Optical: systems and elements – Lens
Reexamination Certificate
2000-02-01
2001-11-06
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
C359S479000, C359S725000, C359S637000, C359S554000, C359S815000, C359S753000, C244S003170
Reexamination Certificate
active
06313951
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 must 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 in 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 required 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 window having a curved outer surface and a curved inner surface, and an optical corrector adjacent to the curved inner surface of the window. The optical corrector has an optical corrector shape which is selected responsive to a shape of the window. The optical corrector has an inner surface and an outer surface, with at least one of the inner surface and the outer surface of the optical corrector having a shape described by a modified Zernike expression of the form
z
=
cr
2
1
+
1
-
(
1
+
k
)
c
2
r
2
+
∑
j
=
1
j
=
66
C
j
+
1
Z
j
where z is a coordinate oriented perpendicular to a reference plane, c is a constant vertex curvature, r
2
=x
2
+y
2,
x and y are Cartesian coordinates lying in the reference plane, k is a conic constant, C
j+1
is a constant coefficient for Z
j
, and Z
j
is the jth Zernike polynomial term. Preferably, both the inner surface and the outer surface of the optical corrector have the shape described by the modified Zernike expression. The optical system further includes an optical train positioned such that the optical corrector lies between the curved window and the optical train, wherein the optical train includes at least one optical element operable to alter an optical ray incident thereon, a movable optical train support upon which the optical train is mounted, and a sensor disposed to receive the optical ray passing sequentially through the window, the optical corrector, and the optical train. The optical train support preferably comprises a gimbal, and most preferably is a roll-nod gimbal.
The optical corrector, having a shape described by a modified Zernike polynomial surface, is well suited to correct the aberrations introduced into the optical ray by its passage through the window. The modified Zernike form is sufficiently flexible to define both shallow and deep surfaces. (A shallow surface has an axial component or depth which is relatively small compared to its maximum lateral extent, and a deep surface has a depth relatively large compared to its lateral extent.) When the Zernike polynomial surface is a relatively shallow surface, the optical corrector having the modified Zernike shape may be readily manufactured and tested. Other proposed types of correctors have deeper concavity, and are much more difficult to manufacture and test. Even though the modified Zernike-shape optical corrector may be made with a relatively shallow depth, it is still operable to correct aberrations introduced by elongated windows with a high fineness (aspect) ratio. The window may therefore be designed to introduce less drag into the aircraft, resulting in longer range and higher speed.
The window is preferably mounted in a housing having an axis of elongation. The optical corrector, which preferably comprises an arched strip of transparent material having an axial component extending along the axis of elongation and a radial component extending outwardly from the axis of elongation, may be rotatable about the axis of elongation. The optical corrector support and the optical train support may also be movable parallel to the axis of elongation, with each movement independent of the other.
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 Zernike optical corrector may be used in a multiple-corrector system, together with other Zernike optical correctors or optical correctors of other forms, as needed for particular window shapes. 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 ra
Ellis Scott
Knapp David
Manhart Paul K.
Sparrold Scott W.
Lester Evelyn A
Raytheon Company
LandOfFree
Optical system with zernike-shaped corrector does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical system with zernike-shaped corrector, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical system with zernike-shaped corrector will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2575038