Phantom eliminator for signal lights

An improved signal light employing a deep-dish reflector and critically positioned mask or masks for blocking at least some selected light rays having their origin in an external source to reduce, or eliminate, phantom signals. The mask is effective to block out at least a portion of the reflector which reflects light, from an external source, back out of the signal in a direction parallel to the reflector axis. The mask may take the form of a planar ring, a cylindrical surface, a cone or portions thereof. Other shapes may prove expedient or the reflector may be treated in selected and controlled locations to inhibit reflection. The mask may comprise any suitable material such as an opaque material or an electrically controlled optical shutter which is closed when it is desired to inhibit phantom signals and opened to accommodate normal signals.

BACKGROUND OF THE INVENTION 
Selectively illuminated traffic control signals and/or similar devices are 
so commonly used as to be familiar to anyone walking along public streets 
or riding in a vehicle along public highways. Among other things, traffic 
lights are used to regulate the flow of both automotive and railroad 
traffic or a combination thereof. For example, where a highway crosses a 
railroad track, it is common to see a traffic light visible to the highway 
traveler and which serves to warn that a train is approaching the 
intersection. 
As is well known, traffic lights typically employ an incandescent lamp 
together with a reflector system which collects and focuses the light so 
that the emitted light rays are concentrated in a beam which may be most 
readily seen by the pedestrian and/or driver for whose benefit the signal 
light is placed. In order to be energy efficient, it is customary to use 
curved reflectors in association with the lamp to collect and focus the 
light rays so that they are emitted essentially parallel to the axis of 
the reflector, or at a small angle of dispersion with respect thereto. 
Occasionally, circumstances are such that at selected times of the day, or 
year, light from an external source, most commonly the sun, can enter the 
signal through the front, be reflected one or more times by the reflecting 
surface and re-emerge in such a way as to simulate a normal signal from 
the lamp. Such apparent signals resulting from light originating 
externally are customarily referred to as "phantom" signals and can give 
rise to unsafe, confusing and ambiguous conditions. For example, a typical 
traffic light which may have the three colors red, yellow and green, 
indicating stop, caution and go, respectively, may receive sunlight and 
creats a simultaneous phantom signal from each of the three reflectors 
thereby providing a confusing or ambiguous signal to the highway traveler. 
Because phantom signals can cause confusion and accident, considerable 
inventive ingenuity has been exercised in an attempt to eliminate or 
minimize phantom signals. For example, U.S. Pat. No. 2,097,785 issued Nov. 
2, 1937 to O. S. Field and assigned to the same assignee as the present 
invention teaches the use of funnel-shaped members having longitudinal 
corrugations to reflect light to non-parallel paths to thereby minimize or 
eliminate phantom signals. U.S. Pat. No. 2,207,656 issued July 9, 1940 to 
C. H. Cartwright, et al, teaches the treatment of the surface on the front 
lens whereby to render the lens non-reflective. U.S. Pat. No. 2,243,448 
issued May 27, 1941 to W. B. Wells, et al, teaches phantom elimination in 
a specialized light signal having at least two optical systems and 
provides means for preventing light rays entering a signal from a foreign 
external source through one optical system from causing the other optical 
system to display a phantom signal. U.S. Pat. No. 2,286,201 to C. L. 
Ferrin, et al, employs an internal apertured plate and a front element 
comprising a plurality of lenses. U.S. Pat. No. 2,336,680 issued Dec. 14, 
1943 to S. E. Gillespie provides phantom elimination by light-polarizing 
means. U.S. Pat. No. 2,413,127 issued Dec. 24, 1946 to W. B. Wells uses a 
conical roundel to prevent reflection of light from an external source. 
Other patents disclosing structures relating to phantom signals include 
U.S. Pat. Nos. 2,419,444; 2,576,849; 2,750,577; 3,235,863; and 3,377,479. 
SUMMARY OF THE INVENTION 
This invention relates to phantom elimination from traffic control signals 
which employ deep-dish reflectors for more energy efficient light 
transmission. When the signal light is required to be oriented so as to be 
subject to phantoms resulting from light from an external source, the 
structure includes one or more critically positioned masks for blocking at 
least some of the light rays which enter the system parallel to the 
optical axis of the reflector. More specifically, the mask is positioned 
to intercept at least some of the light rays, at some point in their path, 
which might otherwise enter the signal and be reflected out of the signal 
lamp as a phantom signal. 
The mask may take a wide variety of shapes and may comprise at least part 
of a planar disk or ring; or it may comprise at least part of a 
cylindrical surface having an axis coincident with the optical axis of the 
reflector or the mask may comprise at least part of a conical surface 
having an axis coincident with the optical axis of the reflector. Other 
shapes and/or combinations may be used, or the reflector may be 
selectively modified, or treated, to inhibit reflections from selected 
areas. In an alternate structure an electrically controlled shutter could 
be used. 
It is an object of this invention to provide a new and improved signal 
light. 
It is a more specific object of the invention to provide a signal light 
such as a traffic control signal which produces a minimal phantom signal 
in response to light from an external source. 
It is another object of the invention to provide an energy efficient 
traffic signal which may be readily and economically adapted to phantom 
elimination when required by the exigencies of orientation. 
It is an even more specific object of the invention to intercept at least 
some of the light rays entering the signal light from an external source 
and which are parallel to the optical axis of the reflector such that they 
would be reflected by the reflector through the focal point to another 
point on the reflector. 
It is another object of the invention to minimize phantom signals by use of 
an optical mask which is opaque only when the traffic signal light is off.

While parabolic reflectors have been illustrated, it should be understood 
that other classical forms of reflectors may be employed. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The concepts incorporated in this invention, together with the details of 
operation may be more fully understood by considering together the 
following description taken in cooperation with the drawing. In FIG. 1, 
element 101 constitutes a cross section of a reflector surface taken on a 
plane including the axis of the reflector. More specifically, the 
reflector 101, as drawn in FIG. 1, comprises a cross section view of a 
deep-dish parabolic reflector having a surface generated by revolving the 
line 101 about the axis 102. In addition, it should be understood that 
other classical reflector shapes, such as spherical, may be used in 
association with the invention described herein. A reflector 101 of the 
type which is typically used with signal lights has a focal point which is 
designated "F". For the purposes of this description, the focal point may 
be defined as a point on the optical axis 102 of the reflector 101 through 
which any rays of light parallel to the optical axis converge after being 
reflected on the reflector surface 101. For example, light ray A impinges 
on the reflector surface at point A1 and through the focal point F to 
impinge of the reflective surface 101 at point A2 from which it is 
reflected out on line A3 which is parallel to the optical axis 102. In a 
similar manner, light ray B reflects at point B1, passes through focal 
point F and reflects at point B2 for reflection out along line B3. 
However, it will be seen that light ray C which is parallel to the optical 
axis 102 impinges on the reflector at point C1 and is reflected through 
the focal point F and out along ray C3. That is, the light reflected from 
point C1 does not impinge on the reflector 101 a second time and is 
reflected out of the system at an angle other than parallel to the optical 
axis 102. Accordingly, light ray C3 is not visible within the normal 
viewing angle of the signal light and thus does not contribute to an 
objectionable phantom signal. 
It should be observed that in accordance with laws of optics, the angle of 
incidence is always equal to the angle of reflection. By way of 
explanation, this means, for example, that light ray E. which reflects at 
point E1 is reflected therefrom such that the angle 103 which is formed 
between the line E-E1 and a tangent to the arc A1-C1 at the point of E1 is 
exactly equal to the angle 104 formed between the line E1-E2 and the same 
tangent line. In like manner, the angled formed between the ray of 
incidence, and the ray of reflection and the plant tangent to the 
reflector 101 at the point of reflection are equal. 
From the above it will be evident that all light rays between rays A and B 
and which are parallel to the optical axis 102 are reflected out of the 
reflector 101 essentially parallel to their original direction. 
It is the reflected rays such as A3, B3 and E3 which cause phantom signals. 
More specifically, traffic signals are usually oriented so that the 
viewer, who is supposed to see the signal, is approximately on the optical 
axis or a relatively few degrees therefrom. Accordingly, a traffic light 
which is situated such that light rays may enter parallel to the optical 
axis will have light reflected out and parallel to the optical axis 
thereby creating a phantom signal. It should be understood, of course, 
that normal signals are created by means of an incandescent bulb which is 
located at approximately the focal point F and light rays emitted 
therefrom reflect on the surface 101 and out parallel to the optical axis 
102. 
It will be apparent that phantom signals could be avoided by employing a 
reflector 101 which extends only from point B1 to point B2. With such a 
reflector, any light ray such as E. would reflect on the reflector at 
point E1, pass through the focal point F and, since there is no reflector 
portion past point B2, the light ray will not be reflected and will not 
extend along the line E3 to create a phantom signal. However, light 
signals of this type are not energy efficient and give a weak signal or 
require a higher input signal to provide the desired light level. 
Accordingly, since phantom signals are created only when the light signal 
has a predetermined orientation and only at selected times and/or dates, 
it is common practice to provide energy efficient deep-dish reflectors and 
to modigy the system at least those situations wherein phantom signals may 
be produced. 
It will be evident that if a mask 105, which serves to absorb or intercept 
light rays, is placed as illustrated in FIG. 1 that all light rays 
parallel to the optical axis 102 and between light rays A and B will be 
intercepted and will not be able to be reflected out and parallel to the 
optical axis 102. For example, light ray D will strike the mask 105 and be 
intercepted or stopped there instead of extending through the point D1 
where it would otherwise be reflected through the focal point F and 
reflected again at point D2 to emerge from the system parallel to the 
optical axis. It will also be apparent that instead of employing the mask 
105, a mask 106 could be used with similar results. In this case, the 
light ray D reflects at points D1 and D2 but is stopped by the mask 106 as 
point D3. As may be seen from FIG. 1, the mask 106 extends between light 
rays A3 and B3. It will be evident that a wide variety of other masks 
could be employed in other locations to achieve identical results. For 
example, mask 107 could be placed parallel to the optical axis 102, or 
mask 108 could be used at an angle relative to the optical axis 102. 109 
and 110 indicate 2 additional mask locations which would provide the same 
general effect as mask 105. 
FIG. 2 comprises a front view of the mask 105. From this view, it may more 
readily be seen that the mask 105 comprises a planar disk similar to a 
washer having an axis which is concentric with the optical axis 102 which 
passes through the focal point F. 105' and 105" comprise the outer and 
inner limits, respectively, of the mask 105. 
FIG. 3A shows, on a reduced scale with respect to FIG. 1, a view of the 
mask 107 as seen when looking along the axis 102. FIG. 3B comprises a side 
view of the same mask 107. Accordingly, as may be seen, the mask 107 
comprises a portion of a right circular cylinder having an axis coincident 
with the optical axis 102. 
In a similar manner, FIGS. 4A and 4B comprise front and side views, 
respectively, of the mask 108 illustrated in FIG. 1. 
Consideration should now be given to FIG. 5 wherein certain symmetrical and 
non-symmetrical relationships will be observed. The reflector 101, shown 
in cross section, is illustrated as a parabolic reflector comprising the 
surface generated by revolving the line 101 about its axis 102. As with 
FIG. 1, light rays A, B and C, together with their points of incidence and 
reflection, are illustrated. It should be observed that light ray A, which 
enters the reflector 101 at a maximum distance from the axis 102, leaves 
the reflector 101 parallel to the axis 102 but much closer thereto. 
Conversely, light ray C, which enters the system relatively close to the 
axis 102, as compared with light ray A, leaves the system after being 
reflected at points C1 and C2 parallel to the axis 102 but at a greater 
distance therefrom. Examination will show that the minimum distances 
between the axis 102 and light rays A and C3 are identical and that the 
distance between the axis 102 and the light rays A and C3 are identical 
and that the distance between the axis 102 and the light rays C and A3 are 
identical. If light ray B is chosen as that ray which has its reflection 
points B1 and B2 on the same line with the focal point F, it will be seen 
that light rays B and B3 are equidistant from the axis 102. If a mask of 
the type illustrated as 105 in FIGS. 1 and 2 is used in conjunction with 
the system of FIG. 5, it will be evident that the net result is that the 
system of FIG. 5 will, in effect, be converted from a deep-dish reflector 
to a shallow-dish reflector. For the purposes of this discussion, a 
deep-dish reflector may be defined as one wherein the limits of the 
reflector 101 extend beyond a plane which is at right angles to the axis 
102 and includes the point F. A shallow-dish reflector may be defined as a 
reflector which has no portion extending beyond a plane at right angles to 
the axis 102 and including the point F. As may be seen, if a shallow-dish 
reflector is used, there will be no phantom signal, as any light ray 
entering the system between rays B and B3 will be reflected from the 
surface 101 through the focal point F and will not again encounter the 
reflector 101 for reflection out parallel to the axis 102. However, as 
already mentioned, shallow-dish reflectors do not provide an energy 
efficient system and therefore it is expedient to provide deep-dish 
reflectors in at least those situations wherein the occurrence ot phantom 
signals is not expected to occur. 
For those few situations wherein phantom signals may be expected to occur, 
a standard deep-dish reflector may be employed and converted to a 
shallow-dish reflector by means of a mask such as that described in 
connection with FIG. 2. However, as stated, the conversion of the 
reflector from a deep-dish reflector to a shallow-dish reflector results 
in certain energy inefficiency, and the brilliance of the beam emerging 
from the signal in response to the illumination of an interior 
incandescent light bulb (not shown) will result in a weaker signal. 
Considering now more specifically FIG. 6, there will be seen a view looking 
toward the reflector 101 of FIG. 5 and along the axis 102. The outer 
circle designated A and C3 comprises the outer limits of the reflector 101 
and the many different points at which a light ray A may be received or a 
light ray C3 reflected therefrom. In a similar manner, the inner circle 
designated C and A3 represents the many points at which a light ray C may 
enter the system or a light ray A3 be reflected therefrom; and the center 
circle designated B and B3 constitutes the many points at which a light 
ray B may enter the system or a light ray B3 may be reflected therefrom. 
It should be evident that a mask which will convert the reflector 101 to a 
shallow-dish reflector may comprise a ring or washer which will intercept 
all the incoming light rays between the outermost and center circles. Such 
a mask is shown in FIG. 5 and, for convenience of illustration, displaced 
outward from the reflector 101 and designated 121 in FIGS. 5 and 6. There 
is also shown an alternate mask 122 in FIGS. 5 and 6 which, while 
different in size and placement, may be seen to have the same results as 
the mask 121. More specifically, if mask 122 is considered to be in place 
and mask 121 removed, it will be seen that any ray of light entering the 
system between rays and and B will be reflected on the surface 101 and 
through the focal point F to again be reflected from the reflecting 
surface 101 and be intercepted by the mask 122. In a similar manner, any 
light rays entering the system between rays B3 and C3 will reflect from 
the surface 101 through the focal point F and again be reflected from the 
reflecting surface 101 to be intercepted by the upper portion of the mask 
122. 
In summary, either of the masks 121 or 122 will intercept the same light 
rays and therefore should be equally effective in preventing phantom 
signals. However, since mask 122 intercepts an intermediate ring of rays, 
it will be evident that the signal from internally-generated light rays 
(from a light bulb not shown) may be perceived as different by a viewer. 
This mask 122 permits signal dispersion over a larger field, albeit with a 
weak ring portion. 
It will also be apparent that if a sun shade is used and external light can 
never enter selected portions of the signal, corresponding portions of the 
mask 121 or 122 could be eliminated, thereby again increasing the energy 
efficiency. 
Furthermore, if a weak phantom is not objectionable, the mask area may be 
selectively reduced to enhance the normal signal. Empirical tests will 
help to determine optimum mask size and orientation. 
Once it is recognized that the masks 121 and 122 can provide similar 
intercept service, it may be seen that a wide variety of masks may be 
designed which will be equally serviceable for intercepting the light rays 
which are effective to produce the undesired phantom signal. For example, 
one such alternate mask 123 is shown in the shaded section of FIG. 6A. 
Considering FIG. 6A, which is drawn to a reduced scale, with respect to 
FIG. 5, it will be seen that any light rays entering the upper half of the 
system between the boundaries of the locus of light rays A and B will be 
intercepted by the portion of the mask 123 between the outer two circles. 
Similarly, any light rays entering the lower half of the system between 
the locus of point B3 and C3 will be intercepted by that portion of the 
mask 123 residing between the inner and central circles of FIG. 6A. 
Another suitable form of mask 124 is illustrated in FIG. 6B. Another mask 
125 is shown in FIG. 6C. An analysis will show that it offers the same 
theoretical results. Quite obviously, a wide variety of mask designs could 
be employed. 
While each of these designs may have the same theoretical results with 
respect to prevention of phantom signals, it will be appreciated that they 
are not identical with respect to the signals produced in response to the 
illumination of the internal incandescent lamp. 
Experimentation with different forms of masks and different conditions has 
revealed that they have somewhat different effectiveness in eliminating 
phantoms and/or that they have different effects on the light beam as 
projected from the system in response to illumination of the internal lamp 
and as perceived by the viewer. The reasons for the difference may be a 
variety of factors including, but not limited to, accuracy of mask design 
and placement, the efficiency of the mask in absorbing light rays, the 
area of the mask, imperfections in the reflecting surface 101, lack of 
accurate alignment of the incandescent lamp source, the idiosyncracies of 
the human eye in responding to signals of varying brilliance and area, 
and/or a variety of other factors. 
All of the masks illustrated in FIGS. 6 through 6C comprised masks lying in 
a single plane. However, as mentioned in connection with FIGS. 1, 3A, 3B, 
4A and 4b, the mask may take cylindrical or conical shapes. 
Considering now FIG. 7, it will be seen that it is similar to FIG. 5. As 
suggested with respect to FIGS. 1, 3A and 3B, a cylindrical mask might be 
employed. For example, all of the rays entering the system between rays A 
and B or between rays B3 and C3 can be prevented from introducing a 
phantom signal by including a cylindrical mask having an axis 102 
coincident with the axis 102 of the reflecting surface 101 and having the 
left-most boundary determined by a plane at right angles to the axis 102 
and pasing through the focal point F and having a right-hand boundary 
lying in a plane normal to the axis 102 and including the points X and Z' 
wherein X is the locus of all possible light rays A intersecting with the 
last-named plane and which is also coincident with the intersection of the 
locus of all points of light ray C after reflection at point C1 with the 
last-named plane. It may also be seen that similar results may be obtained 
by a cylindrical mask generated by revolving the rectangle Y, Z, X', Y' 
about the axis 102. Furthermore, the described cylindrical masks may have 
different radii of curvatures and different lengths with the length 
increasing as the radius of curvature increases. 
In connection with FIGS. 5 through 6C, it was shown that various 
combinations of the two disk masks 121 and 122 could be used to make a 
mask. In a similar fashion, different portions of the cylinders to the 
left of the line B1-B2 and to the right of the same line of FIG. 7 may be 
used to make a mask. For example, FIG. 8 illustrates one form of mask 
which might be used. By analogy with FIGS. 6 through 6C, other cylindrical 
masks may be readily envisioned. 
A choice of which style of mask is used in any particular application may 
depend upon the variety of factors already listed as well as personal 
preference and/or mounting or supporting techniques. 
In addition to providing masks as illustrated, it will be evident that 
segmented conical masks could be provided. Furthermore, the reflector 
surface 101 could be treated to prevent reflections therefrom. For 
example, and referring now to FIG. 5, it would be apparent that by 
treating the reflecting surface 101 between points A1 and B1 a mask 
corresponding to mask 121 could be produced; and by selectively treating 
various portions of the reflecting surface 101, a mask corresponding to 
any of those illustrated in FIGS. 6A to 6C and/or many others could be 
created. Since the cylindrical masks are the full equivalent of selected 
planar disk masks, it is also evident that treating the reflecting surface 
101 could provide results similar to any of the cylindrical or conical 
masks. 
As thus far described, all masks have been considered opaque for preventing 
the transmission of light rays. However, it will be apparent that if 
circumstances permit, a mask may be made which is not entirely opaque. 
Furthermore, masks could be made of light polarizing material with 
appropriate orientation such that light rays which produce phantom signals 
pass through first and second mask portions before and after first and 
second reflections, respectively, and which are oriented at 90.degree. 
with respect to each other whereby such light rays entering the system are 
prevented from leaving and thereby preventing phantom signals. Such a mask 
could comprise, for example, a mask of the type illustrated in FIG. 6 
wherein the first light polarizing element comprises element 121 and the 
second light polarizing element comprises element 122. Such a mask would 
have relatively little effect on light rays emitted from the internal 
source. 
Another type of mask for eliminating phantom signals without significantly 
reducing the signal level would comprise an electrically controlled 
optical shutter instead of an opaque mask. Such a shutter could take any 
of the aforedescribed configurations or could comprise a planar or curved 
surface covering the entire reflector. An electrical shutter might be 
fabricated of a liquid crystal electrically controlled by the same energy 
which lights an internal incandescent bulb. When the bulb is lit, the mask 
will become transparent, and when the bulb is off the mask becomes opaque, 
thereby preventing the entry of light rays which might generate phantom 
signals. The shutter may be opened when the internal bulb is lit as, at 
such time, the production of a phantom signal merely reinforces the 
desired signal. 
While there has been shown and described what is considered at present to 
be a preferred embodiment of the invention, modification thereto will 
readily occur to those skilled in the related arts. For example, in 
another structure the same principles of mask design and placement could 
be applied for use in systems having reflectors which are not entirely 
symmetrical about an optical axis. It is believed that no further analysis 
or description is required and that the foregoing so fully reveals the 
gist of the present invention that those skilled in the applicable arts 
can adapt it to meet the exigencies of their specific requirements. It is 
not desired, therefore, that the invention be limited to the embodiments 
shown and described, and it is intended to cover in the appended claims 
all such modifications as fall within the true spirit and scope of the 
invention.