- #1
JamesJames
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I am confused by this Abbe theory thing involvign the use of an optical computer.
A laser is focused onto a 10 μm screen with adjustable micrometer screws. A pinhole is provided to avoid any unnecessary dispersion of the laser beam. The first lens converts spherical wavefronts into plane waves which then pass through an object plane (Ronchi rulings). It could be thought of as a condenser that produces a parallel uniformly illuminated beam. The purpose of the second lens is to form the Fourier transform image in its focus (transform plane). Low pass filtering in the Fourier plane is acheived by making use of an aperture. The third lens, when placed at its focal distance from the Fourier plane, converts the Fourier image into the inverted image of the object.
Upon inserting a fine wire mesh into the object plane, a faithful representation of the image is obtained. A 2D periodic object like this generates a T shaped diffraction pattern with a high degree of symmetry.
Now here' s what is really bugging me...There are masks that pass only parts of the pattern. For example, the effect on the image of passing only the vertical principal maxima is horizontal lines using a narrow vertical slit at the plane of the T shapped diffraction pattern. Turning the slit to select only the horizontal maxima yields vertical lines.
What is going on here physically? I am confused about how passing vertical maxima yields a the ("opposite") horizontal lines and vice versa.
I' ve read up on Fourier transforms but don' t understand how this is happening.
James
A laser is focused onto a 10 μm screen with adjustable micrometer screws. A pinhole is provided to avoid any unnecessary dispersion of the laser beam. The first lens converts spherical wavefronts into plane waves which then pass through an object plane (Ronchi rulings). It could be thought of as a condenser that produces a parallel uniformly illuminated beam. The purpose of the second lens is to form the Fourier transform image in its focus (transform plane). Low pass filtering in the Fourier plane is acheived by making use of an aperture. The third lens, when placed at its focal distance from the Fourier plane, converts the Fourier image into the inverted image of the object.
Upon inserting a fine wire mesh into the object plane, a faithful representation of the image is obtained. A 2D periodic object like this generates a T shaped diffraction pattern with a high degree of symmetry.
Now here' s what is really bugging me...There are masks that pass only parts of the pattern. For example, the effect on the image of passing only the vertical principal maxima is horizontal lines using a narrow vertical slit at the plane of the T shapped diffraction pattern. Turning the slit to select only the horizontal maxima yields vertical lines.
What is going on here physically? I am confused about how passing vertical maxima yields a the ("opposite") horizontal lines and vice versa.
I' ve read up on Fourier transforms but don' t understand how this is happening.
James
