Kanik Palodhi

Bio: Kanik Palodhi is an academic researcher from Loughborough University. The author has contributed to research in topics: Coherence scanning interferometry & Point spread function. The author has an hindex of 3, co-authored 4 publications receiving 73 citations.

Coherence scanning interferometry: linear theory of surface measurement

Abstract: The characterization of imaging methods as three-dimensional (3D) linear filtering operations provides a useful way to compare the 3D performance of optical surface topography measuring instruments, such as coherence scanning interferometry, confocal and structured light microscopy. In this way, the imaging system is defined in terms of the point spread function in the space domain or equivalently by the transfer function in the spatial frequency domain. The derivation of these characteristics usually involves making the Born approximation, which is strictly only applicable to weakly scattering objects; however, for the case of surface scattering, the system is linear if multiple scattering is assumed to be negligible and the Kirchhoff approximation is assumed. A difference between the filter characteristics derived in each case is found. However this paper discusses these differences and explains the equivalence of the two approaches when applied to a weakly scattering object.

Application of linear systems theory to characterize coherence scanning interferometry

Abstract: This paper considers coherence scanning interferometry as a linear filtering operation that is characterised by a point spread function in the space domain or equivalently a transfer function in the frequency domain. The applicability of the theory is discussed and the effects of these functions on the measured interferograms, and their influence on the resulting surface measurements, are described. The practical characterisation of coherence scanning interferometers using a spherical reference artefact is then considered and a new method to compensate measurement errors, based on a modified inverse filter, is demonstrated.

Determination of the point spread function of a coherence scanning interferometer.

Abstract: Coherence scanning interferometry (CSI) combines the lateral resolution of a high power microscope with the axial resolution of an interferometer and provides a rapid and convenient means to measure surface topography. Although CSI measurements of features, such as step heights, have been demonstrated with nanometre accuracy, systematic errors have been reported in the measurement of more general features, such as sinusoids, where surface gradient can be significant. Recently, methods of three-dimensional imaging systems, including digital holography, confocal microscopy and CSI have been analysed as linear shift invariant processes that are characterised by their point spread function (PSF). In this paper, it is shown that that the linear theory leads to a straightforward model of fringe formation in CSI. The influence of the PSF on measurements of a sinusoidal surface profile and a method to measure the PSF of a CSI instrument is then discussed.

Measurement of complex refractive index using microellipsometry and its relevance to surface measurement using coherence scanning interferometry.

Abstract: Coherence scanning interferometry (CSI) is an increasingly popular method to provide high resolution measurements of surfaces. In essence sub-nanometre axial resolution is achieved by measuring the phase of the field scattered from the surface of interest. The phase is not simply a function of surface height, however, but depends on the complex refractive index of the surface material. The phase contribution due to materials properties is small and approximately constant across the field of view for the case of homogenous surfaces. It can lead to significant measurement errors, however, when composite micro- structures of conducting and non-conducting media are examined. This paper discusses the measurement of complex refractive index using a single-point micro-ellipsometer based on a Michelson interferometer. It shows how the optical properties of the substrate are related to the phase of the scattered components at different spatial frequencies. A whole-field implementation of the method using CSI is then discussed.

Application of porosities in the transparent electrode layer of a perovskite solar cell for performance enhancement.

Abstract: Substitution of monocrystalline or polycrystalline silicon as active materials in photovoltaics with highly efficient perovskite materials is quite common. Although perovskite materials offer better flexibility, are cost-effective, and have higher conversion efficiency, they still require structural modifications for better performance. This study quantitatively investigates how mesoporous top surfaces improve the performance of methylammonium lead iodide (C H 3 N H 3 P b I 3) perovskite solar cells. In fact, both the diameter and the depth of the pores have been tuned to achieve better performance. The performance is further optimized by replacing mesoporous active material with planar active material coated with mesoporous indium tin oxide (ITO). We have demonstrated that the proposed structure achieves the maximum conversion efficiency (η) of 27.43% with an open-circuit voltage (V O C ) of 1.07 V and a short circuit current density (J S C ) of 29.09m A/c m 2, with a fill factor (FF) of 88.10%.

Characterisation of Areal Surface Texture

Abstract: Introduction to surface topography.- The areal field parameters.- The areal feature parameters.- Areal filtering methods.- Areal form removal.- Areal fractal methods.- Choosing the appropriate parameter.- Characterization of individual areal features.- Multi-scale signature of surface topography.- Correlation of areal surface texture parameters to solar cell efficiency.- Characterisation of cylinder liner honing textures for production control.- Characterization of the mechanical bond strength for copper on glass plating applications.- Inspection of laser structured cams and conrods.- Road surfaces.

Principles of interference microscopy for the measurement of surface topography

Abstract: Interference microscopy plays a central role in noncontact strategies for process development and quality control, providing full 3D measurement of surface characteristics that influence the functional behavior of manufactured parts. Here I briefly review the history and principles of this important technique, then concentrate on the details of hardware, software, and applications of interference microscopy using phase-shifting and coherence scanning measurement principles. Recent advances considered here include performance improvements, vibration robustness, full color imaging, accommodation of highly sloped surfaces, correlation to contact methods, transparent film analysis, and international standardization of calibration and specification.

Calibration of the scales of areal surface topography-measuring instruments: part 1. Measurement noise and residual flatness

Abstract: In this paper, we present methods for determining the measurement noise and residual flatness of areal surface topography-measuring instruments. The methods are compliant with draft international specification standards on areal surface texture. We first introduce the international standards framework and then present current methods based on averaging and subtraction to isolate the measurement noise and residual flatness from the sample surface topography. These methods are relatively difficult to apply and time consuming in practice. An alternative method is presented based on thresholding and filtering techniques. This method is simple to apply in practice. Traceability and measurement uncertainty are discussed.

Determination of the transfer function for optical surface topography measuring instruments—a review

Abstract: A significant number of areal surface topography measuring instruments, largely based on optical techniques, are commercially available. However,implementation of optical instrumentation into production is currently difficult dueto the lack of understanding of the complex interaction between the light and the component surface. Studying the optical transfer function of the instrument can help address this issue. Herea review is given of techniques for the measurement of optical transfer functions. Starting from the basis of a spatially coherent, monochromatic confocal scanning imaging system, the theory of optical transfer functions in three-dimensional (3D) imaging is presented. Further generalizations are reviewed allowing the extension of the theory to the description of conventional and interferometric 3D imaging systems. Polychromatic transfer functions and surface topography measurements are also discussed. Following presentation of theoretical results, experimental methods to measure the optical transfer function of each class of system are presented, with a focus on suitable methods for the establishment of calibration standards in 3D imaging and surface topography measurements.