Feliksas Ivanauskas
Vilnius University
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Featured researches published by Feliksas Ivanauskas.
Applied Physics Letters | 2002
A. Žukauskas; Rimantas Vaicekauskas; Feliksas Ivanauskas; R. Gaska; M. S. Shur
A stochastic method of optimization of a white-light source that relies on additive color mixing of the emissions from colored light-emitting diodes (LEDs) was developed. The method allows for finding the optimal wavelengths of LEDs in order to obtain the best possible trade off between luminous efficacy and the general color rendering index (CRI) of the white source for an arbitrary number of primary LEDs. Optimal solid-state lamps composed of two, three, four, and five different LEDs were analyzed. We show that a dichromatic LED lamp can only provide high efficacy with a general CRI close to zero, whereas trichromatic and quadrichromatic lamps are able to cover the entire range of reasonable general CRI values. The optimization of quintichromatic LED lamps and lamps with a higher number of primary color LEDs yields a negligible benefit in improving CRI but provides for quasicontinuous spectra that might be required for special lighting needs.
Applied Physics Letters | 2008
A. Žukauskas; Rimantas Vaicekauskas; Feliksas Ivanauskas; Henrikas Vaitkevičius; M. S. Shur
We apply an optimization scheme based on rendering of all colors of the enhanced Munsell palette to phosphor-conversion (PC) light-emitting diodes (LEDs). This approach yields combinations of peak wavelengths and bandwidths for white PC LEDs with partial and complete conversion that enable lighting with better quality than that obtained using designs based on the standard color-rendering assessment procedure.
Langmuir | 2012
Gintaras Valincius; Tadas Meskauskas; Feliksas Ivanauskas
The electrochemical impedance spectra (EIS) of tethered bilayer membranes (tBLMs) were analyzed, and the analytical solution for the spectral response of membranes containing natural or artificially introduced defects was derived. The analysis carried out in this work shows that the EIS features of an individual membrane defect cannot be modeled by conventional electrical elements. The primary reason for this is the complex nature of impedance of the submembrane ionic reservoir separating the phospholipid layer and the solid support. We demonstrate that its EIS response, in the case of radially symmetric defects, is described by the Hankel functions of a complex variable. Therefore, neither the impedance of the submembrane reservoir nor the total impedance of tBLMs can be modeled using the conventional elements of the equivalent electrical circuits of interfaces. There are, however, some limiting cases in which the complexity of the EIS response of the submembrane space reduces. In the high frequency limit, the EIS response of a submembrane space that surrounds the defect transforms into a response of a constant phase element (CPE) with the exponent (α) value of 0.5. The onset of this transformation is, beside other parameters, dependent on the defect size. Large-sized defects push the frequency limit lower, therefore, the EIS spectra exhibiting CPE behavior with α ≈ 0.5, can serve as a diagnostic criterion for the presence of such defects. In the low frequency limit, the response is dependent on the density of the defects, and it transforms into the capacitive impedance if the area occupied by a defect is finite. The higher the defect density, the higher the frequency edge at which the onset of the capacitive behavior is observed. Consequently, the presented analysis provides practical tools to evaluate the defect density in tBLMs, which could be utilized in tBLM-based biosensor applications. Alternatively, if the parameters of the defects, e.g., ion channels, such as the diameter and the conductance are known, the EIS data analysis provides a possibility to estimate other physical parameters of the system, such as thickness of the submembrane reservoir and its conductance. Finally, current analysis demonstrates a possibility to discriminate between the situations, in which the membrane defects are evenly distributed or clustered on the surface of tBLMs. Such sensitivity of EIS could be used for elucidation of the mechanisms of interaction between the proteins and the membranes.
Optical Science and Technology, SPIE's 48th Annual Meeting | 2004
Arturas Zukauskas; Rimantas Vaicekauskas; Feliksas Ivanauskas; G. Kurilčik; Zenius Bliznikas; Kestutis Breive; Jevgenij Krupic; Andrius Rupsys; A. Novičkovas; Pranciškus Vitta; Alvydas Navickas; Vytautas Raskauskas; M. S. Shur; Remis Gaska
White light with high color rendering indices can be produced by additive color mixing of emissions from several light-emitting diodes (LEDs) having different primary colors. White Versatile Solid-State Lamps (VSSLs) with variable color temperature, constant-chromaticity dimming, and efficiency/color-rendering trade-off can be developed using pulse-width modulation (PWM) driving technique. However, such lamps exhibit chromaticity shifts caused by different temperature and aging coefficients of the optical output for primary LEDs of different colors. To overcome this drawback, we developed a polychromatic white solid-state lamp with an internal digital feedback. The lamp features a quadrichromatic (red-amber-green-blue) design based on commercially available high-power LEDs. The design is optimized to achieve high values of the general color rendering index (69 to 79 points) in the color-temperature range of 2856 to 6504 K. A computer-controlled driving circuit contains a pulse-width modulator and a photodiode-based meter. The software performs periodical measurement of the radiant flux from primary LEDs of each color and adjusts the widths of the driving pulses. These VSSLs with feedback found application in phototherapy of seasonal affective disorder (SAD).
Journal of Mathematical Chemistry | 2003
Romas Baronas; Feliksas Ivanauskas; Juozas Kulys; Mifodijus Sapagovas
A two-dimensional-in-space mathematical model of amperometric biosensors has been developed. The model is based on the diffusion equations containing a nonlinear term related to the Michaelis–Menten kinetic of the enzymatic reaction. The model takes into consideration two types of roughness of the upper surface (bulk solution/membrane interface) of the enzyme membrane, immobilised onto an electrode. Using digital simulation, the influence of the geometry of the roughness on the biosensor response was investigated. Digital simulation was carried out using the finite-difference technique.
SIAM Journal on Numerical Analysis | 1999
Feliksas Ivanauskas; M. Radziunas
The first boundary value problem for a nonlinear Schrodinger equation is investigated. The conditional convergence and stability on the initial data of the explicit three-level difference scheme of DuFort--Frankel type in C and W_2^1
Journal of Mathematical Chemistry | 2002
Romas Baronas; Feliksas Ivanauskas; Juozas Kulys
norms are proved. Grid analogues of energy conservation laws and grid multiplicative inequalities are used.
Applied Mathematics and Computation | 2009
Feliksas Ivanauskas; Tadas Meskauskas; Mifodijus Sapagovas
A mathematical model of amperometric biosensors has been developed. The model is based on non-stationary diffusion equations containing a non-linear term related to Michaelis–Menten kinetic of the enzymatic reaction. Using digital simulation, the influence of the substrate concentration as well as maximal enzymatic rate on the biosensor response was investigated. The digital simulation was carried out using the finite difference technique. The model describes the biosensor action in batch and flow injection regimes.
Applied Physics Letters | 2008
A. Žukauskas; Rimantas Vaicekauskas; Feliksas Ivanauskas; Henrikas Vaitkevičius; M. S. Shur
The stability of difference schemes for one-dimensional and two-dimensional parabolic equations, subject to non-local (Bitsadze-Samarskii type) boundary conditions is dealt with. To analyze the stability of difference schemes, the structure of the spectrum of the matrix that defines the linear system of difference equations for a respective stationary problem is studied. Depending on the values of parameters in non-local conditions, this matrix can have one zero, one negative or complex eigenvalues. The stepwise stability is proved and the domain of stability of difference schemes is found.
International Symposium on Optical Science and Technology | 2001
Arturas Zukauskas; Feliksas Ivanauskas; Rimantas Vaicekauskas; M. S. Shur; Remis Gaska
We present a new approach to the optimization of sources of white light based on colored light-emitting diodes (LEDs) and show that a pentachromatic source composed of LEDs with the equidistant peak wavelengths at about 450, 500, 550, 600, and 650nm renders all 1269 colors of the Munsell palette almost indistinguishably from daylight.