Optical: systems and elements – Lens
Reexamination Certificate
2000-02-01
2001-10-30
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
C359S479000, C359S725000, C359S637000, C359S554000, C359S815000, C359S753000, C244S003170, C244S003230, C250S203100, C250S203600, C250S236000
Reexamination Certificate
active
06310730
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an optical system having a window therein, and in particular to such an optical system having an optical corrector that reduces aberration introduced by the passage of an optical ray through the window.
An optical sensor receives radiated energy from a scene and converts it to an electrical signal. The electrical signal is provided to a display or further processed for pattern recognition or the like. Optical sensors are available in a variety of types and for wavelengths ranging from the ultraviolet, through the visible, and into the infrared. In some applications the optical sensors are fixed in orientation, and in others the optical sensors are movable by pivoting and/or rotational motions to allow sensing over a wide angular field of regard.
The optical sensors generally employ a photosensitive material that faces the scene and produces an electrical output responsive to the incident energy. The photosensitive material and remainder of the sensor structure are rather fragile, and are easily damaged by dirt, erosion, chemicals, or high air velocity. In service, the sensor is placed behind a window through which it views the scene and which protects the sensor from such external effects. The window must be transparent to the radiation of the operating wavelength of the sensor and resist attack from the external forces. The window must also permit the sensor to view the scene over the specified field of regard.
The window would ideally introduce no wavefront aberration at the center of the field of view, other than possibly spherical aberration, particularly if the sensor is an imaging sensor. The thicker and more highly curved is the window, the more likely is the introduction of significant wavefront aberration. A wide variety of sensor windows have been used in various aircraft applications. In many cases such as low-speed commercial helicopters, flat windows are acceptable. Windows that are shaped as segments of spheres are used in aircraft and missile applications, but for these windows the wavefront aberration tends to be high if the gimbal location is not at the spherical window's center of curvature. In all of these window types, if the window must be wide or must project a substantial distance into an airflow to permit a large field of regard, the aerodynamic drag introduced by the window is large.
For applications involving aircraft (including missiles) operating at high speeds, the window should be relatively aerodynamic such that the presence of the window extending into the airstream does not introduce unacceptably high and/or asymmetric aerodynamic drag to the vehicle. A nonspherical or conformal window is therefore beneficial to reducing drag and increasing the speed and range of the aircraft. However, available conformal windows introduce large wavefront aberrations into the sensor beam, particularly for high azimuthal pointing angles of the sensor.
The wavefront aberration may be corrected computationally, but the amount of processing may be great. To reduce the amount of computation or eliminate the need for computation, the wavefront aberration of the image may be minimized optically, either in the optical processing components or by providing a particular shape in the window. Available approaches have not been fully successful in accomplishing this type of correction. Accordingly, there is a need for an improved approach to providing a corrected image in an optical system viewing a scene through an aspheric window. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an optical system and a method for providing corrected optical images using the optical system. The optical system is used with many types of aspheric windows. It may be tailored to provide minimal wavefront aberration over a wide range of azimuthal pointing angles of the sensor of the optical system.
In accordance with the invention, an optical system comprises a curved window, an optical train including at least one optical element (such as a lens, a mirror, or a prism) operable to alter an optical ray incident thereon, and a movable optical train support upon which the optical train is mounted. The optical train support preferably includes a gimbal such as a roll-nod gimbal. The optical train support is operable to point the optical train along a plurality of directions, the plurality of directions including a z axis lying perpendicular to a reference plane having orthogonal x and y axes lying therein. An optical corrector is disposed in an optical path between the window and the at least one optical element of the optical train. The optical corrector has an optical corrector shape responsive to a shape of the window. The optical corrector comprises a transparent body having a shape which is bilaterally symmetric about the z axis in a yz cross section and is not bilaterally symmetric about the z axis in an xz cross section. The optical corrector may optionally be mounted on a movable optical corrector support. A sensor is disposed to receive the optical ray passing sequentially through the window, the optical corrector, and the optical train.
The optical corrector has an inner surface and an outer surface, and preferably at least one of the inner surface and the outer surface of the optical corrector has a shape defined by an asymmetric modified XY polynomial. The shape is desirably defined by
z
=
cr
2
1
+
1
-
(
1
+
k
)
c
2
r
2
+
∑
C
j
x
m
y
n
wherein z is the coordinate oriented perpendicular to the reference plane, c is a constant vertex curvature, k is a conic constant, r
2
=x
2
+y
2
, x and y are the coordinates lying in the reference plane, C
j
is a constant term, m and n are constants, and j=[(m+n)
2
+m+3n]/2+1.
The optical corrector may instead be described as a scoop-shaped piece of transparent material having a curvature different from a curvature of the window.
In a preferred case, the window is mounted to a housing, such as the fuselage of an aircraft, having an axis of elongation coincident with the z axis. Where there is a movable optical corrector support upon which the optical corrector is mounted, the optical corrector support may be movable in a direction parallel to the axis of elongation and/or rotatable about the axis of elongation. Similarly, the optical train support may be movable in a direction parallel to the axis of elongation and/or rotatable about the axis of elongation.
The optical system thus includes the aspheric window, which introduces an aberration into the optical ray that is dependent upon the pointing angle of the sensor through the window, and the optical corrector, which partially or totally negates the aberration. The optical corrector functions as a corrective lens whose position may optionally be rotated about the axis of elongation and/or moved parallel to the axis of elongation. The position of the optical train may also optionally be adjusted along the axis of elongation. These optical components and their adjustability serve to reduce the aberration introduced by the passage of the optical ray through the window.
The asymmetric optical corrector achieves increased optical performance as compared with a symmetric optical corrector, by permitting more terms in the relation defining the shape of the surfaces of the optical corrector. This increased performance is achieved at the cost of the loss of symmetry about the z axis, and the associated need for more support movement in most situations.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
REFERENCES:
patent: 3841585 (1974-10-01), Evers-Euterneck
patent: 4010365 (1977-03-01), Meyer
Knapp David
Sparrold Scott W.
Lester Evelyn A
Raytheon Company
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