical information will have been converted into electrical information. Figure 1.2 shows a standard, active optical scanning image processing system. A plane wave (such as the use of a laser in practice) of frequency

, illuminates a pupil function,

The complex field emerging from the pupil is then projected through the x-y optical scanner in order to scan over the input object specified by a transparency of

.

The photodetector (PD) then accepts all the light to give out an electrical signal, which contains the processed information for the scanned object. If the scanned electrical signal is digitally stored (i.e., in a computer) in synchronization with the 2-D scan signals of the scanning mechanism (such as the x-y scanning mirrors), what is stored as a 2-D record is then a processed image of the scanned object.

Fig. 1.2 An active optical scanning image processing system

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Let's talk about photodetection and see how light information can be converted into electrical information. Assume that the photodetector surface is on the

plane and that the incident

complex field on the detector's surface is given by the photodetector only responds to intensity, i.e.,

as shown in Fig. 1.3 Since , it gives the current, ί, as an

output by spatially integrating the intensity over the active area, D, of the detector:

Eq. (1.1-1)

For example, if the incident field is a plane wave of amplitude A, i.e.,

, the current

output is given by

Eq. (1.1-2) Which is a constant? However, take for instance that if the light has been intensitymodulated, i.e.,

, where

is the modulating signal, the current will then give an

output that varies with the modulation. This is useful for laser communications systems [Pratt (1969)].

Note that since magnitude information, i.e.,

, the output current can only contain the , and the phase information is completely lost. This type of

photodetection is known asoptical direct detection (or optical incoherent detection).

Figure 1.3 Optical direct detection

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Once we comprehend photodetection, we can return to Fig. 1.2 to calculate the current output given after scanning the transparency,

. Instead of modeling the transparency that is

being scanned by an optical beam, as shown by Fig. 1.4, we assume that the transparency, , is moving through the optical beam. In Fig. 1.4, the plane of the photodetector is on the plane and the optical scanning beam specified by a complex field, at the origin of the

, is stationary

plane. By scanning or sampling we mean that successive points (

in transparency coordinates) of of the optical beam in the

are brought into coincidence with the center plane.

Fig. 1.4 Scanning Situation In Fig. 1.4, the arguments, and , of

signify that the transparency is moving or translating

with respect to the optical beam. Therefore, the total complex field reaching the photodetector is,

photodetector collects all the transmitted light and

delivers a current, . According to Eq. (1.1-3),

is given by

Eq. (1.1-3)

Where

and

represent the instantaneous position of the transparency. Alternatively,

scanning imaging can be modeled by moving the optical beam across the transparency, which results in the following equation:

Eq. (1.1-4) VIII

If we let

and

then substitute them into the above equation, we have

Eq. (1.1-5) This is identical to Eq. (1.1-5). We shall use the formulation shown in Eq. (1.1-5) to represent optical scanning throughout the book. Note that for uniform scan speed V, we have

and

.

When we rearrange Eq. (1-1-5), we have

Eq. (1.1-6) Note that this result is interesting because it is an incoherent optical system where only the intensities are processed, i.e.,

is processed by

even though the object

may be complex in nature. Since the beam complex field,

, originally

and the pupil,

are in the

back and the front focal plane of lens L1, respectively, as shown in Fig.1.2, they are related by a Fourier transformation where

Eq. (1.1-7) Figure 1.5 shows a commercially available x-y scanning system from General Scanning™. The mirrors are driven by galvanometers. The figure on the right is a close-up of the x-y scanning mirrors positioned orthogonally to each other (one direction for the x-scanning and the other for the y scanning).

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Fig. 1.5 x-y optical scanning system

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2. Optical Scanning Technologies The scanning techniques and their applications are nowadays in a fast rate development. From the classical solutions of the rotating (plane, polygonal, pyramidal) or oscillating (galvanometric or resonant) mirrors, new solutions emerged: acusto-optic, electro-optic, holographic. From the very first applications, in remote sensing, to the modern ones, from medical to military and from industry to laboratories, the scanners have gained momentum. (See Fig. 2.1)

Fig 2.1 Shows Optical Scanning Technology Here we will talk about two type of optical scanning technology and an example of it, this technology is galvanometric and Polygonal.

2.1 Galvanometric Galvanometer scanners have been used for nearly three (3) decades for laser material processing. They are most commonly used for laser marking and have somewhat less utility in fine machining applications, in particular drilling precision holes and features below 250 microns. This limitation is due to their positional accuracy which is in the range of 2 to 10s of microns, depending upon the galvanometer and the F-theta lens used. Galvanometer-based systems are the simplest and least expensive way to direct a focused laser beam over a wide area. Nevertheless, they lack the “localized” precision for finite features over a large field.

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A conventional multi-mirror galvometric system positions a focused laser beam by moving the beam in vectors. There are no “true arcs” generated for circular features. Instead, a circle is approximated by a series of short vectors. It is very difficult to form precision holes or any arc feature below 100 micron radius. Moreover, the angular resolution of the galvo motors is a further hindrance to the problem of small features and the attainment of high repeatability. Galvo scanners are also subject to limited angular resolution and thermal drift which further restricts the ability of the device to machine precision features over a long period of time, e.g., a single production shift in manufacturing. Non-galvo-based methods such as rotating, offset, wedge pairs (Risley Prisms) allow good precision below 250 microns, but only permit circular features and have a limited dynamic range and tend to be electro-mechanically complex. In the Risley prism design, the offset of the matched wedges causes an angular displacement of the laser beam from the optical axis. This angular deviation causes a lateral displacement of the focal spot when the angularly displaced beam is passed through a focus lens. The difficulty with this technique is that it is hard to coordinate the two wedges precisely at the high rotational speeds or to rapidly change the desired angle of deviation while the wedges are rotating. This approach usually requires a multitude of wedge pairs to cover a wide diameter range. The requirement to change wedge pairs adds significant time to replace and align; it is therefore unsatisfactory for most production processes. Linear stages, in particular air bearing stages, offer a means of high precision. The linear or air bearing X-Y stage moves under a fixed focused laser beam, providing precision and accuracy. However, both air bearing and linear stages devices are expensive and have high inertia arising from moving such stages and the part supported by the stage so the speed of drilling precision features is limited. Yet another approach used over the years is the focus lens itself can be placed offset from the optical axis and rotated or even placed in an open frame X-Y stage used to make all

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conceivable geometries. Mounting a lens in such a way is bulky and limited over the area that can be machined due to common lens aberrations. Given the limitations of existing galvo scanners, the expense and inertia of linear stages, complexity and aberrations of other optical methods, there is clearly a need to have a device which offers the convenience and simplicity of a galvo scanner coupled with the precision of a linear stage.

2.1.1 New Method for Precision Scanning A new, patent pending optical device (NeoScan™ Scanner, see Fig2.2) offers a simple optical, electro-mechanical and software approach to directing a focused laser beam onto materials to machine simple and complex geometries. This novel structure provides the ease of use and simplicity of a galvo system but adds the “localized” precision lacking heretofore.

Fig 2.2 Show NeoScan™ Scanner The innovative concept provides the precision and accuracy comparable to an air bearing XY stage that moves under a fixed focused laser beam but without the high cost and higher

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inertia of moving such a stage and the part. The device in its simplest description demagnifies the scan field by more than two (2) orders of magnitude and likewise the reputability and resolution. A conventional scanner with fair resolution may have a scan field of 50 mm x 50 mm with an F-Theta lens having a focal length of 100 mm. This same scanner has great difficulty providing high accuracy of geometries below 250 micron, due to the angular resolution of the system and the fact that any curved features include a large number of short vectors. In a typical precision a laser galvo scanner that reflects a laser beam over an angular range of plus or minus (+/-) twelve to twenty degrees (12-20º) as the beam passes through a focusing lens, typically an F-theta lens. The angular repeatability of such a galvo is on the order of < +/- 22 μrad, which represents a resolution of ~ +/- 2.2 μm for a scan lens having a 100 mm focal length. The field of such a system will be f*(Tan

), where f is the focal length of the

lens and theta is the angle the beam is reflected before the lens. A laser scanner operating then over a range of plus or minus twelve degree (+/- 12º) with a f=100 mm lens will therefore cover a distance of +/- 21.3 mm. The optical deviation of a beam refracted through a thin optic is determined by the index of refraction of the material and the angle the optic is tilted. Tilting a two millimeter (2 mm) thick optical plate that has an index of refraction of 1.796 over a range of plus or minus twelve degrees (+/- 12º) degrees will cause a laser beam traveling on axis through said plate to deviate from the optical axis by plus or minus 0.188 mm. This increases the resolution of the same galvo by the ratio of 21.2mm/0.188 mm (113:1) which is better than two orders of magnitude. The NeoScan (a refractive scanner) uses the same control of the galvanometer but adds an optical demagnification that essentially maps, for example, a 50 mm x 50 mm field into a 0.2 mm x 0.2 mm field. Figure 2.3 is the NeoScan in its simplest form and is comprised of a pair of inverted positive meniscus lenses. Each lens is mounted to a galvanometer to tilt each lens orthogonally to one another to displace the focus laser spot from the optical axis. The preferred optical material is

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the highest possible index material for the desired laser wavelength. Having a high index allows the thickness of the lenses to be as thin as possible to minimize optical aberrations and minimize the inertia of the galvo.

Fig 2.3 NeoScan refractive scanner with a pair of meniscus lenses mounted to galvos The focal length of the lens combination of the two lens system is defined by 1/f = 1/f1 +1/f2 – t/f1f2, where f1 is the focal length of the first lens, f2 is the focal length of the second lens and t is the separation between lens 1 and lens 2 where it is assumed for simplicity of description that the lenses are thin. The two lenses are tilted and naturally introduce coma, astigmatism and spherical aberration. Accordingly, the design of the lens curvatures, thickness and material are optimized to minimize the lens aberrations at the designed radial displacement from the optical axis. An inverted positive meniscus lens pair produces the fewest aberrations for the optical design as mentioned earlier. As well it is desired to have a high index optical material to facilitate longer radius of curvature surfaces and keep the optical elements as thin as possible to further XV

minimize the aberrations and reduce inertia. Other lens curvatures can be used, e.g., a pair of plano-convex lenses; pair of double convex lenses, etc. Optical modeling has determined that the inverted, positive meniscus lens pair provides minimal aberrations and best optical performance. The tilting of each of the two meniscus lenses causes the laser beam to be displaced in a controlled way from the optical axis. The amount of displacement is dependent upon the power of the designed lenses and the angle that the lenses are tilted about the optical axis and orthogonally to one another. In the system shown in Fig 2.3 where the lens material is sapphire with an index of refraction of 1.796, a combined lens pair focal length of approximately 200 mm and a tilt angle of ten degrees (10º) of each lens, orthogonally, cause a radial shift of the focused spot by > 170 microns. Tilting the lenses beyond ten degrees causes the coma to become too great for usefulness. Fig. 2.4 depicts an enlarged ray trace at the focal point of the lens system on how the light is deviated from the optical axis from the corresponding tilt of first meniscus lens in the X plane in Fig. 2.3.

Fig 2.4 Enlarge view of ray trace of laser spot refracted away from the optical axis (dimensions in mm)

A second variation of the NeoScan refractive scanner (Fig 2.5) incorporates a conventional galvo. The beam exits the F-theta lens of a conventional galvo scanner and passes through a pair of parallel plates, each of which is mounted to a galvanometer.

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Fig. 2.5 Large Field Refractive Scanner The parallel plate galvanometers are orientated orthogonally to one another so that the beam can be offset from the optical axis in a controlled way. The offset of the beam is determined by the angle of the plate, its thickness and the index of refraction of the plate. The parallel plates permit the controlled shift of the laser beam passing through the scanner system and F-Theta lens. The plates are “thin” so the introduced aberration is minor spherical aberration and allows accurate machining of finite features over the large area of the scanner/F-Theta system which can range from a few millimeters to hundreds of millimeters, depending upon the rotation angle of the galvo scan mirror and the focal length of the F-theta lens. Through software control of the galvo pairs, features can be accurately machined over a field range limited only by the scanner/F-Theta system used. The three ray traces (red, green & blue) in Fig 2.5 represent the extremes of beam positioning by the “standard” galvo/F-Theta combination and at each extreme point the parallel plates mounted to galvos provides a higher degree of precision in the smaller field. Clearly a high degree of software synchronization between the mirror galvos and the refractive galvos is needed but would none the less provide a very versatile machining system. Another optical configuration of the NeoScan has a laser beam which pass through a simple, positive lens and then through a pair of plane, parallel windows.

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Each of the plane, parallel windows is mounted to a galvanometer motor and positioned orthogonally to one another. The only difference between this scenario and the previous one mentioned is that the second has a pair of galvanometer mirrors to deflect a laser beam through a scan lens (F-Theta type). In all three scenarios of the NeoScan, the resulting focused light is directed onto a material such as a metal, plastic, glass or ceramic for machining; with the NeoScan refractive optical elements mounted to galvanometers and oriented orthogonally to one another. The NeoScan has a limited field, generally < 500 μm, that is determined by the index of refraction of the optical material and the angle of rotation, but nonetheless provides very high precision capability of features below 500 microns in size in a very simple optomechanical configuration. Figure 2.6 exhibits an array of nominally 155 μm diameter holes with a corresponding sigma of 0.049 machined with NeoScan refractive scanner in Stainless Steel using a 20 watt fiber laser.

Fig. 2.6 NeoScan Drilled Hole Array in Stainless Steel

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In contrast Fig. 2.7 depicts nominally 155 μum diameter with a sigma 2.3 using a conventional galvo scanner (F-Theta Lens = 100 mm) drilled in stainless steel.

Fig. 2.7 Conventional Scanner Drilled Hole Array in SS A comparison of Figs. 2.6 and 2.7 indicates that the refractive scanner consistently produces highly regular circular and repeatable holes over that of a conventional scanner.

Figure 2.8 shows a circle, square, and triangle machined in 80 um thick stainless steel by a conventional and the NeoScan refractive scanner; each having a nominal feature size of 150μm.

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Fig. 2.8 Conventional & NeoScan Scanner Machined Geometrical Shapes in SS

A comparison of the results in Fig. 2.8 indicates that the geometrical shapes formed in stainless steel by the refractive scanner are substantially true to idealized shapes and that the geometrical shapes formed by the conventional scanner are not. The reason for the variance between the scanners is the acceleration and deceleration time in relation to the time to form the feature. In the conventional scanner case the time to form the feature is too close to the acceleration/deceleration time where as in the refractive scanner it is considerably less and therefore not an overriding factor in the formation of the features.

2.1.2 Summary The primary purpose of the refractive scanner is to create an optical system that precisely and repeatedly locates a concentrated laser beam and to manipulate the laser beam in such a way as to remove a wide variety of materials in a controlled way to generate complex geometries with excellent precision and repeatability. This is achieved through three different galvo-

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based refractive optical configurations. The best configuration is dependent upon the type of machining being done and how large a field is required. It is important to note as well that the refractive scanner can compensate for irregularities in the focused laser beam. If a focal spot of the focused laser beam is elliptical, for example, the scanner can be programmed to move in an opposing elliptical manner to compensate and achieve a perfectly round hole despite imperfections in the laser beam.

2.2 Polygonal Polygonal scanners have found a role in a wide range of applications including inspection, laser printing, medical imaging, laser marking, barcode scanning, and displays, to name a few. Ever since the laser was first discovered, engineers have needed a means to move the laser output in a repetitive fashion or scan passive scenes such as used in earlier military infrared systems. The term polygonal scanner refers to a category of scanners that incorporate a rotating optical element with three or more reflective facets. The optical element in a polygonal scanner is usually a metal mirror. In addition to the polygonal scanner other scanners can have as few as one facet such as a pentaprism, cube beam splitter or “monogon.” This section will concentrate on scanners that use a reflective mirror as the optical element. Polygonal scanners share the beam steering market with other technologies including galvanometers, micromirrors, hologons, piezo mirrors and acousto-optic deflectors. Each technology has a niche where it excels. Polygonal scanners excel in applications requiring unidirectional scans, high scan rates, large apertures, large scan angles or high throughputs. The polygonal scanner in most applications is paired with another means for beam steering or object motion to produce a second axis. This creates a raster image with the polygonal scanner producing the fast scan axis of motion.

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2.2.1 Types of Scanning Mirrors There are many types of scan mirrors, but most can be included in the following categories: 1. Prismatic polygonal scanning mirrors; 2. Pyramidal polygonal scanning mirrors; 3. “Monogons”; 4. Irregular polygonal scanning mirrors. Prismatic Polygonal Scanning Mirrors A regular prismatic polygon is defined as one having a number of plane mirror facets that are parallel to, equidistant from, and face away from a central rotational axis (Fig. 2.9). This type of scan mirror is used to produce repetitive scans over the same image plane. It is the most cost-effective to manufacture and therefore finds its way into the vast majority of applications including barcode scanning and laser printing. An illustration of why the manufacturing cost can be lower than other types of scan mirrors is shown in Fig. 2.10 Here we see a stack of mirrors that can be moved through the manufacturing process as a single piece resulting in less handling, more consistency, and less machining time.

Fig. 2.9 Regular prismatic polygonal scanning mirror

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Fig. 2.10 Mirror stack reduces fabrication costs Pyramidal Polygonal Scanning Mirrors A regular pyramidal polygon is defined as one having a number of facets inclined at the same angle, usually 458, to the rotational axis (Fig. 2.11). This type of polygon is expensive to manufacture since one cannot stack mirrors together to process at the same time as is done with regular prismatic polygons. A significant feature of the 458 pyramidal polygon is that it can produce half the output scan angle of a prismatic polygon for the same amount of shaft rotation. Prismatic polygons are used primarily with the input beam perpendicular to the rotation axis whereas pyramidal polygons are used primarily with the input beam parallel to the rotation axis (Fig. 2.12). This feature can be used to the system designer’s advantage by reducing data rates for a given polygon rotation speed.

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Fig. 2.11 Regular pyramidal polygonal scanning mirror

Fig. 2.12 Scan angle vs. rotation angle

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Monogons “Monogons” are scan mirrors where there is only one facet centered on the rotational axis. Because there is only one facet, a “monogon” is not a true polygon but they are an important subset of the scan mirror family. “Monogons” are also referred to as truncated mirrors and find application in internal drum scanning. In a typical system employing a monogon, the laser is directed toward the monogon along the rotation axis and the output sweeps a circle on an internal drum as the scanner rotates. This type of scan system can produce very accurate spot placement and very high resolution and finds application in the pre-press market. An example of a monogon scan mirror is shown in Fig. 2.13.

Fig. 2.13 “Monogon.” Irregular Polygonal Scanning Mirrors An irregular polygonal scanning mirror is defined as one having a number of plane facets that are at a variety of angles with respect to, and face away from, the rotational axis (Fig. 2.14). The unique feature of this type of scan mirror is that it can produce a raster output without a second axis of motion. The resulting output scans are nonsuperimposing if the facets are at different angles.

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This type of scanner finds its way into coarse scanning applications such as: . point-of-sale barcode readers; . laser heat-treating systems; . intrusion alarm scanning systems. These polygons typically cost significantly more than regular polygons because their asymmetry prevents any cost savings from stacking. Another disadvantage of these scanners is the inherent dynamic imbalance of the polygon during rotation. This limits their use to lowspeed applications. A special case where equal and opposing facets are used on each side of the polygon helps with the balance problem. The result is the scan pattern is generated twice each revolution.

Fig. 2.14 Irregular polygonal scanning mirror

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3. Optical Scanning Holography Applications 3.1 Digital Holographic Microscopy Digital Holographic Microscopy provides quantitative phase contrast imaging that is suitable for high resolving investigations on reflective surfaces as well as for marker-free analysis of living cells. Results from engineered surfaces and living cells demonstrate applications of digital holographic microscopy for technical inspection and life cell imaging. Holographic interferometric metrology techniques are established tools in many industrial application areas. There are also important application fields in Biophotonics, Life Sciences and Medicine as these techniques can be applied non-destructively, marker-free, “full-field” (no scanning required) and online simultaneously. With these features the described digital holographic microscopy concept permits a high resolution, multi focus representation of engineered surfaces and living cells. In order to establish digital holography in microscopy, the combination with common microscopy techniques is of particular advantage. In this case flexible and compact digital holographic microscopy modules for the integration into modern microscopy systems are required. Furthermore, for an automated evaluation of the measurement data, it is necessary to implement robust algorithms for the numerical reconstruction of digital holograms.

3.1.1 Principle of Digital Holographic Microscopy Digital holography is based on the classic holographic principle, with the difference that the hologram recording is performed by a digital image sensor, e.g. a CC D or CMOS camera. The subsequent reconstruction of the holographic image that contains the information about the object wave is carried out numerically with a computer. Fig. 3.1 depicts the schematics of two “off-axis” setups for digital holographic microscopy that are particularly suitable for the modular integration into commercial microscopy systems.

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The coherent light of a laser (e.g. a frequency doubled Nd:YAG laser, λ = 532 nm) is divided into object illumination and reference wave, using singlemode optical fibers. Fig. 3.1.a shows an incident light illumination arrangement for the investigation of reflective samples. The set-up in Fig. 3.1.b is designed to investigate transparent specimen such as living cells. For both setups the coherent laser light for the illumination of the sample is coupled into the optical path of the microscope’s condenser by a beam splitter cube. The reference wave is superimposed with the light that is reflected or transmitted by the object by a second beam splitter with a slight tilt against the object wave front in order to generate “offaxis” holograms which are recorded by a CC D camera. After hologram acquisition, the data is transmitted by an IEEE1394 („FireWire“) interface to a PC based image processing system, thus avoiding cost intensive frame grabber cards with hardware specific software. The modular add-on approach provides the advantage that common commercial microscope lenses with high numerical aperture (e.g. water and oil immersion) can be used in combination with an optimized (Koehlerlike) illumination of the sample. Furthermore, the integration of the additional optical components for digital holography does not restrict the conventional functions of the microscopy systems.

Fig. 3.1 Schematics for digital holographic microscopy using incident light (reflective) (left) and inverse transmission arrangements (right)

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3.1.2 Numerical Evaluation of Digital Holograms by Non-Diffractive Reconstruction The reconstruction of the digitally recorded holograms is performed numerically with standard computer hardware. In general, Fresnel-transformation based digital holographic reconstruction methods generate not only the information contained in the object wave but also the intensity of the reference wave (“zero order”) and a „twin image“. Furthermore, the size of the reconstructed holographic image depends on the reconstruction distance to the hologram plane. A non-diffractive reconstruction algorithm has been developed that is particularly suitable to digital holographic microscopy. In a first step of the reconstruction process the complex object wave is calculated with a spatial phase shifting algorithm in the plane of the CC D image sensor (hologram plane). Afterwards, if necessary, the object wave is propagated towards the focused image plane in frequency space (“convolution method”). The main advantage of the method is that it can also be applied to the evaluation of holograms when the sample is imaged sharply onto the image sensor. In addition, the scale of the holographic amplitude and phase contrast images is constant for the numerical reconstruction in different focus planes and “zero order” and “twin image” are avoided. Fig. 3.2 illustrates the evaluation process of digital recorded holograms. Figures 3.2.a and 3.2.b show a digital hologram obtained from a living human pancreas carcinoma cell (Patu8988T) with an inverse microscope arrangement in transmission mode (40x microscope lens, NA = 0.65) and the reconstructed holographic amplitude image that corresponds to a microscopic bright field image at coherent laser light illumination. Fig. 3.2.c depicts the simultaneously reconstructed quantitative phase contrast image modulo 2p. The unwrapped data without 2p ambiguity, are shown in Fig. 3.2.d, representing the optical path length changes that are effected by the sample in comparison to the surrounding medium due to the thickness and the integral refractive index. Fig. 2.3.e depicts a pseudo 3D plot of the data in Fig. 3.2.d. Fig. 3.2.f shows the cell thickness along the marked.

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Fig. 3.2 Example for evaluation of digital holograms: (a): digital hologram of a human pancreas carcinoma cell (Patu8988T); (b): reconstructed holographic amplitude image; (c): quantitative phase contrast image (modulo 2p), (d): unwrapped phase distribution; (e): pseudo 3D plot of the unwrapped phase image in gray level representation; (f): calculated cell thickness along the dotted white line in (d). (Cooperation: Dr. Jürgen Schnekenburger, Molecular Gastrointestinal Cell Biology, Department of Medicine B, University of Münster, Germany).

3.2 3D TV and Display The optical principles of multiview auto-stereoscopy have been known for over a century. Practical displays with a high resolution have recently become available. As a result, 3D television is receiving an increasing attention.

3.2.1 Description This kind of display can deliver multiple views without glasses, thus allowing a limited "look-around" (correct motion parallax). Therefore, these displays offer uninhibited viewing, i.e., without glasses, of high-resolution stereoscopic images from arbitrary positions in a viewing zone. Automultiscopic displays include view-dependent pixels with different

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intensities and colors based on the viewing angle. View-dependent pixels can be implemented using conventional high-resolution displays and parallax-barriers.

3.2.2 Types of 3D Displays There are few basic types of 3D displays. Stereoscopic technology separately sends two views of a 3D scene on its screen(s) to the viewer's two eyes. Autostereoscopic 3D displays advance on stereoscopic technology without the need for any special glasses or other head gear by using high resolution flat panels to generate a given number of views of a 3D scene through some sort of pixel redirection technology. This solution gives "ok" quality 3D views from predefined sweet points in front of the display, but leaves a tangled image while in between the sweet points. Continuous 3D light field display developed by Holografika generates a glasses free 3D image with no restrictions on the number of viewers, their position in front of the screen and their movement. Holographic 3D displaying researchers are able to create a light field which is identical to that which emanated from the original scene (for the technology, see Computer Generated Holography). This last technology is capable of reproducing horizontal and vertical parallax at the same time, while stereoscopic and autostereoscopic technologies can create horizontal parallax 3D images only. This may seem a limitation of 3D displays, but the longitudinal location of human eye on the head is in perfect pair with the horizontally multiplied views of these displays. In addition there are volumetric displays, where some physical mechanism is used to display points of light within a volume. Such displays use voxels instead of pixels. Volumetric displays include multiplanar displays, which have multiple display planes stacked up; and rotating panel displays, where a rotating panel sweeps out a volume. Other technologies have been developed to project light dots in the air above a device. An infrared laser is focused on the destination in space, generating a small bubble of plasma which emits visible light. As of August 2008, the experiments only allow a rate of 100 dots per second. One of the issues which arise with this 3D display system is the use of technologies that could be harmful to human eyes.

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3.2.3 Problems However, automultiscopic displays still have some problems, like limited viewing zone and discreteness of motion parallax. Because the width of viewing zone for each view equals the interpupillary distance approximately, the view image does not change by the viewer's movement within the zone. On the other hand, when the viewer moves over the zone, the view image changes suddenly. So, a moving viewer sees disturbing visual artifacts. Secondly, the acquisition of artifact-free 3D images is difficult. Photographers, videographers, and professionals in the broadcast and movie industry are unfamiliar with the complex setup required to record 3D images. There are currently no guidelines or standards for multi-camera parameters, placement, and post- production processing, as there are for conventional 2D television.

3.2.4 Existing Displays A wide range of organisations have developed 3D displays, ranging from experimental displays in university departments to commercially available displays. Companies involved include: •

3d solar

•

3DIcon Corporation

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Alioscopy

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Asus

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Holografika

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Institute for Creative Technologies

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iZ3D

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MIT Media Lab

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NewSight

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Pavonine

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Philips WOWvx

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QinetiQ

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SeeReal Technologies

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Sharp

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Spatial View

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Zalman

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Zero Creative

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4. Types of Optical Scan Systems 4.1 Mark Sense Systems One technology used is optical mark recognition scanners where voters mark their choice in a voting response location, usually filling a rectangle, circle or oval, or by completing an arrow (See Fig. 4.1). During tabulation the optical scan voting system interprets the votes using "dark mark logic", whereby the computer selects the darkest mark within a given set as the correct choice or vote.

Fig.4.1 Shows Mark Sense Card

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The ballot can be immediately tabulated at polling stations allowing for voters to be notified by the voting system of voting errors such as an over vote and can prevent residual votes. One such method can display a digital image of the ballot being submitted and allows the voter to review how their ballots are being read. This is known as a precinct count voting system. Alternately the ballots can be collected in the polling station and tabulated later at a central facility, known as central count voting system. These voting system are called optical readers which are devices found within most computer scanners that captures visual information and translates the image into digital information the computer is capable of understanding and displaying. An example of optical readers is Mark sense systems for elections where voters mark their choice by filling a rectangle, circle or oval, or by completing an arrow. After the voting a tabulating device reads the votes using "dark mark logic", whereby the computer selects the darkest mark within a given set as the correct choice or vote. Mark sense is also used extensively in such areas as lotteries and multiple choice tests.

4.1.1 Security and Concerns Optical scan voting systems are a form of document ballot voting system, meaning that there is a tangible record of the voter's intent (a paper ballot). Like traditional paper ballots these are subject to electoral fraud and ballot stuffing. One form of wholesale fraud possible with optical scan voting systems is during the recording of votes. Douglas W. Jones of the University of Iowa that if a potential attacker were to gain access to the voting system configuration files, they would be able to "credit one candidate with votes intended for another." He found these files are exposed in the computer system used to prepare the election, making them vulnerable to anyone setting up the election. The files are then transferred to the voting system using removable media, and "anyone with access to these media could potentially attack the system." Another form of wholesale fraud is during tabulation. Possible attacks have been demonstrated by Harri Hursti and the University of Connecticut.

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If an attacker is able to obtain a blank ballot (by theft, counterfeit, or a legitimate absentee ballot) the attacker can then mark the ballot her chosen candidates and convince (through intimidation or bribery) a voter to take the pre-marked ballot to a polling station, exchange it for the blank ballot issued and return the blank ballot to the attacker. This is known as chain voting. Some suggest many of these well-known vulnerabilities can be effectively mitigated. Ballot stuffing may be resolved with incorporation of randomly generated ballot identifier for each paper ballot and capturing digital ballot images of scanned ballots as electronic audit. Tabulation fraud and wholesale tampering can also be prevented by adding a cryptographic verification mechanism. This approach is mathematically based, and thus invariant to software attacks or breaches in chain-of-custody of the paper ballots.[12] One such system is Scantegrity.

4.1.2 Benefits of Optical Scan Voting Machines An advantage of these systems is that the voters don't have to learn to use a voting machine. Physically able voters can simply use pen and paper to mark their intent. Some disabled voters could use a machine to print a voted ballot, which can then be fed into the optical scanner along with all the other ballots, thus preserving the secrecy of their ballot. Optical scan voting systems can allow for manual recounting of ballots. Statistically relevant recounting can serve as a tool to detect or deter malfunction or fraud. Once an error in the counting process is suspected a full recount can determine the proper results. An advantage compared to DRE voting machines is that even if the optical scanner fails, voters can still fill out their paper ballot, and leave it to be scanned when the machine is fixed, replaced with a spare, or even counted by hand.

4.2 Electronic Ballot Marker An electronic ballot marker (EBM) or ballot marking device (BMD) is an electronic device that can aid a disabled voter in marking a paper ballot.

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Ballot marking devices provide accessibility features for standard optical scan ballots such as: •

Touch screens (with magnified font option for reading-impaired voters)

•

Sip and puff interface (for quadriplegic voters)

•

Audio ballots (for blind & language impaired)

•

Support for multiple foreign languages

Each voter uses the accessibility interface they need to mark the paper ballot. For example, an audio interface can read the ballot to a blind or visually-impaired voter wearing headphones, and accept the voter's input for each race. It then prints the voter's choices on the standard optical scan ballot which was inserted at the beginning of the process. Vision-impaired individuals can use the ballot-marking device to verify their ballots. When a completed ballot is inserted, the machine reads the ballot and either displays it on the screen in an enlarged font, or provides an audio description of the votes through the headphones. Ballot marking devices provide over-vote and under-vote protection, thus ensuring that the optical scan ballot completed on behalf of any voter is correctly filled in. Any optical scan ballot completed by the ballot-marking device will be readily accepted by the precinct-count optical scanner. Ballot marking devices do not record or count votes electronically, they only mark a paper ballot for the voter. In essence, they are a “computerized marking pen”. The votes are recorded on a standard optical scan ballot, and the completed ballot is read by precinct-based optical scanner. Ballot marking devices do nothing more than assist voters in completing their optical scan paper ballots. They essentially replace a human assistant, who compromises ballot secrecy, with an automated assistant, which does not compromise that secrecy. Unlike DRE voting machines, the ballot marking devices do not store any electronic ballots nor count any votes. Accordingly, they avoid most of the authentication, security, and auditability issues associated with DRE voting machines. An example of a ballot marking device is the Vogue AutoMARK see Fig. 4.2.

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Fig. 4.2 Shows Vogue AutoMARK

4.3 Digital Pen Voting Systems Digital pen voting systems use ballots on digital paper which is recognized by a small camera in the pen while it is marked by the voter. The ballots are collected in a ballot box and the digital pen is returned to an election official for tabulation. See Fig. 4.3 This technology was expected to be used in the 2008 Hamburg state elections, but eventually was decided against due to controversy surrounding the accuracy of voting tallies. The technology was first used by the town of Menstrie, Clackmannanshire Scotland in their 2006 local community council elections. With this system there is no way for an automated recount.

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Fig. 4.3 Shows Voting process with Digital Pen Voting

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5. Optical Scanning Errors In any scanning system there will always be errors associated with the rotation of the optic that will lead to the output beam displaying certain distinguishable errors that can be traced back to optic-related issues.

5.1 Optic-Related Errors Although both polygons and monogons suffer from many of the same errors, their effects on the scanning system need to be examined separately.

5.2.1 Polygons . Mounting. Misalignment of the polygon axis to the spin axis is likely to be the largest part of the total tracking error. Very accurate machining of the polygon bore and the hub on the shaft is required to minimize this tilting effect. Effect: Repeating weave pattern. . Manufacture. Errors include pyramidal (facet-to-datum), facet-to-facet, dividing angle and facet flatness. Effect: Repeatable positional errors. . Dynamic distortion. Loss of geometry due to thermal growth or mechanical stresses. Effect: Positional tracking errors varying with speed and time.

5.2.2 Monogons . Mounting. Will only cause a slight permanent change of facet angle, which will occur on every revolution and will not usually affect the scan process. Effect: Small, permanent positional change of beam. . Manufacture. Errors include facet flatness, deflection angle, wave front distortion, and astigmatism. Effect: Spot quality and focus issues with speed and time. . Dynamic distortion. Change in flatness and astigmatism due to thermal growth or mechanical stresses. Effect: Change in spot quality and focus with speed and time. XL

6. Error Correction 6.1 Polygons Mounting errors can be minimized by machining the polygon hub in situ on the shaft running in its own bearings. This helps to correct for many of the synchronous bearings related errors as well as the mechanical errors. Alternatively, an adjustable mount can be used to fine tune the tilt of the polygon to bring it on spin axis. The mount can be manufactured from a thermally insulating material to stop thermal effect reaching the polygon. With synchronous errors, an active correction system can be employed to slightly modify the beam path prior to striking the polygon facet to compensate for the error about to be put into the beam. This can be permanently preprogrammed in, or in more complicated systems, a facet error detector must be incorporated to constantly update the error compensation system.

6.2 Monogons Error correction is more limited in monogon optic systems. To correct for dynamic mechanical optic distortion, biased optics can be used that will deform to the correct shape over a small specific running speed range, but this only usually refers to open facet mirrors, not prisms. However, many of the synchronous errors are not so noticeable as they occur on every scan line and in general will not cause banding. The use of a pentaprism can dramatically reduce wobble errors generated in the bearings and is ideally suited for ball bearing scanners where wobble is a major problem in higher accuracy designs.

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Index B ballot marking device (BMD)

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D Digital pen voting systems

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E electronic ballot marker (EBM)

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M Monogons

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O optical readers Optical scanning holography

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P polygonal scanner polygonal scanning mirror prismatic polygon pyramidal polygon

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Glossary 3D Imaging Stereoscopy, stereoscopic imaging or 3-D (three-dimensional) imaging is any technique capable of recording three-dimensional visual information or creating the illusion of depth in an image. The illusion of depth in a photograph, movie, or other two-dimensional image is created by presenting a slightly different image to each eye. Many 3D displays use this method to convey images. It was first discovered by Sir Charles Wheatstone in 1840. Stereoscopy is used in photogrammetry and also for entertainment through the production of stereograms. Stereoscopy is useful in viewing images rendered from large multi-dimensional data sets such as are produced by experimental data. Modern industrial three dimensional photography may use 3D scanners to detect and record three dimensional information. The three-dimensional depth information can be reconstructed from two images using a computer by corresponding the pixels in the left and right images. Solving the Correspondence problem

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in the field of Computer Vision aims to create meaningful depth information from two images. Traditional stereoscopic photography consists of creating a 3-D illusion starting from a pair of 2-D images. The easiest way to create depth perception in the brain is to provide the eyes of the viewer with two different images, representing two perspectives of the same object, with a minor deviation similar to the perspectives that both eyes naturally receive in binocular vision. If eyestrain and distortion are to be avoided, each of the two 2-D images preferably should be presented to each eye of the viewer so that any object at infinite distance seen by the viewer should be perceived by that eye while it is oriented straight ahead, the viewer's eyes being neither crossed nor diverging. When the picture contains no object at infinite distance, such as a horizon or a cloud, the pictures should be spaced correspondingly closer together. 3D display Is any display device capable of conveying three-dimensional images to the viewer. We call automultiscopic display at a new type of three-dimensional display recently introduced on the market holds great promise for the future of 3D visualization, communication, and entertainment. It is desired to acquire images of real-world 3D scenes and display them as realistic 3D images. Fluorescence Is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of a photon with a longer (less energetic) wavelength. The energy difference between the absorbed and emitted photons ends up as molecular rotations, vibrations or heat. Sometimes the absorbed photon is in the ultraviolet range, and the emitted light is in the visible range, but this depends on the absorbance curve and Stokes shift of the particular fluorophore. The term 'fluorescence' was coined by George Gabriel Stokes in his 1852 paper; the name was given as a description of the essence of the mineral fluorite, composed of calcium fluoride, which gave a visible emission when illuminated with "invisible radiation" (UV radiation). Fluorescence Microscope Is a light microscope used to study properties of organic or inorganic substances using the phenomena of fluorescence and phosphorescence instead of, or in addition to, reflection and absorption. Incoherent Holography Technique Is a photographic record of the telescope entrance pupil through a rotation interferometer

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Acousto-optics Is a branch of physics that studies the interactions between sound waves and light waves, especially the diffraction of laser light by ultrasound or sound in general. Heterodyne In radio and signal processing, heterodyning is the generation of new frequencies by mixing, or multiplying, two oscillating waveforms. It is useful for modulation and demodulation of signals, or placing information of interest into a useful frequency range. This operation may be accomplished by a vacuum tube, transistor, or other signal processing device. Mixing two frequencies creates two new frequencies, according to the properties of the sine function: one at the sum of the two frequencies mixed, and the other at their difference. Typically only one of these frequencies is desired—the higher one after modulation and the lower one after demodulation. The other signal is either not passed by the tuned circuitry that follows, or may be filtered out.

Photodetector (PD) A transducer capable of accepting an optical signal and producing an electrical signal containing the same information as in the optical signal. [2196] Note: The two main types of semiconductor photodetectors are the photodiode (PD) and the avalanche photodiode (APD).

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Resources 1. Handbook of optical and laser scanning, by marcel dekder, 2004 2. Optical scanning holography with matlab, by dr. ting-chung poon, 2007 3. www.wikipedia.org/wiki/Voting_machine 4. www.wikipedia.org/wiki/Optical_scan_voting_system 5. www.berkeley.edu/news/media/releases/2001/10/01_ballot.html 6. www.amazon.com/Optical-Scanning-Engineering-Marshall/dp/0824784731 7. http://www.fec.gov/pages/marksnse.htm 8. http://scholar.lib.vt.edu/theses/available/etd-11597153644/unrestricted/Bradphd.pdf 9. http://www.wiley-vch.de/berlin/journals/op/07-02/OP0702_S41_S44.pdf 10. http://www.its.bldrdoc.gov/fs-1037/dir-027/_3989.htm 11. http://www.haaslti.com/Images/PDF%27s/High%20Precision%20Refractive %20Scanner%5B1%5D.pdf

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