Background of the Invention Three-light traffic signals are used worldwide to regulate the flow of pedestrian and vehicular traffic through an intersection in a road. These signals typically have stacked green, yellow, and red lights that cast light toward a desired area of the road in response to an electrical signal. When a person sees one of the three lights illuminated, he or she can determine whether to proceed through the intersection, proceed with caution, or slow to a stop.

Figure 1 illustrates a conventional incandescent signal light arrangement, including a clear bulb 10 and a parabolic reflector 12 that reflects the light through lens 14. Light ray R is illustrative of a light ray emanating from the light source, reflecting off reflector 12, and passing through lens 14. Lens 14 has a smooth outer surface 16 and a faceted inner surface 18 that directs the light toward a specified"footprint"on the road. The boundaries of the footprint are measured relative to mutually perpendicular vertical and horizontal optical axes, which bisect each other at the center of the light source indicated at C. The angular boundaries extend 27.5 degrees to both the left and right of the vertical optical axis, and from-2. 5 to-17. 5 degrees below the horizontal optical axis, as specified in the standard entitled"Vehicle Traffic Control Signal Heads" by the Institute of Transportation Engineers (ITE), 525 School St. S. W., Washington, D. C. 20024.

Conventional signal lights with parabolic reflectors are generally capable of a relative target efficiency of 100%. Relative target efficiency is defined in"Optical and Energy Efficiency of Signal Lights"by Ian Lewin et al, as the total number of lumens that a given light directs to the ITE footprint divided by the total lumens specified by the

ITE footprint. Stated differently, conventional signal lights generally meet, but do not exceed, the requirements of the ITE footprint specification.

Another signal light system is described in U. S. Patent No. (Gardner), which includes a bulb, spherical reflector, an inner Fresnel lens that collimates the light and an external lens that redirects light toward the footprint on the road. Although useful for some applications, it is believed that the signal light described in the'418 patent would not provide uniform light distribution, and may thus be less perceptible than desired.

Any light not directed toward the ITE footprint is considered"lost"for purposes of evaluating the efficiency of the signal light, and a substantial amount of light is lost by conventional signal lights. Because it currently costs approximately $1000 to power the signal lights for an average intersection for one year, the collective cost of this loss or inefficiency is quite large. In view of these and other disadvantages of known signal light systems, it is desirable to provide an improved signal light system with increased energy efficiency and uniform light distribution across the light face. It is also desirable to provide such a system in a form that would enable it to be retroactively installed in existing signal light heads.

Summarv of the Invention The traffic signal light system of the present invention includes a light source having an optical axis, an inner lens, an outer lens for directing light in a desired direction, a reflector positioned to receive light from the light source and reflect it toward the inner lens, and at least one reflective side wall adjacent the periphery of the inner lens, and having a major axis substantially parallel to the optical axis, for reflecting incident light toward the inner lens. The reflector is preferably spherical. The reflective side wall may include a plurality of linear grooves that reflect light by total internal reflection.

Another aspect of the present invention is a lens for a light, including a circular front face, a circumferential side wall, and structure associated with the side wall for reflecting light incident on said structure toward the front face.

It is believed that the signal light of the present invention has a relative target efficiency of 124%, meaning that it is 24 percent more efficient than conventional signal lights with parabolic reflectors.

Brief Description of the Drawings The present invention is described in greater detail with reference to the appended drawings, in which: Figure 1 is a cross-sectional view of a conventional signal lamp arrangement; Figure 2 is a cross-sectional view of a signal lamp according to the present invention; Figure 3 is an exploded view of a second embodiment of a signal lamp according to the present invention; Figure 4 is perspective view of a portion of the outer lens; Figure 5 is a graphical comparison between the distribution of light across the diameter of the outside of a conventional signal light (curve A), and across the diameter of the outside of the light of one embodiment of the present invention (curve B); and Figure 6 is an expanded sectional view of the inner lens and outer lens shown in Figure 2.

Detailed Description of the Invention The traffic signal light system of the present invention is more efficient than conventional signal lights, and provides better light distribution than such signal lights, by providing a reflective side wall around the periphery of the Fresnel lens. The reflective side wall extends generally perpendicular to the optical axis of the light, and redirects light that might otherwise not reach the Fresnel lens back toward that lens to provide a more uniform distribution of light. These and other features will be described in greater detail below.

Figure 2 illustrates an embodiment of the traffic signal light system of the present invention. The system 100 includes a light source 102, a reflector 104, inner lens 106, outer light-directing lens 108, reflective side walls 110. Other components include reflector spring 112 and bulb socket 114. The outer and inner lens may be joined to the remainder of the assembly at their respective peripheries, as shown in Figure 2. The assembly preferably includes a gasket 115 (as shown in Figure 3), O-ring 117 (as shown in Figure 2), or other seal that prevents water and other contaminants from entering the signal light.

A. The Light Source The preferred light source is a full spectrum incandescent bulb of the type specified in the standard entitled"Traffic Signal Lamps,"also by ITE. One such bulb is available from General Electric Lighting of Cleveland, Ohio under the designation P-25.

The light source typically includes a horseshoe-shaped 2.54 cm (1 inch) filament, typically designated"C-9,"within a clear bulb that is screwed into a powered bulb socket 114. The bulbs are typically approximately 65 watts when used with 20.3 cm (8 inch) diameter signal light, and either 135 or 150 watts when used with a 40.6 cm (12 inch) diameter signal light, and are manually replaceable. Although 20.3 cm (8 inch) diameter signal lights have been the most common size for many years, 40.6 cm (12 inch) diameter signal lights are increasingly popular because they are more easily seen by motorists. In particular, larger signal lights are favored for intersections where vehicles are traveling at high speeds when approaching the intersection. In such larger lights, 150 watt bulbs are typically used, but 135 watt bulbs may be preferable because they emit less heat than bulbs of higher wattage.

The light source is compatible with bulb socket 114, which is wired in accordance with requirements. Typically, the wiring is two-color wiring with 18 gauge or larger, 600 volt appliance wiring material (AWM), with 30 mil insulation minimum rated at 150° C. The wire leads may be approximately 914 cm (36 inches) in length.

The bulb socket has a 200° C continuous heat rating.

B. The Reflector The reflector surface of the present invention is preferably spherical, and it redirects incident light toward inner lens 106. A spherical reflector, as that term is used in the art, generally means that at least a portion of the reflector is substantially spherical relative to the light source. The reflector is preferably made from aluminum or an aluminum alloy such as 3002 aluminum, brightened or polished, and coated with a protective coating such as"ALZAK,"available from the Reflek company of Massachusetts. Other reflector materials may be used if they provide sufficiently good optical performance and are resistant to environmental effects such as the heat generated by the light source. One such material is Apec HT High-Heat Polycarbonate available from the Bayer Company of Pittsburgh, Pennsylvania under the designation DP9-9330, other grades of which are also available. Polymeric reflectors are typically injection molded, although any suitable manufacturing process could be used.

The reflector is preferably spherical, although conventional parabolic reflectors may also be used. Chapter 3, entitled"Geometric Foundations of Reflector Design, found in"The Optical Design of Reflectors" (Wiley & Sons, 2d ed.), discusses the conic sections on which reflectors are based, as do other sources familiar to one of skill in the art. The reflector is designed to reflect light back through a reimage area 130, centered approximately 2.54 mm (0.1 in) forward of the light source and toward the inner lens.

The reflector is designed to direct light rays toward the inner and outer lens, and thus toward the desired footprint.

C. The Inner Lens The inner lens is a Fresnel lens, meaning that it has very small concentric circular peaks and valleys and a predetermined focal point. More specifically, the lens is an exit Fresnel lens, because the peaks and valleys are provided on the outer surface of the lens relative to the light source. The inner surface of the lens is preferably smooth. In one embodiment, the depth of the grooves varies from 0.0127 mm near the center of the lens to 0.6782 mm near the outside (0.0005 to 0.0267 in), and the pitch is 0.635 mm (0.025 in). The included groove angle is preferably 90°.

An appropriately constructed lens of this type refracts light by conventional principles of refraction, and thus the lens requires no additional coating or other layers, although a V* wavelength thick, antireflective, optically transparent coating may be used to reduce reflection and increase transmission of light, especially near the outermost grooves. One publication describing Fresnel lenses is"Seeing the Light,"from Optics in Nature, Photography, Color, Vision & Holography, Harper & Row (1986) pp 93-94.

The inner lens is preferably transparent and colorless, and in the case of a 20.3 cm (12 inch) diameter signal has a focal point that is approximately 11 cm (4.34 in) forward of the light source. The function of the lens is to receive light both directly from the light source and as reflected by the reflector 104 through reimage area 130, and direct it to the outer lens. The inner lens may be flat, but is preferably curved to increase efficiency, and may have a radius of curvature of approximately 74.7 cm (29.41 inches).

The inner lens is preferably relatively close to the light source, but not so close as to heat the lens and cause damage.

The Fresnel grooves in the inner lens may be machined (preferably in acrylic) or molded (preferably in polycarbonate), and it is preferred that the peaks and valleys of the Fresnel grooves be as sharp as possible to minimize light leaks. A preferred method of making the inner lens is to machine a master mold, and use the mold in an injection molding process, as known in the art. The peaks and valleys should be as sharp as possible, to prevent light from being misdirected by the inner lens, and may be provided by heating either or both of the liquid polymer and the mold to a sufficiently high temperature to enable the polymer to fill the mold, by applying pressure, or by any other molding expedient known to those of skill in the art.

The inner lens may be made from any suitable material, one example of which is APEC HT High-Heat Polycarbonate sold by the Bayer Company of Pittsburgh, Pennsylvania under the designation DP9-9330.

D. The Outer Lens The outer lens receives essentially collimated light from the inner lens and redirects it in a desired direction. That direction is typically defined by the boundaries of the footprint onto which the light should fall on the road, such as the footprint established by the U. S. ITE specification referred to above. Light-directing outer lenses of this type are well known, particularly in the automotive industry, and may be referred to as pillowed lens arrays. One reference describing such lenses is Optical and Energy Efficiency of Signal Lights, cited above. The outer surface of the outer lens is preferably smooth so that it may easily be cleaned, and substantially flat so that it may be more easily manufactured.

The inside surface of the outer lens has an array of very small protrusions, as shown at 120 in Figure 4. The protrusions, or pillows, have a first radius of curvature at their bottom edges and a second radius of curvature at their top edges, and the bottom edges protrude farther than do the top edges. For purposes of obtaining relatively uniform light distribution, it is desirable to provide the smallest possible pillows, so that the human eye does not perceive the light as emanating from individual pillows. For example, pillows having a length L and a width W of 64 microns (0.0025 inches) are possible, and an array of such pillows is perceived by an observer as being uniform. The height H is preferably 18.39 microns (0.00072 in) or less, measured at the bottom of the pillow. As with the edges of the grooves in the Fresnel lens discussed above, sharper edges on the pillows provides better optical performance, and thus the pillows should be machined or molded as sharp as possible, in accordance with known molding processes.

E. The Reflective Side Walls One or more reflective side walls are also provided, and in the illustrated embodiment the side wall is unitary with the inner lens 106. One way of providing such side walls with reflectivity is to provide linear grooves with 90 degree included angles, as shown at 107 in Figure 3, that are generally parallel to the axis 22 that extends through the center of the light source. The dimensions of the linear grooves may be selected as desired, and in one embodiment the depth of the grooves is 0.1778 mm (0.007 in), the pitch is 3.7338 mm (0.147 in), the grooves have a 90° included angle, and

the grooves are regularly spaced around the periphery of the side wall at 0.147° intervals. Light striking the side wall is reflected toward the inner lens, collimated, and transmitted to the outer lens, which directs it toward the desired footprint.

A film or other reflective substance could also be applied to the side walls, so long as it were able to withstand the elevated temperature and other environmental conditions to which the signal light is subjected. Metallic coatings, vapor coatings, metallic sheetings, and multilayer optical films (of the type available from Minnesota Manufacturing Company (3M) of St. Paul, Minnesota) may be used to provide reflective side walls under appropriate conditions.

The reflective side walls direct light toward the outer edges of the front face of the lens, which is normally the darkest portion of the signal light, and thus significantly contributes to even light distribution across the face of the lens. The reflective side wall may be provided inexpensively by cutting or broaching grooves into the tooling that molds the outer surface of a molded plastic lens. Thus, light would pass through the side wall, be reflected off the surfaces of the grooves in accordance with known principles of total internal reflection, and emerge on a path toward the outer lens.

Grooves such as these could also be molded into the lens, if desired, and preferably have 90 degree corners as is known in the art.

The side wall or walls could also be provided separately from the inner lens. For example, the side wall or walls could extend from the periphery of the reflector, or could be a discrete component of the system.

The side walls need not be entirely reflective, and it may be desirable under some circumstances to make only a portion of the side walls reflective. For example, it may be preferable to make only the front half of the side walls reflective, so that light is not reflected by the back half of the side walls toward the middle of the lens where the reflected light focuses on and therefore heats the lens.

The side walls, when provided with the inner lens, may have a small draft angle to facilitate removal of the inner lens from a mold. A one to two degree draft angle is sufficient, and a one and one-half degree draft angle is preferred.

F. Additional Components and Features Additional components may be provided if desired. For example, it may be desirable to limit the amount of light that is reflected from the side walls toward the center of the inner lens, so that the inner lens doesn't become hot, resulting in warping.

One way to do this is to provide a circumferential stop 122, as shown in Figures 2 and 3, that blocks the light that would otherwise be reflected toward the center of the inner lens. The size of the opening in the stop, if relatively small, can reduce the perception of sun phantoms, as generally described in U. S. Patent No. (Gardner).

Another feature is that of coloring the light emitted by the signal light. The exact color ranges for each of the three signal lights that comprise a signal light head are set forth in the ITE standard cited above, although any desired or required colors could be used. Coloration could be provided by a transparent coating or coatings applied to the bulb surface, the inner lens, the outer lens, or another surface through which the light passes, although it is preferred that color is provided by color filters on the outer lens.

For example, a transparent red polycarbonate filter could be used for the red signal light.

G. Results The signal light system described herein is believed to provide more uniform, efficient transmission of light than conventional systems. In prototype testing, signal light systems of the present invention had a relative target efficiency of approximately 124% efficient, which enables the signal light either to provide a signal light that is as bright as conventional signal lights but uses less energy, to provide a brighter signal light that uses the same amount of energy, or to provide one that is both brighter and uses less energy. For example, if the power supplied to the bulb is reduced by 6%, it can double the life of the bulb, which is desirable not only because of lower bulb costs, but also because of the savings in labor otherwise associated with bulb replacement. Bulb

life is typically about 8000 hours, or one year of usage, so that bulbs used with the system of this invention may only have to be replaced every two years.

The signal light system of the present invention may also be packaged as a unit for retroactive installation in existing signal light heads. The existing reflector, socket, bulb and lens can be removed and the system of the present invention installed, and the bulb may be reused if of an appropriate type.

Another advantage of the signal light system of the present invention is believed to be better light distribution than provided by conventional signal lights having parabolic reflectors, which is believed to be provided by the reflective side walls. Figure 5 shows a comparison between the distribution of light across the diameter of the outside of a conventional light (curve A), and across the radius of the outside of the light of the present invention (curve B). The difference between the two curves represents the difference in optical efficiency of the two systems. The data represented by Figure 5 were gathered at the ITE test point 7.5 degrees down from horizontal and 2.5 degrees off center, which is one of the test points to which the most light is directed.

The present invention has now been described with reference to several embodiments thereof. However, many variations of these embodiments will be apparent to one of ordinary skill in the art, and thus the scope of the invention is limited not to the embodiments disclosed, but by the following claims.