In recent years, optical coherence tomography (OCT) became gained importance in medical disciplines like ophthalmology, due to its noninvasive optical imaging technique with micrometer resolution and short measurement time. It enables e. g. the measurement and visualization of the depth structure of the retina. In other medical disciplines like dermatology, histopathological analysis is still the gold standard for skin cancer diagnosis. The EU-funded project VIAMOS (Vertically Integrated Array-type Mirau-based OCT System) proposes a new type of OCT system combined with micro-technologies to provide a hand-held, low-cost and miniaturized OCT system. The concept is a combination of full-field and full-range swept-source OCT (SS-OCT) detection in a multi-channel sensor based on a micro-optical Mirau-interferometer array, which is fabricated by means of wafer fabrication. This paper presents the study of an experimental proof-of-concept OCT system as a one-channel sensor with bulk optics. This sensor is a Linnik-interferometer type with similar optical parameters as the Mirau-interferometer array. A commercial wavelength tunable light source with a center wavelength at 845nm and 50nm spectral bandwidth is used with a camera for parallel OCT A-Scan detection. In addition, the reference microscope objective lens of the Linnik-interferometer is mounted on a piezo-actuated phase-shifter. Phase-shifting interferometry (PSI) techniques are applied for resolving the conjugate complex artifact and consequently contribute to an increase of image quality and depth range. A suppression ratio of the complex conjugate term of 36 dB is shown and a system sensitivity greater than 96 dB could be measured.
The EU-funded project VIAMOS1 proposes an optical coherence tomography system (OCT) for skin cancer detection, which combines full-field and full-range swept-source OCT in a multi-channel sensor for parallel detection. One of the project objectives is the development of new fabrication technologies for micro-optics, which makes it compatible to Micro-Opto-Electromechanical System technology (MOEMS). The basic system concept is a wafer-based Mirau interferometer array with an actuated reference mirror, which enables phase shifted interferogram detection and therefore reconstruction of the complex phase information, resulting in a higher measurement range with reduced image artifacts. This paper presents an experimental one-channel on-bench OCT system with bulk optics, which serves as a proof-of-concept setup for the final VIAMOS micro-system. It is based on a Linnik interferometer with a wavelength tuning light source and a camera for parallel A-Scan detection. Phase shifting interferometry techniques (PSI) are used for the suppression of the complex conjugate artifact, whose suppression reaches 36 dB. The sensitivity of the system is constant over the full-field with a mean value of 97 dB. OCT images are presented of a thin membrane microlens and a biological tissue (onion) as a preliminary demonstration.
In this paper, we demonstrate how short coherence digital holography with a pulsed fiber laser frequency comb may be
used for multi-level optical sectioning. For the proof of the principle, a conic object having a size of few centimeters is
used. The object shape is obtained by digitally reconstructing and processing a sequence of holograms recorded during
stepwise shifting of a spherical mirror in the reference arm of the holographic set-up. First experimental results are
presented.
Chromatic confocal spectral interferometry (CCSI) is a hybrid method for fast topography measurement, which
combines the advantages of the interferometric gain and accuracy with the robustness of confocal microscopy. The
CCSI-principle provides a single shot measurement of depth while offering a higher lateral resolution than commonly
used spectral interferometers. This contribution is focused on the modeling and simulation of a CCSI-sensor for
measuring rough surfaces, based on sequential and non-sequential ray-tracing. With the simulation, the influence of
surface roughness, surface reflectivity, and surface contamination on reliability of the sensor can be estimated.
Fringe projection is a commonly used method for 3D surface metrology. Numerous applications have demonstrated a
measurement field from a few millimeters to several meters. To enable the measurement of micro systems with this
method, a zoom stereo microscope from Leica was used as the basis for the implementation of a fringe projection
microscope. A state of the art twisted nematic WUXGA LCD was used for flexible fringe generation. The high fill factor
of this reflective LCoS in combination with a 500 Lumen LED and a 12 bit CCD camera delivers fringe patterns with
high contrast. This allows us to measure objects with both a strong reflectivity variation and a low reflectivity. The
second main objective was to increase the measurement field and the depth of field. Using the zoom system and
exchangeable microscope objectives, the measurement fields could be changed quickly from 4 cm2 to less than 1 mm2.
Depending on the measurement field, the depth of field was between 5.22 mm and 0.018 mm. However, this was often
not sufficient to measure the complete depth of a 3D-object. The microscope system also features an integrated high
precision motor stage, which is already used for system calibration. Based on this, we implemented a new z-stitching
method where n measurements at different well determined z-positions of the motor stage were performed. The n
resulting topography maps can be stitched together to get the complete depth map of the entire object. Thus the depth
measurement range is only limited by the mechanics of the z-stage.
In this paper, we report on the recent development of a novel low coherence interferometry technique for the purpose of 3D-topography measurements. It combines the well established techniques of spectral-interferometry (SI) and chromatic-confocal microscopy (CCM). Measuring the optical interference in the spectral-domain allows for the detection of a reflecting or scattering object's depth position, without the necessity of a mechanical axial-scan. Focusing the white-light detection field with a microscope objective combined with a diffractive optical element leads to an expansion of the axial-range of the sensor beyond the limited depth-of-focus, imposed by the numerical aperture (NA) of the focusing objective. Focusing with a high NA objective and confocally filtering the detection light field causes the reduction of the lateral dimension of the area sampled upon the object. By this, the lateral resolution of the sensor is enhanced and due to the high NA, a high light collection-efficiency is achieved as well. The attained interferometric signals consist of high-contrast wavelets, measured in the optical-frequency domain. The depth position of an investigated point of the object is given by the modulation-period of the wavelets. Therefore, unlike in CCM, positionwavelength referencing is not necessary.
White light interferometry is a promising tool for industrial quality inspection. Since modern cameras offer a frame rate far above video-rate, the speed of these systems could be increased in order to fulfill the strong temporal constraints of inline inspection, i.e. the monitoring of every single part during the production process in just a few seconds. Its accuracy up to the sub-μm range enables even the detection of smallest defects like holes with a diameter of only a few microns and thus ensures a fast, contactless and high precision quality inspection. Due to the replacement of the mechanical phase shifting by a spatial phase shift, the commonly known white light interferometers could be extended to a one-shot line-profiling sensor. The main benefit of such a line-profiling technique is that also critical surfaces are
accessible that deviate strongly from a plane shape, like rotary welds on cylindrical parts. It can be shown that the accuracy of the proposed system is comparable to the accuracy of conventional white light interferometers even on rough surfaces. Other parameters like lateral resolution and measuring range strongly depend on the optical setup and will be discussed in the following sections.
In the present paper, we address a hybrid technique which combines the method of spectral interferometry with chromatic confocal microscopy. On the basis of some proof-of-principle experiments, it is shown that with this new concept, the axial detection range of the sensor is decoupled from the limited depth-of-focus of the employed microscope objective, and a high numerical aperture objective can be employed for detection. The attained interferometric signals consist of high-contrast wavelets, measured in the λ-domain. The position of an investigated object is measured by analyzing the spectral-phase of the attained wavelets. In particular, chirp-effects as well as the significant role of confocal filtering are discussed.
Depth-scanning is an established technique in macroscopic and microscopic 3-D metrology. Representative in this context are the confocal technique and the white-light interferometry. A new fast depth-scanning technique has been applied to a confocal point sensor to be used in a laser-welding application for in-process measurement. The depth measurement range can be extended to about +/-1 mm at about 1500 measurement cycles per second. The possibilities and the potential of these techniques are described. Another principle of depth-scanning is the chromatic confocal technique. In connection with a new approach, an innovative confocal setup enables the parallelization of the complete depth-scan for the complete measurement of a line cut of moved objects. In the macroscopic scale, the new measurement techniques of depth-scanning fringe projection (DSFP) was introduced recently. In the microscopic scale, it has been implemented successfully in a stereo microscope.
KEYWORDS: Calibration, 3D metrology, 3D acquisition, CCD cameras, Ronchi rulings, Distortion, Mathematical modeling, Clouds, Signal processing, Imaging systems
We present a measurement setup for the acquisition of topographic and 3-D point cloud data using the depth-scanning fringe projection technique (DSFP). We describe the signal generation, its processing using techniques known from short coherence interferometry and discuss a direct 3-D calibration method. Our measurement system delivers an absolute phase map of the scene under measurement. Calibration procedures for macroscopic measurement methods like fringe projection and / or photogrammetry consider the principal distance (that is to say the distance between the center of projection and the image plane) as a constant. This is feasible as long as no focusing and zooming are performed during measurement. Consequently the depth of the measurement volume is limited by the depth of sharpness of the imaging system. By focusing through the whole depth of the measurement volume, our system overcomes this problem, and offers a virtually unlimited measurement depth. However, we have to take the issue of focusing into consideration in order to calibrate our system. The well-known direct calibration method has been adapted to our DSFP setup in order to deal with the problem of geometrical aberrations and to provide a 3-D point cloud. It has been completed to a set of three polynomial transformations, which allow to include the depth-scanning principle in the calibration of the system.
The chromatic confocal approach enables the parallelization of the complete depth-scan of confocal topography measurements. Therefore, mechanical movement can be reduced or completely avoided and the measurement times shortened. Chromatic confocal point sensors are already commercially available but they need lateral scanning in x- and y-direction in order to measure surface topographies. We achieved a further parallelization in the x-direction by realizing a chromatic confocal line sensor using a line focus and a spectrometer. In a second setup, we realized an area measuring chromatic confocal microscope, which is capable of one-shot measurements without any mechanical scanning. The depth resolution of this setup can be improved by measuring in a small number of different heights. Additional information about the color distribution of the object is gained.
When conducting three-dimensional measurements with fringe projection, the quality of the grating applied for the generation of the fringes is very important. It has a direct influence on the achievable height resolution when phase-shifting algorithms are used. Hence, the created fringes should have an ideal sinusoidal intensity profile. In the past, Ronchi gratings, placed in a defocused position, or gratings written in nematic liquid crystal displays (LCDs) or generated with digital micromirror devices (DMDs) have been used. The latest developments in the field of ferroelectric liquid-crystal-on-silicon (F-LCOS) displays make these devices interesting as the fringe generating element. They offer both high speed operation and high flexibility. Unlike other devices,
F-LCOS displays can also be operated under oblique incidence, still generating sufficient fringe contrast. We report on the characterization of a F-LCOS display and its application in two different setups. A comparison to Ronchi gratings and gratings written in transmissive twisted nematic LCDs is given. The achievable measurement resolutions as well as the measurement times are discussed. Results of measurements conducted on technical and medical surfaces are presented.
We report on the depth-scanning fringe projection technique (DSFP) which is an innovative triangulation method for absolute 3-D profiling of macroscopic scenes. This measurement principle combines the confocal principle with the fringe projection and phase evaluation techniques known from white light interferometry. Scanning of the focal plane and additional lateral shifting allow the phase to be determined for any desired depth range of the measurement volume. Hence, the limitations of the depth of sharpness occurring with projected light techniques can be overcome. A 3-D calibration method has been implemented in order to deal with the problem of geometrical aberrations and to provide a 3-D point cloud. The well-known direct calibration method has been adapted to our DSFP setup. It has been completed to a set of three polynomial transformations which allow to include the depth-scanning principle in the calibration of the system.
Fringe projection is an often used optical technique for 3-D surface measurements of industrial and biological objects. Classical fringe projection has the disadvantage to determine the surface topgraphy in a quite small unambiguity range only. Gray code techniques are used to solve this problem, but the spatial resolution is reduced due to a small numerical aperture. In a new approach we want to extend the measurement range and we will increase the spatial and the vertical resolution too. This is achieved by a depth-scanning approach together with high numerical apertures for the illumination as well as for the detection. With the aid of an additional z-scanning stage either the object, the sensor, or the image plan are scanned to obtain data over the whole measurement volume. We call this technique depth-scanning fringe projection. It is especially advantageous for the measurement of medium sized objects and can achieve resolutions of less than 0.2micrometers .
We are currently introducing a new scanning triangulation method for absolute 3D profiling of a macroscopic scene based on fringe projection technique. A scanning focal plane allows the phase to be determined for any desired depth range of the measurement volume. Furthermore, the limitations of the depth of focus occurring by projected light techniques will be overcome, allowing a large aperture and therefore better use of light. Two different systems based on this technique will be shown: System I uses both vertical and lateral translation of a Ronchi grating. System II uses an LCD element for generation of different fringes which has to be translated vertically, only. The basic principle of the new method is explained. First measurement results of both systems demonstrate the efficiency of the newly developed algorithms and the innovative measurement arrangements.
Interferometers with grazing incidence and visible light offer a fast and practicable solution for waviness and roughness testing in mechanical engineering without the necessity of scanning. Therefore, the possibility to use the grazing incidence of light onto the specimen is considered. The experimentally proved new optical layout for the grazing incidence interference microscope with a piezo-driven mirror for phase-shifting is based on a special optical stage with a hologram as a beam deflector. The object surface is sharply imaged onto the hologram, where an interference pattern is caused by the topography of the surface under test. Then, the diffracted light of the first order is imaged by a second optical stage onto a CCD-camera chip and the fringe pattern evaluation is carried out with a phase- shifting algorithm. Additionally a 2(lambda) -algorithm was implemented in the evaluation software. The necessary variation of (lambda) is generated by a small computer- controlled variation of angle of incident of light ont he specimen surface. First measurement results of objects with continuous and discontinuous surfaces demonstrate the possibilities and the limitations of the new experimental approach.
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