Lighting optical system

An optical system useful in lighting fixtures for uniformly illuminating large areas, includes a plurality of reflectors situated about a light source to cause light emitted by the light source to be concentrated in a first directional range measured from downward vertical, and to provide decreasing intensity of projected light with changes in the angle of projection from the first directional range to downward vertical. In one preferred form, four reflectors, each being a surface of revolution and spaced from the others to preclude any horizontal overlap, are utilized to concentrate the light emitted into a first directional range of about 65 degrees to 75 degrees. All of the light directed into this first directional range is either singly or doubly reflected. Unreflected light passes between a lower reflector and a central reflector to illuminate a middle directional range, and singly reflected light, which is redirected by a portion of an upper reflector, fills a lower directional range. Moreover, the reflectors are situated to prevent the escape of any light above a pre-determined cut-off angle, normally less than 90 degrees, to minimize light pollution and glare.

BACKGROUND OF THE INVENTION 
This invention relates generally to lighting fixtures, and, more 
specifically, to highly efficient optical systems having multiple 
reflectors which are shaped and positioned to produce an even distribution 
of light over a broad illuminated area without glare. 
The efficiency of lighting fixtures for outdoor illumination purposes is 
not merely a function of the number of lamp lumens emitted per unit of 
power consumption, but rather is a combination of several interrelated 
considerations. For example, an outdoor lighting fixture should illuminate 
as large an area as possible, and provide uniform lighting throughout the 
illuminated area. Further, visibility throughout the illuminated area 
should be maximized while eliminating unneeded glare. This can be 
accomplished, in part, by minimizing the amount of light thrown into the 
eyes of drivers or pedestrians, and by preventing the projection of light 
above the horizontal. Moreover, economic factors such as electric power 
consumption per illuminated square foot and the cost of lighting fixtures 
and poles, must be considered in determining the overall efficiency of a 
system. 
It is especially important that rapid changes in the illumination levels 
over the lighted area be reduced or eliminated to enhance visibility. The 
effect of providing an even transition from brightly illuminated areas to 
less well lighted areas is to enhance seeing. Also, a lighting system 
should direct virtually all of the available light to the intended area of 
illumination, causing a fairly steep fall-off of illumination at the outer 
edge of the lighted area. 
A number of lighting fixtures have been designed which attempt to satisfy 
this criteria by providing a plurality of reflectors which intercept light 
flux from a lamp and shape it into a cone of light which has its maximum 
candle power directed at a relatively high angle, taken from downward 
vertical, but less than 90 degrees. Nested reflectors have been used 
extensively to shape this light cone and to prevent any light from being 
projected upwardly, with the principle aim of uniformly illuminating a 
generally circular area. Such prior approaches have usually required the 
use of a multiplicity of reflectors and/or the use of reflectors having 
reflecting surfaces of different contours, resulting in light fixtures 
which are difficult and expensive to manufacture. 
Although the size of the lighting fixture can be minimized when many 
reflectors are utilized, the required multiple reflections of light 
between the small spaces separating the reflectors reduces the lighting 
efficiency of the fixture, and results in a luminaire which is difficult 
to clean. On the other hand, if too few reflectors are utilized, either 
the luminaire becomes very large for a given performance or it is 
necessary for the light to be reflected back across the luminaire where 
the light source and/or its supporting elements interfere with the light 
and reduce the efficiency of the fixture. Thus, a builder of lighting 
fixtures is faced with a number of seemingly conflicting and 
irreconcilable design constraints. 
Generally, lighting fixtures should have a very tightly controlled beam for 
best performance. Besides concentrating the intensity of the beam to 
provide a higher maximum candle power throughout a selected range, a 
tightly controlled beam can be more precisely directed at higher vertical 
angles than less tightly controlled beams to give the fixture a wider area 
of coverage without increasing glare. This feature results from tight 
beams having a more clearly defined cut-off angle, which permits their use 
in wide area illumination coverage in a manner reducing the amount of 
light directed at nearby drivers and pedestrians. 
With point light sources, the beam width can theoretically be made as tight 
as desired. However, the high intensity discharge lamps used in many 
outdoor lighting fixtures are not point sources, but instead have light 
emitting arc tubes of varying lengths. When redirecting light beams by 
means of one or more reflectors, the tightness of the beam depends on the 
length of the arc tube and on the distance between the arc tube and the 
beam forming reflector. To obtain a tight beam with a relatively long arc 
tube, it is often necessary to provide a larger reflector situated at a 
greater distance than is desirable or acceptable. As a result, common 
practice has been to utilize a plurality of reflectors, and to reduce beam 
width by limiting the portion of the light emitted which is redirected by 
each reflector. But, as mentioned above, such a practice results in a less 
than ideal lighting fixture. 
Accordingly, there has been a need for a novel lighting fixture which is 
capable of uniformly illuminating large areas in a highly efficient and 
economical manner. The improved fixture should utilize an optical system 
having a minimum number of reflectors for redirecting the emitted light, 
and be able to advantageously use unreflected and singly reflected light, 
as well as doubly reflected light, to form the illuminating beams. It 
would be preferable that both oppositely facing surfaces of each reflector 
be manufactured to have the same contour and curvature, and that the 
reflectors be spaced far enough apart to facilitate cleaning and 
maintenance. Finally, an aesthetically pleasing design which can form a 
tight beam and minimize glare would be highly desirable. The present 
invention fulfills these needs and provides other related advantages. 
SUMMARY OF THE INVENTION 
The present invention resides in an improved optical system which is 
capable of uniformly illuminating large areas in a highly efficient and 
economical manner. The optical system comprises generally a plurality of 
reflectors situated about a light source, which are capable of forming a 
tight beam and directing that tight beam at a high angle from downward 
vertical, but less than 90 degrees. Such a tight beam permits the 
concentration of greater amounts of light far from the light source 
without increasing glare, by providing a relatively sharp illumination 
cut-off angle. Moreover, consistent with the provision of a tight beam 
which concentrates light at the outer perimeter of the area to be 
illuminated, the optical system of the present invention minimizes changes 
throughout the illuminated area by utilizing a combination of doubly 
reflected, singly reflected and unreflected light. 
In a preferred form of the invention, the plurality of reflectors includes 
a central reflector which has planar inner and outer reflecting surfaces. 
This central reflector is positioned to surround the light source and 
intercept all of the horizontally emitted light from the light source. The 
planar inner reflecting surface directs the intercepted, horizontally 
emitted light downwardly, where it is subjected to further redirection by 
a lower reflector. 
The lower reflector forms an upward and outwardly facing concave reflecting 
surface which is shaped and positioned to form a tight beam, and directs 
that beam into a first directional range toward the perimeter of the area 
being illuminated. Further, the central and lower reflectors are spaced 
from one another to form a gap through which unreflected light can pass 
into a middle directional range having a greater downward slope than the 
light projected into the first directional range. This effective 
utilization of unreflected light to illuminate the middle range of the 
illuminated area helps to maximize the efficiency of a lighting fixture by 
minimizing losses through multiple reflections. 
An intermediate reflector is positioned above the central reflector in a 
manner allowing it to intercept a portion of the light emitted by the 
light source which is directed above the central reflector, and prevent 
the escape of any unreflected light between the intermediate and central 
reflectors. The intermediate reflector includes a concave lower reflecting 
surface, which forms its intercepted light into a tight beam and directs 
that beam toward the planar outer reflecting surface of the central 
reflector. The beam is then simply redirected by the planar outer 
reflecting surface into the first directional range to further concentrate 
light near the outer edge of the illuminated area. 
Finally, an upper reflector is situated above the intermediate reflector 
and positioned to intercept all of the light emitted by the light source 
which is directed above the intermediate reflector in a manner preventing 
the escape of any light between the upper and intermediate reflectors. The 
upper reflector includes a central, downwardly facing, planar reflecting 
surface, and a peripheral concave reflecting surface contiguously 
extending outwardly from the central reflecting surface. The central 
reflecting surface directs its intercepted light through the gap between 
the lower and central reflectors, and into a lower directional range 
extending substantially from downward vertical up to the middle 
directional range. On the other hand, the peripheral reflecting surface is 
shaped to form a tight beam directed into the first directional range. 
Due to the positioning and contouring of the reflectors, no light is 
permitted to escape from the system above a predetermined cut-off angle, 
normally less than 90 degrees, to minimize light pollution and glare. This 
can be accomplished with reflectors having similarly contoured surfaces, 
and without the need to nest adjacent reflectors in a manner causing the 
lower edge of an upper reflector to horizontally overlap the upper edge of 
a lower reflector. Moreover, by concentrating the greatest candle power 
into the first directional range at a high angle from downward vertical, 
lighting fixtures utilizing the optical system of the present invention 
can be spaced much further apart than conventional lighting fixtures. This 
can be done while maintaining acceptable levels of uniform ground 
illumination. 
Other features and advantages of the present invention will become apparent 
from the following more detailed description, taken in conjunction with 
the accompanying drawings which illustrate, by way of example, the 
principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawings for purposes of illustration, the present 
invention is concerned with a novel optical system, generally designated 
by the reference number 10. In a preferred form of the invention, the 
optical system 10 is incorporated into a lighting fixture 12 for the 
efficient and economical illumination of large outdoor areas. As best 
illustrated in FIG. 1, the lighting fixture 12 is supported at a 
predetermined height above the ground by a relatively short pole tenon 14 
extending upwardly from an elongated vertical supporting pole 16. The 
upper end of the pole tenon 14 is securely attached to a fitter plate 18 
forming the base of the lighting fixture 12, and this fitter plate in turn 
supports the remainder of the lighting fixture. 
Above the fitter plate 18, a ballast 20 is provided for a conventional high 
output lighting source 22. Such as exemplary lighting source 22 is shown 
in the drawings in the form of a lamp having a clear envelope 24 which 
surrounds an elongated, vertically extending arc tube 26 responsible for 
the emission of light when energized. FIG. 4 schematically illustrates 
that such a lamp 22 is normally supported within a socket 28 at its lower 
end, and through this socket electrical energy is provided to the lamp by 
an upwardly extending power cord 30. 
In accordance with the present invention, and as best shown in FIGS. 1, 2 
and 4, the optical system 10 includes a plurality of reflectors situated 
about the light source 22, which are capable of forming a tight beam and 
directing that tight beam at a high angle from downward vertical, but less 
than 90 degrees. Such a tight beam permits the concentration of greater 
amounts of light far from the light source 22 without increasing glare, by 
providing a relatively sharp illumination cut-off angle. Moreover, 
consistent with the provision of a tight beam which concentrates light at 
the outer perimeter of the area to be illuminated, the optical system 10 
minimizes changes throughout the illuminated area by utilizing a 
combination of doubly reflected, singly reflected and unreflected light. 
A central reflector 34 is positioned to surround the source of the emitted 
light, which in this case is the arc tube 26, and intercept all of the 
horizontally emitted light from that source. This central reflector 34 
forms a surface of revolution about a vertical axis 36 coaxially located 
with the arc tube 26, and includes a planar reflective plate 38 and 
integral upper and lower reflective skirts 40 and 42 situated, 
respectively, at the upper and lower edges of the planar reflective plate. 
These skirts 40 and 42 are included in the illustrated embodiment 
primarily for improving the aesthetic appearance of the lighting fixture 
12, and also to insure that no light is projected from the optical system 
10 above a predetermined cut-off angle, as will be more fully explained 
below. Moreover, the central reflector's planar reflective plate 38 
includes oppositely facing, planar inner and outer reflecting surfaces 44 
and 46. The planar inner reflecting surface 44 directs the intercepted, 
horizontally emitted light downwardly, where that light is subjected to 
further reflection by a lower reflector 48. 
Like the central reflector 34, the lower reflector 48 also forms a surface 
of revolution about the vertical axis 36. This lower reflector 48 has an 
upward and outwardly facing concave reflecting surface 50, which is shaped 
and positioned to receive the light directed downwardly by the central 
reflector 34 and form a tight beam 52 of that light. The outwardly facing 
concave reflecting surface 50 directs that tightly formed beam 52 into a 
first directional range toward the perimeter of the area to be 
illuminated. This first directional range is ideally directed at an angle 
of 70 degrees from downward vertical, and practically all of the light 
directed into the first directional range is projected from the optical 
system 10 between 65 and 75 degrees above downward vertical, with a total 
cut-off point preventing any light from being reflected at an angle 
greater than 75 degrees. Additionally, the lower reflector 48 also has an 
integral, downwardly extending skirt 54 at its lower edge which is 
provided primarily to enhance the appearance of the lighting fixture 12. 
The central and lower reflectors 34 and 48 are spaced from one another far 
enough to form a sufficiently large gap between them through which 
unreflected light can pass into a middle directional range. In fact, these 
two reflectors 34 and 48 are spaced so that the upper edge of the lower 
reflector 48 is below the lower edge of the central reflector 34, or, in 
other words, so that there is no horizontal overlap of the reflectors. The 
unreflected light passing through this gap into the middle directional 
range has a greater downward slope than the light projected into the first 
directional range. 
More specifically, this middle directional range extends from approximately 
the first directional range, or 70 degrees, down to an angle of about 25 
degrees above downward vertical. Due to the natural distribution of the 
high intensity discharge lamp 22, the intensity in candela of this 
unreflected light will gradually decrease with a decreasing angle toward 
downward vertical, thus providing uniformity of illumination over the 
lighted area when combined with the light directed into the first 
directional range, and fill light directed below 25 degrees which will be 
more fully discussed below. This effective utilization of unreflected 
light to illuminate the middle range of the illuminated area helps to 
maximize the efficiency of the lighting fixture 12 by minimizing losses 
through multiple reflections. 
Next, an intermediate reflector 56 is positioned above the central 
reflector 34 in a manner allowing it to intercept a portion of the light 
emitted by the light source 22 which is directed above the central 
reflector, and prevent the escape of any unreflected light between the 
intermediate and central reflectors. The intermediate reflector 56 is 
similar to the central reflector 34 in that it forms a surface of 
revolution about the vertical axis 36, and includes a central curved 
portion 58 bounded at the upper and lower edges by skirts 60 and 62. These 
skirts 60 and 62 perform the same function as those found on the central 
reflector 34. 
The central curved portion 58 of the intermediate reflector 56 has 
similarly curved lower concave and upper convex reflecting surfaces 64 and 
66. The lower concave reflecting surface 64 forms the light intercepted by 
the intermediate reflector 56 into a tight beam 68, and directs that beam 
toward the planar outer reflecting surface 46 of the central reflector 34. 
The beam 68 is then simply redirected by the planar outer reflecting 
surface 46 into the first directional range, to combine with the doubly 
reflected light redirected by the lower reflector 48 and further 
concentrate light near the outer edge of the illuminated area. Although 
the upper convex surface 66 is normally specularly reflective, this type 
of construction is not necessary for the proper operation of the optical 
system 10 because it is not utilized to reflect any light. 
Finally, situated directly above both the light source 22 and the 
intermediate reflector 56 is an upper reflector 70 which is positioned to 
intercept all of the light emitted by the light source which is directed 
above the intermediate reflector. Like the intermediate reflector 56, the 
upper reflector 70 importantly prevents the escape of any light between 
those two reflectors, and accomplishes this without any horizontal overlap 
between adjacent reflectors. Further, the upper reflector 70 forms a 
generally circular surface of revolution about the vertical axis 36, and 
this vertical axis passes through the center of the upper reflector. 
For redirecting the intercepted light, the upper reflector 70 includes a 
central, downwardly facing, planar reflecting surface 72, and a peripheral 
concave reflecting surface 74 contiguously extending outwardly from the 
central, horizontally planar reflecting surface. The central reflecting 
surface 72 directs its intercepted light 76 through the gap between the 
lower and central reflectors 48 and 34, and into a lower directional range 
extending substantially from downward vertical up to approximately the 
lower extent of the middle directional range. This lower directional range 
typically extends from about 8 to 25 degrees from downward vertical. On 
the other hand, the peripheral reflecting surface 74 is shaped to form a 
tight beam 78 of singly reflected light directed into the first 
directional range. 
All four of the reflectors 34, 48, 56 and 70 are supported about the 
vertical axis 36 by a pair of support rods 80 in a conventional manner. 
The upper reflector 70 is easily removable from the support rods 80 to 
provide convenient access to the lamp 22 for maintenance purposes and the 
like. Moreover, the optical system 10 is usually housed within a 
transparent enclosure 82 shaped substantially like a sphere to protect the 
reflectors 34, 48, 56 and 70, the light source 22, and the associated 
fixture members from the elements. 
In the present invention, the number of reflectors is reduced to a 
practical minimum while still providing control of the light emitted so 
that the light directed by the reflectors can be combined with unreflected 
light to uniformly illuminate a given area. Highly efficient lighting is 
achieved by maximum use of unreflected light, and by minimizing the number 
of multiple reflections to which the light is subjected. Further, the lamp 
position and the size of the arc tube 26 is carefully correlated with the 
reflectors to provide a well defined cut-off angle above which the 
projection of light is prevented, without the necessity of using the 
reflectors as merely delimiting shields. This characteristic of the novel 
optical system 10 desirably allows wider spacings between adjacent 
reflectors, which in turn facilitates cleaning and maintenance of the 
lighting fixture 12. 
The single contours of the oppositely facing surfaces on the intermediate, 
central and lower reflectors 56, 34 and 48 further economize the optical 
system 10 of the present invention. The contours of these three reflectors 
are correlated so that both the top and bottom surfaces of each reflector 
can have the same overall shape. This type of reflector is much easier and 
cheaper to produce than other multiple contour reflectors found in prior 
lighting fixtures. 
Referring now to the lighting characteristics of a fixture utilizing the 
optical system 10 of the present invention, a typical candle power 
distribution curve 83 for the design is shown in FIG. 3. In that Figure, 
the radially extending lines 85 indicate the angle of projection of light 
from downward vertical. It should be noted that the beam of maximum candle 
power is directed generally at an angle of 70 degrees, and the width of 
this beam is approximately 10 degrees. This very tight light control, 
combined with the high efficiency of the optical system 10, allows a 
single unit at a given mounting height to illuminate an area about 40 
percent greater than most prior units, to the same minimum illumination 
level. This results in an energy savings of roughly 30 percent, and a 
savings in fixture and installation costs of a similar magnitude. If the 
light source 22 used is a 150 watt, clear, high pressure sodium lamp rated 
at 16000 lumens, the five arced candle power curves 84, 86, 88, 90 and 92 
would be labeled, respectively, 1000 to 5000. 
This candle power distribution curve 83 also indicates that there will not 
be a sharp line of demarcation between a maximum intensity at the outer 
edge of the area to be illuminated, and an unlighted area. Rather, there 
is a fall off of illumination at the edge which, while not abrupt, is 
fairly steep. 
From the foregoing it is to be appreciated that due to the positioning and 
contouring of the reflectors, no light is permitted to escape from the 
system above a predetermined cut-off angle, to minimize light pollution 
and glare. This is accomplished with reflectors having similarly contoured 
surfaces, and without requiring any horizontal overlap of adjacent 
reflectors. By concentrating the greatest candle power into the first 
directional range at a high angle from downward vertical, lighting 
fixtures utilizing the optical system 10 can be spaced much further apart 
than conventional lighting fixtures. Moreover, this can be done while 
maintaining acceptable levels of uniform ground illumination. 
Although a particular embodiment of the invention has been described in 
detail for purposes of illustration, various modifications may be made 
without departing from the spirit and scope of the invention. For example, 
notwithstanding the fact that the normal utility of the above-described 
luminaire is for the symmetrical distribution of light, the addition of 
various reflectors or lens elements internally or externally to produce 
asymmetrical distributions should not be precluded. Accordingly, the 
invention is not to be limited, except as by the appended claims.