Simple Techniques For Frequency Analysis Of Images

J.A. J van Leunen

doi:10.1117/12.941586

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14 May 1984 Simple Techniques For Frequency Analysis Of Images

J.A. J van Leunen

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Proceedings Volume 0467, Image Assessment: Infrared and Visible; (1984) https://doi.org/10.1117/12.941586
Event: Image Assessment Infrared and Visible, 1983, Oxford, United Kingdom

Abstract

A straightforward way to arrive at the spatial frequency content of an image is to digitize the image and then to compute the Fourier transform. This method involves expensive instrumentation and a powerful computer. If spatial frequency analysis is applied as a 100 % image assessment test in a production line it is neither suitable nor necessary to perform a complete frequency analysis. Commonly the spatial frequency response at a single well-chosen spatial frequency suffices to characterize the performance of the test specimen for adequate production control. By using fixed or tunable large-area spatial bandpass filters it is possible to build rigid and relatively inexpensive measuring instruments that can be used in a production line. For example, if the modulation transfer function has only to be measured at a single spatial frequency it is possible to use a checkerboard pattern which acts as a spatial triangular wave-filter on the image of the object line. If the line image is unsharp enough the higher harmonics of the filter have negligible influence. To arrive at this situation a well-defined amount of unsharpness can be arranged by the introduction of movement blurring. Using this concept we have built modulation-transfer-factor measuring instruments that are accurate to within two percent. The instruments described are used in the production lines for X-ray image intensifiers. This measurement replaces the measurement of the more subjective limiting resolution. A more detailed description of the method can be found in an earlier paper by the author (1). It is fairly easy to generate a tunable triangulare wave filter just by putting two bar patterns of equal frequency on top of each other with a small angle between the bars. The spatial frequency of the resulting triangular wave Moiree pattern varies with this angle. This principle is used in some rather expensive instruments for the measurement of the modulation transfer function. In these instruments the higher order harmonics of the triangular wave are filtered out by electronic means. Due to the electronic signal processing the tunable spatial filter acts effectively as a tunable spatial sinewave filter. There are other ways of producing tunable spatial sinewave filters. These and the application of such a filter in the measurement of the Wiener spectrum of photographs are the main subjects of this paper.

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J.A. J van Leunen "Simple Techniques For Frequency Analysis Of Images", Proc. SPIE 0467, Image Assessment: Infrared and Visible, (14 May 1984); https://doi.org/10.1117/12.941586

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