Chromatic confocal microscopy with a finite pinhole size.

Chromatic confocal microscopy has the advantage of short measurement times because of its parallel depth scan. As most white-light sources have limited optical output power, light-efficient setups are necessary. Using an extended detection pinhole is one way to improve light efficiency. We have calculated the effect of extended pinholes in chromatic confocal setups. We found that, for certain pinhole sizes, the FWHM of the confocal signal is nearly constant over a large wavelength interval.

Signal evaluation for high-speed confocal measurements.

The confocal-detection principle is open especially for use in medical applications. For inspection systems applications for technical objects in reflection confocal setups are of growing importance. For such applications the confocal measurements need to have a very short measuring time. A fast detection system is needed and to satisfy this requirement only a small number of height levels are measured and a fast-evaluation algorithm is used. Drawbacks of the reduction of height levels are a greater influence of noise and additional systematic errors on the measured heights. Study the effects of the reduction are calculated, different evaluation algorithms are analyzed, and the optimization of the parameters is discussed.

Highly Sensitive Pulsed Digital Holography for Built-in Defect Analysis with a Laser Excitation.

A highly sensitive method is presented for noninvasive defect analysis on thin structures with a Q-switched double-pulsed ruby laser with frequency doubling (347 nm). In our research we feature an all-optical arrangement, where a focused laser pulse derived from the same ruby laser (694 nm) acts as a built-in synchronous excitation source for digital holographic interferometry. The recordings are made with a CCD camera for capturing two holograms (two states of the specimen) corresponding to the two UV laser pulses with a short time separation (10-50 mus). Subtraction of the phase distribution in two digital holograms gives a fringe phase map that shows the change in deformation of the specimen between the recordings. The advantage of the proposed method is two fold. First, the use of a shorter wavelength results in a higher sensitivity. Second, owing to the induced synchronous built-in optical excitation, the specimen is not subjected to any external physical excitation devices. Experimental results are presented on identification and evaluation of defects in thin metal sheets.

Comparative study of various endoscopes for pulsed digital holographic interferometry.

A comparison of several endoscopes as object image carriers in pulsed digital holography is presented. Three multicore flexible fiber endoscopes of different spatial resolution and one rigid endoscope are investigated. The four endoscopes are integrated in a setup for the recording of digital holograms on a CCD camera. A double-pulsed ruby laser is used as the light source. A spatial carrier is introduced by an off-axis reference beam, which permits quantitative evaluation of the phase difference between two holograms recorded with a short time separation (5-600 micros). From reported studies it may be inferred that the quality of the phase maps so derived from digital holographic interferometry has a strong correlation to the spatial resolution of the multicore fiber used in these endoscopes. With the endoscopic technique combined with pulsed digital holography a number of useful applications (in areas such as medical endoscopy, micromechanics, and microelectronics) are envisaged for which access to the objects of interest is otherwise difficult.

Theoretical limits of scanning white-light interferometry signal evaluation algorithms.

Scanning white-light interferometry is widely used for the microstructure analysis of technical and biological specimens. For each pixel in the focal plane of the apparatus a white-light interferogram is acquired and evaluated by means of an algorithm. We discuss some properties of mathematically optimal evaluation methods and the best possible achievable resolution derived therefrom depending on the setup parameters. A comparison of the results to one of the algorithms described in the literature is given.

Pulsed digital holographic interferometry for dynamic measurement of rotating objects with an optical derotator.

A method for measuring dynamic deformations of rotating objects with pulsed digital holography is described. An optical derotator is used to compensate for the rotation. A CCD camera is used to record two holograms with a short time separation (20 mus). Results of deformations between the recordings are obtained after subtraction of the phase distribution between the two digital holograms. Fringe phase maps of the phase subtraction of two holograms compensated by the derotator and recorded with a Q-switched double-pulsed ruby laser are presented. A flat disk and the blades of a fan were investigated. We used an optical arrangement that allowed us to improve laser illumination and energy efficiency. Experimental results on quantitative evaluation of dynamical out-of-plane deformations are presented.

Heterodyne temporal speckle-pattern interferometry.

In temporal speckle-pattern interferometry deformation information is extracted by a Fourier transform technique from the speckle pattern that is recorded over a period of time as the object is deformed. A limitation of the experimental arrangements reported to date is that the direction of the deformation cannot be determined. We propose removing this limitation by using the heterodyne principle. Some experimental results that were obtained by use of a rotating half-wave-plate frequency shifter are presented.

Pulsed digital holographic interferometry with 694- and 347-nm wavelengths.

A method for deformation analysis and shape measurement based on digital holography is presented. Two wavelengths, 694 and 347 nm, are used. The object is illuminated with the two wavelengths at the same time, and digital holograms are recorded on a CCD chip. The information corresponding to the two wavelengths is separated in the Fourier domain, and the phase corresponding to the wave fronts is calculated. By recording holograms with two different wavelengths at the same time, we can get measurements of deformations or shape with different sensitivities. Experimental results are presented.

Fast algorithms for data reduction in modern optical three-dimensional profile measurement systems with MMX technology.

Confocal microscopy and white-light interferometry are two promising methods for the three-dimensional microstructure analysis of technical and biologic specimens. For both methods the specimen is scanned through the focus position by means of an actuator. A large series of intensity frames is acquired. These data are used for the final calculation of the topography. We demonstrate that the multimedia extended (MMX) instruction set, which is implemented in modern Intel microprocessors, can be used for effective real-time preprocessing and for fast evaluation algorithms. So this new technique enables the implementation of more-complex algorithms with acceptable run times even on standard computer technology. The possibilities of the MMX instruction set are discussed for confocal microscopy and for white-light interferometry.

Pulsed digital holography for deformation measurements on biological tissues.

Digital holograms are recorded of biological tissues by use of a Q-switched double-pulsed ruby laser. An image-plane digital holography setup is used with a CCD camera for capturing two holograms with a short time separation (20-800 micros). Subtraction of the phase distribution in two digital holograms yield a fringe phase map that shows the change in deformation of the tissue surface between the recordings. Experiments are performed on tissue from a pig that was excited by a short-shock pulse and on a human hand that was excited by sinusoidal stimulation. Results when the object is imaged through an endoscope are also presented. The technique could be an approach for measuring parameters like elasticity on biological tissues.

Edge localization of subwavelength structures by use of polarization interferometry and extreme-value criteria.

A polarization interferometric method is presented for the quantitative microscopy of topographical structures with subwavelength linewidths. A liquid-crystal phase shifter is inserted into the imaging optics of a reflected-light microscope, and the principles of phase-shifting interferometry are applied to measuring the phase and the contrast of the TE-polarized image (E parallel edge) with the TM-polarized image (E perpendicular edge) as the reference. This common-path interferometric method provides selective edge detection for line structures because the polarization difference is localized at the structure edges. Two different threshold criteria for linewidth determination are discussed: distance of the contrast minima and distance of the points of the steepest phase change. Linewidths as small as 300 nm were measured at a 635-nm wavelength. The dependence on the illumination numerical aperture, as well as on the material, the width, and the depth of the structure, is investigated both experimentally and by rigorous numerical simulations.

Application of a liquid-crystal spatial light modulator for brightness adaptation in microscopic topometry.

Three-dimensional optical topometry of technical surfaces becomes increasingly important for the control of industrial processes. However, the local reflectance of the surface of the investigated sample often varies within a wide range, making accurate measurements by fringe projection difficult. We demonstrate the use of a liquid-crystal spatial light modulator as the fringe-generating element in a standard stereo microscope. With this device the brightness of the projected patterns can be adapted pixelwise. This technique leads to a significant improvement of the results of our measurements with a phase-shifting algorithm.

Phase-shifting polarization interferometry for microstructure linewidth measurement.

We report on a novel procedure to measure the linewidth of steep microstructures, based on the polarization anisotropy caused by the structure edges. A liquid-crystal phase shifter and a polarizer are introduced into the imaging optics of a reflection-mode microscope. We apply the principle of phase-shifting interferometry to measure the phase and contrast of a TM-polarized image with the TE-polarized image as reference. The method provides selective edge detection because the polarization difference is localized at the structure edges.

Optical particle trapping with computer-generated holograms written on a liquid-crystal display.

Computer-generated holograms written on a liquid-crystal display can be used to generate dynamic light fields of arbitrary shape. This method was used to simultaneously trap polystyrene particles laterally and to displace them independently of one another.

Speckle interferometry with temporal phase evaluation: influence of decorrelation, speckle size, and nonlinearity of the camera.

Recently, a new method to measure object shape and deformation with temporal evolution of speckles in speckle interferometry was reported. In this method, certain parameters, sensitive to shape or deformation are changed continuously, and the fluctuations in the irradiance of each speckle is recorded. The information over the whole object deformation is retrieved by Fourier-transformation techniques. We present a detailed theory and analyze the influence of decorrelation due to longitudinal and lateral size of the speckles. It is also shown that the method can be used to measure small deformations (less than 5 microm) with higher resolution. Further, the nonlinearity of the camera is shown to enhance the sensitivity.

Optical roughness measurements with fringe projection.

The characterization of roughness of engineering surfaces over an area is an important task for different applications as well as for manufacturing processes. The surface roughness is in particular an important factor in determining the performance of a workpiece. We demonstrate that the fringe projection technique allows very fast three-dimensional surface inspections. The inspection time for an entire measurement is reduced to less than 5 s with standard hardware. Based on a zoom stereo microscope setup, we demonstrate a modular measuring instrument. The magnification-dependent vertical resolution can be as high as 0.1 microm. The special properties for roughness measurements are demonstrated, especially the comparability with a tactile sensor and with other optical sensors, which is discussed in connection with amplitude parameters.

Pulsed digital holography for high-speed contouring that uses a two-wavelength method.

A two-wavelength method for a fast shape measurement by use of a pulsed ruby laser is presented. The wavelength change is produced by alteration of the distance between the plates of the laser's output etalon. One plate of the etalon is mounted on a vibrating piezoelectric element; this allows a fast wavelength change. Two holograms at different wavelengths are recorded in a few microseconds by use of a CCD. The holograms are reconstructed digitally, and the wave-front phase is calculated. The shape is obtained by subtraction of the phases of the wave fronts recorded at different wavelengths. Environmental disturbances at low frequencies, such as air turbulence, vibrations, and object drift, have no influence on the measurement. Experimental results are presented.

Simultaneous three-dimensional dynamic deformation measurements with pulsed digital holography.

The three deformation components x, y, z of a vibrating object are measured simultaneously by use of digital holography with a double-pulse ruby laser source. The object is illuminated from three different directions, each optically path matched with three reference beams such that three independent digital holograms are formed and added incoherently in one single CCD image. The optical phase difference between the two recordings taken for each hologram is quantitatively evaluated by the Fourier-transform method so that a set of three phase maps is obtained, representing the deformation along three sensitivity vectors. The total object deformation is obtained as a vector resultant from the data of the three phase maps. To give the full three-dimensional (3-D) description, the shape of the object is measured by the two-wavelength contouring method. Experiments are performed with a cylinder as the test object, transiently and harmonically excited. The 3-D deformation and shape measurement results are presented graphically.

Topograph for inspection of engine cylinder walls.

The microstructural inspection of engine cylinder walls is an important task for quality management in the automotive industry. Until recently, mainly tactile methods were used for this purpose. We present an optical instrument based on microscopic fringe projection that permits fast, reliable, and nondestructive measurements of microstructure. The field of view is 0.8 mm x 1.2 mm, with a spatial sampling of 1100 x 700 pixels. In contrast to conventional tactile sensors, the optical method provides fast in situ three-dimensional surface characterizations that provide more information about the surface than do line profiles. Measurements are presented, and advantages of this instrument for characterization of a surface are discussed.