Dimensional measurement technique using optical filtering

Claim 1

In a dimensional measurement apparatus including a coherent light source, a first lens spaced from said light source, a second lens coaxial with said first lens and spaced therefrom, a spatial filter coaxial with and located between said two lenses, and means for measuring the energy of the light transmitted through said lenses and said filter, the improvement wherein said spatial filter is a bandpass filter having low frequency and high frequency cut-offs, said filter comprising a transparent annulus having an inner radius ri and an outer radius ro such that ri ui lambda f/2 pi Ro where ui is such that the Bessel function Jo (ui) Jo(uo) for uo 2 pi Roro/ lambda f and where lambda wavelength of the coherent light source f focal length of the first lens Ro radius of hole being measured ro outer radius of the annulus ri inner radius of the annulus.

Claim 3

An apparatus for measuring the size of a hole in an object under test, said apparatus comprising a first lens, means for holding the object under test substantially at the object focal plane of said first lens, spatially coherent light source means for exposing said object to coherent light radiation so as to transmit said radiation through said hole, a second lens coaxial with and spaced from said first lens, bandpass spatial filter means coaxial with and located between said two lenses at the object focal plane of said second lens and at the image focal plane of said first lens for filtering out from said transmitted radiation all frequency components outside a predetermined passband, and means located substantially at the image focal plane of said second lens for measuring the intensity of the filtered light radiation transmitted thereto, said filter means comprising a plurality of concentric transparent annuli.

Claim 4

An apparatus for measuring the size of a hole in an object under test, said apparatus comprising a first lens, means for holding the object under test substantially at the object focal plane of said first lens, spatially coherent light source means for exposing said object to coherent light radiation so as to transmit said radiation through said hole, a second lens coaxial with and spaced from said first lens, bandpass spatial filter means coaxial with and located between said two lenses at the object focal plane of said second lens and at the image focal plane of said first lens for filtering out from said transmitted radiation all frequency components outside a predetermined passband, and means located substantially at the image focal plane of said second lens for measuring the intensity of the filtered light radiation transmitted thereto, said filter means comprising a plurality of concentric transparent annuli.

Claim 5

An apparatus for measuring the size of a hole in an object under test, said apparatus comprising a first lens, means for holding the object under test substantially at the object focal plane of said first lens, spatially coherent light source means for exposing said object to coherent light radiation so as to transmit said radiation through said hole, a second lens coaxial with and spaced from said first lens, bandpass spatial filter means coaxial with and located between said two lenses at the object focal plane of said second lens and at the image focal plane of said first lens for filtering out from said transmitted radiation all frequency components outside a predetermined passband, and means located substantially at the image focal plane of said second lens for measuring the intensity of the filtered light radiation transmitted thereto, said bandpass spatial filter means comprising an opaque circular area surrounded by a plurality of concentric annular circular transparent areas.

Claim 6

An apparatus for measuring the size of a hole in an object under test, said apparatus comprising a first lens, means for holding the object under test substantially at the object focal plane of said first lens, spatially coherent light source means for exposing said object to coherent light radiation so as to transmit said radiation through said hole, a second lens coaxial with and spaced from said first lens, bandpass spatial filter means coaxial with and located between said two lenses at the object focal plane of said second lens and at the image focal plane of said first lens for filtering out from said transmitted radiation all frequency components outside a predetermined passband, and means located substantially at the image focal plane of said second lens for measuring the intensity of the filtered light radiation transmitted thereto, said bandpass spatial filter means comprising an opaque circular area surrounded by a plurality of concentric annular circular transparent areas.

Claim 7

An apparatus for measuring the size of a hole in an object under test, said apparatus comprising a first lens, means for holding the object under test substantially at the object focal plane of said first lens, spatially coherent light source means for exposing said object to coherent light radiation so as to transmit said radiation through said hole, a second lens coaxial with and spaced from said first lens, bandpass spatial filter means coaxial with and located between said two lenses at the object focal plane of said second lens and at the image focal plane of said first lens for filtering out from said transmitted radiation all frequency components outside a predetermined passband, and means located substantially at the image focal plane of said second lens for measuring the intensity of the filtered light radiation transmitted thereto, said filter means comprising a transparent annulus having an inner radius ri and an outer radius ro such that ri ui lambda f/2 pi Ro where ui is such that the Bessel function Jo (ui) Jo (uo) for uo 2 pi Ro ro/ lambda f and where lambda wavelength of the coherent light source f focal length of the first lens Ro radius of hole being measured ro outer radius of the annulus ri inner radius of the annulus.

Claim 8

An apparatus for measuring the size of a hole in an object under test, said apparatus comprising a first lens, means for holding the object under test substantially at the object focal plane of said first lens, spatially coherent light source means for exposing said object to coherent light radiation so as to transmit said radiation through said hole, a second lens coaxial with and spaced from said first lens, bandpass spatial filter means coaxial with and located between said two lenses at the object focal plane of said second lens and at the image focal plane of said first lens for filtering out from said transmitted radiation all frequency components outside a predetermined passband, and means located substantially at the image focal plane of said second lens for measuring the intensity of the filtered light radiation transmitted thereto, said filter means comprising a transparent annulus having an inner radius ri and an outer radius ro such that ri ui lambda f/2 pi Ro where ui is such that the Bessel function Jo (ui) Jo (uo) for uo 2 pi Ro ro/ lambda f and where lambda wavelength of the coherent light source f focal length of the first lens Ro radius of hole being measured ro outer radius of the annulus ri inner radius of the annulus.