System for measuring the total integrated scatter of a surface

• US 5,661,556 A
• Filed: 03/20/1996
• Issued: 08/26/1997
• Est. Priority Date: 11/27/1995
• Status: Expired due to Term

• Current Assignee: Schmitt Measurement Systems, Inc. (Schmitt Industries, Inc.)
• Original Assignee: Schmitt Measurement Systems, Inc. (Schmitt Industries, Inc.)
• Inventors: Schiff, Tod F., Bernt, Marvin L.
• Primary Examiner(s): PHAM, HOA Q
• Application Number:
  US08/618,977
• Time in Patent Office: 524 Days
• Field of Search: 356/236, 356/371, 356/445, 356/446, 250/228, 359/387
• US Class Current: 356/236
• CPC Class Codes: G01B 11/303 using photoelectric detection means
• CPC Class Codes: G01N 21/474 Details of optical heads therefor, e.g. using optical fibres

Abstract

A system and method for characterizing a surface are disclosed. The system includes a light source and source optics which direct a beam of light toward the surface. A first optical integrating device is positioned and configured to receive a first portion of the scattered light which corresponds to a first range of spatial frequencies. A second optical integrating device is positioned and configured to receive a second portion of the scattered light corresponding to a second range of spatial frequencies. In one embodiment, an integrating sphere is employed as the first optical integrating device. The sphere includes a sampling aperture which is surrounded by a light absorption region on the interior of the sphere. Total integrated scatter data is generated for each range of spatial frequencies and is used to approximate the spectral scatter function of the surface. RMS roughness is then approximated for any range of spatial frequencies.

Claims

1. A system for measuring the total integrated scatter of a surface, comprising:

a light source for producing an incident beam of light at a known wavelength range;

source optics for directing the incident beam at an incident angle (θ

_i);

a hollow sphere having a radius (R_s) and configured with an input aperture, a sampling aperture, and an output aperture,

the light source, source optics, and sphere positioned such that the incident beam is directed through the input aperture, through the sampling aperture, and onto the surface and such that the specular beam reflected off the surface is directed out of the sphere through the output aperture,

the interior surface of the sphere including an absorption region surrounding the sampling aperture,

the interior surface of the sphere outside the absorption region comprising a reflective region,

the absorption region having a reflectance less than the reflectance of the reflective region over the wavelength range of the light source;

a scatter detector positioned for detecting the intensity of light within the sphere; and

a specular detector positioned for detecting the intensity of the reflected specular beam.

2. A system for measuring the total integrated scatter of a surface as defined in claim 1, wherein the light source produces the beam of light at a known wavelength (λ

).

3. A system for characterizing a surface as defined in claim 2, wherein the incident angle of the beam of light (θ

_i) is less than about 10 degrees and wherein the output aperture has a circular perimeter with a radius (r₀) approximately equal to f_min 2R_s λ

, where f_min is the minimum spatial frequency of the total integrated scatter measurement of the surface.

4. A system for characterizing a surface as defined in claim 3, wherein the source optics are configured such that the incident beam has a circular cross section with a radius (r₁) as it leaves the source optics and wherein the sampling aperture a circular perimeter with a radius (r_s) which satisfies the following relationship:

##EQU18## where ##EQU19## where a equals the distance along the beam of light from the source optics to the input aperture of the hollow sphere.

5. A system for characterizing a surface as defined in claim 4, wherein the scatter detector is positioned within the integrating sphere such that the angle (θ

_D) between the specular beam and the line between the sampling aperture and the specular detector is greater than θ

_max, where θ

_max equals tan^-1 (r_s /h), where h is the distance from the surface to the interior surface of the hollow sphere at the sampling aperture along a line normal to the surface.

6. A system for measuring the total integrated scatter of a surface as defined in claim 1, wherein the reflectance of the absorption region is less than about 10 percent and the reflectance of the reflective region is greater than about 90 percent.

7. A system for measuring the total integrated scatter of a surface as defined in claim 6, wherein the reflectance of the absorption region is less than about 5 percent and the reflectance of the reflective region is greater than about 95 percent.

8. A system for measuring the total integrated scatter of a surface as defined in claim 1, wherein the absorption region comprises black glass.

9. A system for measuring the total integrated scatter of a surface as defined in claim 1, wherein the source optics are configured to focus the specular beam at the output aperture of the hollow sphere.

10. A system for measuring the total integrated scatter of a surface as defined in claim 9, wherein the source optics are configured such that the incident beam has a circular cross section with a radius (r₁) as it leaves the source optics and wherein the input aperture has a circular perimeter with a radius (r_i) which satisfies the following relationship:

##EQU20## for values of K≧

1, where a equals the distance along the beam of light from the source optics to the input aperture of the hollow sphere.

11. A system for characterizing a surface as defined in claim 10, wherein the light source produces a laser beam and K≧

√

2.5.

12. A system for measuring the total integrated scatter of a surface as defined in claim 1, wherein the source optics are configured such that the incident beam has a circular cross section with a radius (r₁) as it leaves the source optics, the input aperture has a circular perimeter with a radius (r_i), and wherein the absorption region has a circular perimeter with a radius (r_bo) which satisfies the following relationship:

##EQU21## where a is the distance along the beam of light from the source optics to the input aperture of the hollow sphere.