Stent quality control is a highly critical process. Cardiovascular stents have to be inspected 100% so as no defective stent is implanted in a human body. However, this visual control is currently performed manually and every stent could need tenths of minutes to be inspected. In this paper, a novel optical inspection system is presented. By the combination of a high numerical aperture (NA) optical system, a rotational stage and a line-scan camera, unrolled sections of the outer and inner surfaces of the stent are obtained and image-processed at high speed. Defects appearing in those surfaces and also in the edges are extremely contrasted due to the shadowing effect of the high NA illumination and acquisition approach. Therefore by means of morphological operations and a sensitivity parameter, defects are detected. Based on a trained defect library, a binary classifier sorts each kind of defect through a set of scoring vectors, providing the quality operator with all the required information to finally take a decision. We expect this new approach to make defect detection completely objective and to dramatically reduce the time and cost of stent quality control stage.
Stent quality control is a critical process. Coronary stents have to be inspected 100% so no defective stent is implanted into a human body. We have developed a high numerical aperture optical stent inspection system able to acquire both 2D and 3D images. Combining a rotational stage, an area camera with line-scan capability and a triple illumination arrangement, unrolled sections of the outer, inner, and sidewalls surfaces are obtained with high resolution. During stent inspection, surface roughness and coating thickness uniformity is of high interest. Due to the non-planar shape of the surface of the stents, the thickness values of the coating need to be corrected with the 3D surface local slopes. A theoretical model and a simulation are proposed, and a measurement with white light interferometry is shown. Confocal and spectroscopic reflectometry showed to be limited in this application due to stent surface roughness. Due to the high numerical aperture of the optical system, only certain parts of the stent are in focus, which is a problem for defect detection, specifically on the sidewalls. In order to obtain fully focused 2D images, an extended depth of field algorithm has been implemented. A comparison between pixel variance and Laplacian filtering is shown. To recover the stack image, two different methods are proposed: maximum projection and weighted intensity. Finally, we also discuss the implementation of the processing algorithms in both the CPU and GPU, targeting real-time 2-Million pixel image acquisition at 50 frames per second.
In this paper we introduce a new optical technique for the measurement of aspheric and free-form optics and moulds.
This technique, called confocal tracking, consists on tracking the focus on the sample while it is moved along the
horizontal XY axes. Unlike all single-point based techniques, confocal tracking images the surface, which makes it
possible to determine the best in focus position within every field of view and to correct the residual tracking errors for
each measured point. As a result, confocal tracking provides shape measurements with nm-level accuracy and
acquisition speeds of 1 mm/s typically. Depending on sample geometry, high NA objectives can be used, with which it is
possible to measure slopes as high as 65°. In addition, because confocal tracking is not a single-point but an imaging
technique, it is possible to center the surface to be measured with a very quick procedure that can be automated easily.
This step may be particularly relevant for optics with symmetry of revolution. The confocal tracking profiler is a
proprietary technology of UPC and Sensofar and can be considered the optical equivalent of a high-accuracy contact
profiler.
Optical profiling techniques, mainly confocal and white light interferometry, have demonstrated to be suitable
techniques for characterization of transparent thick films. Measurements are carried out by vertically scanning the
upper and lower film interfaces. Thickness of the layer is determined from the two peaks in the confocal axial response
or from the two sets of interference fringes developed during the vertical scan. The 3D topographies of the upper and
lower interfaces of the film can also be obtained. Measurements of photoresists or oxide coatings are typical examples
of thick film characterization. On the other hand, measurement of thin films is considered to be a very difficult
application to carry out with most optical imaging profilers. A film should be considered as thin when the two peaks
obtained along the vertical scan become unresolved. We introduce new methods based on confocal techniques, which
make it possible to measure sub-micrometric layers on structured samples. These techniques are based on the
comparison between the axial responses obtained in areas where the film is present and those in other areas where only
the substrate is present. This method has been successfully used for thickness assessment of several samples, such as a
set of calibrated Si-SiO2 layers.
New material applications and novel manufacturing processes are driving a systematic rise in market demands concerning surface inspection methods and the performance of non-contact profilers. However, analysis of the specifications and application notes of commercial optical profilers shows that no single system is able to offer all the features a general purpose user would like simultaneously. Whereas white light interferometers can achieve very fast measurements on the micro and nano-scale without any range limitation, they can not easily deal with steep smooth surfaces or structured samples containing dissimilar materials. PSI techniques allow the user to perform shape and texture measurements even below the 0.1 nm scale, but they have an extremely short measurement range. Imaging confocal profilers overcome most of these difficulties. They provide the best lateral resolution achievable with an optical profiler, but they have a resolution limit, which is dependent on the NA and cannot achieve the 0.1 nm vertical resolution. In this paper we introduce a new dual-technology (confocal & interferometer) illumination hardware setup. With this new sensor head it is possible to choose between standard microscope imaging, confocal imaging, confocal profiling, PSI and white light interferometry, by simply placing the right objective on the revolving nosepiece.
One of the most challenging applications of optical metrology is measuring the shape of the inner surfaces of nozzles such as those of fuel injectors, wiring dies and printheads. A current non-contact solution is confocal laser scanning microscopy (CLSM). However, the inner urfaces of the nozzles behave as though they were optically polished, which gives rise to very weak, backscattered light signals. Therefore, measuring with CLSM is a very slow process and the uncertainty of the results is very high. Moreover, new nozzle designs are moving towards even steeper walls, which means that CLSM may well become useless in the near future. In this paper, we introduce a new method based on a proprietary unfolded confocal arrangement, which uses the light that is reflected onto the inner surfaces and that passes through the nozzle instead of the backscattering signal. The setup and implementation of this new method and the attendant profiling algorithms are explained. With regard to real applications, we focus on measuring the 3D topography of conical nozzles drilled into organic polymer films with excimer lasers. These films are used in the manufacture of the orifice plates, which are attached to the printheads of thermal inkjet cartridges. Fast measurements and accurate results obtained for nozzles of 25 micrometers in diameter and wall angles close to 17º are demonstrated.
One of the applications, which is considered to be very difficult to carry out with most optical imaging profilers, is the shape and texture measurements of structured surfaces obtained from the superposition of various micro or sub-micrometric layers of dissimilar materials. Typical examples are the architectures of microelectronics samples made up of Si, SiO2, Si3N4, photoresists and metal layers. Because of the very different values of the index of refraction of the involved materials, visible light is reflected in the various interfaces. As a result, some reflected wavefronts are superposed giving rise to interference patterns, which are difficult to understand in terms of surface topography and layer thickness.
In this paper we introduce a new method based on non-contact confocal techniques to measure the shape of structured samples. The method is based on the comparison of the axial responses obtained in areas of the surface where there is a layer and in other areas where there is just the substrate. To our knowledge, this approach enables the confocal profilers to measure the thickness of layers on the sub-micrometric scale for the first time.
Recently, telescopes with segmented primary mirrors are becoming increasingly popular due to their ability of achieving large apertures without the inconveniences caused by the fabrication and handling of monolithic surfaces with 8m (or over) in diameter. The difference in position of each pair of adjacent segments along the local normal of their interface (called piston hereafter), however, needs to be precisely measured in order to provide a diffraction- limited image. If a system yielding the nanometric accuracy required in piston measurements worked in daylight hours, the resultant saving in observation time would be an important advance on a majority of the state-of-the-art piston measurement systems. An interferometric piston measurement instrument accomplishing such objectives has been designed starting from the usual Michelson configuration at the CD6 (Terrassa, Spain), and its final test has been carried out in the test workbench of the Instituto de Astrofisica de Canarias (IAC, Canary Islands, Spain). Its optical layout relies on projecting the reference arm of the interferometer onto one of the segments of the pair considered, along the direction of the local normal to the surface while the measurement arm is projected onto the interface which divides the pair of segments considered. The field of view and its illumination are calculated to be equivalent in both segments. The lateral shift of the fringes in both interferograms determines the piston error present. A combination of monochromatic and white light is used, in order to remove the (lambda) /2 phase ambiguities present in piston measurements without losing the required resolution in the measurement. In this paper, the optical design of this interferometric piston measurement instrument will be presented. The particular configuration used in the interferometer, the implementation of an imaging system allowing to see both the interface of the segments and the interference fringes, the effect of the extension of the source and the use of both monochromatic and whit light will be discussed. A detailed study of the wavefront errors embedded in the wavefront at different stages of the interferometer is performed. The analysis shows the validity of the design of the instrument, and some experimental results obtained at the test workbench are provided to demonstrate the ability of the instrument to perform nanometric piston measurements under daylight conditions.
One of the objectives of surface metrology is to obtain a better and faster assessment of the micro- or nanogeometry of component surfaces. In this way the innovative concept of the profiler is changing towards non-contact modular computer- controlled systems for measuring and analyzing shape and texture of a surface. In this paper we present a new instrument which is based on the concept of confocal microscopy. In this instrument (which may be used for measurements on smooth and rough surfaces) a pattern of slits is imaged by a very high numerical aperture optical system on the surface of the sample to be measured. The reflected or diffused light is observed with a CCD array and analyzed with different digital image processing algorithms. In addition to the replacement of the existing stylus systems there are also important new potential applications for this type of instrument. We present the results obtained in micro- or nanomeasurements of high precision optical surfaces, texture assessment of non-homogeneous liquid depositions and metrology of microstructures such as master gratings and certified calibration standards. The obtained results show that the confocal profiler is robust enough to provide a surface topography with spatial resolution lower than 0.5 micrometer and uncertainty of about 10 nm.
Laser heating of glass samples is a simple and versatile method for obtaining polished surfaces of optical quality. Since laser beam intensity non-uniformity can translate into significant variations in the induced surface temperature, the success of the laser surface-polishing process strongly depends on obtaining uniform intensity profiles or flat-top distributions at the sample plane. In this paper we present a comparison between large-area CO2 laser-polishing experiments carried out in optical glass substrates following two different approaches: (1) A reshaped beam obtained by an active integration method is swept over the glass surface. (2) A static beam reshaped by means of both a multifaceted mirror and a square pipe light guide is applied.
The surface metrology market toady is moving towards non- contact modular computer-controlled system for measuring and analyzing roughness, contour and topography. In this paper we present a new optical instrument based on the concept of confocal microscopy. In this instrument, which is especially suitable for measurements on smooth surfaces, either a pinhole or a structured light pattern in imaged by a very high numerical aperture optical system on the surface of the sample to be measured. The reflected light is observed wit a CCD array and analyzed with different image data processing algorithms. Two different experimental prototypes were developed to allow the measurement not only of surfaces with good accessibility but also of those with intricate geometries, difficult access and small dimensions. Various samples such as high precision optical surfaces, master gratings, and diamond drawing dies were measured. All the results obtained show that the confocal optical profiler is robust enough to provide a surface topography with spatial resolution lower than 1 micrometers and uncertainty about 10 nm. In addition to the replacement of the existing stylus system, there are also important new potential applications for this kind of instrument.
The surface metrology market today is moving towards non- contact modular computer-controlled systems for measuring and analyzing roughness, contour and topography. Since most of these systems require a great accessibility to the surface to be measured they can not be used for the metrology of surfaces with intricate geometries, difficult access and small dimensions. In this paper a new instrument concept for measuring the profiles of such difficult-to- access surfaces is demonstrated. Two different prototypes based on the confocal microscopy arrangement have been developed. A visible laser beam is focused with a high numerical aperture optical system onto the component surface that has to be measured. Depending on the prototype configuration, either the retroscattered or the reflected light signals are measured with a CCD array and analyzed with an image data processing algorithm. As an example of the application of these confocal prototypes, over 20 diamond wiring dies with different sizes and geometries have been measured and their inner profiles displayed and analyzed. All the results obtained show that the measurement method is robust enough to provide sub-micron repeatability and in the worst case the precision achieved is one order to magnitude better than the manufacturing tolerances of wiring dies.
In laser materials processing applications it is often necessary to work with uniform intensity distributions. This goal is quite difficult to achieve when dealing with high power laser beams, and becomes critical for a successful application involving surface heat treatment of non-metallic materials. We have designed and tested a very simple beam shaper for transforming the initial intensity distribution of a CO2 laser beam mode into a more uniform intensity profile. The beam shaper is a tow-faceted mirror for active integration of high power coherent laser beams. After reflection in the faceted mirror, a TEM00 or TEM01 CO2 laser beam is divided into two beamlets that overlap to give a more uniform intensity distribution. A sharp interference pattern due to the high spatial coherence of the incident beam appears. This interference pattern is actively integrated by a high-frequency longitudinal displacement of one of the facets. This provides a change in the relative phase of the two beamlets, and consequently the interference pattern vibrates and its contribution to the intensity distribution averages out. When sweeping this distribution over a sample, a uniform amount of energy is deposited at every point of its surface. It must be emphasized that unlike multifaceted mirrors, out two-facet integrator may provide uniform intensity profiles over any working distance. Finally, as in other integration devices an imaging system may be used to obtain a spot of the shape and the size desired for a particular application.
The development of new optical glass polishing methods that improve the production efficiency of standard components is of key interest for the optical industry. Laser polishing is a very attractive method since it requires no mechanical abrasives or surface-adapted polishing tools. Moreover, it can be applied to surfaces of very complicated topography that are difficult or even impossible to polish using more traditional techniques and provides the possibility of automatic processing. We demonstrate rapid generation of large area polished optical surfaces by high power CO2 laser irradiation. Results focus on glasses with high expansion coefficients conventionally used in the optical industry such as TRC-33. The technique is applied to preheated glass samples with initial rms roughness up to 500 nm. To find out the conditions for successful and reliable use of the proposed laser polishing method, the laser-driven heating process is monitored by means of the surface and depth temperature distributions. Whereas the former is determined in-situ from the IR radiation emitted by the glass surface, the latter is obtained by comparison of the IR radiation emitted by the bulk sample with a theoretical model. Laser polishing of 5000 mm2 glass surfaces is reported and processes involved in the surface modification of irradiated samples are described and discussed.
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