Apparatus for detecting a polarization altering substance on a surface

• US 5,841,538 A
• Filed: 07/10/1996
• Issued: 11/24/1998
• Est. Priority Date: 06/13/1995
• Status: Expired due to Fees

• Current Assignee: Rosemount Aerospace, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Original Assignee: Robotic Vision Systems, Inc. (Acuity Brands, Inc.)
• Inventors: Schoeffler, Daniel, Stern, Howard, Costa, Pat V.
• Primary Examiner(s): Pham, Hoa Q.
• Application Number:
  US08/677,785
• Time in Patent Office: 867 Days
• Field of Search: 356/364-370, 250/225, 250/339.11, 250/339.12, 250/341.8, 244/134 R, 244/134 F, 340/583, 340/580, 340/581, 340/602, 340/962, 348/61, 348/143
• US Class Current: 356/369
• CPC Class Codes: B64D 15/20 Means for detecting icing or initiating de-icing
• CPC Class Codes: B64F 5/20 Ground installations for de-icing aircraft
• CPC Class Codes: G01J 4/00 Measuring polarisation of light
• CPC Class Codes: G01N 2021/8848 Polarisation of light
• CPC Class Codes: G01N 21/21 Polarisation-affecting properties G01N21/19 takes precedence
• CPC Class Codes: G01N 21/88 Investigating the presence of flaws or contamination
• CPC Class Codes: G08B 19/02 Alarm responsive to formation or anticipated formation of ice

Abstract

An apparatus for detecting a polarization altering substance, such as ice, on a surface includes a polarizing filter on the surface between the surface and the polarization altering substance. When the polarizing filter includes alternating regions having orthogonal polarizing properties, only one viewing of the surface through a blocking filter is required. When light, either polarized or unpolarized, reflects off the surface, it passes through the polarizing filter and becomes polarized. Reflected light that additionally passes through ice after leaving the polarizer becomes unpolarized. When viewed through a blocking polarizer filter, polarized light passing through ice appears bright due to the unpolarizing effect of ice. On the other hand, polarized light not passing through ice retains its polarization and appears dark when viewed through a blocking filter. Since the polarizing filter is between the surface and the viewer, the surface can be metallic, dielectric, or painted without affecting the results. If the proper blocking orientation for the viewer is not known in advance, the Stokes coefficients can be calculated if views are taken through a series of specified polarizing filters. The ratio of polarized light returned to the viewer compared to the unpolarized light returned to the viewer can then be calculated from any arbitrary position. A retroreflective substance on the surface further enhances the effect for systems employing an active illumination source located coaxially with or adjacent to the imaging system.

Claims

1. An apparatus for detecting a polarization altering substance, comprising:

a retroreflector on a surface;

a first polarizing filter on said retroreflector;

said first polarizing filter having at least first and second regions adjacent one another;

each of said at least first and second regions of said first polarizing filter having different polarizing characteristics;

a light source for illuminating said retroreflector through said first polarizing filter;

said retroreflector is effective to produce a returned light returned toward said light source when said light source illuminates said retroreflector;

a second polarizing filter in a path of said returned light;

said second polarizing filter having a characteristic that operates differently on said returned light from each of said at least first and second regions of said first polarizing filter; and

means for detecting said returned light through said second polarizing filter, whereby a presence of said polarization altering substance is detected by a change in a relationship of said returned light passing through each of said at least first and second regions of said first polarizing filter, polarized light and ambient nonpolarized light reflected from said surface; and

said means for detecting includes viewing means for viewing said retroreflective substance through said second polarizing filter effective for determining a presence of said polarization altering substance.

2. An apparatus for detecting a polarization altering substance on a surface, comprising:

a retroreflector on said surface;

a first polarizing filter on said retroreflector;

means for illuminating said retroreflector through said first polarizing filter to produce a reflected light;

said reflected light passes through said first polarizing filter to produce a polarized reflected light;

said reflected light passes through said first polarizing filter and also passes through said polarization altering substance to produce a nonpolarized reflected light,

a second polarization filter having a first polarization characteristic;

first means for detecting, through said second polarizing filter, said polarized reflected light and said nonpolarized reflected light reflected from said retroreflector through said first filter;

said first means for detecting includes first viewing means for viewing said retroreflector through said second polarizing filter;

said second polarizing filter being a blocking filter;

a third polarizing filter having a second polarization characteristic;

second means for detecting, through said third polarizing filter, polarized reflected light and nonpolarized reflected light reflected from retroreflector through said first filter;

said second means for detecting including second viewing means for viewing said retroreflector through said third polarizing filter;

said third polarizing filter being an non-blocking filter; and

means for comparing a response to said first means for detecting and said second means for detecting to determine a presence of said polarization altering substance on said surface.

3. A system for detecting a polarization altering substance on a surface, comprising:

a retroreflector on said surface;

at least first and second polarizing filters adjacent each other on said retroreflector;

said first polarizing filter having a first polarizing characteristic;

said second polarizing filter having a second polarizing characteristic;

a light source for illuminating said retroreflector through said at least first and second polarizing filters;

said retroreflector producing a reflected light when illuminated by said light source;

said reflected light being projected from said retroreflector through said at least first and second polarizing filters toward said light source along a path;

a polarizer in said path of said reflected light;

said polarizer having a characteristic that operates differently on said reflected light from each of said at least first and second polarizing filters; and

means for receiving said reflected light from said polarizer, whereby said presence of said polarization altering substance on said surface is detectable by a change in a relationship of said reflected light passing through said polarizer from said at least first and second polarization filters.