Archive of Medical Research

Visualization of two-dimensional phase gradients by subtraction of a reference periodic pattern.

We describe a simple method for visualization of phase objects. The phase object is placed between a printed two-dimensional periodic pattern and a CCD camera. The ray deflection that is due to the phase object distorts the image of the pattern. This image is subtracted from a reference image and then, by squaring and low-pass filtering, a measurement of the two-dimensional refractive-index changes is obtained. Because the optical system does not require special alignment or illumination, the method presented has potential application for detection of gas leaks in industrial environments.

Perciante CD, Ferrari JA.

Phase retardometer: a proposed device for measuring phase retardance.

The realization of a standard device for measuring phase retardance has been retarded by the lack of a simple underlying physical concept that can be formulated in a simple mathematical expression to provide quick, direct, and accurate results. A proposed device that meets these requirements and simple formulas for retardance measurements are presented.

Nagib NN.

Anisotropic incoherent reflection model for spectroscopic ellipsometry of a thick semitransparent anisotropic substrate.

An anisotropic incoherent reflection model for the Mueller matrix elements of an optically thick uniaxial anisotropic semitransparent substrate with its anisotropy axis along its surface normal is developed. The Mueller matrix elements are measured by phase-modulated spectroscopic ellipsometry (SE) and compared with incoherent reflection model simulations. In the case of a sapphire substrate the oscillations observed are accurately modeled, and, in addition, the oscillating degree of polarization is correctly predicted. A straightforward generalization of the optical model, in the case of an arbitrary stack of layers containing a thick anisotropic semitransparent substrate, is also proposed and experimentally validated. The model is further applied to study the anisotropic dielectric function of a semi-insulating 4H-SiC wafer. An approximation based on a simple variation in the optical transition element is proposed to model the SiC birefringence. In conclusion, SE is shown to be a powerful alternative for investigating and predicting the behavior of optically thick birefringent materials.

Ossikovski R, Kildemo M, Stchakovsky M, Mooney M.

Optical plastic refractive measurements in the visible and the near-infrared regions.

Some new refractometric results are obtained in the visible and the near-infrared spectral regions. The main optical plastics are analyzed: poly(methyl methacrylate), polystyrene, polycarbonate, and styrene acrylonitrile. New materials, such as methyl methacrylate styrene copolmer, CTE-Richardson, Zeonex, Optorez, and Bayer are examined. The refractive indices are measured for wavelengths from 435.8 to 1052 nm with a new device. Abbe constants and dispersion coefficients are calculated. The measured and computed data is intended for designers and technologists.

Nikolov ID, Ivanov CD.

Method for measuring veiling glare in high-performance display devices.

An experimental method for measuring the veiling glare characteristics of display devices is presented. The measured veiling glare ratio (G) is taken to be the luminance in the surrounding bright field divided by the luminance in a dark circle. The method is based on a collimated conic probe that minimizes signal contamination from bright surroundings allowing for measurements of low luminance in a circular dark spot of a test pattern. A correction factor computed with test patterns having opaque spots is introduced. The factor is expressed as a bivariate function of the dark-spot radius and the distance between the probe and the emissive surface. We studied the uncertainty introduced by the method by measuring veiling glare test patterns printed on radiographic film for which the transmission of the dark spots was determined experimentally. Performance characterization measurements show that signal contamination is less than 10(-4) of the bright field surrounding a dark circle. Our results show that G of a few hundred can be measured with an uncertainty of a few percent, and ratios of approximately 10(3) can be reported within 10%. Finally, we demonstrate the method by measuring G for a high-performance monochrome cathode-ray tube display.

Badano A, Flynn MJ.

Optimization of resolution and color reproduction by controlling luminance and contrast levels of a liquid-crystal display monitor.

The image quality of a color liquid-crystal display monitor is studied by changing the contrast and the luminance levels. The resolution is assessed with modulation transfer function area (MTFA) that is measured at nine different combinations of the luminance and contrast. At these combinations, we measure the chromaticity coordinates and luminance of red-green-blue channels as a function of the digital-analog convert value. We analyze the relations among MTFA value, constant-channel chromaticity, color gamut, and maximum luminance of a white pattern. From the results, the combination of levels is found that provides both high resolution and good color reproduction.

Kim TH, Lee YW, Lee IW, Choi SC.

Model for quantifying photoelastic fringe degradation by imperfect retroreflective backings.

In any automated algorithm for interpreting photoelastic fringe patterns it is necessary to understand and quantify sources of error in the measurement system. We have been considering how the various components of the coating affect the photoelastic measurement, because this source of error has received fairly little attention in the literature. Because the reflective backing is not a perfect retroreflector, it does not preserve the polarization of light and thereby introduces noise into the measurement that depends on the angle of obliqueness and roughness of the reflective surface. This is of particular concern in resolving the stress tensor through the combination of thermoelasticity and photoelasticity where the components are sensitive to errors in the principal angle and difference of the principal stresses. We have developed a physical model that accounts for this and other sources of measurement error to be introduced in a systematic way so that the individual effects on the fringe patterns can be quantified. Simulations show altered photoelastic fringes when backing roughness and oblique incident angles are incorporated into the model.

Woolard D, Hinders M.

Control of the characteristics of a long-period grating by cladding etching.

We have analyzed the effects of the fiber cladding radius on the characteristics of long-period fiber gratings. By etching the cladding of a common single-mode fiber, we verified the characteristics experimentally. When by etching we reduce the cladding radius of a common single-mode fiber on which a long-period refractive-index modulation has been imposed, the coupling strength of the core and the cladding modes increases. In addition, the difference in the propagation constants (for a fixed wavelength) between the core mode and the cladding modes increases; hence the resonant transmission dip wavelengths shift to longer wavelengths. The proposed method can be useful in making and detuning long-period fiber grating filters.

Kim S, Jeong Y, Kim S, Kwon J, Park N, Lee B.

Propagation of light trapped within a set of lowest-order modes of graded-index multimode fiber undergoing bending.

We describe experimental results and a theoretical analysis for propagation in graded-index multimode fiber when diode laser light is launched into the lowest-order propagation modes and the fiber undergoes severe bending perturbations. Experimentally, near-field modal interference images and transmission loss measurements were obtained for different loop diameters. The data indicate that, when the fundamental mode is excited, the light remains in lowest-order modes even for small bend diameters. This is consistent with analysis which predicts that, in a parabolic-index multimode fiber subject to constant diameter bending, the light tends to oscillate between lowest-order modes and remains trapped therein rather than diffusing to high-order modes. Implications of these results for an intrusion-resistant communication system with graded-index multimode fiber are discussed.

Asawa CK, Taylor HF.

Multiplexed continuous-wave diode-laser cavity ringdown measurements of multiple species.

Rapid cavity ringdown measurements of multiple broadband absorbing species (methanol and isopropanol) in gas mixtures have been recorded with two multiplexed continuous-wave distributed-feedback diode lasers operating near 1.4 mum. A measurement sensitivity of 2.4 x 10(-9) cm(-1) for a 4.3-s averaging time was achieved in a 39.5-cm-long static cell with 99.94% reflectivity mirrors. This corresponds to a water-vapor detection limit of less than 2 ppb (parts in 10(9)) for the strong H(2)O lines near 1.4 mum. The shot-to-shot noise of the decay time constant tau was approximately 0.3-0.7%, and ringdown acquisition rates as great as 900 Hz were achieved.

Totschnig G, Baer DS, Wang J, Winter F, Hofbauer H, Hanson RK.