U.S. patent number 3,833,285 [Application Number 05/362,653] was granted by the patent office on 1974-09-03 for retrodirective reflector visible over wide range of observation angles.
This patent grant is currently assigned to Amerace Esna Corporation. Invention is credited to Sidney A. Heenan.
United States Patent |
3,833,285 |
Heenan |
September 3, 1974 |
RETRODIRECTIVE REFLECTOR VISIBLE OVER WIDE RANGE OF OBSERVATION
ANGLES
Abstract
The reflector is constructed of transparent material and has a
plurality of reflector elements at the rear and a light-receiving
face at the front. Each reflector element has three faces
intersecting at three edges, three dihedral angles being
respectively defined by the intersection of adjacent faces. Two of
the dihedral angles of all of the reflector elements are
substantially 90.degree.. The third dihedral angle of at least some
of the reflector elements is substantially greater than the angle
of the other two dihedral angles, so that light reflected by the
reflector is diverged into an elongated pattern.
Inventors: |
Heenan; Sidney A. (Park Ridge,
IL) |
Assignee: |
Amerace Esna Corporation (New
York, NY)
|
Family
ID: |
23426988 |
Appl.
No.: |
05/362,653 |
Filed: |
May 22, 1973 |
Current U.S.
Class: |
359/551 |
Current CPC
Class: |
G02B
5/124 (20130101); G02B 5/122 (20130101) |
Current International
Class: |
G02B
5/12 (20060101); G02B 5/122 (20060101); G02B
5/124 (20060101); G02b 005/12 () |
Field of
Search: |
;350/97-109
;404/9-16 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wibert; Ronald L.
Assistant Examiner: Tokar; Michael J.
Attorney, Agent or Firm: Prangley, Dithmar, Vogel, Sandler
& Stotland
Claims
What is claimed is:
1. A retrodirective reflector for retrodirectively reflecting light
in an elongated pattern, said reflector comprising a body of
transparent material having a light-receiving front face, and a
plurality of retrodirective reflector elements at the rear of said
body, each of said reflector elements having first and second and
third faces intersecting to form first and second and third
dihedral angles, the edges respectively defined by said first
dihedral angles lying in parallel first planes, said second and
third dihedral angles of each reflector element being substantially
90.degree., said first dihedral angle of at least some of said
reflector elements being substantially greater than the associated
second and third dihedral angles, whereby the light reflected by
said reflector is diverged to a greater extent in planes
perpendicular to said first planes than in planes parallel to said
first planes.
2. The retrodirective reflector set forth in claim 1, wherein said
first dihedral angle of others of said reflector elements is
substantially 90.degree..
3. The retrodirective reflector set forth in claim 1, wherein said
first dihedral angle of said some reflector elements is about
90.degree.30'.
4. The retrodirective reflector set forth in claim 1, wherein said
first dihedral angle of said some reflector elements is such as to
cause the light reflected thereby to have a peak specific intensity
at an angle of about 1.3.degree. in planes perpendicular to said
first planes.
5. The retrodirective reflector set forth in claim 1, wherein said
faces have cylindrical formations thereon to modify the amount of
light spread furnished by said reflector.
Description
BACKGROUND OF THE INVENTION
A cube-corner retrodirective reflector operates to reflect incident
light substantially back to the source of the light. Theoretically
a beam emanating from the source and striking such a cube-corner
reflector will travel back toward the source essentially along the
path of the incident light. If such an "ideal" reflector were
mounted on a roadway to be impinged by light emanating from a
vehicle head lamp, the reflected light would be directed
substantially back to the head lamp. The reflector would appear
dark to the driver of the vehicle, since no light would be directed
to his eyes.
However, a cube-corner reflector does not have such perfect
characteristics but rather, the reflected light takes the form of a
narrow cone. This conical pattern is due to inaccuracies in the
reflector elements, particularly curvature in the cube-corner faces
thereof. The cone is defined by an angle of divergence (angle
between cone element and cone axis) at any point within which the
specific intensity of the reflected light exceeds a selected
value.
The observation angle is defined as the angle between a viewer's
line of sight to the reflector and a line from the source to the
reflector. In certain instances, it is necessary that a cube-corner
reflector reflect more light at substantial observation angles,
such as 1.5.degree.. Adding curvature to the cube-corner faces to
increase the divergence angle to 1.5.degree. is not satisfactory,
since the intensity of light at observation angles of between
0.degree. and 0.5.degree. would be much too low. One solution has
been to place prismatic elements or cylindrical surfaces on the
front surface of the reflector, which are respectively aligned with
selected ones of the cube-corner elements on the back surface.
These prismatic elements serve to change the axis along which a
peak response is achieved, that is, the nominal divergence axis,
from 0.degree. to another value, such as 1.3.degree.. One
disadvantage in this approach is that precise registry between the
prismatic element and its associated cube-corner reflector element
is necessary, but is difficult to achieve. The use of cylindrical
surfaces modifies the divergence angle, with the attendant
disadvantage above noted.
SUMMARY OF THE INVENTION
It is, therefore, an important object of the present invention to
provide a retrodirective reflector which has high reflectivity at
greater-than-usual observation angles.
Another object is to provide a retrodirective reflector which has
high reflectivity at greater-than-usual observation angles but does
not require the use of prismatic formations or the like on the
front surface.
Still another object is to provide a reflector which has good
response at small observation angles, specifically between
0.degree. and 0.5.degree., yet has an improved response at greater
observation angles, such as 1.5.degree..
Yet another object is to provide a reflector in which the reflector
elements are constructed to have nominal divergence axes at
different angles, that is, the nominal divergence axes of some
reflector elements are arranged at an angle of 0.degree., the
nominal divergence axes of other reflector elements are arranged at
another angle, say 1.3.degree., and perhaps the nominal divergence
axes of still other reflector elements are arranged at other
angles.
In summary, there is provided a retrodirective reflector for
retrodirectively reflecting light in an elongated pattern, the
reflector comprising a body of transparent material having a
light-receiving front face, and a plurality of retrodirective
reflector elements at the rear of the body, each of the reflector
elements having first and second and third faces intersecting to
form first and second and third dihedral angles, the edges
respectively defined by the first dihedral angles lying in parallel
first planes, the second and third dihedral angles of each
reflector element being substantially 90.degree., the first
dihedral angle of at least some of the reflector elements being
substantially greater the associated second and third dihedral
angles, whereby light reflected by the reflector is diverged to a
greater extent in planes perpendicular to the first plane than in
planes parallel to the first plane.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of the rear surface of a reflector
incorporating the features of the present invention, some of the
reflector elements being shaded to denote those having nominal
divergence axes at angles other than substantially 0.degree.;
FIG. 2 is a schematic view of the reflector of FIG. 1, being
impinged by incident light and illustrating the manner in which the
reflected light strikes a receiving member, such as a sheet of
film;
FIG. 3 is an enlarged fragmentary view of a portion of the rear
surface of the reflector of FIG. 1 on an enlarged scale and showing
a group of the reflector elements;
FIG. 4 is a greatly enlarged view of the reflector element in the
circle marked 4 of FIG. 3;
FIG. 5 is a view in cross section, taken along the line 5--5 of
FIG. 4;
FIG. 6 is a view in cross section, taken along the line 6--6 of
FIG. 4;
FIG. 7 is a view in cross section, taken along the line 7--7 of
FIG. 6;
FIG. 8 depicts a pattern produced by light reflected from a
"standard" reflector element;
FIG. 9 depicts a pattern produced by light reflected from a
"unique" reflector element incorporating one of the features of the
present invention;
FIG. 10 depicts a curve plotting specific intensity against
observation angle for the reflector of FIG. 1; and
FIG. 11 is an exploded view of a portion of the curve of FIG.
10.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to the drawings and more particularly to FIG. 1
thereof, there is shown a reflector 20 incorporating therein the
features of the present invention. The reflector 20 comprises a
body 21 of transparent material formed of a synthetic organic
plastic resin, the preferred resin being methyl methacrylate. The
body 21 has a smooth front face 22 which is also flat in the
embodiment shown. The body 21 is provided with a configurated rear
23 schematically shown in FIG. 1. As will be described presently,
the configurated rear 23 is made up of a multiplicity of
retrodirective reflector elements 40 (represented by shaded and
unshaded squares) which serve to return the incoming ray back
toward the source. In the specific form illustrated, the reflector
20 has 320 unshaded reflector elements 40 (hereinafter
characterized as "standard" reflector elements) and 30 shaded
reflector elements 40 (hereinafter characterized as "unique"
reflector elements).
Turning to FIG. 2, the manner in which the reflector 20 operates
will be described. There is schematically depicted a source of
light 30 which emits a ray 31. The ray 31 passes through a hole 33
in a sheet of film 32. The ray 31 passes through the front face 22
of the reflector 20, through its body 21 to strike the configurated
rear 23. Because of imperfections in the reflector 20, particularly
in the flatness of the faces which make up the reflector elements,
some of the light, as represented by the rays 31a and 31b,
diverges. Thus, the returning light beam is in the form of a cone.
The cone is defined by an angle of divergence (angle between an
element of the cone and its axis) at any point within which the
specific intensity of the reflected light exceeds a selected value.
Depending upon the quality of the reflector, the angle of
divergence will vary.
Reference is now made to FIGS. 3-6 which illustrate the details of
each of the reflector elements that make up the configurated rear
23 of the reflector. The reflector element is designated by the
number 40 and includes three faces 41, 42, and 43 which intersect
along edges 44, 45, and 46. The faces 41, 42, 43 are inclined away
from a common peak or apex. 47. The reflector element 40 has an
axis 48.
Each reflector element 40 is square, although the outline could be
rectangular, hexagonal, etc. Each reflector element 40 has one side
50 which is recilinear and is contained by the face 41. The end of
the edge 44 divides a second side 51 into shorter and longer
portions; the edge 45 intersects a third side 52 and divides it
into portions of equal length; and the edge 46 intersects a fourth
side 53 and divides it into longer and shorter portions. In each
reflector element 40, the angle between the faces 41 and 42 is
substantially 90.degree.; similarly, in each reflector element 40,
the angle between the faces 41 and 43 is also substantially
90.degree.. Finally, most of the reflector elements 40 also have an
angle of substantially 90.degree. between the faces 42 and 43,
which are the "standard" reflector elements 40 that are unshaded in
FIG. 1. An angle can exceed 90.degree. by as much as 6' or 7' and
still be "substantially" 90.degree..
The "unique" reflector elements 40 (those shaded in FIG. 1) have an
angle between the faces 42 and 43 substantially greater than the
angle between the faces 41 and 42 and the angle between the faces
41 and 43; for example, the angle between the faces 42 and 43 can
be 90.degree.30'. The angle between the faces 41 and 42 and the
angle between the faces 41 and 43 remains substantially 90.degree..
In either event, because the edge 45 is and remains substantially
perpendicular to the face 41, the above-described swivel of the
face 42 does not affect the angle it forms with the face 41.
Referring specifically to FIG. 7, the angle 55 between the faces 42
and 43 is substantially greater than the angles between the faces
41-42 and 41-43. The phantom line marked 56 represents an end view
of a plane passing through the edge 45 and the element axis 48. The
face 42 forms an angle 57 with the plane 56; and the face 43 forms
an angle 58 with the plane 56. If the angle 55 is 90.degree.30',
the angles 57 and 58 may each be 45.degree.15', in which case both
faces 42 and 43 would have been swiveled in opposite directions
about the edge 45. Alternatively, one of the faces, for example,
43, may remain fixed, so that it forms an angle of 45.degree. with
respect to the plane 56 and the other face 42 is swiveled about the
edge 45 to furnish the desired angle. If the angle 55 was
90.degree.30', the angle 57 would be selected to be
45.degree.30'.
Reference is now made to FIGS. 8 and 9 to describe the manner in
which the reflected light pattern is affected by changing the angle
55. FIG. 8 illustrates a piece of film 32 which has been exposed to
a light beam reflected by a "standard" reflector element 40. An
irregular area 60 centered about the hole 33 represents light
reflected from the reflector 20. Since the light diverges, as
exemplified by the rays 31a and 31b in FIG. 2, the light will not
be concentrated at the center of the hole 33, but, rather, will
have some finite size. The intensity at the center of the returning
beam will have a maximum value; the farther from the center, the
lower the intensity of the beam. Accordingly, the size of the area
60 will be determined by the time the film 32 is exposed. Because
the angles between the faces 41-42, 42-43, and 41-43 are
substantially 90.degree., the "nominal divergence axis" along which
the intensity of the returning light beam is greatest, is at an
angle of substantially 0.degree.. The light reflected by a
"standard" element is in the pattern of a narrow, cone having a
given angle of divergence.
On the other hand, for the "unique" reflector elements 40 in which
the angle 55 between the faces 42 and 43 is, for example
90.degree.30' (the angles between the faces 41-42 and 41-43 are
substantially at 90.degree.), the pattern illustrated in FIG. 9
results. The exposed areas are displaced in a direction normal to
planes containing the edges 45 and the element axes 48. If these
planes are horizontal, which is the orientation illustrated in FIG.
3, then the displacement takes place vertically. The reflected
light will cause an area 61 displaced upwardly a distance 62 from
the center of the hole 33. Similarly, the reflected light will
cause an area 63 displaced downwardly from the center of the hole
33 a distance 64. By measuring the distances 62 and 64 and knowing
the distance the film 32 is from the reflector 20, the angle of the
nominal divergence axis can be calculated. With the angle between
the faces 42 and 43 about 90.degree.30', the angle of the nominal
divergence axis in each direction is about 1.3.degree.. It is at
this angle (up and down) where the peak light response is achieved.
In much the same way as the irregular area 60 in FIG. 8 represented
varying intensities of the beam about a nominal divergence area at
0.degree., the areas 61 and 63 represent varying intensities of the
beam about a nominal divergence axis at an angle of
1.3.degree..
Turning now to FIG. 10, further details of the above-described
operation will be discribed. FIG. 10 depicts a curve, plotting
specific intensity, measured in candle power per foot candle of
incident light, against observation angle for the reflector 20. The
reflector 20 furnished 130 candle power per foot candle of incident
light at an observation angle of 0.degree. (line of sight aligned
with nominal divergence axis of "standard" reflector elements 40).
The specific intensity decreased to 20 at an observation angle of
0.5.degree.. Thus, the nominal divergence axis of the 320
"standard" reflector elements 40 in FIG. 1 is at an angle of
0.degree..
The presence of the "unique" reflector elements 40 having one
90.degree.30' angle provides a second peak in specific intensity at
1.3.degree.. FIG. 11 is a vertically expanded version of the curve
of FIG. 10 in the region of observation angles from 0.7.degree. to
1.5.degree.. In other words, the nominal divergence axes of the
"unique" reflector elements 40 are at an angle of 1.3.degree.. The
reflected light is in the form of cones centered about such axes.
The value of the specific intensity at 1.3 is dependent on the
number of "unique" reflector elements. Thus, if the number of
"unique" reflector elements were increased, the specific intensity
at 1.3.degree. would increase. Assuming a fixed area accommodating
350 reflector elements 40 in all, increasing the number of "unique"
elements by 30 would result in a decrease by 30 of the number of
"standard" reflector elements. The latter would result in a
decrease in specific intensity of about 10% at the lower
observation angles.
The reflector 20 thus has a peak response at about 0.degree., due
to the "standard" reflector elements and another peak response at
1.3.degree., due to the "unique" reflector elements.
The fact that only the angle 55 between the faces 42 and 43 is
increased is significant in that the spread or divergence of the
light beam takes place in only one direction (in planes
perpendicular to the planes containing the edges 45).
The combination of the "standard" and "unique" reflector elements
results in an elongated light pattern, the top and bottom of which
is defined by the exposed areas illustrated in FIG. 9. The central
portion of this pattern will have an area corresponding to the
darkened area in FIG. 8. It should be appreciated that, without the
presence of the "unique" reflector elements, the value of specific
intensity at a divergence angle of 1.5.degree. would be
substantially less than the value of two candle power per foot
candle incident light indicated on the graph of FIG. 11.
It is to be understood that the total number of reflector elements
40 in the reflector 20, and the ratio in the number of "standard"
elements to the number of "unique" elements control the values of
specific intensity. The curves illustrated in FIG. 10 and 11 are
merely exemplary. Thus, while the "standard" reflector elements in
the instant embodiment have angles of substantially 90.degree., so
as to furnish a nominal divergence axis at an angle of
substantially 0.degree., all three angles can be as much as a few
minutes greater than 90.degree., so that the nominal divergence
axis angle would be as much as a few tenths of a degree. Similarly,
the angle 55 associated with the "unique" reflector elements 40 can
have any value substantially greater than the other two angles,
depending upon at what angle the nominal divergence axis or peak
value of specific intensity is required.
In essence the "unique" reflector elements 40 have a divergence
angle characteristic similar to the divergence angle characteristic
of the "standard" reflector elements, except the nominal divergence
axis has been shifted from 0.degree. to some other value such as
1.3.degree.. The result is that the "standard" reflector elements
40 enable the reflector 20 to be visible at the usual small
observation angles, while the "unique" reflector elements 40 enable
the reflector 20 to be visible at greater observation angles. If
desired, reflector elements having nominal divergence axes at other
angles may be employed. For example, reflector elements 40 having a
nominal divergence axis at an angle of 0.8.degree. may be provided
to increase the response of the reflector 20 at that angle.
Reflector elements having nominal divergence axes at several
intermediate angles will flatten out the curves of FIGS. 10 and
11.
The angle of divergence or spread of the curves in FIGS. 10 and 11
in the region of the peak response can be controlled by modifying
the faces of the reflector elements 40. By adding some cylindrical
curvature thereto, the spread may be increased.
The reflector 20 may be mounted so that planes containing the edges
45 of the reflector elements 40 are arranged horizontally. In that
case displacement of the nominal divergence axes occurs in vertical
planes.
It is believed that the invention, its mode of construction, and
many of its advantages should readily be understood from the
foregoing without further description, and it should also be
manifest that while a preferred embodiment of the invention has
been shown and described for illustrative purposes the structural
details are, nevertheless, capable of wide variation within the
purview of the invention as defined in the appended claims.
* * * * *