Spectral interferometric microscope with tandem liquid-crystal Fabry-Perot interferometers for extension of the dynamic range in three-dimensional step-height measurement

The maximum measurable range of a spectral interference microscope depends on the coherence length of the light transmitted by its tunable spectral filter. To achieve a large range in step-height measurement we have developed a new tunable spectral filter that uses tandem liquid-crystal Fabry-Perot interferometers (LC-FPIs), which can simultaneously attain both a high spectral resolution and a large tuning range. Fringe visibility measurements were carried out, and it was found that the coherence length of the light transmitted through tandem LC-FPIs is two times larger than that transmitted through a single LC-FPI. Using this novel tunable spectral filter, we developed a new spectral interference microscope for the measurement of three-dimensional shapes of discontinuous objects. Experimental results of step-height measurements both with a single LC-FPI and with tandem LC-FPIs are presented for a combination of standard steel gauge block sets with 1-, 99-, and 100-microm steps. A large range (1-100 microm) of measurement with submicrometer resolution was achieved with tandem LC-FPIs that was not possible with our previous system in which a single LC-FPI was used.     

Two-wavelength Talbot effect and its application for three-dimensional step-height measurement

The phenomenon of Talbot self-image shift by changing the wavelength of the illuminating light is described and demonstrated experimentally. A periodic grating is illuminated by light with wavelengths lambda1 and lambda2 generated by two lasers, and the Talbot self-images are recorded along the longitudinal direction at individual wavelengths. The Talbot self-image shift due to the change in the wavelength of light is implemented for the measurement of the three-dimensional step height of a large discontinuous object without any phase ambiguity problem. Fourier-transform fringe analysis was used to determine the maximum contrast of the high-visibility bands for the measurement of the step height of the object. The main advantages of the proposed system are nonmechanical scanning, high stability because of its common path geometry, compactness, and a wide range of measurement as compared to interferometric three- dimensional profilers.     

Step-height measurement by means of a dual-frequency interferometric confocal microscope

A novel instrument, the dual-frequency interferometric confocal microscope (DICM), which facilitates the measurement of step features, is investigated. It combines the advantages of the high resolution (subnanometer) of heterodyne interferometry and the relatively large measurement range (approximately 5 microm) of confocal microscopy. The axial response curves of the confocal microscopy system are compared in experiments in which microscopic objects with various numerical apertures and magnifications are used. The results prove that the variation in light intensity is enough to permit discrimination of different orders of interference fringes. The DICM has been successfully utilized to measure the step height of a standard mask, and the experimental results agree well with those measured by scanning probe microscopes. The results also show that the system has good repeatability, with a maximum deviation of 5 nm.     

Surface morphologies of sputter-deposited aluminum films studied using a high-resolution phase-measuring laser interferometric microscope

Sputter-deposited aluminum (Al) film surface morphologies were studied with a new nondestructive method that incorporates a high-resolution phase-measuring laser interferometric microscope. Good correlation is obtained between rms roughness and reflectivity for various conditions of temperature and argon gas pressure. It should be noted that the rms roughness is much more sensitive than reflectivity when reflectivity exceeds 90%. A drastic change is observed in the temperature dependence of the rms roughness and the skewness at 200 °C. As a result there are changes in Al grain sizes and surface morphologies based on concomitant scanning electron microscope observations. We found that the rms roughness value depends on the resolution of the objective especially when the Al grain sizes are comparable to the resolution.     

Highly-sensitive and high-resolution all-fiber three-dimensional measurement system

A practical all-fiber three-dimensional measurement system is demonstrated with an incoherent interferometer at the eye-safe wavelength of 1.55 mum. The sensitivity and axial resolution are as high as 102 dB and 1.4 mum from a few meters' distance, respectively. A rotating scanner is developed for axial scanning, and a wide longitudinal scanning range of 54 mm is demonstrated. The high resolution images of a few samples are clearly obtained at the speed of 52 points/s. Moreover, the resolution, sensitivity, speed, and angle dependence are discussed for measurement of a 100 yen Japanese coin.     

Optical tomographic microscope for quantitative imaging of phase objects

We describe a tomographic microscope, for imaging phase objects, that makes use of the transport-of-intensity equation to estimate the phase of the transmitted light through the object. The wave-front data from optical fibers are reconstructed with an algorithm that incorporates correction for the ray bending. The reconstructed refractive-index cross sections of the fibers are found to be in agreement with the available values specified in the catalogs.     

Improved three-dimensional imaging with a digital holography microscope with a source of partial spatial coherence

A digital holographic technique is implemented in a microscope for three-dimensional imaging reconstruction. The setup is a Mach-Zehnder interferometer that uses an incoherent light source to remove the coherent noise that is inherent in the laser sources. A phase-stepping technique determines the optical phase in the image plane of the microscope. Out-of-focus planes are refocused by digital holographic computations, thus considerably enlarging the depth of investigation without the need to change the optical focus mechanically. The technique can be implemented in transmission for various magnification ratios and can cover a wide range of applications. Performances and limitations of the microscope are theoretically evaluated. Experimental results for a test target are given, and examples of two applications in particle localization and investigation of biological sample are provided.     

Examining a transparent object with a linnik tomographic microscope

Various applications are described for a Linnik tomographic microscope to biological and engineering phase objects. An interferometric microscope has been applied to human lymphocytes and to the measurement of birefringence on rabbit muscle fibers, as well as in nondestructive quality testing on microlens rasters and embossed holograms. Translated from Izmeritel'naya Tekhnika, No. 1, pp. 46–49, January, 1999.     

Theoretical analysis of three-dimensional imaging and recognition of micro-organisms with a single-exposure on-line holographic microscope

Single-exposure on-line (SEOL) digital holography is a recently proposed technique for monitoring, visualization, and recognition of three-dimensional (3D) objects. In contrast to traditional multi-exposure on-line digital holography, it uses only one exposure, which makes it particularly suitable for imaging and recognizing moving micro-organisms. However, the cost of using only one exposure is the superposition of a conjugate image on the desired reconstructed image. The influence of the conjugate image on the visualization and recognition performance is investigated. The conditions for which the cross-talk noise induced by the conjugate image is negligible are derived. It is demonstrated that with conditions common in imaging of microscopic 3D biological objects, SEOL digital holography is highly tolerant of cross-talk noise induced by the conjugate image.     

Simultaneous spatial and spectral imaging of lasing droplets

We present an experimental technique that allows the simultaneous spatial imaging and spectral analysis of falling droplets that exhibit lasing. Single droplet investigations serve as, among other purposes, a preliminary study for spray and combustion researchers. The described setup provides a valuable tool for the evaluation of microdroplet investigations with laser-spectroscopic techniques that rely on laser-induced fluorescence (LIF) or similar spectroscopical phenomena. The emphasis is that both spatial and spectral information are obtained from single-shot images of a falling droplet. Furthermore, combining spatial imaging and a spatially resolving optical multichannel analyzer makes a pointwise rastering of the droplets spectrum possible. This allows for the (almost) unambiguous determination of sources of influence on the spectrum of these droplets-such as geometrical distortion and lasing, nondissolved tracer lumps, and similar phenomena. Although the focus is on the experimental technique itself, we supplement detailed studies of lasing in falling microdroplets. These results were obtained with the aim of developing a system for measuring temperature distributions in droplets and sprays. In the light of these results the practice of calibrating a droplets spectrum by use of a bulk liquid sample needs to be critically reviewed.     

Simultaneous measurement of foveal spectral reflectance and cone-photoreceptor directionality

An instrument for simultaneous measurement of foveal spectral reflectance and cone-photoreceptor directionality is described. The key element is an imaging spectrograph (spectral range of 420-790 nm) with its entrance slit conjugate to the pupil plane of a human eye. A 1.9-deg spot on the retina is sampled in 1 s. Video observation of the retina and the pupil facilitates proper alignment. Measurements were performed on 21 healthy subjects. Model analysis of the spectra provided densities of photostable ocular absorbers. As an example, macular pigment and melanin are discussed in more detail. Spatial profiles exhibited the optical Stiles-Crawford effect, reflecting cone-photoreceptor directionality.     

Multimodal spectral imaging of cells using a transmission diffraction grating on a light microscope

A multimodal methodology for spectral imaging of cells is presented. The spectral imaging setup uses a transmission diffraction grating on a light microscope to concurrently record spectral images of cells and cellular organelles by fluorescence, darkfield, brightfield, and differential interference contrast (DIC) spectral microscopy. Initially, the setup was applied for fluorescence spectral imaging of yeast and mammalian cells labeled with multiple fluorophores. Fluorescence signals originating from fluorescently labeled biomolecules in cells were collected through triple or single filter cubes, separated by the grating, and imaged using a charge-coupled device (CCD) camera. Cellular components such as nuclei, cytoskeleton, and mitochondria were spatially separated by the fluorescence spectra of the fluorophores present in them, providing detailed multi-colored spectral images of cells. Additionally, the grating-based spectral microscope enabled measurement of scattering and absorption spectra of unlabeled cells and stained tissue sections using darkfield and brightfield or DIC spectral microscopy, respectively. The presented spectral imaging methodology provides a readily affordable approach for multimodal spectral characterization of biological cells and other specimens.     

Increasing the range of unambiguity in step-height measurement with multiple-wavelength interferometry--application to absolute long gauge block measurement

An instrument for step-height measurement by multiple-wavelength interferometry is described. The addition of a 1152-nm wavelength to a multiple-wavelength scheme applying wavelengths of 633, 612, and 543 nm relaxes the tolerance range of the required preliminary measurement to +/- 140 microm, if the total uncertainty in the fringe fraction measurement can be kept below 2%. For larger fringe fraction measurement uncertainty, numerical simulations show that the integer number of interference orders can still be determined unambiguously if the range in the preliminary knowledge of the length has been correspondingly reduced. The interferometer instrument is described, and experimental data are presented in the context of long gauge block calibration at the National Research Council of Canada.     

Measurement of three-dimensional temperature fields with interferometric tomography

Optical computerized tomography (OCT) technology is used to reconstruct the asymmetric three-dimensional temperature field generated by radiators and electronic chips. First, the OCT method is described. Second, the reconstructed results are tested by a double-cylinder radiator model. Finally, OCT is applied to reconstruction of the temperature field above the surface of a CPU. The air-temperature field above a CPU circuit can be imaged with an OCT system that reflects whether the heat production from different parts of the CPU is even; therefore possibly the technique can be used to determine whether the integrated-circuit design in the CPU is smart.     

Geometrical tomographic imaging of refractive indices through turbid media by a wavelength-scanning heterodyne interference confocal microscope

A wavelength-scanning heterodyne interference confocal microscope has proved to provide the tomographic image of the refractive indices of transparent and turbid media on the scale of geometrical depth when weakly reflected light with an optical power as low as of the order of 10(-14) W is used. The refractive indices of the transparent object and the turbid media were measured with accuracies of -0.5% and approximately 3%, respectively. This imaging method is advantageous for evaluating quantitative refractive indices and internal structures.     

Analysis of computed tomographic imaging spectrometers. I. Spatial and spectral resolution

Computed tomographic imaging spectrometers measure the spectrally resolved image of an object scene in an entirely different manner from traditional whisk-broom or push-broom systems, and thus their noise behavior and data artifacts are unfamiliar. We review computed tomographic imaging spectrometry (CTIS) measurement systems and analyze their performance, with the aim of providing a vocabulary for discussing resolution in CTIS instruments, by illustrating the artifacts present in their reconstructed data and contributing a rule-of-thumb measure of their spectral resolution. We also show how the data reconstruction speed can be improved, at no cost in reconstruction quality, by ignoring redundant projections within the measured raw images.     

Simultaneous tomographic reconstruction and segmentation with class priors

We consider tomographic imaging problems where the goal is to obtain both a reconstructed image and a corresponding segmentation. A classical approach is to first reconstruct and then segment the image; more recent approaches use a discrete tomography approach where reconstruction and segmentation are combined to produce a reconstruction that is identical to the segmentation. We consider instead a hybrid approach that simultaneously produces both a reconstructed image and segmentation. We incorporate priors about the desired classes of the segmentation through a Hidden Markov Measure Field Model, and we impose a regularization term for the spatial variation of the classes across neighbouring pixels. We also present an efficient implementation of our algorithm based on state-of-the-art numerical optimization algorithms. Simulation experiments with artificial and real data demonstrate that our combined approach can produce better results than the classical two-step approach.     

Chemical imaging with a confocal scanning Fourier-transform-Raman microscope

Traditional approaches in confocal microscopy have focused on techniques to generate volumetric intensity or phase images of an object. In these different imaging modes the scattered optical-field properties depend on local refractive index and absorption, properties not unique to a given material. We report here on a confocal microscope that uses Raman scattered light to generate volumetric chemical images of a material. We designed and built a prototype instrument, called a confocal scanning laser Raman microscope, that combines a confocal scanning laser microscope with a Fourier-transform-Raman spectrometer. The high depth and lateral spatial resolution of the confocal optics design define a volume element from which the Raman scattered light is collected, and the spectrometer analyzes its spectral content. The sample is scanned through the microscope probe volume, and a chemical image isgenerated based on the content of the Raman spectrum extracted from each scan position in the sample. The results inclu e instrument characterization measurements and examples of confocal chemical imaging.     

Absolute interferometric distance measurement using a FM-demodulation technique

We propose an interferometric method for measuring absolute distances larger than the wavelength. A laser diode is used as a light source. The principle of operation is based on multiple-wavelength interferometry that uses a modulated light source. This method uses the fact that the wavelength of light emitted by the laser diode can be varied by means of the injection current. The modulation of the injection current in combination with the optical heterodyne technique causes a high-frequency phase-modulated detector signal. The phase deviation of the signal is a measure of the optical path difference in the interferometer. By FM demodulation of the detector output with a phase-locked loop demodulator, the optical path difference can be determined directly without the classical ambiguity problem of interferometry. The measuring range in the experiments was limited to 50 mm by the maximum travel range of the used specimen translation stage. Because of the inherent light sensitivity of the method described, the rangefinder can be used for three-dimensional profile measurements on a wide variety of objects, even on diffuse scattering surfaces.     

Laser interferometer with a high-resolution optical microscope for measuring small line scales

The optical train of an interferometer to certify small-scale measures in the range 1–200 μm with a high-resolution modulation optical microscope for sighting the lines on the measure is described. The circuit of a interference-fringe recording unit is presented. Some results of line scale measurements are reported. Translated from Izmeritel'naya Tekhnika, No 9, pp 27–29, September, 1997