Optics: measuring and testing – By polarized light examination
Reexamination Certificate
1999-10-07
2003-05-13
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
By polarized light examination
C356S369000, C250S341300, C250S332000
Reexamination Certificate
active
06563582
ABSTRACT:
BACKGROUND—FIELD OF INVENTION
The invention relates to optical components particularly with respect to controlling the polarization of optical radiation.
BACKGROUND—DESCRIPTION OF THE PRIOR ART
In addition to having color, light waves have the attribute of polarization. Light is a superposition of two orthogonal states of polarization. We can consider one of these states to be an oscillation of the wave up and down along a vertical direction. The other state will be an oscillation left and right along a horizontal direction. If the two states are oscillating in phase, then, at a given point in space, the electric field vector of the light wave traces, in time, a segment along a straight line. The light is said to be linearly polarized. When the two states are oscillating partially out of phase, then the electric vector traces an ellipse. In particular, when they are out of phase by 90 degrees and the oscillation amplitudes are equal, then the electric vector traces a circle, and the light is circularly polarized. Light is circularly polarized, right-handed or left-handed, when the two states are out of phase by +90 or −90 degrees respectively. The properties of polarized light are described in detail in D. Clarke et al.,
Polarized Light and Optical Measurement,
(Pergamon Press, Oxford, 1971).
Scientists understand that when linearly polarized light is incident on the surface of a material, the light which is reflected has a component which is circularly polarized. Such sources of linearly polarized radiation are pervasive in the visible and infrared wavelengths. Some examples are the polarization found in light from the daytime blue sky, in the thermal emission from the ocean surface, and in scattered light underwater. It is surprising then, that imaging sensors which are sensitive to circular polarization have not been developed for object detection and recognition. Recent measurements reported in K. P. Bishop et al., “Multi-spectral polarimeter imaging in the visible to near IR,” in
Targets and Backgrounds: Characterization and Representation V,
W. R. Watkins, D. Clement, and W. R. Reynolds, Eds, Proceedings of SPIE Vol. 3699, 49-57(1999), suggest that, in the visible and near infrared wavelength range, as much as 5 percent of ambient light is circularly polarized. Another advantage of using circular polarization images is that the sign and magnitude of the circular polarization can potentially be used to reveal the spatial orientation, material, and surface roughness of the object's surface.
The generation by reflection of circularly polarized light is enhanced when the surface is smooth or the surface material is electrically conductive. These characteristics are more common in man-made surfaces than in natural surfaces. A sensor imaging circular polarization would be able to detect man-made objects in a background almost free of clutter. Such a sensor could have applications in automobiles to alert drivers of the presence of other vehicles, especially at night, in fog, or in rain. Military applications include the detection of vehicles placed among trees and shrubs.
In ambient scenes, there is potentially as much variety and information in polarization images as there is in color images. However, a practical and reliable method of imaging circular polarization has not been developed.
A method to detect circular polarization must distinguish between right-handed and left-handed polarized light. Most photodetectors are insensitive to polarization. So a filter must be placed in front of the detector which is able to transmit only right-handed or only left-handed polarized light. In practice, such a filter is a combination of a quarter-wave retarder and a linear polarizer.
As light travels through the retarder, the phase of the horizontal state of oscillation is shifted by 90 degrees relative to the vertical state of oscillation. Consider the case when right-handed circularly polarized light is incident on the retarder. The retarder tranforms the light into a linearly polarization state with oscillations along a direction rotated 45 degrees from horizontal.
The light then enters the linear polarizer. If the linear polarizer is oriented to transmit light oscillating along the direction rotated 45 degrees from horizontal, then the light will be incident on the photodetector. However, if the light originally had left-handed polarization, the light exiting the retarder would be linearly polarized along the direction rotated −45 degrees from horizontal and would not be transmitted by the linear polarizer. Hence, the combination of the quarter-wave retarder, linear polarizer, and photodetector will only respond to light containing right-handed circular polarization.
In contrast, if the linear polarizer is rotated so that it transmits light with oscillations along the direction rotated −45 degrees from horizontal, then the combination will respond only to left-handed circular polarized light. Similarly, the linear polarizer can remain fixed and the retarder rotated. At certain retarder orientations the combination will respond only to right-handed polarized light, and at certain other orientations only to left-handed polarized light. In a circular polarization sensor, the mechanism for rotating the linear polarizer or retarder introduces weight and cost and makes the sensor less reliable. In addition, the frequency at which the images can be updated, i.e. the frame rate, is limited by the rotation rate.
The simplest retarder is a plate, referred to, in the art, as a waveplate or phase plate, made of a birefringent material. Birefringent materials have a fast axis and a slow axis. Light waves, with oscillations along the direction of the fast axis, propagate with higher velocity than light waves, with oscillations along the slow axis. Because of this velocity difference, as light waves traverse a birefringent material, their two states of oscillations can be shifted in their relative phase. The phase shift as the light exits the waveplate is specified by choosing the thickness of the birefringent material.
Linear polarizers suitable for use in imaging sensors are know in the art. However, quarter-wave retarders with suitable characteristics have not been developed. The technology of retarders, are reviewed in J. M. Bennett et al., “Polarization,” in
Handbook of Optics,
W. G. Driscoll, Editor (McGraw-Hill, New York, 1978).
For the purpose of imaging circular polarization of light in ambient scenes, a retarder should have the following characteristics.
First, retarders should have achromatic response. Ambient light contains a range of wavelengths. However, quarter-wave retarders in the art are able to transform light with a phase shift of 90 degrees in only a very narrow range of wavelengths. For use in imaging circular polarization, a quarter-wave retarder, is needed which is achromatic over a wavelength range matching the wavelength range of the photodetector.
An infrared retarder, which in the art is considered to be achromatic, is described in U.S. Pat. No. 4,961,634 to Chipman et al. (1990). However, measurements that show this device is only approximately achromatic are reported in Sornsin et al. “Alignment and calibration of an infrared achromatic retarder using FTIR Mueller matrix spectropolarimetry,” in
Polarization: Measurement, Analysis, and Remote Sensing,
D. H. Goldstein et al., Eds, Proceedings of SPIE Vol. 3121, 28-34 (1997). These measurements show that over the wavelength range from 3 to 14 micrometers, the phase shift varies in a range from 74 to 98 degrees.
The device of Chipman et al. uses a combinations of two bulk crystals, cadmium sulfide (CdS) and cadmium selenide (CdSe). We use the term, bulk crystal, to refer to a macroscopic crystal as distinguished from a material formed as a thin film of microscopic thickness using methods related to the fabrication of integrated circuits.
An achromatic retarder which is a combination of two waveplates is described in M. G. Destriau et al., “Réalisation d'un quart d'onde quasi
Lauchman Layla
Stafira Michael P.
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