Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1998-11-05
2001-01-30
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C359S642000, C244S003170
Reexamination Certificate
active
06180938
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an optical system having a window therein, and in particular to such an optical system used in an aircraft or missile wherein the window is a conformal 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. Optical sensors are used in a variety of commercial and military applications. In some applications the optical sensors are fixed in orientation, and in others the optical sensor is movable such as by a pivoting motion to allow sensing over a wide angular range.
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 center of the window. 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 and 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 conformal window is therefore beneficial to reducing drag and increasing the range of the aircraft. Some existing conformal windows introduce large wavefront aberrations into the sensor beam, particularly for high azimuthal pointing angles of the sensor.
An important consideration in achieving acceptable cost of the optical system is that the conformal window must be easily tested for its accuracy of shape, and must also be readily aligned upon mounting in the flight vehicle. The more complex the shape of the conformal window, the greater the challenge in testing and alignment.
There is a need for an improved window to be used in conformal window applications in high-speed missiles and aircraft. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an optical system including a window whose shape is selected to be conformal for aerodynamic purposes and capable of optimization to achieve excellent optical properties. The window is designed to a preselected nominal shape, and the actual fabricated shape is readily determined and compared to the nominal shape to assess whether the actual window is within specified manufacturing tolerances and also whether any inaccuracies may be compensated for with optical compensation systems.
In accordance with the invention, an optical system comprises a window made of a curved piece of a transparent material having an inner surface and an outer surface. The inner surface has a nominal inner surface conicoidal shape whose shape is defined by a first conic sag relationship. The first conic sag relationship may preferably be expressed in the mathematical form
z=c&rgr;
2
/(1+(1−(1
+k
)
c
2
&rgr;
2
)
½
,
where z is the distance along an axis of symmetry of the surface, &rgr; is the distance from the centerline to the surface, and k and c are constants. Other equivalent expressions for a conicoidal shape may be used to describe the shape of the inner surface.
The outer surface has a nominal outer surface shape of a general aspheric form, but which may for many useful cases be defined as a second conic sag relationship modified by at least one aspheric term. The second conic sag relationship, which may be modified by at least one aspheric term, is preferably expressed in the mathematical form
z′=c′&rgr;′
2
/(1+(1
+k′
)
c′
2
&rgr;′
2
)
½
+Ap ′
4
+Bp ′
6
+Cp ′
8
+Dp ′
10
,
where z′ is the distance along an axis of symmetry of the surface, &rgr; is the distance from the centerline to the surface, and k′, c′, A, B, C, and D are constants. Many other mathematic relationships may used to express a general aspheric shape. For the present purposes, such other general aspheric mathematical forms are equivalent to those expressed herein.
Far less desirably, the outer surface may be defined by a first conic sag relationship and the inner surface may be defined by a second conic sag relationship modified by at least one aspheric term. This approach would, however, negate some of the testing and alignment advantages discussed subsequently.
One surface of the window, preferably the inner surface, is therefore necessarily conicoidal to facilitate the testing and alignment described subsequently. The other surface of the window, preferably the outer surface, is selected to have another shape which, in combination with the conicoidal surface of the window, will impart to the window the desired net refraction as part of the optical system. That is, the selection of the one surface as conicoidal is a key to the invention in order to facilitate testing and alignment, and the shape of the other surface is selected in conjunction with the shape of the conicoidal surface to achieve the desired optical performance.
The optical system preferably includes a sensor sensitive to energy of an operating wavelength. The sensor is positioned interiorly to the window, that is, closer to the inner surface of the window than to the outer surface. The transparent material is transparent to energy of the operating wavelength. There is typically in addition an optical train positioned between the inner surface of the window and the sensor to direct the optical beam onto the sensor.
The window is designed so that the nominal inner surface shape is conicoidal in form to facilitate testing and subsequent alignment of the window in an aircraft or other structure. The fact that the conicoidal shape has two focal points, an adjacent focus close to the inner surface and a remote focus further from the inner surface, is used in the testing and alignment. The testing is required because, even though the nominal inner surface shape is designed to a particular nominal relationship, manufacturing operations usually result in some variations in the shape from the idealized nominal shape that is desired. To assess these variations and determine whether they are within acceptable tolerances, the window is conveniently tested by passing a test beam of a two-beam interferometer through the remote focus, reflecting the beam from the inner surface toward the adjacent focus, reflecting the beam from a spherical mirror at the adjacent focus back along generally the same ray path (but which may not be perfectly the same ray path due to defe
Crowther Blake G.
McKenney Dean B.
Mills James B.
Sparrold Scott W.
Collins David W.
Le Que T.
Lenzen, Jr. Glenn H.
Luu Thanh X.
Raytheon Company
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