Дослідження методів знешумлення інтерферограми білого світла
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Date
2015
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Видавництво Львівської політехніки
Abstract
Проаналізовано джерела спотворень інтерферограми білого світла. Обґрунтовано методику та досліджено
ефективність знешумлення синтезованого сигналу інтерферограми з використанням цифрових фільтрів
та вейвлет-перетворення. Результати апробовано на реальній інтерферометричній картині. Проанализированы источники искажений интерферограмы белого света. Обоснована методика и
исследована эффективность фильтрации синтезированного сигнала интерферограмы с использованием
цифровых фильтров и вейвлет-преобразования. Результаты апробированы на реальной
интерферометрической картине. White light interferometry (WLI) is a non-contact measurement technique which is commonly used in determining the
mechanical quantities such as geometric dimensions, position, and surface topography of the object. The main areas of
use of the white light interferometers are micro- and nanotechnology, biomechanics, polymer chemistry, semiconductor
equipment, and others.
The measuring channel of optical interferometer includes the optical part and the computer unit. The accuracy of
surface reconstruction depends on the quality of interferogram registration (optical part) and metrological properties of
the reconstruction algorithm (computer unit). In practice interferogram registration is accompanied by different
distortions, including optical nonlinearities and noise. Reduce of the distortions destabilizing effect can be achieved by
processing the obtained interferogram in a computing unit by special algorithms. The aim of the work is analysis of
methods for white light interferogram denoising.
The article analyzes the distortion sources of white light interferogram and the ways of their processing. The technique
of researchis based on analyzing the effectiveness of denoising the synthesized white light interferogram with signal-tonoise
ratio of 30 dB. Quantitative parameter for evaluating the filtering effectiveness is signal-to-noise ratio. Firstly the
researches were conducted on one-dimensional data (for the central line of interferometric pattern).
This article examines the possibility of WLI denoising with digital filters (frequency domain) and wavelet
transform (time-frequency domain). When using digital filters, the non-recursive filters were selected, as they are
characterized by a linear phase response that will keep the shape signal. Filtering of interferogram signal using nonrecursive
filter has not produced satisfactory results, as in one case with noise suppression the part of useful signalis
damaged, and the signal-to-noise ratio was 15 dB. By increasing the cutoff frequency of the filter, signal shape does not
undergo distortion, but there are some artifacts, especially in its central part. The reason is that the signal lies partly in
the frequency dominated, where the noise predominates. The signal-to-noise ratio was 35 dB.
Wavelet transform allows to decompose a signal to approximating and detailing components, with higher levels of
detailing component responsible for scheduling and signal noise can be set to zero without signal distortion. Application
of wavelet transformation made it possible to achieve better denoising results compared to digital filters with signal-tonoise
ratio about 50 dB. Taking into account these results, the wavelet transform method is adapted to the two-dimensional data (interferometric pattern). Compared to the one-dimensional data the denoising is slightly lower, as
evidenced by the signal-to-noise ratios about 40 dB. This is due to the fact that the interferogram signal intensity
decreases as the distance from its center.
The effectiveness of the denoising method based on wavelet transform was investigated on a real white light
interferogram obtained for a spherical surface. Defects observed in the interferometric pattern after denoising are caused by optical distortions.
Description
Keywords
інтерферометрія білого світла, спотворення інтерферограми, знешумлення сигналу, цифрові фільтри, вейвлет-перетворення, відношення сигнал/шум, интерферометрия белого света, искажение интерферограмы, фильтрация сигнала, цифровые фильтры, вейвлет-преобразование, отношение сигнал/шум, whitelight interferometry, interferogram distortion, signal denoising, digital filters, wavelet-transform, signal-to-noisе ratio
Citation
Хома А. Дослідження методів знешумлення інтерферограми білого світла / Анна Хома // Вимірювальна техніка та метрологія : міжвідомчий науково-технічний збірник / відповідальний редактор Б. І. Стадник. – Львів : Видавництво Національного університету "Львівська політехніка", 2015. – Випуск 76. – С. 74–80. – Бібліографія: 11 назв.