APPARATUS AND METHOD FOR ELIMINATING INTERFERENCE ERRORS IN DUAL-BEAM INFRARED REFLECTION MEASUREMENTS ON A DIFFUSELY REFLECTING SURFACE BY GEOMETRICAL ELIMINATION OF INTERFERENCE-PRODUCING SPECULARLY-REFLECTED RADIATION COMPONENTS

• US 3,693,025 A
• Filed: 11/28/1969
• Issued: 09/19/1972
• Est. Priority Date: 11/28/1969
• Status: Expired due to Term

• Current Assignee: Process Automation Business Incorporated
• Original Assignee: Donald C. Brunton
• Inventors: Brunton, Donald C.
• Primary Examiner(s): Lawrence, James W.
• Assistant Examiner(s): Frome, Morton J.
• Application Number:
  US04/880,543
• Time in Patent Office: 1,026 Days
• Field of Search: 250/83IR,83.3D,219,222TH 356/161,209,210,211,212
• US Class Current: 250/340
• CPC Class Codes: G01B 11/0633 using one or more discrete wavelengths
• CPC Class Codes: G01N 21/8422 Investigating thin films, e.g. matrix isolation method

Abstract

Interference error elimination is provided for infrared reflection measurement of a film having a diffusely reflecting surface by apparatus and method which utilizes geometrical arrangement of apparatus components for geometrical selection of only diffusely reflected radiation components and elimination of specularly reflected radiation components that would produce interference error. A specific application of this measurement technique is in connection with a two-layer, sheet-form product having a film of infrared-radiation-transmissive material formed on a substrate or base layer with the interface surface being diffuse. A radiation source forms and directs two beams of infrared radiation of discrete wavelengths in angularly incident relationship toward an exposed, specular surface of the film producing reflective-components at both the specular surface and the diffuse interface surface. Only diffusely reflected components of incident beams of radiation are detected by a radiation-responsive sensor which is disposed in a particular geometrical arrangement such that only the diffusely reflected beam components are incident to a radiation receptor surface of the sensor. Thus, interference errors are eliminated as the specularly reflected beam components will not be incident to the radiation sensors receptor-surface.

Claims

1. A dual-beam infrared reflection measurement method for determining a parameter of a radiation-transmissive film where radiation incident to the film is both specularly and diffusely reflected comprising the steps of A. generating first and second beams of infrared radiation of respectively different wavelengths, one of which is selected to exhibit substantially more absorption with respect to the film material than the other, and directing said beams toward the film in angularly incident relationship to a first specular surface thereof with components of each beam specularly reflected from The first surface at a single predetermined angle and with other components of each beam entering the film to be diffusely reflected and subsequently exit the film at a number of angles including said predetermined angle, B. sensing and detecting by a radiation responsive sensor only those components of each beam diffusely reflected at angles greater than said predetermined angle and forming a signal related to the magnitude of each respective beam of radiation thus detected, and C. determining the ratio of the signals for each beam thereby providing an indication of the film parameter.

2. The method of claim 1 for measurement of a polyethylene film having a thickness greater than 0.0,005 inch and utilizing radiation of wavelengths in the range of 2.30 - 2.60 microns for one of the beams to exhibit absorption with respect to polyethylene material and utilizing a relatively close wavelength for the other beam which does not exhibit absorption.

3. The method of claim 2 wherein the other beam wavelength is 2.25 + or - 0.02.

4. The method of claim 1 for measurement of polyethylene film having a thickness less than 0.001 inch and utilizing radiation of 3.45 + or - 0.02 micron wavelength for one of the beams to exhibit absorption with respect to polyethylene material and utilizing a relatively close wavelength for the other beam which does not exhibit absorption.

5. The method of claim 4 wherein the beam wavelength is either 2.65 + or - 0.02 or 3.75 + or - 0.02 micron.

6. Apparatus for effecting dual-beam, infrared reflection measurement of a parameter of a radiation-transmissive film where radiation incident to the film is both specularly and diffusely reflected comprising A. a radiation source generating first and second beams of infrared radiation of respective, different wavelengths, one of which is selected to exhibit substantially more absorption with respect to the film material than the other, B. means directing said beams of radiation toward the film in angularly incident relationship to a first specular surface thereof with components of each beam specularly reflected from the first surface at a single predetermined angle and with other components of each beam entering the film to be diffusely reflected and subsequently exit the film at a number of angles including said predetermined angle, C. a radiation responsive sensor interposed in the path of reflected components of the beam and angularly oriented to detect only those components diffusely reflected at angles greater than said predetermined angle of specularly reflected radiation, said sensor forming a respective signal related to the magnitude thereof for each beam thus detected, and D. means responsive to the respective beam signals thus formed for each beam and forming an output signal which is a ratio of the beam signals and indicative of a parameter of the film.

7. Apparatus according to claim 6 wherein the wavelength of the first radiation beam exhibits absorption with respect to polyethylene material and is selected to be within the range of 2.30 - 2.60 micron and the wavelength of the second radiation beam is selected to be relatively close.

8. Apparatus according to claim 7 wherein the wavelength of the second radiation beam is selected to be 2.25 + or - 0.02 microns.

9. Apparatus according to claim 6 wherein the wavelength of the first radiation beam is selected to exhibit absorption with respect to polyethylene material and is 3.45 + or - 0.02 micron and the wavelength of the second radiation beam is selected to be relatively close.

10. Apparatus according to claim 9 wherein the wavelength of the second radiation beam is selected to be 2.65 + or - 0.02 or 3.75 + or - 0.02 microns.