This PDF file contains the front matter associated with SPIE Proceedings Volume 10747, including the Title Page, Copyright information, Table of Contents, Introduction, and Author and Conference Committee lists.
The Sine Condition Test has been experimentally demonstrated as an effective tool for measuring linearly field dependent aberrations in simple optical systems. Simulations have also shown that it can be used to provide feedback as part of an alignment procedure for more complex systems. In this paper we show how the Sine Condition Test was used as part of the process for aligning a three mirror telescope. We present the basic concept of the Sine Condition Test, how it was implemented in our system and the experimental results from multiple alignments. Finally, we compare our experimental results to simulated results.
Axicon gratings are computer generated holograms of equally spaced concentric circles printed on a plane substrate. When illuminated by a point source of light they create axes in space defined by a line between the point source and the center of the grating pattern. The axis can be viewed in either transmission or reflection with an autostigmatic microscope. The axis created by the grating can be located to less than 1 um in translation and depending on distance from the grating to less than 1 microradian in angle. Several examples of such a use in alignment are explained.
A zoom lens has been designed for proton radiography applications. Radiographic images are recorded at the end of an accelerator, where protons exit an aluminum vacuum window producing a shadowgraph image onto an LYSO (lutetium yttrium orthosilicate) scintillator. Emission from this 5-inch-square scintillator reflects off a pellicle and is then collected by a zoom lens located 24 inches away. Proton radiography can make high-speed, multi-frame radiographs or radiographic movies. This zoom lens provides 2X magnification for viewing different object sizes. The zoom lens incorporates eleven lenses, including a moving doublet that changes the magnification. Refocus of the camera is required when zooming. Only one moving doublet lens is required to change magnification. The stop was anchored to the moving doublet and its diameter is unchanged throughout magnification changes. The entire lens system is housed in a cylindrical tube. This lens will be used with a 10-frame camera with a 44 × 44 mm square image format and 1100 × 1100 pixel resolution. Stray light suppression is most important in this lens system. Radial compensation is controlled by two locking micrometers on element 9, which relaxes the mechanical tolerancing. A helical cam barrel using a linear rail controls the movement of the doublet. Alignment of the mechanical gears will be discussed.
The tendency in the manufacture of large telescopes that working in visible light, has improve the manufacture of large mirrors by the use off-axis segments, which must be aligned with very high precision in order of fractions of wavelength for the optimal operation. In this paper we present a new method for alignment segments of a big telescope, where the positions of the images produced by each off-axis segment, of a point source placed at center of curvature, the primary mirror. These positions can be known by exact rays tracing, considering the aberrations produce in each off-axis segment. Consequently, each point source is not on the optical axis of the system. Some numerical simulations are shown to establish the accuracy achieved with the proposed method.
The manufacturing errors of an optical system are in different types of tolerance categories. Among these tolerance categories, the optical surface figure error and the surface / element alignment errors are the two most important factors in determining the optical system performance. In the large optics, the optical surface figure error is typically the dominating factor while the alignment are, for most of time, the most critical factor in the miniature optics. However, the optical testing of the miniature optics alignment is difficult and sometimes nearly impossible to be achieved properly. The misaligned lens element induce the unsymmetrical optical aberration spanning throughout the optical field. This type of aberrations usually dominates near the edge of the field if the misalignment is near its tolerance limit. We proposed a novel method utilizing an ultra-high dynamic range wavefront sensor (UHDR-WFS) to measure the off-axis aberrations of a miniature lens near its field edge. The unsymmetrical aberration is measured by spin-scanning the optical field of the tested miniature lens along the azimuthal direction. A F/2.0 optical beam out of a miniature lens is measured directly without using any collimator. The preliminary test result showing that the constant coma aberration are the dominating aberration of a misaligned miniature lens. The measurement result does matches our expectation from the simulation in the ray tracing software.
Multiple wavelength interferometry has long been considered an option for the measurement of large aspheric slope departures. In particular, a synthetic wavelength offers a variety of approaches by which large phase excursions can be unwrapped. Using multiple wavelengths can create collimation and magnification mismatch errors between the individual wavelengths that arise during beam expansion and propagation. Here, we present and analyze alignment and calibration methods for a dual-wavelength interferometer that can significantly reduce both misalignment errors and chromatic aberrations in the system. To correct for misalignment, a general method is described for the alignment of a dual-wavelength interferometer, including the alignment of lasers, beam expanders, beam splitters for combining beams and for compensating errors in the reference surface, and the fringe imaging system. A Fourier transform test at the detector surface was conducted to validate that there is essentially no magnification difference between two wavelengths resulting from misalignment of optical system. For the chromatic aberration introduced by the optical elements in the system, a ray-trace model of the interferometer has been established, to simulate the chromatic effect that optical elements will have on the measurement results. As an experimental test, we examine an off-axis spherical mirror in a non-null condition using a highly aliased interferogram. The above alignment methods and the results are analyzed based on the simulated system errors. Using this method, we demonstrate a measured surface profile of deviation of λ/25 which is comparable to a direct measurement profile of the surface on axis using a Fizeau interferometer.
The concavity of an initially flat wavefront typically increases after each reflection of the ten-reflection beam transport system at the Navy Precision Optical Interferometer (NPOI). Ideally, the exiting wavefront contour from the beam transport system preserves the original contour that enters. The beam transport system is common to and separate from the front-end, which includes primary light collectors such as siderostats or telescopes, and the back-end which includes major subsystems such as the optical delay lines, beam combiners and detectors. The beam transport system should have minimal influence on the interferometer. However, manufacturing tolerances and mount-induced deformations of each mirror collude to alter each reflected wavefront. All beam transport mirrors at the NPOI are slightly concave and each reflection adds to the concavity in the resultant wavefront. To improve the flatness of the resultant wavefront, we counter-deform a single mirror in the ten-reflection transport system. Previous analytical work using finite element analysis demonstrated the feasibility of this approach. In the present work, we have undertaken the task of verifying this approach experimentally. We set up a nine-reflection system of NPOI transport mirrors and measured the resultant beam wavefront contour. We applied a single actuator to the backside of one of the mirrors in the system and measured the contour of the exiting wavefront. Additionally, we compared the reduced concavity of the exiting wavefront to our finite element method results from the previous work, and excellent agreement was observed. In this paper, we describe our wavefront improvement approach, experimental method and results, and recommendations.
Focal length is a fundamental lens parameter. The main methods to measure the focal length in the optical shop include nodal bench and image magnification. On the other hand, high accuracy methods involve diffraction and interferometric setups. We propose in this work the use of adaptive photodetectors based on the non-steady state photo-electromotive force effect and the Talbot effect to determine the focal length of a lens; in the range of few meters. As the adaptive photodetectors produce an electrical current proportional to the square of visibility of the light pattern which impinges on them, there is no image processing involved in the detection process. Adaptive photodetectors has been used to determine with high accuracy the positions of maximum and minimum visibility of the light patterns diffracted by a vibrating grating (Talbot effect or self-imaging phenomenon). Therefore, if we place the lens under test close to the diffraction grating, the positions of maximum and minimum visibility will be shifted. By measuring the value of this shift the focal length of the lens under test can be readily determined. Preliminary experiments demonstrate that is possible to determine focal lengths up to 3 meters with an uncertainty about 0.1 %.
The EUV Snapshot Imaging Spectrograph (ESIS) is a slitless, tomographic imaging spectrograph for observing the solar transition region in extreme ultraviolet (EUV) at 63nm wavelength. An array of concave diffraction gratings re-image from the telescope prime focus to our CCD detectors. The instrument is aligned and focused in visible light, using substitute diffraction gratings ruled for the red HeNe laser line. To transfer precise alignment and focus of the visible gratings to the EUV gratings, we have developed a minitaturized, three point, noncontact measurement system, TEA (Transfer ESIS Alignment). TEA locates the grating surface using confocal microscopy, with three independent channels scanned together on a single stage, to specify the position and orientation of the spherical surface. Challenges for this measurement include the small size of the ESIS gratings (~ 16X20μm), their curved surfaces, diffraction effects, the alignment of tiny optics within TEA, and the mechanics used to repeatability mount the gratings. Our testing shows that the intrinsic repeatability of our measurement apparatus is approximately 2μm. In practice, however, our error is dominated by the process of mounting the grating subsystem in TEA, which introduces 12μm differences between subsequent runs. This level of repeatability meets our requirements for ESIS.
The SPIE short course “Introduction to Optical Alignment Techniques” has been taught for over twenty years at many SPIE conferences. It has always been one of the well attended ones, indicating a need for the subject matter. Reviews of first order concepts, and the basic optical aberrations set the stage for understanding how optical systems become misaligned, how to recognize the aberration type and diagnose what is wrong. This will suggest solutions to the alignment problem, enabling failing optical assemblies to be corrected. The course closes with a discussion of the alignment issues that impact off-axis parabolas and higher-order aspheres.
There is a long history of non-contact profilometry of mirrors performed using an autocollimator. An autocollimator is scanned over a mirror resulting in a set of local slope measurements that are then integrated to produce a measurement of the mirror profile. Such profilometers are often associated with the testing of x-ray optics. In this application a small beam is usually created by placing an aperture near the mirror under test. The W2-AM uses a reticle with a circular target. The noise, sensitivity and error characteristics of the W2-AM instrument is assessed relative to the needs of profilometry of x-ray optics.
One of the most common pieces of opto-mechanical hardware is the one inch, tip-tilt, mirror mount. Though there are numerous manufacturers of these mounts the majority of the designs follow the familiar theme of a corner pivot point and two adjustable actuators located orthogonally. This type of mount (often referred to as simply a “kinematic” mirror mount) can be found in many sizes and has been tailored for many different applications. While the simple and robust design has proven effective in many circumstances there are times when it isn’t ideal. Common issues with these mounts include long term stability, temperature dependent pointing and sensitivity to external shock and vibration. These shortcomings have led many to develop alternatives, which is especially true when designing for OEM applications. Though many alternatives have been devised and implemented they often lack the convenience and simplicity of the well-known form and are never broadly adopted.
This paper details an alternative design that has the potential to meet the needs of laboratory use while being equally well suited for demanding OEM applications. The design explored, addresses the common shortcomings listed while keeping to a compact and convenient form factor. The mirror mount has a virtual pivot point centered on the front surface of the mounted mirror and is fully lockable. The resulting approach is compared directly to the features and function of the familiar “kinematic” mirror mount form.
A tolerance analysis of an illumination system is an iterative process used to determine the bounds on the tolerances which result in an acceptable range of performance measures. Tolerances are parameters in an optical system that could deviate from their nominal value during manufacturing and assembly of optical systems (eg. curvature, conic constant, aspheric coefficients, the position or alignment of an optical element, surface roughness etc). Performance measures are properties of an optical system that encapsulate requirements like total power, or uniformity of illuminance, manufacturing cost etc.
Modern optical systems for illumination use Monte-Carlo simulations for design and analysis . Often, these optical Monte-Carlo simulations for illumination systems are computationally expensive and could take multiple hours to complete. A tolerance analysis can require multiple optical Monte-Carlo simulations, which can then take days to complete.
To reduce the time to perform a tolerancing analysis, we used a multidimensional fit. This fit allows us to modify the tolerance distributions and estimate the resulting performance measures without having to rerun lengthy ray trace simulations. Two illumination examples, one with color analysis and the other with a TIR lens are used to illustrate the advantages and limitations of this approach.
 Koshel, R.J., ed. [Illumination Engineering: Design with Nonimaging optics], Wiley-IEEE Press, New York (2013).
There are two distinct problems in determining optical surface defects. On one hand, we have a measurable area-based destruction of the wave front. This can be easily classified by optical designers based on contrast loss. On the other hand, we have brightness-based and magnification-dependent aesthetic imperfections. They have to be classified according to their visibility. So far only a dimensional classification of imperfections has been possible in the ISO standard. The integration of ANSI "Scratch & Dig" into the ISO standard offers the possibility to classify aesthetic imperfections with standardized reference plates. This paper will give an introduction to both methods.
In the design of optical assemblies, emphasis is placed on tolerancing the surface irregularity, which is a driving factor in price and manufacturing prices and time during polishing. Quite often, the default irregularity tolerance in modeling software is assumed to be a 50:50 split between astigmatism and 3rd order spherical aberration (i.e. symmetric zonal errors). In this paper, we reviewed the irregularity of over 1,000 custom fabrication optical surfaces. We looked at the relationship between the spherical and astigmatism aberrations and found generally that a surface will be either astigmatic or spherical, but rarely a mixture of the two. We also looked at the PV and rms of the surfaces and how that compares to the model and the general knowledge. One striking result of our analysis came from a closer analysis of how the optical modeling software package handles ‘power’ errors in the irregularity tolerance. It is possible that there is a mismatch between the model and the optical manufacturer.
Proc. SPIE 10747, Increasing system stability with tighter tolerances following the Taguchi Robust Design approach on short-pulsed laser helical drilling of stainless steel, 107470H (19 September 2018); doi: 10.1117/12.2317939
Helical drilling promises high quality laser drilling geometries together with a controllable shape (taper, diameter) in multiple materials. Latest research has shown that the use of a short pulsed laser source and an ultra-short pulsed laser source increase those benefits while the process time is increased. However reproductively and stability still remain an issue on these application due to the complex optical alignment and the dynamic beam rotation. The research presented covers a full fractional experimental parameter study of the controllable parameters such as laser power, optical angle and optical offset. Manufacturing deviations and process capability are derived. In addition to that a Taguchi experimental setting with a noise array (temperature, focus drift, change in process gas pressure) has been carried out. The results of five-level factor plan L25(5^9) with 9 parameters (6 control parameters and 3 noise parameters) is analyzed using ANOVA and Taguchi Robust Design. The outcome is used in an optimization process to identify any parameter combinations that result in decreased deviations from the quality remark (e.g. entrance diameter). Afterwards the control parameter are used to adjust the quality remark to the targeted value. It is shown that this optimized parameter set offers higher stability and therefore an increased process capability.
Multi-camera networks are becoming ubiquitous in a variety of applications related to medical imaging, education, entertainment, autonomous vehicles, civil security, defense etc. The foremost task in deploying a multi-camera network is camera calibration, which usually involves introducing an object with known geometry into the scene. However, most of the aforementioned applications necessitate non-intrusive automatic camera calibration. To this end, a class of camera auto-calibration methods imposes constraints on the camera network rather than on the scene. In particular, the inclusion of stereo cameras in a multi-camera network is known to improve calibration accuracy and preserve scale. Yet most of the methods relying on stereo cameras use custom-made stereo pairs, and such stereo pairs can definitely be considered imperfect; while the baseline distance can be fixed, one cannot guarantee the optical axes of two cameras to be parallel in such cases. In this paper, we propose a characterization of the imperfections in those stereo pairs with the assumption that such imperfections are within a considerably small, reasonable deviation range from the ideal values. Once the imperfections are quantified, we use an auto-calibration method to calibrate a set of stereo cameras. We provide a comparison of these results with those obtained under parallel optical axes assumption. The paper also reports results obtained from the utilization of synthetic visual data.
We have designed a SWIR(Short Wavelength Infrared) optical zoom system for surveillance. The focal length of the optical zoom system was 50mm to 500mm for a 640x512 pixel detector. The spot diameter of the optical system was about less than 30um concerning the pixel pitch of the SWIR detector. The MTF of the optical system was about 0.3 at the edge field of view. In field test we have proved to be able to get the image of a target in 1.5 times longer distance than visibility range by using our optical system with a SWIR detector at the focal length of 300mm.
In this paper, the position error modeling of the three-element Risley-prism beam scan system is established and the exact expressions of the beam pointing errors are derived by ray tracing based on the Snell’s law. The impacts of the tilt error of three-element prisms and the tilt error of the bearing position on the beam pointing accuracy are graphically presented with analytical and numerical results. According to the given pointing accuracy requirements, the allowable limit values of assembly errors are calculated, which is of practical significance for improving the design level of threeelement prisms. The analysis method in this paper is universal for the analysis of the assembly error of other cascade prisms.