Note: Descriptions are shown in the official language in which they were submitted.
Applicants herein have made the discovery that any type of focusing
device in combination with a surface, exhibiting any degree of reflectivity
and positioned near the focal plane of the device, acts as a retroreflector.
A retroreflector is defined as a reflect~~r wherein incident rays or radiant
energy and reflected rays are parallel .:or any angle of incidence within the
field-of-view. A characteristic of a retroreflector is that the energy
impinging thereon is reflected in a very- narrow beam, herein referred to as
the retroreflected beam. This phenom<:non is termed retroreflection.
It is herein to be noted that the term radiant energy includes light
energy, radio frequency, microwave energy, acoustical energy, X-ray
energy, heat energy and any other type:. of energy which are part of the
energy spectrum and which are capable of being retroreflected by the device,
instrument or system sought to be detecaed.
One type of optical device which exhibits this phenomenon, and thus
is a particular type of retroreflector, i;~ a corner reflector consisting of.
three mutually perpendicular reflecting planes. However, this type of retro-
reflector is both difficult and expensive to fabricate.
Due to the applicants discovery , it has now become possible to
accomplish a great many feats heretofore considered impossible, as will
become more apparent from the discussion to follow hereinafter. In this
context it should be noted that the eyes of human beings , as well as those of
animals, operate as retroreflectors. Also, any optical instrument which
includes a focusing lens and a surface h<~.ving some degree of reflectivity,
no matter how small, positioned near the focal point of the lens, act as a
retroreflector, whereby any radiant energy from a radiant energy source
directed at these instruments is reflected back towards the source in a
substantially collimated narrow beam.
It is therefore the primary object of the present invention to provide
a method and apparatus for detecting objects exhibiting retroreflection
It is another object of the present invention to provide a method and
apparatus to detect objects having retrcreflection characteristics by illumi-
nating the same with a radiant energy s ~urce.
It is a more particular object of the present invention to provide a
method and apparatus for scanning an area to detect the presence of optical
instruments such as binoculars, telescopes, periscopes, range finders,
cameras, and the like.
It is a further object of the present invention to provide means and
apparatus for determining the characteristics of a device exhibiting retro-
reflection characteristics from a remote location.
It is a further object of the present invention to provide a method
and apparatus for detecting optical instruments for rendering the instruments
ineffective and for neutralizing humans utilizing said instruments by employ-
ing lasers or similar high energy sources.
It is yet another object of the poesent invention to provide a method
and apparatus for transmitting and receiving radiant energy utilizing con-
These and other objects, features and advantages of the present
invention will become more apparent from the following detailed discussion
considered in conjunction with the accorr..panying drawings , wherein:
Figure 1 is a diagram showing :~, retroreflection system consisting
of a lens and a reflector wherein the source radiation is parallel to the
axis of the lens.
Figure 2 is a diagram of a retrc~reflection system similar to that of
Figure 1, wherein the source radiation is not parallel to the optical axis of
the lens .
Figure 3 is a diagram of a retroreflection system similar to Figure 1
wherein the lens is imperfect so as to fo;-m an image rather than focusing at
a single point.
Figure 4 is a diagram of a retrareflection system wherein the
reflector is obliquely positioned with respect to the optical axis of the
Figure 5 is a diagram of a human eye, wherein there is depicted
the retroreflection characteristics thereof.
Figure 6 is a schematic representation depicting a beam splitting
optical system for transmitting and receiving radiant energy.
Figure 7 is a schematic representation depicting a concentric optical
system for transmitting and receiving :radiant energy.
Figure 7a is a schematic representation of another embodiment of
the concentric optical system depicted in Figure 7.
Figure 7b is a schematic representation of still another embodiment
of the concentric optical system depicted in Figure 7.
Figure 8 is a schematic representation depicting an ordinary tele-
scope as an image forming system having retroreflection characteristics.
Figure 9 is a schematic representation depicting one half of an
ordinary binocular as an image forming system having retroreflection.
Figure 10 is a schematic representation depicting an ordinary peri-
scope as an image system having retrooeflection characteristics.
Figure 11 is a schematic reprf~sentation depicting an ordinary
camera as an image forming system ha~;ring retroreflection characteristics.
Figure 12 depicts a system for scanning an area to detect the
presence of optical instruments by utili:,ing the retroreflection characteris-
tics thereof and for neutralizing observers using said optical instruments ,
and/or rendering the instruments ineffective.
Figure 13 is a diagram of a radar system, and more particularly
of a radar antenna which is to be detected in accordance with the principles
of the present invention.
Figure 14 depicts the waveforrr~s obtained during the detection of the
radar system shown in Figure 13.
Jn accordance with the general principles of the present invention an
optical system consistix~g of a focusing lens and a reflective surface
near the focal plane of said lens has radiant energy rays supplied thereto by
radiant energy transmitter. The radiant energy rays reflected by the optical
system due to its retroreflection characteristics are recovered by a radiant
energy receiver to thereby detect the Fresence and relative position of said
optical system. The output of the radi~~.nt energy receiver may be applied to
a utilization means for determining the characteristics of the retroreflE:ctor
or for directing a weapon means.
Referring now to the drawings and more particularly to FIG. 1
thereof, there is shown an optical syst<~m consisting of a lens 20 and a
reflective surface 22, which herein is m mirror, positioned in the focal plane
24 of the lens 20, Rays of radiation 26 and 28, respectively, are directed
towards the system, and more particul;~.rly towards the lens 20, from a
radiation source (not shown); the incident rays in the present illustration
parallel to the optical axis 30 of the lens. It is herein to be noted that for
purpose of clarity the incident rays are herein shown as being confined to the
top half of the lens 20. The incident rays 26 and 28 are refracted by the lens
and focused at the focal point 32 of the lens , which focal point lies on the
mirror 22. The rays are then reflected by the mirror so that the angle of
reflection equals the angle of incidence, and are returned to the lower half
of the lens where they are again refracted and emerge therefrom as retro-
reflected rays 26R and 28R. The rays .:6R and 28R are returned to the
radiation source parallel to the incident rays 26 and 28 thereof. However,
20 as shown in the drawing, the relative positions of the rays 26 and 28 are
inverted so that the image returned to tx.e radiation source is also inverted.
In the optical system depicted in FIG. 2, similar parts are denoted
by similar reference numerals, In this system the rays 34 and 36 are not
parallel to the optical axis 30A of both the lens 20A and the mirror 22A, the
mirror 22A being positioned in the focal plane 24A of the lens. The rays 34
and 36 are refracted by the lens 20A and focused at a point 37 removed from
the optical axis but still on the focal plar:e. The rays 34 and 36 are
by the mirror. Both of the rays 34 and :36 would normally emerge from the
lens as retroreflected rays 34R and 36R , after refraction by the lens , and
would be returned to the source of the rays 34 and 36 in a direction parallel
thereto. However, since the lens 20A is of finite size, the reflected ray 34R
will miss the lens and will not be retrore~.flected. The loss of reflected
in this manner is called "vignetting".
In the system depicted in FIG. 3 wherein similar parts are denoted
by similar reference numerals, the lens 20B is assumed to be imperfect;
i.e. , it has aberrations. In this case the rays 38 and 40 are parallel to the
optical axis 30B but are not focused at w single point on the focal plane 24B,
and instead form an image on the mirrcr 22B, which image is referred to as
the circle of confusion. In most practi<:al optical systems there are circles
of confusion and the mirror is normally positioned at the plane of least
of confusion, herein depicted by the reference numeral 42. Thus,_ the image
formed on the mirror by means of the rays 38 and 40 can be considered to be
a radiant source, and the retroreflected rays 38R and 40R exit from the lens
20B substantially parallel to each other., This is possible since each emerg-
ing ray can be paired with a parallel inc ident ray which radiates from a
common point of the image source located at the mirror 22B.
In the system depicted in FIG. 4, the reflecting surface or mirror
22C, and its axis 44, is tilted with respect to the optical axis 30C of lens
However , the ray 48 is again retrorefle ~ted by the system and the retrore-
flected ray 48R is returned parallel to tae incident ray 48. The
ray 46R, due to the ray 46, is lost beca~~se of vignetting.
The concept set forth herein above in conjunction with FIG. 3, that
the retroreflected rays be considered a:. radiating from a source on the image
plane, is highly significant. With this concept in mind, it will be readily
apparent that even if the retroreflecting surface is dispersive, curved, or
tilted, (as shown in FIG. 4), the system will still exhibit retroreflective
perties for any and all rays which are rEaurned to the lens by the reflecting
The rays retroreflected by the optical systems depicted in FIGS. 1
to 4 are in the form of a narrow, substantially collimated beam having a high
radiant flux density. It is to be noted that there is an actual increase in
radiant flux density of the retroreflected beam due to the narrowing thereof.
This increase in radiant flux density is herein termed optical gain.
For example, if the irradiance produced by the radiating source at
the collecting lens in FIG. 3 is 100 watts /cm2 and the area of the lens is
100 cm2, then the radiant flux at the image or focal plane (circle of
100 watts X 100 cm2 , or 104 watts.
It is a characteristic of a retroreflector to return the retroreflected
energy or rays in a very narrow beam. The dimensions of the retroreflected
beam is a function of the angular resolution of the retroreflector which
includes the lens and the reflecting surf~.ce.
The solid angle into which the incident radiant flux will be retro-
reflected is.determined by the square of the angular resolution of the retro
reflector. If, for example, the resolution of the optical system is 10'4
radians , the solid angle into which the retroreflected beam will be returned
is 10'8 steradians. One steradian being the solid angle subtended at the
center of a sphere by a portion of the surface of area equal to the square of
the radius of the sphere. Thus at a distance of 104 cm from the focal plane
the area of the retroreflected beam is or~ly 1. 0 cm2. The retroreflector , by
radiating into such a small solid angle, iaas radiant intensity of
10 watts , or 1012 watts /steradian.
10- steradian ;
In order to obtain a measure of the optical gain we must compare
the retroreflector to a standard or reference. This reference has been taken
to be a diffuse surface known in the art as a Lambertian radiator. If the
104 watts of incident radiant flux were simply re-radiated in a Lambertian
manner; i. e. , into a solid angle of 3. 14 (~) steradians , the radiant
104 watts , or 3. 1 x 103 watts/steradian.
3. 14 steradians
Thus , the retroreflector has an overall o~~tical gain equal to
1012watts /steradian ~ or 3. 14 x 108
3. 1 x 103watts /steradian
Although there is no actual increase in radiant flux, the retrore-
Elector has a radiant intensity which is 3. 14 x 108 greater than that of a
Lambertain radiator which emits the same radiant flux. Thus , for example ,
a telescope having a collecting area of 7.00 cm2 and an angular resolution of
0. 1 milliradian would appear similar in size to about 3.5 x 108 cm2 of a
diffuse or Lambertian radiator.
It should be noted that in almost all cases, the retroreflector will
be disposed within an environment that i~roduces background radiation in a
Lambertian manner. Thus, the radiant intensity of the retroreflector is so
much greater than that of a Lambertian radiator that it is easily discernible
from the background, even when, (as shown in Fig. 2) a large percentage of
the retroreflected radiant flux is lost due to vignetting.
It is herein to be noted that the radiant intensity of the retroreflected
beam is dependent upon the characteristics of the optical system employed.
If an optical system of the type shown in FIGS. 1, 2, and 4 were possible and
there were no loss of energy (power) entering the system, then the radiant
intensity gain would be almost infinite s:.nce the energy would be
in an almost perfectly collimated beam, i, e. a retroreflected beam whose
divergence angle is almost zero. Howe~rer , almost all optical systems re-
semble that shown in FIG . 3 and the factor which determined the divergence
angle of the retroreflected beam is the size of the circle of confusion and
particularly, the least circle of confusion. The size of the least circle of
confusion is dependent upon the resolution of the system and in particular
upon the resolution of the focusing lens. Thus, the less aberrations in the
lens, the better the resolution, the smaller the circle of least confusion,
smaller the divergence angle of the retroreflected beam, and thus the greater
the optical gain.
Referring to FIG. 5, there is shown a magnified cross-sectional
view of a human eye denoted generally b~r the reference numeral 50. The eye
includes a cornea 52, an anterior chamber 54, a lens 56, and a retina 58.
The retina has a small portion or point 60 thereon termed the yellow spot or
macula lutes, which is approximately 2 tnm in diameter. At the center of
the macula iutea is the fovea centralis 62 whose diameter is approximately
0.25 mm. The acuity of vision is greatest at the macula lutea and more
particularly at the fovea centralis. Thas, the eye is always rotated so that
the image being examined or the rays entering thereon fall on the fovea 62.
As seen in FIG. 5, rays 64 and 66 enter the eye and pass through the cornea
52 and the anterior chamber 54 and are refracted by the lens 56 and focused
on the fovea centralis portion 62 of the retina 58. The rays are then
passing through the lens 56, anterior chamber 54 and cornea 52 and emerge
therefrom as retroreflected rays 64R and 66R which are parallel to the :rays
64 and 66, Thus, it is seen that even the human eye acts as a retroreflector.
Referring now to FIG. 6, therf~ is shown an optical system for
transmitting and receiving radiant ener,~y, the more particularly a beam
splitter for transmitting radiant energy and for receiving or recovering a
portion of said radiant energy.
The beam splitter includes an optical bench 70 having an optical
system consisting of a lens 72 and a rotating pattern or reticle 74, which may
also be a modulator , said system being placed on said bench. The beam
splitter also includes a radiant energy source 76, a collimator 78, a thin
plate of glass 80 having a semi-reflecti~~e coating thereon, a detector 82.
In the operation of the beam splitter, the radiant energy from the source 76
is collimated to form a beam by the collimator 78 and the beam is directed
upon the glass plate 80, a portion of the energy in the beam being reflected
and a portion of the energy in the beam being transmitted by the glass plate.
The energy is then transmitted down the optical bench 70 where the lens
refracts the transmitted energy and focL.ses the beam upon the reticle 74
from whence it is retroreflected back to the glass plate. A portion of the
retroreflected energy passes through thcs glass plate and is lost, and a
thereof is reflected by the glass plate and detected by means of the detector
and the output thereof is then fed to the utilization means 83. The detector
82 i.s thus effectively positioned within o:r concentric with the
energy beam without affecting the transmission of radiant energy from the
source to the optical system. The ener~;y obtained by the utilization means
can be used to obtain the spectral and temporal characteristics of the retro-
reflected beam, and the same may then be compared with the transmitted
beam to determine various characteristics of the optical system being in-
vestigated. It will be apparent that the use of this test instrument makes
possible the investigation and characterization of optical systems in terms
of recording the retroreflective characteristics thereof.
The rotating pattern or reticle 74 can be replaced with a reflective
surface and a modulator placed on the light incident side of the lens 72. The
modulator can then be tilted so that none: of the light reflected from its
returns to the beam splitter 80 to be reflected to the detector 82. The only
light then returning to the detector 82 will be that modulated by the
and reflected back from the reflective s~zrface replacing the reticle.74.
Figure 7 depicts a folded concentric optical system for transmitting
and receiving radiant energy - also kno«n as an optical transceiver. The
optical transceiver 84 includes a primary mirror 86 having a substantially
parabolic shape, a secondary mirror 88 having a planar configuration, a
radiant energy source 90, a detector 92 and a utilization means 94. The
primary mirror has an aperture 96 concentric with its principal axis and the
principal axis of the secondary mirror is aligned so as to be coaxial
The light source and detector are also aligned with the mirrors so that all of
the aforesaid elements are concentrically disposed with respect to each other.
The light source is positioned adjacent to the nonreflecting surface of the
primary mirror while the detector is positioned adjacent to the nonreflecting
surface of the secondary mirror.
In the operation of the transcei~~er 84, rays 98 and 100 are emitted
by the radiant energy source 90, and im~~inge upon the secondary mirror 88,
from whence they are reflected and impinge upon the primary mirror 86.
The rays are then reflected by the primary mirror and directed towards an
optical instrument 102 which exhibits rei.roreflective characteristics. The
incident rays are retroreflected by the optical instrument 102 and are
as retroreflected rays 98R and 1008. T:ze rays 98R and 1008 return in a
direction parallel to the rays 98 and 100 and impinge upon the primary
- 10 -
mirror 86 and are reflected thereby to«vards the detector 92 where they are
detected, and the detector output signal is then fed to the utilization means
As discussed previously, the term optical instruments exhibiting
retroreflective characteristics include the eyes of animals and humans.
A second embodiment of a folded concentric optical transceiver is
shown in Figure 7a, wherein similar parts are denoted by similar reference
In this embodiment the light source 90A is positioned adjacent to
the nonreflecting surface of the secondary mirror 88A and the detector 92A
is positioned adjacent to the nonreflecting surface of the primary mirror 86A.
In the operation of the transceiver 84A, rays 104 and 106 are emitted
by the radiant energy source 90A, and inpinge upon the primary mirror 86A,
from whence they are reflected towards the optical instrument 102A. The
rays are retroreflected by the optical instrument and are returned as retro-
reflected rays 1048 and 1068. The ray,; 1048 and 1068 return in a direction
parallel to the rays 104 and 106 and impinge upon the primary mirror and
are reflected thereby towards the secondary mirror through the aperture 96A
to the detector 92A, and the output signal of the detector is then fed to the
utilization means 94A.
A third embodiment of a folded concentric optical transceiver is
depicted in Figure 7b, wherein similar Farts are denoted by similar reference
In this embodiment, the detector 92B is once more positioned adja-
cent to the nonreflecting surface of the secondary mirror 88B and the radiant
energy source 90B is positioned between the reflecting surfaces of the primary
mirror 86B and the secondary mirror 88B. There is also included a collector
108, which may be an elliptically shaped mirror for collecting the spurious
radiation rays from the source 90B and zeflecting back upon the source,
wherefrom they are directed upon the secondary mirror and ultimately
directed toward the optical instrument 102B.
In the operation of the transceiver 84B, energy from the radiant
energy source 90B impinges upon the secondary mirror 88B, and more
particularly rays 110 and 112 so impinge. These rays are reflected by the
secondary mirror towards the primary mirror, from where they are once
more reflected towards the optical instrument 102B. The incident rays 110
and 112 are then retroreflected by the optical instrument and returned as
retroreflected rays 1108 and 1128. The rays 1108 and 1128 return in a.
direction parallel to the rays 110 and 112 and impinge upon the primary
mirror and are reflected thereby towards the detector 92B where they are
detected and the output thereof is then f<~d to the utilization means 94B.
It is herein to be noted that although the folded optical transceivers
depicted in Figures 7 , 7a , and 7b have t>een shown as being concentric , :it
also possible to employ the above type of transceivers wherein their optical
characteristics are not concentric. However, it has been found from the
viewpoint of efficiency and efficacy that the concentric optical transceivers
are more desirable.
Figure 8 is an optical schematic representation of a telescope having
an objective lens 116, a reticle 118, a pair of erector lenses 120 and 122, a
field lens 124, and an eyelens 126.
Thus, when rays 128 and 129 a~~e directed towards the objective lens
116, they are focused on the reticle 118 and retroreflected thereby to produce
retroreflected rays 1288 and 1298 resp<:ctively, whose direction is opposite
and parallel to that of the incident rays :128 and 129. Thus , the combination
of the objective lens 116, and the reticle 118 form a retroreflective optical
instrument, in and of themselves.
It is herein to be noted that eve:z if the reticle 118 is merely plain
glas s , as in most cases it is , it still exhibits some degree of
which reflectivity gives rise to the retroreflected rays 1288 and 1298.
It is herein also to be noted that: incident rays passing through the
telescope to the eye of the observer are also retroreflected by the eye of the
observer. Thus , there is in effect, two retroreflective optical systems and
thus two retroreflective signals.
Figure 9 is an optical schemati~~ representation of one half of a
binocular and comprises an objective ler..s 132, a first porro prism 134, a
- 12 -
second porro prism 136, a reticle 138, a field lens 140, and an eyelens 142.
When a ray such as 144 is incident on the objective lens 132, it is focused
thereby on the reticle 138, after passing through the porro prisms 134 and
136. It is herein to be noted that although the ray 144 is directed along a
which is not straight; i, e. , there are several right angle bends therein,
entire path is still part of the focal path of the instrument. Thus , the ray
is focused on the reticle 138, causing tl!:.e same to be retroreflected as ray
1448 which then goes through a path similar to that of ray 144 and emerges
from the objective lens 132 in a direction which is opposite and parallel to
that of the incident ray 144. It is to be voted that the description herein
describing a single ray is for purposes ~~f simplicity of explanation.
Figure 10 is an optical schemai:ic representation of a periscope,.
The periscope includes a window 146, an objective prism 148, an objective
lens 149, an amici prism 150, an erecting prism assembly 152, a reticle 154,
a field lens 156, an eyelens 158, and a filter 160, An incident ray 162 enters
the periscope through the window 146, then passes through the prism 148,
objective lens 149, amici prism 150, and erecting prism assembly 152 to the
reticle 154 whereon the incident ray is reflected and emerges from the peri-
scope as retroreflected ray 1628 whose direction is opposite and parallel to
the incident ray 162. Again it is to be noted that the description above
describing a single ray is merely for the purpose of simplicity of
Figure 11 is an optical schematic representation of a camera. The
camera includes a lens 164, a shutter 166 , and film 168. In the operation of
the camera when a picture is taken the shutter opens and incident rays 170
and 171 are focused on the film 168 throL~gh an aperture 172 in the shutter,
means of the lens 164. These rays are then reflected by the film and emerge
from the lens as retroreflected rays 17071 and 1718.
It is to be noted that most, if not all, optical systems will have a
reflecting surface such as a reticle , a le ns , or a pr ims in the focal
and the incident radiation will be retrore:flected by any such surface.
Referring now to Figure 12, there is shown one embodiment of a
system for detecting the presence of an optical instrument, for tracking said
- 13 -
instrument, and for neutralizing observers utilizing said instrument and/or
rendering the instrument ineffective.
The system includes a scanner 180, including an optical searching
means 182, such a source of infrared light, a detector 184, and a laser 186.
It is herein to be noted that the search rzeans 182 and the detector 184 may
combined in the form of a transceiver as described hereinbefore in conjunc-
tion with Figures 7, 7a, and 7b. The scanner 182 is controlled by a scanning
and positioning means 188, which includes a servo motor (not shown). The
scanning and positioning means 188 is powered by a power and control means
190, which means also supplies power for the scanner 180, and a utilization
In the operation of the system, the scanner 180 is caused to scan a
preselected area by means of the scanning and positioning means 188, the
means 188 being programmed by the utilization system 192. The optical
searching means emits rays 194 and 195 , when these rays impinge upon an
optical instrument 196 exhibiting retrore~flective characteristics , as herein-
before described, they are retroreflected as retroreflected rays 1948 and
1958 respectively, and detected by the detector 184 and the detector output is
then fed to the utilization system 192. 7'he utilization system may be pra-
grammed to merely track the instrumen~: 196, in which case, this information
would be fed to the scanning and positioning means 188 and thence to the
scanner 180 causing it to track said inst:-ument. However, if it is desired to
neutralize the observer using the instrurnent, or to render the instrument
ineffective, then the utilization system 192 will feed a signal to the laser
activating the same and causing a high intensity laser beam to be directed at
the instrument, thereby accomplishing the aforementioned objects.
It is herein to be noted that although the present system has been
described as employing a laser, it is also possible to use any other high
energy system, weapon, or weapon system.
With the present system, it will be readily apparent to those skilled
in the art, that a hostile satellite orbitin;~ the earth and employing optical
surveillance equipment to monitor a cour~try's activities can be detected and
ifs surveillance capability destroyed.
- 14 -
It is herein again to be noted that the aberrations in almost all optical
instruments cause a small divergence cf the retroreflected rays, the amount
of said divergence being governed by the resolution of the retroreflector. As
a practical matter the angular resolution of optical systems such as
periscopes, telescopes, cameras, and ~~ptical systems carried by missiles will
be between about 10'3 and 10-5 radians ~c~hich produce retroreflected beams
of 10-6 to 10-10 steradians. At a ranges of 1 , 000 feet the area of these
would be 1.0 and 10-4 ft2 respectively. This divergence is so small so that
the retroreflected rays are substantialhT collimated.
It is herein to be noted that in microwave application corner reflec-
tors have been utilized for retroreflecting purposes. However, the present
invention enables the detection of microwave apparatus , such as antennas and
the like which do not have a corner reflf~ctor as an integral part thereof, by
utilizing the inherent retroreflection characteristics of the apparatus as
inbefore discussed. Thus , this apparatws and systems exhibiting the retro-
reflection phenomenon can be similarly detected by the use of radio frequency,
microwave, X-ray, acoustical or any similar types of energy directed thereat.
In many microwave antenna systems , microwave lenses are utilized
in place of reflectors for the purposes of obtaining wide angle scanning as
compared with the system bandwidth. These microwave lenses exhibit
characteristics which are equivalent to the optical lenses hereinbefore dis-
cussed, and thus a detailed explanation of the retroreflection of microwave
and similar types of energy by these lenses, in conjunction with a reflective
surface , will be readily apparent to those skilled in the art.
In this connection, Figure 13 is an illustration of a radar system
which is to be detected by means of the r etroreflection principles of the
present invention. The radar system is generally indicated by the reference
numeral 200 and includes a parabolic disk antenna 202 having a feed 204 whose
impedance mismatch is lowest at the operating frequency of the radar system
When the radar system 200 is iii an off condition, the resonant fre-
quency of the antenna feed 206 can be detected by beaming swept frequency
- 15 -
microwave energy at the system such as by utilizing a variable frequency
klystron (not shown) or the like.
The pulses produced by the klystron are indicated as 210 in the
waveforms shown in Figure 14. The wave energy 210 is retroreflected by
the parabolic disk antenna 202 because the parabola focuses the energy at
the feed horn which in turn is mismatched. Hence, the energy reflected from
it is recollimated by the parabola similar to the optical system described
heretofore. The energy is detected in a suitable manner and produces the
waveforms indicated at 212 in Figure 14, until such time that the frequency
of the klystron is equal to the operating frequency of the feed 206. When this
occurs, the energy beamed to the radar system is focused on the feed horn,
absorbed by the feed 206 and is therefore not retroreflected. This results
in the waveform indicated as 214 in Figure 14. The dip or drop in power
level indicates absorption of the beamed energy and thus the frequency of the
operation of the radar system is now known. By further analysis of the
retroreflected waves it is possible to obtain even more information concerning
the electrical and mechanical characteristics of the radar system 200, such
as the type of antenna system being utili:;ed, its scan angle, its beamwidth,
its gain, etc.
It will be apparent to those skilled in the art that if the antenna were
a sonar disk and acoustical energy were directed thereat, the acoustical
energy would be retroreflected and the r~aroreflected acoustical energy would
be capable of detection.
It is thus again reiterated that although only a few types of radiant
energy have herein been discussed, any type of energy which can be retra-
reflected may be employed.
While we have shown and described various embodiments of our
invention, there are many modifications, changes, and alterations which may
be made therein by a person skilled in th~~ art without departing from the
spirit and scope thereof as defined in the appended claims.