Semi-transparent Objects Research Articles

Accurate quantitative phase imaging using generalized phase contrast

The generalized phase contrast (GPC) method is explored for improving the accuracy in quantitative reconstruction of two-dimensional phase distribution from images of semi-transparent objects viewed with a common-path interferometer (CPI). We propose a novel optical scheme for highly accurate determination of the object-dependent complex synthetic reference wave (SRW) in a CPI. Using a simple 4f imaging optical setup, GPC provides an analytic model of the SRW profile that is shown here to increase phase measurement accuracy over the entire output aperture. The improved accuracy due to the GPC model can exceed one order of magnitude compared to that of the conventional plane wave model of the reference beam. Furthermore, we describe a novel method for accurate derivation of the strength of the phase object's zero spatial frequency component based on the intensity of the traditionally ignored halo region encompassing the interferogram. Combining this information with three inteferometric measurements, full-field phase images with unconstrained phase strokes are obtained accurately.

Absorption-free discrimination between semitransparent objects

Absorption-free (also known as ``interaction-free'') measurement aims to detect the presence of an opaque object using a test particle without that particle being absorbed by the object. Here we consider semitransparent objects that have an amplitude $\ensuremath{\alpha}$ of transmitting a particle while leaving the state of the object unchanged and an amplitude $\ensuremath{\beta}$ of absorbing the particle. The task is to devise a protocol that can decide which of two known transmission amplitudes is present while ensuring that no particle is absorbed by the object. We show that the probabilities of being able to achieve this are limited by an inequality. This inequality implies that absorption-free distinction between complete transparency and any partial transparency is always possible with probabilities approaching 1, but that two partial transparencies can only be distinguished with probabilities less than 1.

Recovery of Temperature and Species Concentration Profiles in Flames Using Low-Resolution Infrared Spectroscopy

The temperature and species distributions in semi-transparent gaseous axisymmetric objects are retrieved by carrying out the inversion of their directional low-resolution spectral transmission and/or spectral emission in the infrared by using methods based on the solution of the radiative transfer equation. The validity domains of the hypothesis on which the low resolution inversion methods are based are discussed by comparison with the corresponding exact radiative properties generated by line-by-line descriptions. A propane-air laminar premixed flame is experimentally studied. The infrared data are collected for the (ν3) band of CO2, either by a directional scanning (at a fixed frequency) or by a frequency scanning (in a fixed direction). Depending on the data acquisition methods, two different reconstruction techniques are used. For spatial scanning, a generalized Abel transformation or the regularized Murio’s method, is utilized. For spectral scanning the recovered results are carried out by the Chahine-Smith method. For both techniques, profile reconstructions are worked out accounting for a spectral data bank based on the narrow statistical band model. Finally, thermal and CO2 concentration mappings of the premixed flame are presented.

An interactive visualization and navigation tool for medical volume data

In order to make direct volume rendering practicable convenient visualization options and data analysis tools have to be integrated. For example, direct rendering of semi-transparent volume objects with integrated display of lighted iso-surfaces is one important challenge especially in medical applications. Furthermore, explicit use of multi-dimensional image processing operations often helps to optimize the exploration of the available data sets. On the other hand, only if interactive frame rates can be guaranteed, such visualization tools will be accepted in medical planning and surgery simulation systems. In this paper we propose a volume visualization tool for large scale medical volume data which takes advantage of hardware-assisted 3D texture interpolation and convolution operations. We demonstrate how to use the 3D texture mapping capability of high-end graphics workstations to display arbitrary iso-surfaces which can be directly illuminated to enhance the spatial relations between objects. Back-to-front 3D texture slicing is used to simultaneously display semi-transparent material densities. In order to enable on-the-fly data analysis, first approaches using hardware-assisted-convolution operations have been integrated. An implementation of the proposed method based on the OpenInventor rendering toolkit is described offering interactive frame rates at high image quality including sophisticated user interactions.

Optical interaction-free measurement of semitransparent objects

An interaction-free measurement system with a polarization-based Mach-Zehnder interferometer structure is analyzed for semitransparent objects. Here, it is shown that the interaction-free measurement of such objects with 100% detection efficiency is possible in principle if the system loss is negligible. The influence of two system imperfections, the system loss and the low quantum efficiency of the photon detectors, on the interaction-free measurement is also analyzed. Although the two system imperfections limit the maximal detection efficiency that can be achievable, the quantum efficiency of the photon detectors is less important when the system loss becomes small.

Shadow generation for volumetric data sets

-buffer technique for fast and efficient shadow generation. Volumetric data contain information about the grid points only. Such data do not provide surface information that could be projected immediately onto the shadow map. To solve this problem, we have implemented two techniques. The first uses a modified adaptive version of the well-known marching cubes algorithm for the special characteristics of medical data sets. The algorithm uses material properties for a precise representation of object boundaries, generating volumetric objects quickly and effectively. There are two representations of the same data set: we use a view-independent approximation to display shadows and the original representation of the volume for object visualization in full precision. The second algorithm uses a ray-tracing approach to create shadow maps. The same routine is used for object rendering, but is restricted to depth-value generation. Semitransparent objects are handled by storing an intensity profile in addition to the depth value.

Virim: A massively parallel processor for real-time volume visualization in medicine

Architecture and applications of a massively parallel system currently developed are described, which allows real-time visualization using volume oriented visualization algorithms. Volumes of 256 × 256 × 128 voxels can be visualized with a frame rate of 10 Hz. The system is scalable and modular, and will allow a multi-user access over high-speed networks. 3D-rotation around arbitrary rotation axis, perspective, zooming and arbitrary grey value mapping are provided in real-time. A volume oriented algorithm is used that is tailored to the requirements in medicine [H.-P. Meinzer et al., IEEE Comp. Graph. Appl., 34 (Nov. 1991)]. With this algorithm, small structures without defined surfaces, e.g., tumours, can be visualized as well as semi-transparent objects. One planned application of the system is heart surgery.

The role of early mechanisms in motion transparency and coherence

Perceptual dissociation of moving plaid patterns into independently moving bar gratings occurs most readily when the grating signals are combined as if the bars were semi-transparent objects (Ramachandran, V. S. (1990) in: AI and the Eye. Wiley, Chichester, pp. 21-77. Stoner, G. R., Albright, T. D. and Ramachandran, V. S. (1990) Nature 344, 153-155). These and other examples of motion transparency are exploited to constrain the set of viable models for human motion processing. For example, one may exclude any fixed recombination of local motion signals into a plaid motion signal. Broad classes of linear and non-linear mechanisms for tracking blobs, corners, and other unambiguous plaid motion cues can also be ruled out because they fail to reproduce the experimental results even qualitatively. The shifting balance between the 'coherent plaid' and 'sliding gratings' percepts are attributed to processing stages before any integration or combination of local motion signals. The first essential stage is a roughly logarithmic nonlinearity before orientation filtering. In general, the resulting cross-products (at the intersections) code unambiguously for the true plaid motion vector, but these signals will be nulled for multiplicatively combined plaid components. Supporting evidence for this idea is obtained in our measurements of detection thresholds for the 'plaid' and 'sliding' percepts. The essential element of the second stage consists of 'end-stop' cells which detect the nullable intersection signal, and so produce a plaid-motion signal with the required characteristics. Finally, it is argued that the ecological role of the proposed mechanism lies in the ability to handle movement of patterned objects in lighting conditions dominated by complicated cast shadows.

Quantitative measurement of radiation properties for opaque and semi-transparent greybodies

Because of the lack of theory and methods of measuring the ambient radiation in real situations, it is impossible to quantify the self-radiation properties of a target. Our earlier paper, (13) proposed a theory, “Equivalent Blackbody Radiation Theory,” and a design method for infrared systems that can measure quantitatively the ambient radiation and self-radiating properties of opaque targets. We now extend our treatment to the more general case which includes opaque, semi-transparent and transparent objects. As examples, the ambient radiation in different situations, the self-radiation properties of an opaque metal plate and a semi-transparent crystal, and the true temperature difference of human breath during inspiration and exhalation are measured quantitatively and successfully.

Three-dimensional visualization methods for confocal microscopy.

Three-dimensional images of microscopic objects can be obtained by confocal scanning laser microscopy (CSLM). The imaging process in a CSLM consists of sampling a specific volume in the object and storing the result in a three-dimensional memory array of a digital computer. Methods are needed to visualize these images. In this paper three methods are discussed, each suitable in a specific area of application. For purposes where realistic rendering of solid or semi-transparent objects is required, an algorithm based on simulation of a fluorescence process is most suitable. When speed is essential, as for interactive purposes, a simple procedure to generate anaglyphs can be used. Both methods have in common that they require no previous interpretation or analysis of the image. When the study of an object imaged by CSLM involves analysis in terms of a geometrical model, sophisticated graphics techniques can be used to display the results of the analysis.

Bright-field microscopy of semitransparent objects.

We examine theoretically and experimentally the characteristics of in-focus and out-of-focus images of simple, well-defined phase objects. Theoretical calculations are based on the theory of partial coherence, and a simple calculation for imaging with coherent light demonstrates distinctive aspects of bright-field images. Experiments are performed with a well-corrected microscope, equipped for the precise control of illumination conditions and focus position. Theoretical and experimental results agree, although the contrast in the experimental images is often lower than expected. Also verified by experiment is a (to our knowledge) previously uninvestigated linear response in the intensity modulation of defocused, coherent images of thin, phase objects. The near-focus behavior of phase object images differs in symmetry from the more-familiar behavior of opaque object images.

PYROMETRIC MEASUREMENT OF GLASS PLATES WITH NON-UNIFORM TEMPERATURE DISTRIBUTION

During the forming of glass articles by a variety of different processes, it is important to be able to accurately measure the temperature of the glass. Also, to be practical for production applications, the temperature measuring technique must not interfere with the process or disturb the product. Only infrared radiation pyrometry is capable of meeting these requirements. Narrow-band radiation pyrometers are non-contacting sensors that (depending on the operating wavelength) either measure the surface temperature or some weighted average of the surface and internal temperatures of semi-transparent glass objects without significantly affecting the product or its heat exchange with the environment. When attempting pyrometer temperature measurements on glass, it must be recognized that the incident radiation originates not only from the surface, but also from the interior which may be at a different temperature. However, a knowledge of the directional spectral volume emissive power of the glass object can b...

Apodisation for coherent imagery of extended objects — Semi-transparent phase edges

A class of apodisation functions for use in coherently illuminated systems is applied to the imagery of semi-transparent phase edge objects. It is shown that the image intensity distribution is either a monotonic function or at most has a single minimum.

Comparison of the diffraction theory of image formation with the three-dimensional, first Born scattering approximation in lens systems

The Kirchoff diffraction theory of image formation is compared with the tree-dimensional, first Born scattering approximation for semitransparent objects imaged through a lens system. The comparison reveals the spatial frequency range over which the ‘equivalent two-dimensional, flat object’ represents the actual three-dimensional object in the imaging process.

Determination of the Amplitude and the Phase of Scattered Fields by Holography*

It is shown how both the amplitude and the phase of a scattered field may be determined by holography. It is estimated that information about details down to about nine wavelengths of light can be obtained by this technique. The result is of importance for unambiguous determination of the three-dimensional structure of semitransparent objects, such as are frequently encountered, for example, in biology.

Three-dimensional structure determination of semi-transparent objects from holographic data

A solution is presented to an inverse scattering problem that arises in the application of holography to the determination of the three-dimensional structure of weakly scattering semi-transparent objects. This solution, together with a result obtained in another recent publication, relating to the determination of the complex amplitude distribution of scattered fields from measurements of the intensity transmission functions of holograms, makes it possible to calculate the distribution of the (generally complex) refractive index throughout the object. In general many holograms are needed. corresponding to different directions of illumination of the object.

On the Fraunhofer (Far Field) Diffraction Patterns of Opaque and Transparent Objects with Coherent Background

The diffracting objects considered are opaque and semi-transparent objects of known cross section which are situated in an otherwise transparent area and illuminated with a collimated quasi-monochromatic beam of light. The plane in which the resultant diffraction pattern is considered is at a sufficient distance from the object for the approximation of Fraunhofer (far field) diffraction to be made whilst still remaining in the near field of the surrounding transparent area. The resultant intensity distribution is investigated both theoretically and experimentally for objects which present both circular and rectangular cross sections to the beam.