A. R. Ganesan

Real-time comparative Digital Speckle Pattern Interferometry

Abstract A comparative Digital Speckle Pattern Interferometric technique which allows compensation of fringes is presented in this paper. The technique employs two identical objects, the master and the test, that are identically loaded to detect the difference in their displacement profiles. The advantage of using DSPI is that real-time compensation of fringes can be achieved precisely and defect detection becomes much easier. Theory and experimental results are presented.

Classification of indoor daylight enhancement systems

In a dense urban environment, sky obstruction caused by high rise buildings causes minimal daylighting for interiors on lower floors and in deep interior spaces. Innovative daylighting systems can be used to enhance daylighting indoors using available sunlight. This paper presents an overview of recent developments of innovative daylighting systems. The daylighting systems, which are classified as light guiding, light transport, light diffracting, hybrid and integrated systems, have been reviewed and a comparison has been made with respect to ease of integration, durability, maintenance, availability, efficiency, light output and transportation. Suitability of climate has been identified with respect to each daylighting system. Recent research and development involves hybrid and integrated systems which have the advantage of providing lighting irrespective of external sky conditions. The advantages and disadvantages of each system are summarized. Compared to active systems, passive systems are environmentally friendly and their efficiencies are also higher. It is inferred that each system has its own unique principle of light transmittance and is suitable for certain climates. More research is required on developing daylighting systems which are environment friendly, less energy intensive, cost-effective and having ease of integration.

Conceptual design and assessment of a profiled Fresnel lens daylight collector

This paper describes design of daylight collectors using multiple Fresnel lenses, each facing the sun for a particular period of time in a day. The influence of solar movement on flat Fresnel lens was studied. The influence of groove density of the lens was also determined by which the suitable Fresnel lens was selected. Passive and profiled north-facing and south-facing collectors with multi-lens system have been designed based on the solar altitude and azimuth angle of a geographical location. Performance of the north- and south-facing collector at different times of a day with an acrylic dome indicated better efficiency and uniformity of lighting throughout the day time for the proposed profiled Fresnel collectors.

Daylight enhancement using laser cut panels integrated with a profiled Fresnel collector

Performance of conventional vertical light pipes with acrylic domes is highly dependent on the solar altitude since for lower solar elevation the acceptance angle is limited due to the small inlet aperture. A profiled Fresnel collector with a multi-lens system is useful in enhancing light in a light pipe at low solar altitudes but suffers losses due to multiple reflections during transmission. In this paper we show that further enhancement of light during the morning and evening hours is possible by using a profiled Fresnel collector integrated with graded laser cut panels, i.e. laser cut panels with bands of varying depth to width ratio that can redirect low altitude sun rays axially down the light pipe. The effect of various depths to width ratios of the laser cut panel with respect to variation in incident light on the performance of a light pipe is also presented in this paper. The graded laser cut panel performed better than other laser cut panels. Optimum tilts of the laser cut panel at which maximum light is deflected for various incident angles of the source were identified. Maximum light output for the graded laser cut panel was for lower incidence of solar altitude angle. The graded/profiled laser cut panel was integrated with Fresnel north and south facing collectors and the performance was assessed in daylight. Good redirection of light was observed for the Fresnel collector in the morning and evening hours when compared to a light pipe with an acrylic dome or a Fresnel collector without a laser cut panel. The light is redirected more parallel to the axis of the pipe thus minimizing the reflections on the internal lining and thereby increasing the performance of the light pipe.

Effective Medium-Based Plasmonic Waveguides for Tailoring Dispersion

We propose a waveguide configuration with a plasmonic grating for tailoring dispersion characteristics. The unit-cell of the plasmonic grating encompasses the subwavelength distribution of metal nanowires forming a highly resonant effective medium. The configuration enables independent control of the coupling between the plasmonic and waveguide modes via the resonant strength of the effective medium. Numerical simulations show that the line shapes of the coupled modes can be varied from Fano to electromagnetically induced transparency-like. Furthermore, we use the structure to enhance the group index from 250 to 850 and to broaden the associated band from 40 to 180 meV.

Design and Development of a Closed Loop Adaptive Optics System for Wavefront Sensing and Control

Atmospheric distortion limits the quality of images obtained with ground-based telescopes. Similarly, laser beam propagation through turbulent atmosphere results in discontinuity in satellite optical links. Adaptive Optic techniques provide compensation for the aberrations introduced due to the random refractive index fluctuations of the atmosphere. In this paper, we present the practical details of the successful design and development of a low-cost Shack-Hartmann wavefront analyzer. as well as a closed loop Adaptive Optics system which compensates the global till and the local phase fluctuations of a wavefront. The Shack-hanmann sensor measures the wavefront profile in real time and generates correction signals. Global tilt measurements to accuracy of 19μ radians and phase profiles up to λ/50 have been acheived. Dynamic corrections have been made with tip-till mirror and a deformable mirror. The Opto-mechanical system, software design, and implementation are described. Experimental results of wavefront measurements and correction of aberrations are presented.

Increasing the sensitivity for tilt measurement using a cyclic interferometer with multiple reflections

Abstract. Measurement of tilt plays an important role in metrological applications and consequently, several methods have been proposed in the recent past. Classical interferometric methods can measure angles with high accuracy but are easily susceptible to external turbulences. We propose to use a cyclic interferometer to measure tilt in which the sensitivity to tilt measurement is double when compared with that of the classical Michelson interferometer. Since the counter propagating beams travel identical paths, the interferometer is insensitive to external vibrations and turbulence and thus can be used under harsh environmental conditions. The novelty in the technique lies in creating multiple reflections in the tilt mirror to enhance the measurement accuracy by the way of increasing the sensitivity. This paper presents the basics of the interferometer and experimental results to quantify the increase in sensitivity. By increasing the number of reflections, it is shown that sensitivity can be further improved to measure tilt angles below 5  μrad.

Study on decentration-induced optical aberrations in an optical system using Shack Hartmann wavefront sensor

Alignment of optical components is one of the important requirements in any optical system. Decentration of a component, like a lens, in the path of the beam, would introduce aberrations of various types. This would affect the measurement accuracy in the optical system such as an interferometer. In this work, we have analyzed the influence of decentration of an optical component on the wavefront in an optical system. The various aberrations caused due to the shifting of the axis of a lens in the path of an optical wavefront have been measured using a Shack Hartmann Wavefront Sensor and their influence studied. One of the lenses in the optical system is moved or decentered in transverse direction by 500 μm in steps of 50 μm. Decentration was done for all four quadrants. For each step, wavefront data is been taken and data was analyzed. Defocus, horizontal coma, vertical coma and spherical aberration were analyzed, apart from peak-to-valley and RMS values. Results showed that the error introduced is minimal up to 300 μm decentration, above which the aberrations were quite large. The experimental results and analyses are presented.

Analysis of refractive errors in the eyes of young Indians

The refractive errors of normal, healthy eyes were measured using an indigenously developed Shack–Hartmann aberrometer. Measurement was made in both right and left eyes after dilation for a 6 mm pupil size. The power vector method was used to represent the sphero-cylindrical errors. Analysis was done for astigmatism with the rule, oblique astigmatism and defocus between the right and left eyes of the subjects, which showed a negative, positive and zero correlation respectively. No correlation could be detected for RMS values between right and left eyes, though Zernike between right and left showed bilateral symmetry in our subjects. It was found that with an increase of spherical aberration, defocus decreased slightly. The validity and repeatability of our Shack–Hartmann aberrometry in measuring the refractive error was analysed and repeatability coefficient calculated. Optimal correction for greater retinal image quality has been discussed and far-point vergence for detecting the point of maximum retinal image quality is suggested.

A spot pattern test chart technique for measurement of geometric aberrations caused by an intervening medium?a novel method

Accurate surface metrology and transmission characteristics measurements have become vital to certify the manufacturing excellence in the field of glass visors, windshields, menu boards and transportation industries. We report a simple, cost-effective and novel technique for the measurement of geometric aberrations in transparent materials such as glass sheets, Perspex, etc. The technique makes use of an array of spot pattern, we call the spot pattern test chart technique, in the diffraction limited imaging position having large field of view. Performance features include variable angular dynamic range and angular sensitivity. Transparent sheets as the intervening medium introduced in the line of sight, causing aberrations, are estimated in real time using the Zernike reconstruction method. Quantitative comparative analysis between a Shack–Hartmann wavefront sensor and the proposed new method is presented and the results are discussed.