Intensity spatial filter having non-uniformly spaced filter elements

Claim 3

Apparatus for inspecting non-periodic errors in a two-dimensional pattern containing a plurality of nominally identical and regularly spaced elements, arranged in a planar periodic array, which comprises: means for direcging a spatially coherent beam of lighT at the plane of the pattern so that the light is diffracted thereby; a first lens positioned to focus the light diffracted by the pattern; a planar optical filter consisting of distribution of discrete substantially equally sized opaque regions on a transparent field, the spacing between adjacent regions, along at least one axis of the filter, increasing from region-to-region from the centermost region to the edges of the filter, wherein the distance X of any given region, on said at least one axis, from said centermost region is given by the formula: X f tan (sin 1 (n lambda /d) ) where, f the focal length of the Fourier transforming lens; lambda the wavelength of said beam; n the order of the spatial harmonic; d the step-and-repeat distance of said two-dimensional pattern; the filter being positioned at the focal plane of the first lens for spatially modulating the intensity of the light focused thereon by the first lens; a second lens positioned to reimage the light transmitted by the filter to form a visual image of the non-periodic errors in the pattern of the image display means; and means for projecting the visual image onto the image display means.

Claim 4

Apparatus for inspecting non-periodic errors in a two-dimensional pattern containing a plurality of nominally identical and regularly spaced elements, arranged in a planar periodic array, which comprises: means for direcging a spatially coherent beam of lighT at the plane of the pattern so that the light is diffracted thereby; a first lens positioned to focus the light diffracted by the pattern; a planar optical filter consisting of distribution of discrete substantially equally sized opaque regions on a transparent field, the spacing between adjacent regions, along at least one axis of the filter, increasing from region-to-region from the centermost region to the edges of the filter, wherein the distance X of any given region, on said at least one axis, from said centermost region is given by the formula: X f tan (sin 1 (n lambda /d) ) where, f the focal length of the Fourier transforming lens; lambda the wavelength of said beam; n the order of the spatial harmonic; d the step-and-repeat distance of said two-dimensional pattern; the filter being positioned at the focal plane of the first lens for spatially modulating the intensity of the light focused thereon by the first lens; a second lens positioned to reimage the light transmitted by the filter to form a visual image of the non-periodic errors in the pattern of the image display means; and means for projecting the visual image onto the image display means.

Claim 5

Apparatus according to claim 2 wherein said image display means and said projecting means comprises: a television camera focused on said visual image; control means, connected to said camera, for supplying deflection signals and power to said camera, said camera scanning across said visual image to detect said non-periodic errors; and counting means, connected to the video output of said camera, for counting the number of non-periodic errors so detected.

Claim 6

A spatial filter, which consists of: a matrix-like array of discrete substantially equally sized opaque elements on a transparent field, the spacing between adjacent elements, along at least one axis of the array, increasing from element-to-element, from the centermost element outwards towards the edges of the array, wherein the distance X of any given element on said at least one axis, from said centermost element, is given by the formula: X f tan (sin 1 (n lambda /d) ) wherein, f the focal length of the Fourier transforming lens; lambda the wavelength of the light forming said image; n the order of the spatial harmonic; d the step-and-repeat distance of the workpiece; whereby, when said filter is positioned to intercept the Fourier transform of the image of a workpiece comprising a matrix-like array of nominally identical features, said opaque elements inhibit further transmission of substantially all periodic information in said transform.

Claim 7

A spatial filter, which consists of: a matrix-like array of discrete substantially equally sized opaque elements on a transparent field, the spacing between adjacent elements, along at least one axis of the array, increasing from element-to-element, from the centermost element outwards towards the edges of the array, wherein the distance X of any given element on said at least one axis, from said centermost element, is given by the formula: X f tan (sin 1 (n lambda /d) ) wherein, f the focal length of the Fourier transforming lens; lambda the wavelength of the light forming said image; n the order of the spatial harmonic; d the step-and-repeat distance of the workpiece; whereby, when said filter is positioned to intercept the Fourier transform of the image of a workpiece comprising a matrix-like array of nominally identical features, said opaque elements inhibit further transmission of substantially all periodic information in said transform.