Optical: systems and elements – Signal reflector – 3-corner retroreflective
Reexamination Certificate
2001-12-12
2004-06-01
Dunn, Drew (Department: 2872)
Optical: systems and elements
Signal reflector
3-corner retroreflective
C342S007000
Reexamination Certificate
active
06742903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Retro reflectors for electromagnetic waves, acoustic waves, and other wave phenomena have been known since the Fessenden patent (1,384,014) in 1921. These devices reflect the wave energy preferentially in the direction back toward the source. This creates a much stronger reflection of energy back toward the source than would occur if the reflection were to be equally strong in all directions.
2. Description of the Related Art
A retro reflector is a device which generally produces a strong return in the direction of the source, while the strength of this return may depend on the orientation of the reflector. The Luneberg Lens is completely insensitive to the direction of incidence of the waves. This is often a desirable property. However, the Luneberg Lens is unsuitable for many applications for reasons such as cost and weight. Cost and weight are also a limitation for the Van Atta Array, which generally is sensitive to the direction of incidence.
An alternative which is often cheaper to build and lighter in weight is a corner reflector consisting of two or three substantially perpendicular surfaces, similar to those used in the Fessenden patent. Retro reflectors may also consist of an array of corner reflectors, each oriented differently. These retro reflectors have received much attention. They originally were used primarily for radar and more recently have become especially important also for light such as from lasers. They are also useful for sonar. However, in prior art they generally have had a significant limitation. While they give a strong return towards the source over many or most incident directions of the waves, they generally give a weak return over some ranges of incident directions. This can be a significant limitation. For example, small boats often use radar reflectors so that they can be seen on a large ship's radar in foggy conditions. This is an aid in preventing collisions. However, if there are certain directions where the backscattered return is weak, then for ships approaching from these directions they might not be seen from a distance and a collision might occur.
The importance of a uniformly strong backscattered return for all (or for a large range of) incidence angles has been widely recognized. Boating magazines and the sales literature for radar reflectors for small boats have often included graphs of this strength for several commercially available radar reflectors from different manufacturers. One set of test results that is often quoted is the Admiralty Surface Weapons Establishment tests from England.
Test results have shown again and again that a corner formed by three mutually perpendicular reflecting surfaces gives an effective retro reflector. It produces a very strong return for all incidence angles which look into the corner and which see a significant area for all three surfaces. However, there are some incidence directions which see the interior of a corner, while this incidence direction is also nearly parallel to one of the three surfaces. That is, for these directions the projected area of that one surface (projected onto a plane perpendicular to the incidence direction) is very small. In this case, when looked at from the source, one of the surfaces of the retro reflector is viewed nearly end on. For these directions, the retro return may not be strong. This results in a serious limitation in prior art.
Often, an array of retro reflectors is formed by three perpendicular (or nearly perpendicular) intersecting reflecting surfaces, where each surface continues past the line of intersection. This produces eight interior corners, each of which functions as a retro reflector for some angles of incidence. We will call this the standard array of corner reflectors.
FIG. 1
gives an example of the standard array of corner reflectors. Four of the interior corners are visible in FIG.
1
and there are four more corners which are not visible in this view.
The standard array of corner reflectors has a significant limitation. For waves incident in a direction nearly parallel to any one of its surfaces, the retro return may be small. For some applications using the standard array, the directions of incidence that are important are all substantially horizontal directions. For example, for a boat in conditions of limited visibility it is important to appear on the radar of other boat and ships. When the standard array is used, it may be oriented in different ways. The orientation has a significant affect on the strength of the retro-return. The orientation with one surface horizontal and two surfaces vertical will be called the “spill water” orientation. The orientation with one corner pointed upwards will be called the “hold water” position. The hold water orientation is illustrated in FIG.
1
. In this orientation, one might think of the upper corner as holding water that is poured into it. This language will be used even though the surfaces of the retro reflector might not contain water. For example, they may have large holes or they may be made from a mesh or a porous material. The hold water orientation is often considered as the preferred orientation. For incidence from substantially horizontal directions, it is less likely that a direction of an incident source will be nearly parallel to one of its surfaces than for the spill water orientation. However, for some horizontal directions this does still occur.
The importance of producing a large retro return in all directions has led to attempts to “finesse” the problems that a non omnidirectional retro reflector creates. For example, a retro reflector can be made to spin. Two examples of this are given in the Norwood Patent (#2,746,035) and the Matson patent (#2,702,900).
There have been many attempts at making the return of a retro reflector more even as a function of the incident direction. Taking just one example of many, the Aw Patent (#5,097,265) describes an array of twenty corner reflectors (twenty interior corners). It is possible to arrange an array of reflectors by the “Aw” method or by many other methods so that for all angles of incidence there is at least one corner reflector such that its interior and all three of its interior surfaces can be seen. However, this is not sufficient to ensure a strong retro return for all angles of incidence. Wave phenomena involve interference effects. The return from two or more corner reflectors may interfere constructively or destructively. Destructive interference can cause two returns to cancel or to nearly cancel each other, resulting in a very weak total return.
One might also attempt to make the return more even as a function of incidence direction by use a large number (in some cases as large a number as twenty or more) of corner reflectors all oriented differently. Certain theorems in statistics, related to the central limit theorem, then suggest the properties that result. It is unlikely that destructive interference would give a very weak return for any angle of incidence. However, this method generally produces a somewhat uniform return for all angles, which is uniformly moderate, not strong and not weak.
The amount of energy returned to the source by a retro reflector depends on many factors, such as the design and orientation of the reflector, the distance to the source, and the strength of the source. In some cases polarization also matters. If all of these factors are kept constant, then the energy returned will increase as a larger retro reflector is used. The amount of energy that a retro reflector intercepts will be proportional to the square of its linear dimension. However, for a well designed retro reflector the amount of energy returned to the source will be approximately proportional to the fourth power of its linear dimension. The reason for this is that larger retro reflectors direct the reflected energy into a narrower beam. This beam is narrower in both directions transverse to the direction of propagation. Thus, for larger retro reflectors more ene
Dunn Drew
Pritchett Joshua L
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