Tuukka Prykäri

Study on the use of optical coherence tomography in measurements of paper properties

This study proposes a new method for the characterization of paper surface and bulk structure: optical coherence tomography (OCT). Although having been used in medicine for years, this technique is still relatively unknown in the paper-making industry. First, we provide a brief description of a conventional PC-controlled measurement system for the OCT imaging of paper. In this set-up, a powerful superluminescent diode is used to illuminate a Michelson interferometer with a free-space configuration, and a piezo-transducer is placed in the optical delay line to modulate the measurement signal for optical heterodyne detection. The set-up is then applied to demonstrate the ability of the technique to visualize the surface structures of widely disparate paper samples using a single device. Next, the paper provides 3D images of a fibre network and of typical copy paper. The results prove that OCT is applicable not only to the 3D imaging of simple wood fibre networks, but also to the imaging of complex commercial paper, provided that an appropriate clearing agent is used. Finally, the effect of filler on the OCT signal slope calculated by averaging several A-scans from the sample is demonstrated, showing that increasing filler content produces a corresponding decrease of decay of the A-scan in depth.

Optical and terahertz measurement techniques for flat-faced pharmaceutical tablets: a case study of gloss, surface roughness and bulk properties of starch acetate tablets

Surface and bulk properties of flat-faced starch acetate tablets were studied. For surface quality inspection optical coherence tomography and recently developed diffractive glossmeter were utilized. Both these optical devices together provide local information on surface roughness and gloss of a tablet over a measured area. The concepts of mean topography and mean gloss profile for surface quality of a tablet are introduced. It was observed that the surface quality of the tablet varies, and compression at high pressure may not guarantee a good surface quality of the tablet. Using novel statistical parameters for gloss and relevant surface roughness parameter, it is possible to get more comprehensive quantitative data on the surface condition of a tablet. THz spectrometer was utilized for detection of THz pulse delay in transmission measurement mode from the tablets. The delay time and thickness ratio of the tablet are consistent with the porosity of the tablet as a function of compression pressure. We suggest that the multimeasurement scheme using three different devices helps tablet makers to better assess bulk and surface quality of their products.

Skin phantoms with realistic vessel structure for OCT measurements

We present here a novel phantom for optical coherence tomography (OCT) made of polyvinyl chloride-plastisol (PVCP). The optical properties of PVCP were estimated by the Mie theory and deduced from OCT measurements. Titanium dioxide (TiO2) powder and black plastic colour (light-absorbing plastic ink) were used to introduce scattering to the phantom and create capillary structure, respectively.

Application of LabVIEW: Complex Software Controlling of System for Optical Coherence Tomography

This article briefly describes the system for optical coherence tomography (OCT), an interferometric imaging technique and focuses on software controlling of this complex apparatus and data acquisition (DAQ) by means of LabVIEW. It states advanced LabVIEW techniques for parallel DAQ, multi-instrument communication, and postprocessing needed to assure OCT measurements being carried out at the University of Oulu.

Diffractive-optical-element-based glossmeter and low coherence interferometer in assessment of local surface quality of paper

The surface microroughness of paper has an important role on its gloss. Unfortunately, commercial glossmeters do not provide information on the local gloss of paper. In this study a low-coherence interferometer was employed for the assessment of the average surface roughness of fine, supercalendered, and Xerox papers by means of recorded topography maps. Furthermore, the local and average gloss were measured by a diffractive-optical-element-based glossmeter. This is the first time that the measurement of the local gloss of paper has been accomplished. The information on both surface roughness and gloss, obtained by the two devices in this study, should help papermakers in their research and development of optimal paper surface quality, which is crucial to optimal ink absorption in printing.

Flow velocity profile measurement of scattering liquid using Doppler optical coherence tomography

The aim of this study was to measure the velocity and velocity profile of 0.3% Intralipid mixture in a 1.5-mm thick glass capillary using Doppler Optical Coherence Tomography (DOCT). First, while still empty, the dimensions of the capillary were measured. The outer diameter was 1.50 mm ± 0.01 mm while the lumen diameter was 1.01 ± 0.01 mm. Then, having filled the capillary with 0.3% solution, the lumen diameter was measured again. The mean refractive index of the solution was calculated and turned out to be 1.36 ± 0.01 mm. During the next stage, flow measurements were performed at an angle of 88° between the illuminating beam and the velocity vector of the fluid. The velocity profile, based on a set of 10 measurements, was calculated from the DOCT signal using a discrete Fourier transform in 32 sections of the capillary. The maximum velocity, located in the middle part of the capillary, was 0.256 ± 0.035 m/s. The results show that the flow velocity profile of 0.3% Intralipid solution can be obtained from a glass capillary.

Evaluation of microfluidic channels with optical coherence tomography

Application of time domain, ultra high resolution optical coherence tomography (UHR-OCT) in evaluation of microfluidic channels is demonstrated. Presented study was done using experimental UHR-OCT device based on a Kerr-lens mode locked Ti:sapphire femtosecond laser, a photonic crystal fibre and modified, free-space Michelson interferometer. To show potential of the technique, microfluidic chip fabricated by VTT Center for Printed Intelligence (Oulu, Finland) was measured. Ability for full volumetric reconstruction in non-contact manner enabled complete characterization of closed entity of a microfluidic channel without contamination and harm for the sample. Measurement, occurring problems, and methods of postprocessing for raw data are described. Results present completely resolved physical structure of the channel, its spatial dimensions, draft angles and evaluation of lamination quality.

Comparing low coherence interferometry with conventional methods of measuring paper roughness

This paper introduces optical non-contact paper surface characterization based on Low Coherence Interferometry (LCI). Using this technique, the roughness of two different types of fine paper series are measured and the obtained results are compared to those of two air leak methods, PPS and Bendtsen, which are used as reference methods.

Optical coherence tomography evaluating the random tissues based on dynamical processing the stochastic low-coherence interference fringes

Interferometers with a low-coherent illumination allow non-contact evaluating random tissues by locating the visibility maxima of interference fringes. The problem is the light scattering by a tissue, it is why interference fringe parameters are randomly varied. Other problem consists in the need to process large amount of data obtained in optical coherence tomography (OCT) systems. We propose to use a stochastic fringe model and Kalman filtering method for noisy low-coherence fringe processing. A fringe signal value is predicted at a next discretization step using full information available before this step and a prediction error is used for dynamic correction of fringe envelope, frequency and phase. The advantages of Kalman filtering method consist in its noise-immunity, high-speed data processing and optimal evaluation of fringe parameters. Specially fabricated random tissues have been measured with a low-coherence interferometer. The obtained data from the tissue internal structure are evaluated using a dynamic stochastic fringe processing algorithm applied to fringe signal samples series. Nonlinear Kalman filtering method was applied to measure scattering liquid velocity profile in the Doppler OCT. The measurement results are in good agreement with the results obtained by the Fourier transform method.

Comparison of artificial neural network and multilinear regression analysis models in estimation of pulp flow speed from low coherence Doppler flowmetry measurement data

Low Coherence Doppler Flowmetry (LCDF) measurement produces a signal, which frequency domain characteristics are in connection to the speed of the flow. In this study performances of Artificial Neural Network (ANN) and Multilinear Regression (MLR) methods in prediction of pulp flow speed from the LCDF measurement data were compared. In the study the pulp flow speed was estimated distinctly from consecutive frequency bands of the LCDF data with both methods. The smallest estimation error in flow speed with the ANN method was 20% and with the MLR method 30%, depending on the selected frequency band. The results indicate the relationship between characteristics of the LCDF measurement and pulp flow speed includes remarkable number of nonlinear components. The result is in line with theoretical calculations about the Doppler shifts occurrence in the LCDF data.