Tomi Pitkäaho

Digital Fresnel hologram watermarking

We present a method of digital hologram watermarking using a Fresnel hologram of a real-world 3D object and the fractional Fourier transform. A watermark is encrypted using double random phase fractional Fourier domain encoding technique and then encoded into the digital hologram. The hologram is watermarked in a plane at some known distance from the object so that even if a new hologram is generated from the original hologram the watermark can always be traced by propagating the new hologram back to the object and then onto the watermark plane. The watermark is retrieved successfully using the correct encryption parameters. We consider both numerical (full complex field) and optoelectronic (phase-only) reconstruction methods. We obtain the watermark from different windows of the hologram corresponding to different reconstruction perspectives.

Calculating depth maps from digital holograms using stereo disparity

Depth extraction is an important aspect of three-dimensional (3D) image processing with digital holograms and an essential step in extended focus imaging and metrology. All available depth extraction techniques with macroscopic objects are based on variance; however, the effectiveness of this is object dependent. We propose to use disparity between corresponding points in intensity reconstructions to determine depth. Our method requires a single hologram of a scene, from which we reconstruct two different perspectives. In the reconstruction the phase information is not needed, which makes this method useful for in-line digital holography. To our knowledge disparity based 3D image processing has never been proposed before for digital holography.

Partially coherent digital in-line holographic microscopy in characterization of a microscopic target

Digital holographic microscopy enables the capture of large three-dimensional volumes. Instead of using a laser as an illumination source, partially coherent alternatives can be used, such as light-emitting diodes, which produce parasitic reflection and speckle-free holograms. Captured high-contrast holograms are suitable for the characterization of micrometer-sized particles. As the reconstructed phase is not usable in the case of multiple overlapping objects, depth extraction can be conducted on a reconstructed intensity. This work introduces a novel depth extraction algorithm that takes into consideration the possible locations of multiple objects at various depths in the imaged volume. The focus metric, the Tamura coefficient, is applied for each pixel in the reconstructed amplitude throughout the volume. This work also introduces an optimized version of the algorithm, which is run in two stages. During the first stage, coarse positions of the objects are extracted by applying the Tamura coefficient to nonoverlapping window blocks of intensity reconstructions. The second stage produces high-precision characterizations of the objects by calculating the Tamura coefficient with overlapping window blocks around axial positions extracted in the first stage. Experimental results with real-world microscopic objects show the effectiveness of the proposed method.

Plug-and-play mechanism for plain transducers with wired digital interfaces attached to wireless sensor network nodes

The ability to connect sensors to the Wireless Sensor Network WSN nodes without the need for physical device configuration has many advantages: application development is simplified, network deployment and service is easier, and sensors can be swapped or added on-the-fly. The existing solution for sensor Plug-and-Play P&P for WSN nodes is the IEEE 1451 set of standards developed for smart transducers. The serious drawback of this solution is that it cannot be used with the most widespread plain transducers without adding multiple external components. Therefore, in this paper, we introduce a novel mechanism that allows implementation of P&P connection to WSN nodes for commercially available off-the-shelf sensors with the most widespread wired plain digital interfaces SPI, I2C, 1-wire etc. without any single external component utilisation.

Numerical reconstruction of digital holograms for conventional 3D display

True hologram video displays are currently under development, but are not yet available. Because of this restriction, conventional 3D displays can be used with digital holographic data. However when using conventional 3D displays, holographic data has to be processed correctly to meet the requirements of the display. A unique property of digital holograms, namely that a single hologram encodes multiple perspectives, can be used to achieve this goal. Reconstructions from digital holograms at different perspectives are processed further to meet the requirements of the conventional 3D display, which are typically based on stereoscopic images of the scene.

Evaluation of perceived quality attributes of digital holograms viewed with a stereoscopic display

Holography is a well-known technique for sensing and displaying real-world three-dimensional (3D) objects. Digital holograms have been successfully displayed on conventional stereoscopic displays allowing research into perception of quality of 3D holographic data. We do know that quality is enhanced if reconstructions of digital holograms are displayed with conventional stereoscopic displays rather than with a regular two-dimensional (2D) screen. However, it is not known how different attributes (e.g. noise, blur, and perceived depth) and the viewers subjective perception of quality are related. In this study, we show how 13 viewers evaluated these attributes and the visual quality of five holograms displayed stereoscopically.

Using disparity in digital holograms for three-dimensional object segmentation

Digital holography allows one to sense and reconstruct the amplitude and phase of a wavefront reflected from or transmitted through a real-world three-dimensional (3D) object. However, some combinations of hologram capture setup and 3D object pose problems for the reliable reconstruction of quantitative phase information. In particular, these are cases where the twin image or noise corrupts the reconstructed phase. In such cases it is usual that only amplitude is reconstructed and used as the basis for metrology. A focus criterion is often applied to this reconstructed amplitude to extract depth information from the sensed 3D scene. In this paper we present an alternative technique based on applying conventional stereo computer vision algorithms to amplitude reconstructions. In the technique, two perspectives are reconstructed from a single hologram, and the stereo disparity between the pair is used to infer depth information for different regions in the field of view. Such an approach has inherent simplifications in digital holography as the epipolar geometry is known a priori. We show the effectiveness of the technique using digital holograms of real-world 3D objects. We discuss extensions to multi-view algorithms, the effect of speckle, and sensitivity to the depth of field of reconstructions.

Stereo vision based approach for extracting features from digital holograms

With digital holography one can record and reconstruct real world three-dimensional (3D) objects [1,2]. The recorded interference pattern includes information about both amplitude and phase of a wavefront reflected from or transmitted through the object. However, some of the hologram capture setups pose a problem for the reliable reconstruction of quantitative phase information. This can be because the twin image or noise corrupts the reconstructed phase. In such cases it is usual that only amplitude is reconstructed and used as the basis for metrology. A focus criterion is often applied to this reconstructed amplitude to extract depth information from the sensed 3D scene [3,4]. In this paper we present an alternative technique based on applying conventional computer stereo vision algorithms to amplitude reconstructions. We show the effectiveness of our technique using digital holograms of both macroscopic and microscopic real-world 3D objects. We discuss sensitivity to the depth of field of reconstructions, and which hologram capture setups are, and which are not, suitable for the technique.

Speckle Noise Reduction in Michelson Digital Holography Using Known or Unknown Reference Linear Phases and Image Processing

Speckle is caused by the illumination of an optically rough surface by coherent light [1] which can cause problems in digital holography. Speckle in a reconstruction reduces visibility of details and may impede important measurements of an object. Speckle can be reduced by capturing multiple holograms and summing the reconstructed intensities [2-4]. Digital signal processing methods, which are applied after capture, have also been proposed [5–8].

RFID and Wireless Sensor and Actuator Networks in Advanced Production Applications

In this study we focus on adding wireless intelligence to machines and systems to be used in production applications. The key enabling technologies of piloted mechatronic systems were RFID (Radio-Frequency Identification) and WSAN (Wireless Sensor and Actuator Network). This work is mainly done in a project “Ubiquitous Computing in Maintenance Using Sensors and RFID Tags”. There were several industrial partners in the project. The main goal is to develop solutions that are suitable for industry. Several systems piloted in harsh industrial environments are considered in the paper.