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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains generally to correcting for viewing
aberrations produced by electromagnetic radiation passing through a
surface, and, more particularly, to correcting for refraction of radiation
that has passed through an axisymmetric surface.
2. Description of Related Art
There are many situations in which electromagnetic energy (e.g., infrared
light) must pass through a surface (e.g., window) before actuating other
apparatus. In the control of a missile, for example, infrared energy from
an external target on being sensed at the missile serves as a means for
identifying and locating the target to enable guiding the missile to the
target. Where the windows used have a substantial curvature to them
(so-called domes), this tends to produce a prohibitively large refraction
of the electromagnetic energy when the angle of radiation incidence varies
significantly from 90 degrees, giving the impression to the control
apparatus that the source of the energy is located at a different position
than where it is in actuality.
A known technique for solving this problem at the present time is to
construct the window of either a flat plate or a plate having a spherical
surface. In addition, there are other approaches in which a fixed
correction is built into an axisymmetric window according to an assumed
direction that target information can be expected to take which, of
course, means that a variable amount of error will have to be tolerated,
dependent upon how different the actual target direction is from the
assumed direction. The use of flat or spherical surfaces, however,
produces an undesirably high aerodynamic drag if they are allowed to be
exposed directly to the environment, so that if used will require a
removable shroud which is positioned over the window until quite late in
the missile trajectory when it is jettisoned. In the latter case, the
shroud precludes use of, say, an infrared sensor at long range, and the
jettison mechanism for removing the shroud involves additional weight, is
awkward and restricts operation somewhat.
OBJECTS AND SUMMARY OF THE PRESENT INVENTION
It is therefore a primary aim and object of the present invention to
provide means and method for reducing image distortion when radiation must
pass through an axisymmetric window.
A typical optical system of a guidance or tracking system includes a curved
reflector that focuses radiation from a target onto a secondary mirror and
which, in turn, redirects the radiation onto a sensor. The two reflectors
and sensor are gimbal mounted and servo driven to be centered on a
principal ray coming from the radiation source. The system then drives the
missile along the optical axis to the target. A transparent window is
located in front of the two mirrors and sensor for protection of the
enclosed apparatus.
Where the window between a source of radiation to be sensed (e.g., target)
and the sensor, for example, is formed outwardly convex for aerodynamic
purposes, incoming radiation that is angled off the normal to the window
surface will tend to be refracted angularly away from the line of sight.
Also, of course, the amount of refractive aberration will vary depending
upon the precise location of the source and the angle that it forms with
the window outer surface.
In accordance with a first embodiment of the invention, a domelike window
is provided having a generally convexly curved outer surface that is also
symmetric about an axis (i.e., axisymmetric), both for aerodynamic reasons
and, as well, in that this makes aberration correction less complex. The
inner surface of the window or dome is specially shaped so that the inner
surface area at a point of principal ray exit will be parallel to the
external surface area where the ray is incident. This special surface
arrangement establishes correction in the "pitch" plane or Y-Z' where Z'
is the new Z axis after roll has been taken into account. In this manner
the refraction of the ray upon leaving the inner surface of the dome is
lessened over that which it would have been subjected to in a dome of
normal construction having parallel inner and outer surfaces measured
along a normal through the dome material. This choice of coordinates also
permits the inner surface to be circular in cross-section.
It is also necessary to correct for refractive aberration in the "roll"
plane, i.e. the plane X'-Y where X' is the X axis shifted after roll is
accomplished. This is accomplished in accordance with the invention by
constructing the focusing reflector to be an electrically alterable
element such as a piezo-electric plate. More particularly, the
electrically alterable reflector includes a plurality of electrodes
located behind a piezo-electric reflector and by selective energization of
the electrodes the shape and curvature of the reflector are changed
causing a corresponding change in reflection of incident radiation. The
voltages to be applied to the various piezo-electric electrodes may be
obtained from an empirical table of voltages for the electrodes determined
for each pitch angle. Alternatively, the voltages may be obtained by first
applying a dither on each voltage and sensing the sharpness of the image
in the various regions of a scanned beam from which an adaptive scheme may
be implemented.
BRIEF DESCRIPTION OF THE DRAWING
In the accompanying drawing:
FIG. 1 is a sectional elevational partially fragmentary view of a
conventional curved dome enclosing a radiation sensor;
FIG. 2 is a sectional, fragmentary enlarged view of the dome material;
FIG. 3 is a side elevational, sectional partially fragmentary view of part
of a dome modified in accordance with the present invention to correct for
refractive aberration of light passing therethrough;
FIG. 4 shows the electrically variable reflector plate of the invention in
its unenergized stage and in one stage of energization; and
FIG. 5 shows the coordinate systems as applied to the radiation sensor
optical elements of FIG. 3.
DESCRIPTION OF A PREFERRED EMBODIMENT
With reference now to the drawings and particularly FIG. 1, there is shown
a radiation sensor 10 mounted on a gimbal 12 and which sensor is typically
controlled, when part of a guidance system, for example, in order to be
directed at a target 14 from which radiation 16 is being sensed or
detected. More particularly, a principal ray 16 coming from the target 14
reflects off a focusing reflector 18 onto a secondary reflector 20 and
then onto the sensor 10. By a servo drive system (not shown), the gimbal
mounted sensor is aimed directly at the target and remains so oriented.
For aerodynamic reasons, it is desirable to enclose the sensor, reflectors
and gimbal system within a transparent dome 22 having a specially curved
outer surface to reduce drag. However, radiation coming from the target 14
along the path 16 on passing through the dome wall is refracted a certain
amount dependent upon the angle of incidence and, as well, the material
from which the dome is made. Accordingly, the refracted radiation beam,
instead of traveling straight to the sensor along the line 16 of the
principal ray moves along a refracted line 24 which is angularly displaced
from the beam path 16. Of course, resetting the gimbal in order to point
the sensor directly at the target will then be subject to an error
depending upon the refraction or aberration of the beam passing through
the dome.
Because of this error, according to one technique in the past, radiation
windows have been made flat of a uniformly thick material which has the
undesirable result of being detrimental to the missile aerodynamics in
producing substantial drag. Also, where a flat window was used and it was
desired to improve aerodynamic properties, the window was covered with a
shroud having a curved outer surface prior to actual use of the window in
order to reduce the amount of aerodynamic drag. Such shrouds, however,
must be jettisoned before target sensing is initiated which is awkward and
poses a certain amount of restriction to normal operation.
Alternatively, spherical outer surfaces have been provided on domes
achieving some relief from the aerodynamic drag difficulty, but, in
actuality, only providing a partial correction of image distortion
resulting from refraction.
As shown best in FIG. 5, the coordinate system as applied to the radiation
sensor and dome of FIG. 2 is seen to consist of a conventional XYZ set of
orthogonal axes. The two planes of correction are essentially those in
which "roll" takes place exemplified by the angle .PSI. and in the plane
of so-called "pitch" exemplified by the angle .phi.. In each case, Y
represents the boresight of the sensor. More particularly, the inertial
coordinate system consists of X, Y, Z whereas the coordinate system for
the sensor is X', Y, Z'.
As will be more particularly described, the error correction means of the
present invention is different for roll than for pitch. However, error
correction in both the pitch and roll planes must be achieved in order to
result in satisfactory correction that would enable the use of a dome
having a surface other than a flat or spherical shape.
Returning to FIG. 1, a conventional curved dome 22 is seen to have a
cross-sectional geometry that is symmetrical to the dome longitudinal axis
and is constructed such that the outer and inner surfaces of the dome are
parallel to one another in the region of a normal line drawn through the
two surfaces (FIG. 2). However, when the principal ray 16 enters the dome
material at an angle substantially skewed from the normal, it will leave
the dome material at an internal surface 26 that is not parallel to the
outer surface beam-entrance area. This lack of surface parallelism between
the ray entrance and exit points on the dome is a major contributor to the
radiation beam aberration sought to be corrected by the present invention.
With reference now to FIG. 3, a corrected dome 28 to remove the aberration
and provide a correction in the pitch plane (Y, Z') is shown. The dome 28
has a smooth, curved, non-spherical outer surface designed primarily to
reduce aerodynamic drag. For reasons that will be explained later, it is
desirable that in cross-section the dome be axisymmetric (e.g., circular).
More particularly, image distortion or refraction correction is provided
by forming the interior surface 30 of the dome in such a way that the
surface area at which a ray is emitted into the interior of the dome
(e.g., area 31) is parallel to the surface area 33 at the point of
principal ray incidence on the exterior surface. That is, there is, in
effect, a shift of the inner surface with respect to the dome outer
surface from that encountered in the conventional domes. With this
modified dome interior surface, a principal ray passes into the dome
interior along a line path substantially parallel to the ray path that
would be followed by the principal ray if no dome were present. This
change permits the dome inner surface to be circular in cross-section
resulting in correction only having to be made for changes in the pitch
angle .phi..
Since the dome 28 is axisymmetric, all principal rays coming from any
forwardly located target at a given angular position about the dome
longitudinal axis will be corrected in the same manner as described and to
the same extent. For the ensuing description of that part of the invention
for correcting aberrations in the roll plane, reference is made
simultaneously to FIG. 4. As shown, the focusing reflector 18 is
constructed of a piezo-electric plate 32 confined within an edge rim 34
and having a concavely curved front surface provided with a conductive
reflective layer 36. On the back surface of the plate 32, there are
affixed a plurality of spaced apart electrodes 38. A selective switching
means 40 is interconnected with the electrodes 38, the reflective layer 36
and a voltage source 42.
In a way well known in the art, the switching means can be selectively
actuated to establish a voltage connection to any one or all of the
electrodes 38 to produce a corresponding change in the reflector
curvature. By selecting the correct electrode or electrodes a curvature
change can be obtained which will correct for an aberration in the roll
plane. Selection of the correct electrode or combination of electrodes can
be controlled by a digital processor 44 including stored correction values
obtained from earlier calibration or test runs.
Alternatively, the processor 44 can be programmed to "dither" energization
of the electrodes 38 and sensing sharpness of the image in different
regions of a given scene enable implementing an adaptive scheme.
Although the described embodiments have been referenced for use in a
missile guidance system, it is contemplated that the invention can be
advantageously employed in any number of different situations where
imaging is accomplished through a window whether or not the image is used
for control purposes.
Although the invention has been described in connection with a preferred
embodiment, it is to be understood that modifications may be made which
will be within the spirit of the invention and the ambit of the appended
claims. For example, an opto-refractive element could be used to achieve
the correction provided by the modified focusing reflector 30.
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