Search

Coherence scanning interferometry is one of the most accurate surface measuring technologies, and it is increasingly applied to challenging surface structures, such as additive manufactured parts and transparent films, directly in environments that resemble production areas more than metrology labs. Environmental disturbances may further compromise measurement accuracy. Data acquisition strategies to reduce measurement noise in coherence scanning interferometry include averaging a sequence of repeated topography measurements or increasing the sampling frequency of the fringe signal during a single data acquisition—sometimes referred to as oversampling. In this paper, we improve the understanding of the mechanisms of the two noise reduction methods and compare their effects on surface topography measurement in the presence of environment-induced vibration. The results provide guidance for good practice in the reduction of uncertainty in surface measurement for a wide range of applications.

Holography was invented 75 years ago by Dennis Gabor. Meanwhile it is much more than a fascinating technique for the recording and reconstruction of 3D images. The paper addresses the outstanding peculiarities and the big variety of applications of holography.

The capability of optical surface topography measurement methods for measurement of steep and tilted surfaces is investigated through modelling of a coherence scanning interferometer. Of particular interest is the effect on the interference signal and measured topography when tilting the object at angles larger than the numerical aperture slope limit (i.e. the specular reflection limit) of the instrument. Here we use theoretical modelling to predict the results across a range of tilt angles for a blazed diffraction grating. The theoretically predicted interference patterns and surface height measurements are then verified directly with experimental measurements. Results illustrate the capabilities, limitations and modelling methods for interferometers to measure beyond the specular reflection limit.

We propose a model for coherence scanning interferometry using familiar Fourier optics methods and the spectrum of plane waves for the case where light source spectral bandwidth limits the fringe contrast as a function of optical path length. The model is straightforward to implement, is computationally efficient, and reveals many of the common error sources related to the optical filtering properties of the imaging system. We quantify the limits of applicability of the model related to the geometrical approximations for Fourier optics, particularly for high numerical apertures, and when using the fringe contrast for determining surface heights. These limitations can be overcome using a three-dimensional imaging model.

A powerful technique for interference fringe analysis uses tunable light sources. Originally developed to solve the problem of phase shifting in large aperture systems, the technique has evolved to the simultaneous measurement of multiple surfaces and optical thickness of optical assemblies and components. Here we review the principles and current state of the art for swept-wavelength interferometry for optical testing, including recent advances in digital holographic refocusing and environmental robustness using model-based data analysis. Applications for swept-wavelength interferometry span the full optical metrology space; we provide examples of the measurement of glass substrates for rigid data storage drives to planar waveguides for augmented and mixed reality.

Optical instruments have long played a role in manufacturing, and strong arguments favor accelerated adoption of fast, non-contact measurements of surfaces, shapes and positions as an enabler for industry 4.0. High-precision techniques such as optical interferometry have advanced considerably and have found applications ranging from semiconductor wafer lithography to automotive engine production. Even though there are clear benefits, there are obstacles to the more widespread adoption of optical techniques for dimensional measurements. Many of these obstacles are technical--such as vibration sensitivity and metrological traceability; but others reflect the cultural gaps between academia, makers of optical instruments, standards organizations and end users. In this talk, I propose that understanding these cultural differences can assist in advancing optical methods for the most critical needs of data-driven manufacturing.

Interferometers for the measurement of topography rely on imaging systems to map surface points to a camera. The response of an interferometer to variations in surface height depends on the filtering properties of the imaging system. Here we provide a simple and practical model of an imaging interferometer using classical Fourier optics, including the effects of partial coherence. The model is useful for understanding basic properties such as lateral resolution and error sources related to measurement principles that make assumptions regarding the fidelity of the instrument response over a range of surface spatial frequencies and light source wavelengths.

Optical non-contact, interferometric measurement strategies for mold pins and plates provide an effective means for improving the consistency of the injection molding of small lenses for consumer devices. In this presentation, we describe high-precision interferometric metrology combined with motion systems and advanced software to address the wide range of length scales and metrology requirements of mold assemblies intended for high-volume production.

Injection molding is the leading high-volume production method for aspheric lenses used in consumer electronics. The design tolerances of these lenses require careful metrology of pins, resultant lenses and the injection molding plates themselves. Pins determine the form of the lenses, while the alignment of the pins to the mold plate determines apex distance, decenter and tilt. One key parameter of the mold plate is the overall flatness, as it is a critical datum for the pins. We demonstrate flatness metrology on a platform capable of sub-micron lateral resolution that can be used for roughness measurements as well as high resolution measurements of sub-millimeter features such as apex centration. The platform is capable of measuring the full 200 mm by 200 mm mold plate in under 30 minutes. The flatness results are correlated with measurements from a laser Fizeau interferometer and demonstrate better than ±1 μm correlation on samples with 20 microns of departure and better than ±0.2 μm correlation on plates with sub-micron departure.

Engineered functional surfaces often feature varying slopes on macro- and micro-scales. When surfaces are mirror-like, the highest surface slope that can be measured by a far-field 3D imaging optical surface measuring instrument isthe arcsine of the numerical aperture (NA) of the objective lens, i.e. the acceptance angle of the lens. However, progress in instrument design has allowed for measurement of non-specular surfaces with slopes steeper than this “traditional” NA limit. Nonetheless, there is currently a lack of understanding about the instrument response to surfaces with steep slopes beyond this limit. It is unclear over what surface spatial frequencies we can expect to accurately report fine surface-feature details. Here we present results demonstrating the capability of a commercial coherence scanning interferometer for measuring surface topography of a roughened flat and a blazed grating with tilt angles greater than the NA slope limit. We show that the surface form, i.e. the tilted plane, can be measured correctly. But, while surface texture information that can appear useful is also obtained, tilting significantly influences the measurement accuracy of micro-scale texture, and for asymmetric gratings, can depend on the tilting direction. A simplified surface scattering model suggests that the loss of scattered power captured by the instrument and a low signal-to-noise ratio causes the reduction of measurement accuracy. However, a rigorous three-dimensional instrument model is needed for a full understanding; we will develop this in our future work.

An important characterization for surface topography instruments is a traceable calibration of the height scale. We calibrate our coherence scanning interference microscopes using a natural spectral emission line in place of a sequence of material measures such as step-height specimens. The uncertainty budget for our approach includes estimates for several error sources associated with long term drift. Here we summarize results collected over 3 years’ experience from our laboratories to provide statistical support for confirming and refining these uncertainty contributions. We find that the source wavelength stability is < 0.005% RMS and the stability of the height scaling factor (the amplification coefficient) is < 0.02% RMS over 900 days. Both values are better than our original estimates. We also show < 0.13% RMS reproducibility of the complete traceable process using acceptance test data for over 100 manufactured instruments. Finally, we report results of 3 years of experience in certifying step-height specimens using the traceable wavelength method.

Interferometers for the measurement of surface form and texture have a reputation for high performance. However, the results for many types of surface features can deviate from the expectation of one cycle of phase shift per half wavelength of surface height. Here we review the fundamentals of imaging interferometry and describe ways of defining instrument response, including the linear instrument transfer function. These considerations define practical regimes of linear behavior that are usually satisfied for traditional uses of interferometers; but that are increasingly challenged by applications involving complex textures and high surface slopes. We conclude by proposing pathways for further improving performance on difficult surface structures using advanced modeling techniques.

The functional behavior of sealing surfaces and bearings depends on the texture direction with respect to the axis of rotation of a machined part, known as the twist or lead angle. We present our lead angle measurement solution using interference microscopy, multi-axis staging, and advanced software for determining surface texture direction and cylinder rotation axis.

An international comparison of surface topography measurement noise was carried out. The comparison involved twelve optical surface topography instruments (focus variation instruments, confocal microscopes and coherence scanning interferometers) from six European research laboratories. The purpose of this comparison was to perform a practical test of the ISO definition and procedures for determining measurement noise on a variety of instruments, and to provide good practice guidance to the user community for quantifying, specifying and interpreting measurement noise. Despite taking steps to enable an optimal comparison by supplying a variety of specimens, where the ‘best’ could be selected and the measurement area was clearly defined, an appropriate comparison of the measurement noise beyond the actual measurement conditions was complex, as the measurement time, speed and lateral resolution were difficult to establish for most of the instruments. The results that could be obtained were: (i) the measurment noise N M can be reliably estimated from two measurements, (ii) noise may vary considerably between instruments, even when they have the same measurement principle, (iii) the noise in a practical measurement may be considerably higher than the noise with a levelled flat, and (iv) the ‘rms repeatability’ has little correlation to the noise determined according to the ISO specification standard, and may lead to values that are more than a hundred times smaller.

For optical measurements of areal surface topography, the instrument transfer function (ITF) quantifies height response as a function of the lateral spatial frequency content of the surface. The ITF is used widely for optical full-field instruments such as Fizeau interferometers, confocal microscopes, interference microscopes, and fringe projection systems as a more complete way to characterize lateral resolving power than a single number such as the Abbe limit. This paper is a comprehensive review of the ITF, including standardized definitions, ITF prediction using theoretical simulations, common uses, limitations, and evaluation techniques using material measures.

The pursuit of low noise in optical instruments for areal surface topography measurement is relevant to many surface types, ranging from super-polished optical surfaces to weakly reflecting or scattering textures that require enhanced signal sensitivity. We clarify the definition and experimental methods for quantifying random noise in areal surface topography measurements. We also propose a parameter, the topographical noise density, that concisely summarizes the effects of measurement bandwidth. To illustrate these ideas, we present results from a commercial phase-shifting interference microscope showing an RMS measurement noise of 0.03 nm for a 1-s data acquisition of 1 million surface topography image points, after application of a 3 × 3-pixel convolution filter. The results follow the expected inverse square root dependence on the data acquisition time for fast averaging of topography maps, resulting in a measurement noise of

We propose a practical theoretical model of an interference microscope that includes the imaging properties of optical systems with partially coherent illumination. We show that the effects on measured topography of a spatially extended, monochromatic light source at low numerical apertures can be approximated in a simplified model that assumes spatially coherent light and a linear, locally shift-invariant transfer function that accounts for optical aberrations and the attenuation of diffracted plane wave amplitudes with increasing spatial frequencies. Simulation of instrument response using this model agrees with methods using numerical pupil-plane integration and with an experimental measurement of surface topography.

Limited light source coherence can be both a complication and a benefit to interferometric optical metrology. Although high-coherence lasers are great for displacement measuring interferometry, holography, and Fizeau interferometry, many instruments rely on low coherence as part of the measurement principle. Examples include systems that separate parallel surfaces of transparent parts, coherence scanning interferometers for surface topography, and coupled-cavity fiber position sensors. In cases where high coherence is essential, there can nonetheless be a benefit to synthesizing reduced coherence to suppressing spurious fringes, coherent noise, and unwanted speckle.

The pursuit of low noise in interference microscopy for topography measurement is relevant to many surface types, ranging from super-polished optical surfaces to weakly-reflecting or scattering textures that require enhanced signal sensitivity. Noise is a random error source that may be quantified using a repeatability test. Here we propose a noise density normalized to the square root of the number of data points per unit time, to evaluate performance independent of measurement speed and areal filtering. Consistent with standardized vocabulary, we also distinguish between measurement noise, which is specific to a part and environmental conditions, and instrument noise, which is an apparatus specification corresponding to measurement noise under the best possible conditions. To illustrate these ideas, we present results from a commercial phase-shifting interference microscope showing an RMS measurement noise of 0.03 nm for a 1-second data acquisition of 1 million surface topography image points, after application of a 3x3-pixel median filter. The results follow the expected inverse square root dependence on the data acquisition time.

Optical fabrication relies on precision metrology over a wide range of lateral scales. Consequently, an important performance parameter for Fizeau interferometers is the instrument transfer function (ITF), which specifies the system response as a function of surface spatial frequency. Advances in test procedures, instruments and automated analysis techniques now enable reliable ITF characterization independent of many traditional sources of error. Results here show the ITF for a commercial 100-mm aperture interferometer with spatial frequency response ranging from 0 to 1500 cycles per aperture

Optical systems for immersive displays incorporate a range of optical components and assemblies that require precision non-contact metrology, including Fizeau interferometry of surface form, new techniques for aspheric microlenses, and interference microscopy for surface structure and texture analysis. Here we consider the problem of evaluating the parallelism and surface form deformation for stacked assemblies of multiple flat glass substrates. Similar structures are common for RGB planar waveguides with slanted sub-wavelength gratings acting as in- and out-couplers. In our experiments, we demonstrate the effectiveness of coherence scanning over a large aperture area using an 100-mm aperture white-light interferometer.

Calibration, adjustment and verification of surface topography measuring instruments are important tasks, often facilitated by precision step-height specimens that have been calibrated using traceable metrology such as interferometry. Although standardized procedures for calculating parameters of the step-height are available for line profiling contact stylus systems, there is inconsistent guidance as to how to interpret step height data for 3D, areal surface topography instruments, such as confocal and interference microscopes. Here we provide definitions for the reference and measurement areas of step-height specimens as well as practical measurement protocols for processing the surface topography map.

Advances in test procedures and analysis techniques now enable reliable instrument transfer function measurements to 1000 cycles per aperture independent of several traditional sources of uncertainty and operator error.

This review gathers together 15 special topics in modern interferometric metrology representing a sampling of historical, current and future developments. The selected topics cover a wide range of applications, including distance and displacement measurement, the testing of optical components, interference microscopy for surface structure analysis, form and dimensional measurements of industrial parts, and recent applications in semiconductor manufacturing and consumer electronics. Techniques range from laser Fizeau systems to dynamic ellipsometry using polarized heterodyne interferometry.

In holography, we create a “holistic” representation of a light wave from the coherent superposition of measurement and reference beams. We shift the light phase to detect the complex amplitude. This sounds a lot like interferometry! Here I explore how advances in digital signal processing are driving a convergence of the two techniques. Article not available.

This article reviews selected topics representative of advancements in the optical measurement of shape, texture, geometric dimensions, and position of objects, with applications ranging from stage metrology to surface texture analysis. The unifying theme of this update is the confluence of two technology trends – (1) rapidly advancing component technology, enabling high performance and new techniques and (2) expanding investment in advanced metrology for high value-added products from consumer electronics to precision-engineered components. Keywords: fiber sensor; optical encoder; deflectometry; focus sensing; confocal; OCT; super resolution; instantaneous interferometry

We propose a new type of interference objective that makes use of two partially-reflective beamsplitter plates arranged coaxially with the objective lens system, in an assembly that is better suited to large fields of view than the traditional Michelson design. The coaxial plates are slightly tilted to direct unwanted reflections outside of the imaging pupil aperture, providing high fringe contrast with spectrally-broadband, spatially extended white light illumination. Examples include a turret-mountable 1.4× magnification objective parfocal with high-magnification objectives up to 100×, and a dovetail mount 0.5× objective with a 34×34mm field for wide-field measurements of surface form.

We propose an in situ method for establishing the amplification coeffici ent (height scale) for an interference microscope as an alternative to the traditional step height standard technique for routine calibration. The method begins by determining the properties of the microscope illuminator equipped with a narrow-band spectral filter, using a spectrometer to provide traceability to the 546.074nm 198 Hg line. A data acquisition with the interference microscope links this wavelength standard to a calibration of the properties of the optical path length scanning mechanism of the interferometer. A capacitance sensor in the scanner maintain s this calibration for subsequent measurements. A targeted k=1 uncertainty of 0.1% is favorable when compared to calibration using physical artifacts, and the calibration procedure is easier to perform and less sensitive to operator error. Keywords: coherence scanning interferometry, interfer ence microscopy, calibration, standardization 1. INTRODUCTION

High-performance data processing algorithms for phase-shifting interferometry accommodate adjustment errors in the phase shift increment as well as harmonic distortions in the interference signal. However, a widely overlooked error source is the combination of these two imperfections. Phase shift tuning errors increase the sensitivity of phase estimation algorithms to second-order and higher harmonics present in Fizeau interference signals. I derive an analytical formula for evaluating these errors more realistically, in part to identify the characteristics of the optimal PSI algorithm. Even for advanced algorithms, it is found that multiple reflections increase the error contribution of detuning by orders of magnitude compared with the two-beam calculation and impose a practical limit of 30% in tuning error for sub-nm metrology in a 4%–4% Fizeau cavity. Consequently, a preferred approach for high precision spherical cavities is to use either wavelength tuning in place of mechanical phase shifting or an iterative solver that accommodates unknown phase shifts as a function of field position. © 2014 Optical

Specifications for confocal microscopes, optical interferometers and other methods of measuring areal surface topography can be confusing and misleading. The emerging ISO 25178 standards, together with the established international vocabulary of metrology, provide a foundation for improved specifications for 3D surface metrology instrumentation. The approach in this paper links instrument specifications to metrological characteristics that can influence a measurement, using consistent definitions of terms, and reference to verification procedures.

The measurement of flat optical components often presents difficulties because the presence of parallel surfaces generates multiple reflections that confuse conventional laser-based interferometers. These same parts have increasingly demanding surface finish tolerances as technologies improve over time, further complicating the metrology task. Here we describe an interferometric optical system for high-accuracy noncontact evaluation of the form and texture of precision flat surfaces based on an equal-optical-path geometry that uses extended, broadband illumination to reduce the influence of speckle noise, multiple reflections, and coherent artifacts by a factor of 10 when compared to laser-based systems. Combined with a low-distortion, fixed-focus imaging system and 4-Mpixel camera, the 100 mm aperture instrument offers surface height resolutions of 0.1 nm over lateral spatial frequencies extending from 0.01 to 10 cycles/mm. The instrument is vibration resistant for production-line testing of flat optics such as glass hard disks for the data-storage industry and flat-panel-display substrates.

Advances in the implementation of coherence scanning interferometry have dramatically extended the range of application for this well-known technique. New data acquisition and data processing methods significantly improve dynamic range, enabling measurements of steeply-sloped surfaces usually considered beyond the reach of high-NA objectives. Hybrid data acquisition incorporating sinusoidally-modulated phase shifting reduces signal-to-noise to the 0.1 nm/√Hz level, extending the technique to super-polished surfaces. OCIS codes: 120.3180 Interferometry; 120.3940 Metrology; 120.6660; Surface measurements, roughness

The ancestral roots of the Fringe conference are in the automatic processing of fringe patterns. When we think of patterns, an image comes to mind of flowing lines beautifully wrapped around surface contours. But automatic processing of fringes is not limited to this kind of pattern: The fringes may be laid out along a line of sight as the time history of an object displacement, captured by a detector and processed to tell us something about how the object has moved, or more generally, where the object is with respect to a reference point in space.

Optical interference microscopy has evolved from a tool for 3D surface roughness to a multi-functional platform for complete surface structure analysis. This review paper considers the evolution, current state of the art and future prospects for this technology.

The interactive self-study programme (called CAVOCA: Computer Assisted VOCabulary Acquisition) is based on generally accepted theories about the mental lexicon (cf. Aitchison's Words in the Mind) and the "levels of processing" theory first advanced by Craik and Lockhart. The programme constitutes a systematic attempt to operationalize the theoretical analysis of the word learning process that recognizes various stages in this same process: learning about the various features of the word, embedding it in various networks, consolidation etc. leading to efficient lexical retrieval. With this end in mind, the words are presented in a variety of L2 contexts, enabling the learner to process the word intensively and facilitating long-term retention. The programme is intended as an alternative to more orthodox word learning techniques such as bilingual word lists with their disappointing long-term retention results. The programme has been contrasted in various experimental (differential treatment) settings with bilingual word lists, and the results confirm the theory concerning the structure of the mental lexicon in that a deeper level of processing results in better long-term retention.

We propose a type of interference objective that extends the range of application for flexible microscope platforms to larger fields of view. The objective comprises a beamsplitter plate and a partially transparent reference mirror arranged coaxially with the objective lens system. The coaxial plates are slightly tilted to direct unwanted reflections outside of the imaging pupil aperture, providing high fringe contrast with spatially extended white-light illumination. Examples include a turret-mountable 1.4× magnification objective parfocal with high-magnification objectives up to 100× and a dovetail mount 0.5× objective with a 34×34 mm field. This design is a practical alternative to the classical Michelson and Mirau type objectives for low magnifications.

We describe a scanning white-light interferometer for high-precision surface structure analysis. Interferograms for each of the image points in the field of view of the instrument are generated simultaneously by scanning the object in a direction perpendicular to the object surface, while recording detector data in digital memory. These interferograms are then transformed into the spatial frequency domain and the surface height for each point is obtained by examination of the complex phase as a function of frequency. The final step is the creation of a complete three-dimensional image constructed from the height data and corresponding image plane coordinates. The measurement repeatability is better than 0·5 nm r.m.s. for a surface height range of 100 μm.

Carbon/epoxy material in the form of a unidirectional composite was loaded under different conditions to determine the frequency content of acoustic emission signals. Special specimens, designed to fail under well-defined modes such as matrix cracking, debonding, fibre pull-out and fibre failure, were tested from zero load to failure. Real-time analysis of the acoustic signals was performed with a Dantec burst spectrum analyser permitting the detection of frequencies in a wide band around a certain central frequency. The signals analyzed ranged from 50 to 600 kHz and were distinguished as being released from four special types of failure. By using different types of specimen it was concluded that matrix cracking released frequencies between 90 and 180 kHz, fibre failure frequencies above 300 kHz, debonding frequencies between 240 and 310 kHz and pull-out frequencies between 180 and 240 kHz. A comparison with published values shows agreement with the frequency bands determined for matrix and fibre failure, while for the debonding and pull-out mechanisms the frequency bands turned out to be interchanged. It can still be concluded that these failure mechanisms generate frequencies between those of matrix failure and fibre fraction.

This brief review of modern interferometric methods of surface measurements describes basic techniques, typical applications and common performance specifications. Recent developments include model based methods, analysis of surface structures that include transparent films and optically unresolved features, and systems designed specifically to be compatible with industrial environments and international standards.

We review definitions of optical resolution and how they relate to the Instrument Transfer Function of surface profiling interferometers. The corresponding optical cutoff provides a selection criterion for a given metrology application (PSD, waviness).

This article deals with an alternative approach to the problem of the selection of words in foreign language teaching at higher levels. Since frequency as a selection criterion is inadequate beyond the first 2.500 words, it is argued that the question as to how many and which words can only be answered on the basis of quantitative data on the relationship between lexical coverage and text comprehen-sion. Only after establishing what coverage goes with what level of comprehension will it be possible to come forward with valid suggestions concerning the number of words required for reading authentic L2 texts. As to which words, it is argued that these should be selected from a corpus of 10.000 words (beyond the first 2.500) to be constructed on the basis of a combination of selection criteria such as frequency, valency etc. Knowledge of any 5.000 words from this corpus (combined with the first 2.500) will yield such a dense coverage (at least 95%) of general L2 texts that the meaning of any remaining unknown words can be deduced via contextual clues utilisation. To substantiate this claim the results of some experimental investigations of the relationship between coverage and text comprehension are reported.