Data processing: measuring – calibrating – or testing – Measurement system – Orientation or position
Reexamination Certificate
2002-03-13
2004-01-13
Nghiem, Michael (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Orientation or position
Reexamination Certificate
active
06678632
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to remote surface-orientation measurement. It is applicable particularly, but not exclusively, to passive, monocular systems for making such measurements.
2. Background Information
In the discussion that follows, the following bracketed codes will be used to refer to the sources identified next to them:
[Born&Wolf1965
] Principles of Optics
, M. Born and E. Wolf, Pergamon Press, (1965).
[Collett1993
] Polarized Light
, E. Collett, Marcel Dekker, (1993).
[Hapke1993
] Theory of Reflectance and Emittance Spectroscopy
, B. Hapke, Cambridge University Press, (1993).
[Iannarilli2000] “Snapshot LWIR hyperspectral polarimetric imager for ocean surface sensing”, F. J. Iannarilli, J. A. Shaw, S. H. Jones, and H. E. Scott,
Proc. SPIE
Vol 4133 (2000).
[IannarilliRubin2002] “Feature selection for multi-class discrimination via mixed-integer linear programming, F. J. Iannarilli and P. A. Rubin, submitted (2002).
[Joubert1995] “3-D surface reconstruction using a polarization state analysis”, E. Joubert, P. Miche, and R. Debrie, J. Optics 26, pp.2-8 (1995) (French journal; no longer published).
[Proll2000] “Application of a liquid crystal spatial light modulator for brightness adaptation in microscopic topometry”, K. P. Proll, J. M. Nivet, Ch. Voland, and H. J. Tiziani, Applied Optics 39(34), 6430-35 (2000).
[Ward1994
] The Optical Constants of Bulk Materials and Films
, L. Ward, Institute of Physics Publishing, Philadelphia (1994).
[Windecker1995] “Topometry of technical and biological objects by fringe projection”, R. Windecker and H. J. Tiziani, Applied Optics 34, 3644-3650 (1995).
[Wolff1991] “Constraining object features using a polarization reflectance model”, L. B. Wolff and T. E. Boult, IEEE Trans. on Pattern Analysis and Machine Intelligence, 13(7), 635-657, (1991).
[Wooten1972
] Optical Properties of Solids
, F. Wooten, Academic Press, (1972).
There are numerous applications for non-contact acquisition of an object's three-dimensional surface geometry or topometry. Consequently, many types of topometric apparatus have become available, each offering relative strengths and realms of applicability. Optical approaches by nature offer the convenience and spatial resolution compatible with typically desired measurement scales. In 3D photography, retrieval of object topometry is attempted by using multiple viewpoints (stereometry), “shape-from-X” (where “X” can be shading, texture and other photometric attributes), or perhaps from illuminant time of flight by employing a synchronized light source and camera shutter. In the similar yet distinct “3D scanning” domain, object topometry can be retrieved by using two or more displaced cameras to triangulate a scanned laser-illuminated spot. Structured (incoherent) light or interferometric (coherent) retrieval techniques are also common.
These various approaches can be categorized in accordance with whether the measurement is based on the differential range to the subtended surface element, its stereometric disparity, or its orientation.
Differential range measurements by nature use a controlled (active, directed) illumination source. This category includes the structured light, interferometric, and “time-of-flight” techniques [Windecker1995]. Some of the practical challenges involve control of, or accounting for, spatial non-uniformity of illumination or surface reflectance [Proll2000]. An implicit requirement is the arrangement for pre-measurement setup, so the approach typically cannot be used spontaneously.
Stereometric-disparity approaches implicitly require that feature correspondence be ascertained. That is, they need to identify the point in the image taken from viewpoint A that represents the same real-world point as a given point in the image taken from viewpoint B. Active time-sequential spot illumination (e.g., by laser) overcomes the “matchpoint” ambiguity suffered by passive imaging, particularly when surfaces lack features. The effectiveness of spot illumination depends on whether enough light is scattered into the direction of the receiving sensor(s) (stationed at fixed viewpoint), so spot illumination works best for diffusely reflecting surfaces and not so well for glossy (quasi-specular) surfaces. Stereometric-disparity methods cannot be used for objects located outside the overlap in the tandem sensors' fields of view.
The photometric (“shape-from-X”) measurements are by nature modulated by the object's surface orientation, which in turn modulates the observable reflectance and surface relief. A strong advantage that such methods (potentially) offer is monocularity, that is, the ability to employ a single sensor from a single viewpoint. This minimizes or eliminates the need for pre-measurement set-up. It thus enables these techniques to be used spontaneously over wide fields of regard. But shape-from-shading and shape-from-texture methods often suffer from ambiguities that typically confine them to ascertaining only relative (differential) rather than absolute surface orientation. One way to overcome the ambiguity is to employ variable-incidence active illumination, but this limits flexibility and measurement spontaneity.
Novel sub-categories of the photometric methods employ radiometric attributes beyond scalar intensity. Perhaps the most evident among these is polarimetry. For ideal smooth surfaces, the Fresnel equations relate the measurable polarimetric attributes, e.g., degree of polarization (“DoP”) and angle of polarization (“AoP”), to surface orientation. This suggests the possibility of passive monocular 3D imaging, where the light sources are ambient illumination and/or thermal self-emission [Wolff1991, Iannarilli2000]. Specifically, the physics suggests a one-to-one mapping from polarimetric (DoP, AoP) to surface-orientation coordinates.
But closer investigation reveals that, without further refinement, such polarimetric imaging may itself be limited to ascertaining relative rather than absolute surface orientation. Now, by making a simplifying assumption, an approach described in [Joubert1995] does make a polarimetric determination of the surface orientation. But it relies on stereometric measurements (i.e., two or more independent measurements from different vantage points) to determine the angle that the line of sight makes with the surface normal. So far, there has been no reasonably robust way to determine three-dimensional surface orientation by passive polarimetry from a single vantage point.
One reason for this is that the degree of polarization for a given surface orientation can vary due to unknown surface roughness. Attributing the proper absolute scale to DoP is also confounded in thermal-infrared applications became the balance between surface self-emission (i.e., temperature) and reflected environmental illumination is unknown. For example, in an application of monochromatic infrared polarimetry to passive retrieval of ocean surface waveslope [Iannarilli2000], the radiometric balance between self-emission from the warm ocean and reflection from an either cold or warm sky varies the scale of DoP by a factor of 10.
To discuss the problem in more detail, we introduce the notation illustrated by
FIG. 1
, which is a diagram of the various planes and angles defined by a polarimetric sensor
28
and the surface
12
to be measured. The sensor may have imaging capability, in which case it resolves a point on surface
12
at viewing-angle coordinates (&bgr;,&agr;) with respect to its reference frame. We define a right-handed Cartesian sensor reference frame
14
such that the Z-axis is coincident with the sensor's optical axis. The Z-axis's positive direction is from the sensor's entrance aperture to its focal plane, i.e. along the direction of light propagation. If the sensor is non-imaging, the viewing-angle coordinates (&bgr;,&agr;
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