High pass filtering has been relatively little used in microscopy, yet it may have application to linewidth measurement and visualization of phase objects. I have designed and built a spatial filtering microscope entirely of conventional microscope objectives. For linewidth measurement, the spatial filter has an optimum width that allows linewidths to be measured within a few percent. Phase lines can also be examined, but phase contrast microscopy may be more suited to weak phase objects such as integrated-optical waveguides.

Spatial filtering microscope for linewidth measurements.

Spatial Filtering Microscope for Linewidth Measurements

Coherent sources of light are easily available to university undergraduate laboratory courses and the demonstration of electro-magnetic wave diffraction is typically made with light. However, the construction of arbitrary patterns for the study of light diffraction is particularly demanding due to the small linear scale needed when using sub-micrometer wavelengths, limiting the possibility to thoroughly investigate diffraction experimentally. We describe and test a simple and affordable method to develop arbitrary light diffraction patterns with first year undergraduate or last year high school students. This method is exploited to investigate experimentally the connection between diffraction and the Fourier transform, leading to the development of the concept of spectral analysis of a (2D) signal. We therefore discuss the possibility of building a teaching unit for first year undergraduate or last year high school students on the interdisciplinary topic of spectral analysis starting from an experimental approach to light diffraction.

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Hands-on Fourier analysis by means of far-field diffraction

A method includes passing an interrogating light beam through a Fourier transform lens and onto the surface of a material to form a Fraunhofer diffraction pattern of one or more surface features of the material. An image of the diffraction pattern is processed to determine the dimensions of the feature.

Linewidth measurement system

Optical Fourier transforms of haddock fish scales were made for fish taken from two different bodies of water. The scales in each group varied in age from 2 to 10 yr. A comparison of the image processed digitized Fourier transforms for the scales was made on a personal computer. Statistical correlation studies of these transforms were performed, and the results show a fairly good age discrimination between the scales in each age group.

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Optical Fourier transform characterization of fish scale age

The classical, most often used, system for performing the optical Fourier transform is by using parallel coherent beam illumination. Lenses used in this method can become quite costly. In this paper we present results obtained using converging beam illumination which is suitable for many applications and is less expensive than the parallel beam method. The input objects for which the Fourier transforms were made are transparencies of snowflakes.

Converging beam optical Fourier transforms

In this paper we describe two methods of constructing optical Fourier transforms for use in image processing. Both the coherent parallel and converging beam Fourier transform methods are presented. The converging beam set-up was used to obtain results presented in another paper (1) at this NATO school but its full discussion at that time was postponed until now. The input objects chosen for this study are those of snow crystals. They are most appropriate since their Fourier transforms contain both some high and low spatial frequencies, not to mention they are beautiful patterns to observe.

Srisuda Puang-ngern

Papers

Converging beam optical Fourier transforms

Optical Fourier Transform Construction