Imaging confocal microscopy (ICM) and focus variation (FV) are two of the most used technologies for 3D surface metrology. Both methods rely on the depth of focus of the microscope objective, which depends on its numerical aperture and wavelength of the light source to compute an optical section. In this paper we study how several methods of structured illumination microscopy affect the metrological characteristics of an areal optical profiler. We study the effect of the projection of different structured patterns, the sectioning algorithms, and the use of high and low frequency components onto the optically sectioned image. We characterized their performance in terms of system noise, instrument transfer function and metrological characteristics such as roughness parameters and step height values.
Most 3D metrological microscopes used today require a scanning through the optical axis, which is time consuming. The common techniques are Coherence Scanning Interferometry (CSI), Imaging Confocal Microscopy (ICM), and Focus Variation (FV). If one technique is good for smooth surfaces, it is not for rough ones, while the good for rough is too noisy for smooth ones. Additionally, high local slopes are also dependent on the scattering properties of the surface, making the Numerical Aperture of the objective the most important property of the microscope. Imaging Confocal Microscopy is the best compromise in terms of surface application range (from smooth to rough), high local slopes on shiny surfaces, highest numerical aperture and highest possible magnification. Unfortunately, any kind of Confocal microscope today (laser scan, disc scan or microdisplay scan) requires an in-plane scanning to build up the confocal image in addition to the vertical scan, increasing the total measuring time in comparison to CSI and FV. This is against the needs of quality control in production environments, where scanning speed must be as short as possible. In this paper, we use a Microdisplay Scanning Microscope for obtaining the confocal image only relying on a single image per plane. We use a structured illumination to project a desired pattern onto the surface with a very well-defined frequency and direction. By means of the Hilbert transform, we digitally shift the projected pattern one or many times to recover the bright field and the optical sectioned images. This new method reduces significantly the measurement time, simplifies the overall cost of the system and eliminates the maintenance of scanning devices, while maintaining the optical sectioning properties of each plane. We also studied the performance of the resulting topography in terms of system noise, accuracy, repeatability and fidelity of the surface using different methods to obtain the confocal image. Finally, we also compared the results with true confocal results and with other techniques that require a single image per plane, such as Active illumination Focus Variation (AiFV).
Imaging Confocal Microscopes (ICM) are highly used for the assessment of three-dimensional measurement of technical surfaces. The benefit of an ICM in comparison to an interferometer is the use of high numerical aperture microscope objectives, which allows retrieving signal from high slope regions of a surface. When measuring a flat sample, such as a high-quality mirror, all ICM’s show a complex shape of low frequencies instead of a uniform flat result. Such shape, obtained from a λ/10, Sa < 0.5 nm calibration mirror is used as a reference for being subtracted from all the measurements, according to ISO 25178-607. This is true and valid only for those surfaces that have small slopes. When measuring surfaces with varying local slopes or tilted with respect to the calibration, the flatness error calibration is no longer valid, leaving what is called the residual flatness error.
In this paper we show that the residual flatness error on a reference sphere measured with a 10X can make the measurement of the radius to have up to 10% error. We analyzed the sources that generate this effect and proposed a method to correct it: we measured a tilted mirror with several angles and characterized the flatness error as a function of the distance to the optical axis, and the tilt angle. New measurements take into account such characterization by assessing the local slopes. We tested the method on calibrated reference spheres and proved to provide correct measurements. We also analyzed this behavior in Laser Scan as well on Microdisplay Scan confocal microscopes.
Confocal microscopes are widely used for areal measurements thanks to its good height resolution and the capability to
measure high local slopes. For the measurement of large areas while keeping few nm of system noise, it is needed to use
high numerical aperture objectives, move the sample in the XY plane and stitch several fields together to cover the
required surface. On cylindrical surfaces a rotational stage is used to measure fields along the round surface and stitch
them in order to obtain a complete 3D measurement. The required amount of fields depends on the microscope’s
magnification, as well as on the cylinder diameter. However, for small diameters, if the local shape reaches slopes not
suitable for the objective under use, the active field of the camera has to be reduced, leading to an increase of the
required number of fields to be measured and stitched. In this paper we show a new approach for areal measurements of
cylindrical surfaces that uses a rotational stage in combination with a slit projection confocal arrangement and a highspeed
camera. An unrolled confocal image of the cylinder surface is built by rotating the sample and calculating the
confocal intensity in the centre of the slit using a gradient algorithm. A set of 360º confocal images can be obtained at
different heights of the sample relative to the sensor and used to calculate an unrolled areal measure of the cylinder. This
method has several advantages over the conventional one such as no stitching required, or reduced measurement time. In
addition, the result shows less residual flatness error since the surface lies flat in the measurement direction in
comparison to field measures where the highest slope regions will show field distortion and non-constant sampling. We
have also studied the influence on the areal measurements of wobble and run-out introduced by the clamping mechanism
and the rotational axis.
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.
KEYWORDS: 3D metrology, Imaging systems, 3D scanning, Confocal microscopy, 3D metrology, Manufacturing, Optics manufacturing, Microscopes, 3D modeling, Interferometry, Inspection, Optical testing, Astatine, Image fusion, Data fusion, Signal to noise ratio, Sensors, Cameras, Objectives, 3D image processing
The most common optical measurement technologies used today for the three dimensional measurement of technical surfaces are Coherence Scanning Interferometry (CSI), Imaging Confocal Microscopy (IC), and Focus Variation (FV). Each one has its benefits and its drawbacks. FV will be the ideal technology for the measurement of those regions where the slopes are high and where the surface is very rough, while CSI and IC will provide better results for smoother and flatter surface regions. In this work we investigated the benefits and drawbacks of combining Interferometry, Confocal and focus variation to get better measurement of technical surfaces. We investigated a way of using Microdisplay Scanning type of Confocal Microscope to acquire on a simultaneous scan confocal and focus Variation information to reconstruct a three dimensional measurement. Several methods are presented to fuse the optical sectioning properties of both techniques as well as the topographical information. This work shows the benefit of this combination technique on several industrial samples where neither confocal nor focus variation is able to provide optimal results.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.