Ini adalah review dari teknologi pengukuran, Dima dalam ilmu pengukuran sangatlah penting di lakukan karena berkaitan dengan geometri dan kegunaan dari suatu produk dan ini adalah produk yang di ukur dalam ukuran nano.
Dalam kesimpulannya maka dapat di lihat di akhir dari tulisan makalah ini OK....
Review of MANUFACTURING AND MEASUREMENT OF FREE-FORM OPTICS
F.Z. Fang (1)a,*, X.D. Zhang a, A. Weckenmann (1)b, G.X. Zhang (1)a, C. Evans (1)c
a State Key Laboratory of Precision Measuring Technology and Instruments, Centre of MicroNano Manufacturing Technology – MNMT, Tianjin University, 300072, China
b Chair Quality Management and Manufacturing Metrology, University Erlangen-Nuremberg, Germany
c University of North Carolina at Charlotte, USA
Abstract
Freeform optics is the next-generation of modern optics, bringing advantages of excellent optical performance and system integration. It finds wide applications in various fields, such as new energy, illumination, aerospace and biomedical engineering. The manufacturing of freeform optics is an integrated technology involving optical design, machining,moulding, measurement and characterization. This paper surveys the current application status and research on major technologies in details.
INTRODUCTION
Freeform surfaces can be defined as surfaces with no axis of rotational invariance (within or beyond the part). Freeform surfaces may appear to have arbitrary shape, and regular or irregular surface structures.
Freeform optics has the following features :
- Increased range of manufacturable surfaces, giving optical designers more flexibility and scope for innovation.
- Enhancing the optical system performance to the maximum extent. For instance, freeform optics enable optical performance otherwise impossible, such as simultaneously correcting aberrations, increasing depth of field and expanding field of view.
- Simplifying system structure with fewer surfaces, lower mass, lower cost, smaller package-size and reduced stray-light.
- Realizing system integration easily, and reducing the difficulty in assembly. For example, multiple optical surfaces can be made on one freeform element.
Aspheric optics can be considered a special case of freeform optics with an axis of rotational invariance. Conventionally, an aspheric surface has an axis, while freeform surfaces do not.
Application
There are numerous applications of freeform optics in reflective, refractive, and diffractive optical systems [36,120]. They can be categorized into several areas, such as high performance imaging, illumination, concentration and other applications.
Freeform optics is adopted to improve the performance of optical systems. Paolo et al. proposed a new layout for an anamorphic collimator based onto two freeform cylinder surfaces, giving diffraction limited images. This collimator can be used to achieve a high resolution spectrograph for large telescopes and an interferometer cavity to test large plano optics.
A freeform mirror solves this problem and yields a cylindrical projection without digital unwrapping, as shown in Fig. 8 [82]. The panoramic annular lens (PAL) is also widely used, which consists of a single piece lens to produce the annular image of the entire 3608. Ma et al. proposed a freeform varifocal PAL design to achieve the zooming effect through a rotation of PAL around optical axis while not moving lenses [137]. Fig. 9 shows the model of this freeform varifocal PAL. Fig. 10 shows a new compact video.
Source mask optimization (SMO) has been identified by industry leaders as the only practical method to reach the 22 nm node of optical lithography [122]. To fully realize the potential of SMO, next-generation source definition optics must create exceedingly precise freeform pupils. Diffractive optical elements (DOEs) are integrated in the illumination systems of process control equipment to shape and control the beam for high accuracy inspection requirements [208]. Special illumination patterns, such as annular, dipole, quasar, and so on are commonly used in optical lithography.
Freeform mirrors have been used to solve the problem of astigmatism in traditional Czerny-Turner spectrometers. Raytracing result has shown a reduction in sagittal spot size from several hundred micrometres to around 10 mm in the wavelength range from 200 nm to 800 nm. The structure of a typical Czerny– Turner reflective grating spectrometer is shown in Fig. 19, where camera mirror M2 is a freeform surface [246].
Design and machining of freeform surfaces require an accurate mathematical description. The most common ways to describe optical freeform surfaces are:
- Freeform surfaces expressed by specific mathematic formulae. For instance, the double sinusoid surface can be described with sine and cosine functions [257].
- Surfaces described by general XY polynomials, as has been used routinely in commercial optical design software.
-Surfaces described using NURBS, suitable for CAD modelling software. Commercial optical design software also supports this data format [18].
Optical design
There are two strategies for optical design of freeform optics: multi-parameter optimization and direct mapping [10]. In multiparameter optimization the optical requirement is defined as a quantitative merit function that is to be minimized by optimization. However, when optimizing freeform optics, the higher order polynomials required for complex surfaces can make optimization slow.
Simultaneous multiple surface (SMS) method
SMS method, an important breakthrough in freeform optics, was invented in 1990 for the 2D non-imaging optical design. The abbreviation of SMS comes from the fact that it enables the simultaneous design of multiple optical surfaces. The original idea came from Minano, and the first implementation to 3D geometry came from Benitez. Therefore, SMS also is called the Minano– Benitez design method [242].
Machining mechanism
Freeform optics can be machined by a variety of methods, including ultra-precision cutting, grinding and polishing. The earliest method used in finishing of optics was polishing. Grinding has been used for centuries for machining optical blanks, but grinding with nanometric precision has not become available before the advent of ultra-precision machines. Grinding is well suited to hard and brittle materials, such as glass, silicon and steel [21] but can have low efficiency, and high cost, and limitations for complex surfaces. Single-point diamond cutting can fabricate optics without the subsequent finishing for some materials.
This amorphous modification can be demonstrated using appropriate characterization methods. Amorphous silicon can be detected by Raman spectroscopy [76], TEM [58] and so on. Fig. 26 shows the TEM analysis of the cutting chips reveals that single crystal silicon undergoes phase transition during machining [57]. The existence of amorphous state indirectly verifies the transition from Si-I to Si-II.
Ultra-precision milling always uses a high-speed spindle for high performance. The easy approach is that the diamond tool is mounted onto the spindle of an ultra-precision turning machine and the part where the tool post is normally situated; this method is called fly-cutting. It only needs the X and Z two-axes motion, and the tool feed direction is perpendicular to the spindle rotational axis for nominally plano surfaces; More complex surface shapes can be produced, particularly on machines with a B axis. Flycutting is commonly used to fabricate the micro-grooves, so that it is also known as diamond grooving. As with textured cylindrical roll turning, burrs are a major problem. An experimental set-up for machine micro-grooving was presented and the burr formation mechanism have been discussed both theoretically and experimentally [56,59].
Diamond machining is well suited to cutting crystal materials such as germanium, zinc sulfide and zinc selenide, as well as polymers such as polymethylmethacrylate, polystyrene, and polycarbonate. Mould inserts, for moulded optics, are manufactured by harder and more brittle materials that cause rapid diamond wear.
Moulding technologies
Many small aspheric lenses, such as camera lenses, are made by the direct moulding of glass or plastic in an aspheric mould. The moulds have the opposite shape of the finished asphere and are
made from materials that can withstand the required high temperatures [8]. These optical components are readily mass produced by the millions with astonishingly good quality. High quality plastic optics is mass produced by injection moulding.
Measurements
Though ultra-precision machining can achieve a very high accuracy, many factors may cause the profile errors, such as environmental factors, machine structural errors, vibration and tool wear. The metrology and compensation are indispensable and fundamental techniques for obtaining better accuracy as shown inFig. 4.
Savio et al. surveyed metrology for general freeform surfaces in 2007, ranging from car body parts to optics [191]. For optical surfaces, metrology requirements extend down to the nanometre range.
Contact measurement
Currently coordinate measuring machines (CMM) are the most frequently applied instruments for measuring freeform parts in contact mode. Conventional CMMs have a large measurement range but an uncertainty in the micrometre range. Some studies were made to improve the precision of CMMs by applying laser interferometers, fulfilling Abbe’s principle in all axes and fundamental
principles of precision machine design [195]. Examples appropriate for measuring freeform optics include nanoCMMs from IBS, SIOS, Werth Messtechnik [100], as shown in Fig. 45. They can be characterized by a measurement range between 25 mm x 25 mm x 5 mm and 400 mm x 400 mm x 100 mm.
Another design was developed at the TU Eindhoven and is now available by Xpress, the ‘‘Gannen XP’’, as shown in Fig. 47 [78]. It is based on piezoresistive deflection detection, allowing a far higher
sensitivity than regular resistive strain gauges. The stylus is connected to a triangular silicon membrane suspended by three small strips containing the strain gauges.
Non-contact measurement
Optical surface measurement methods (including imaging) and interferometric techniques provide non-contact measurement of freeform optics, and may measure the entire surface in a single
measurement. Optical measurements can be performed very fast and with low uncertainty, but are sensitive to environmental influences as well as to disturbances caused by the workpiece itself (colour, roughness, defects, chips from machining, dust, oil or water coats, etc.).
An auto focus laser probe mounted on CMM can measure freeform optics in non-contact mode. Fig. 51 shows an example using this method. With the laser beam focused on a sample surface using the microscope objective lens, the surface reflected light is detected by a four quadrant detector (FQD).
On-machine measurement
In on-machine, or in situ measurement, an appropriate probing system is built into the ultra-precision manufacturing machine, moved by the machine axes, and provides measurement data without removing the workpiece from the machine. The measuring range of the original measuring system is extended by the machine axes and the measuring path can be controlled using the CNC machine. Typically point to point scanning is used in these current On-machine measurements.
Other approaches include mounting a contact-type displacement sensor on a slide, with a ring artefact vacuum-chucked on the spindle surrounding the aspheric object, and two capacitance-type displacement sensors set on the slide to scan the surface of the ring artefact [203]. The surface profile of the ring artefact can be measured accurately by reversal [53]. Three sensors can be applied to measure the motion errors of the probe completely.
A conventional atomic force microscope (AFM) has a very small measuring range. However, Gao et al. mounted an AFM cantilever tip on a diamond turning machine, and a linear encoder with a resolution of 0.5 nm for accurate measurement of the Z-directional profile height in the presence of noise associated with the diamond turning machine (Fig. 54). A superposition of periodic sine-waves
along the X and Y directions (wavelength (XY): 150 mm, amplitude (Z): 0.25 mm) were measured [70]. In freeform grinding machines, on-machine measurement is applied widely. Satisloh integrates the On-Machine-Metrology (OMM) in their CNC grinders. The sensor is built into the tool head
of a CNC machine, moved by the CNC-axes.
Alignment
Before applying alignment methods the collected points must be corrected for known systematic deviations of the measuring system. This includes – for tactile CMM measurements – probe stylus bending effects and the tip ball correction caused because the centre point of the tip ball is captured but the surface point is different.
There are two main methods to align freeform surfaces. The first considers functionally relevant surfaces to define the workpiece coordinate system which is then transformed to the reference coordinate system of the designed surface. The other aligns both surfaces considering all data points, applying either surface or feature based approaches. The form error is obtained from the distance from the measured points to the designed surface. After filtering, reliable characterization results are derived if the influence of systematic errors is eliminated efficiently. Jiang et al. described the data processing technology for 3D surface metrology.
There are also some software packages available in the market, such as Polyworks, RapidForm, Imageware and Geomagic, providing the comparison procedures including the manual initial alignment, suggestions for the registration operations of systems and feature-based alignment. Although some research and achievements in measurement of freeform surfaces have been reported, most of them are still limited to the millimetre or micro/ sub-micro metre scale due to the matching precision.
Filtering
The form errors obtained by matching can be used for shape compensation to reduce the deviation, because they contain the roughness, waviness and real form deviation. Three components should be separated using the filtering methods, which have been widely discussed [for example 104–106, 127, 179]. The earliest filter used in surface metrology is the 2RC filter [4].
Traceability
Traceability is often considered one of the desiderata of modern metrology and seen, often erroneously, as an endorsement that the measurement is of the highest quality. If the goal of the measurement is process control, traceability may not be necessary, provided that there is a relationship between the measurements made and the function of the product or process.
Traceability is defined as a property of the result of a measurement whereby it can be related to stated references through an unbroken chain of comparisons all having stated uncertainties.
Conclusions
Manufacturing of freeform optics is a promising technology in various applications discussed in this paper. The manufacturing of freeform optics requires integrating multiple technologies including design, machining, metrology and evaluation. For the design of freeform optics, multi-parameter optimization has been adopted widely using various commercial software systems. However, the direct mapping method has potential for higher accuracy design, although it still depends on a fitting model method with high accuracy because the discrete points are directly calculated. In design of freeform optics, the tolerances should be a key focus, in order to reduce the precision requirement of machining. Machining method is the core technology of freeform optics, facing several important problems currently as follows.
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Kesimpulan
Manufaktur optik bentuk yang unik adalah teknologi yang menjanjikan dalam berbagai aplikasi yang dibahas dalam makalah ini. Pembuatan optik bentuk yang unik memerlukan pengintegrasian beberapa teknologi termasuk desain, mesin, metrologi dan evaluasi. Untuk desain optik bentuk yang unik, optimasi multi-parameter telah diadopsi secara luas menggunakan berbagai sistem perangkat lunak komersial. Namun, metode pemetaan langsung memiliki potensi untuk desain akurasi yang lebih tinggi, meskipun masih tergantung pada metode model yang pas dengan akurasi yang tinggi karena titik-titik diskrit secara langsung dihitung. Dalam desain optik bentuk yang unik, toleransi harus menjadi fokus utama, untuk mengurangi kebutuhan presisi mesin. Metode Machining adalah teknologi inti optik bentuk yang unik, menghadapi beberapa masalah penting saat ini.
