Note: Descriptions are shown in the official language in which they were submitted.
WO 92/17733 PCT/US91/07373
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L
LAMP AND REFLECTOR ASSEMBLY
Field of the Invention
This invention relates to electric lamps having
reflectors for providing a desired light pattern
and, more particularly, to a lamp assembly including
a reflector having a multiplicity of reflecting
facets and a relatively long, small diameter
filament.
B_ackqround of the Invention
Reflector lamps, commonly known PAR lamps, are
well known in. the art and have been in commercial
use for many years. These lamps are fabricated of
. pressed glass and include a 'reflector having a
reflecting surf ace, a light source and a cover or
lens. The reflecting surface is typically a concave
paraboloid. Although the light source is typically
a tungsten filament or a tungsten halogen lamp
capsule, an arc discharge tube also can be
utilized. The cover may be clear or may be omitted
when a tungsten halogen capsule is used, but most
commercial lamps have had stippled and/or lenticular
..
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- 2 -
configurations in the cover g;.ass to smooth the beam
_ and/or tc provide she reauired beam spread. The
filament is located as close zs possible to the
focal point ef the =eflecting s;arface.
A principal advalztage of PAR lamps is that t =a
reflector and the lens form ogtically Cortrol~ed
light beams ranging from narrow spotlights to widE
floodlights. Beam angles range from a few degrees
ro about 64 degrees. Since the lamps are p=efccused
at the time of manufacture, the problems of
combining separate coznponerts in the field are
avoided. Synce the optical surfaces are sealed
within the .amp. They remain clean and in good
_ conditior_ throughout the life of the .amp.
regardless of environmental conditions., W:minai~es
az~d lampholders do not require any optical
components and are relatively inexpensive and si:,~.ple
to manufacture. Nevertheless, dif~ersnt beam
spreads and beam intensities are possible with a
single luminaire simply by selecting another Pr~.R
lamp.
One of t'~e problems in designing PAR lamps is to
control the beam pattern produced by the lamp for
different sizes and orientations of filaments.
Although the filament is typically located at or
near the focal point of the reflector, bea3n
spreading occurs because the filament has a _iait2
s ize . The dimens i ons of ti:e f i 1 amEnt ar a dict azed
primarily by the v.alzage and wattage ratings of the
lamp. It has beer_ particularly difficult to obtain
a desired light pattern with a lar_g, small diameter
Sll~3.STiTUTE ~~~E~
WO 92/17733 PCT/US91/07373
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filament, which generally corresponds to a
relatively high operating voltage. The beam pattern
produced by a long, small diameter filament mounted
axially in a reflector typically includes a small
central area of high intensity and a large
surrounding area of lower but significant
intensity. The desired pattern is a central region
of uniformly high intensity which smoothly falls off
to an insignificant value outside the central
region.
Prior attempts to overcome the above problem
have included the use of shorter, lower voltage
filaments and mounting the filament transversely
with respect to the reflector axis. Another
technique for controlling the beam pattern from a
reflector lamp involves spreading and smoothing the
light by roughening the reflecting surface
microscopically and/or macroscopically. This
technique provides little control over where the
light is scattered.
Still another technique for controlling the
light pattern is to introduce small local
deformations of the basic reflector surface. The
local deformations can take the form of facets,
peens, ribs, or the like. In this case, the light
is spread by a specular surface through a given
angular range that is established by the geometry of
the local curvature. The advantage of facets or
peens is that the light is spread about the
direction that the light would take in the absence
WO 92/17733 PCT/US91/07373
- 4 -
of the element, and the magnitude of the spread is
determined by the design of the element. No light
is spread beyond the design limit. Thus, control of
the beam shape is maintained, and light is not
scattered out of the beam to reduce beam efficiency
unless such spread is deliberately desired.
A projector lamp reflector having a faceted
surf ace for spreading the image formed by the
reflector into a larger and smoother pattern and
reducing the amount of imaging of the lamp filament
and support posts is disclosed in U.S. Patent No.
4,021,659 issued May 3, 1977 to Wiley.
A headlight reflector having offset facets is
disclosed in U.S. Patent No. 1,394,319 issued
October 19, 1921 to McNeal. The facet rings in the
McNeal reflector appear to have different numbers of
f acets .
A projection lamp with a reflector having facets
and an axially oriented filament is disclosed in
U.S. Patent No., 4,545,000 issued October 1, 1985 to
Fraley et al._
Multifaceted reflectors are also disclosed in
U.S. Patent No. 4.153,929 issued May 8, 1979 to
Laudenschlarger et al; U.S. Patent No. 3,511,983
issued May 12, 1970 to Dorman; U.S. Design Patent
No. 253,195 issued October 16, 1979 to Henkel et al;
U.S. Design Patent No. 61,209 issued July 11, 1922
to Otte; and U.S. Design Patent No. 61,210 issued
July 11, 1922 to Otte. None of the prior art
reflectors known to applicants has been entirely
WO 92/17733 PGT/US91/07373
- 5 -
satisfactory in producing a desirable light pattern
when a long, small diameter filament is utilized.
It is a general object of the present invention
to provide improved reflector lamp assemblies.
It is another object of the present invention to
provide a faceted lamp reflector for use with a
long, small diameter filament.
It is yet another object of the present
invention to provide reflector lamp assemblies that
are operable with relatively high voltages.
It is still another object of the present
invention to provide reflector lamp assemblies that
have a selectable beam width without a surrounding
area of low but significant intensity.
It is still another object of the present
invention to provide reflector lamp assemblies
having uniform beam patterns.
It is a further object of the present invention
to provide reflector lamp assemblies wherein
circumferential variations in the beam pattern are
substantially_eliminated.
It is a further object of the present invention
to provide reflector lamp assemblies that are easy
to manufacture and low in cost.
It is yet a further object of the present
invention to provide a technique wherein
significantly differing beam angles can be formed
without changing the size or basic shape of a
projector lamp reflector.
WO 92/17733 PCT/US91/07373
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Summary of the Invention
According to the present invention, these and
other objects and advantages are achieved in a lamp
assembly comprising a reflector having a concave
reflecting surface and an axis of rotation, and a
light source including a filament mounted at or near
a focal point of the reflecting surf ace and aligned
with the axis of rotation, the filament having a
length to diameter ratio of 6:1 or greater, the
reflecting surface comprising a multiplicity of
reflecting facets, the facets on at least 50% of the
reflecting surf ace having dimensions and curvatures
selected to produce a light pattern wherein a ratio
of filament width spread to filament length spread
produced by the facets is at least 2:1.
The axis of rotation defines an axial direction
and a rotational direction around the axis.
Preferably, the reflecting facets are arranged in
axially adjacent rings that are centered on the axis
and lie in a plane perpendicular to the axis.
Typically, at. least a plurality of the facet rings
each has the same number of facets.
In a preferred embodiment, adjacent rings with
the same number of facets are successively offset in
the rotational direction so as to reduce
circumferential variations in the light pattern.
The total rotational offset of the plurality of
rings is preferably not less than about one half the
angle subtended by one of the facets.
The facets have a curvature and dimension in the
WO 92/17733 PCT/US91/07373
~~~~t~~~~
rotational direction selected to provide the desired
filament width spread. The facets are typically
rectangular in shape and are not substantially
curved in the axial direction so as to limit
filament length spread.
According to another aspect of the invention,
there is provided a lamp assembly comprising a
reflector having a concave reflecting surface and an
axis of rotation that defines an axial direction and
a rotational direction around the axis, and a light
source including a filament mounted at or near a
focal point of the reflecting surface and aligned
with the axis of rotation, the reflecting surface
comprising a multiplicity of reflecting facets
arranged in axially adjacent rings that are centered
on the axis and lie in a plane perpendicular to the
axis, at least a plurality of the rings having the
same number of facets, adjacent rings with the same
number of facets being successively offset in the
rotational direction.
Brief Description of the Drawings
For a better understanding of the present
invention, together with other and further objects.
advantages and capabilities thereof, reference is
made to the accompanying drawings which are
incorporated herein by reference and in which:
FIG. 1 is a schematic diagram of a reflector
lamp assembly showing the reflection produced by an
incremental area of the reflecting surface;
WO 92/17733 PCT/US91/07373
_ g
FIG. 2 shows an image of a long, small diameter
filament produced by the incremental area of the
reflecting surface shown in FIG. 1;
FIG. 3 shows images of a long, small diameter
filament produced by several incremental areas of
the reflecting surf ace shown in FIG. 1;
FIG. 4 is a graph of light intensity as a
function of angle from the reflector axis showing a
beam pattern in accordance with the prior art;
FIG. 5 is a graph of light intensity as a
function of angle from the reflector axis showing a
desired beam pattern;
FIGS. 6A and 6B show axial and off-axis views,
respectively. of a faceted reflector in accordance
with the present invention;
FIG. 7 is a schematic cross sectional view of a
reflector showing facet parameters;
FIG. 8 is an enlarged cross sectional view of
the reflecting surface in accordance with the
invention, showing the details of reflection from a
single facet;.
FIG. 9 is a graph of light intensity as a
function of angle from the reflector axis for an
example of the present invention;
FIGS. l0A and lOB are axial and off-axis views,
respectively, of a reflector in accordance with the
present invention having offset rings of facets;
FIG. 11 is an elevational side view of an
embodiment of a reflector lamp assembly in
accordance with the present invention; and
WO 92/ 17733 PCT/US91 /07373
r:
s~ ~-:
_ g
FIG. 12 is an elevational side view of a
reflector lamp assembly in accordance with another
embodiment of the invention.
Detailed Descri tion of the Invention
As discussed above, it has been difficult to
obtain a desired light pattern in a reflector lamp
having a long, small diameter filament. A reflector
lamp having a long, small diameter filament is shown
in FIG. 1. A reflector 10 of approximately
paraboloidal shape contains a high voltage tungsten
halogen capsule 12 with a long, small diameter
tungsten incandescent filament 14. As used herein,
long, small diameter filaments include those having
a length to diameter ratio of at least 6:1. The
filament 14 is approximately centered on a focal
point 16 of reflector 10. The longitudinal axis of
filament 14 lies on an axis 18 of reflector 10. A
small, incremental element 20 of the reflector
surf ace is located at an arbitrary point on the
reflecting surface and has a normal that intersects
or passes near the reflector axis 18. Light from
the filament 14 arrives at element 20 within the
bounds of a solid angle 22 that is in the form of a
pyramidal cone. A light ray 24 that appears to
leave the focal point and strike element 20 is
reflected as a light ray 26 essentially parallel to
the axis 18 of the reflector. If the element 20 is
sufficiently small, it acts similar to a small plane
mirror that reflects all rays within the incident
WO 92/17733 PCT/US91/07373
- 10 -
solid angle 22 into another geometrically similar
solid angle 28 about the ray 26.
A screen 30 is positioned perpendicular to
reflector axis 18 and at a great distance from the
reflector 10. The axis 18 intersects the screen at
point 32. The screen 30, viewed along axis 18 from
the position of reflector 10, is shown in FIG. 2.
The light within the reflected solid angle 28
strikes the screen within a boundary 34 as an image
of filament 14. The size, aspect ratio and
orientation of boundary 34 depends not only on the
filament dimensions, but also on the location of
element 20 in space relative to filament 14.
The screen 30 is shown in FIG. 3 with light
reflected from incremental elements at several
different locations on the reflector 10 to form
images 35, 36, 37, etc. In general, the light
pattern produced on screen 30 by the reflector lamp
assembly, as shown in FIG. 1, is the sum of
contributions from all the incremental elements of
the reflecting surface. All or nearly all the
elements of the reflector contribute light at points
within a central region 40. With respect to points
successively outward from region 40 to an outer
limit 42, fewer and fewer reflector elements
contribute light.
The light pattern thus produced in a plane
perpendicular to reflector axis 18 is shown
graphically in FIG. 4. Light intensity is plotted
as a function of angle from reflector axis 18. A
WO 92/ 17733 PCT/US91 /07373
'~;,~ - . . .
21as9~~ -~~-
high intensity portion 44 corresponding to region 40
near the beam center is superimposed on a low
intensity portion 46 that spreads over a large
angular range. It can be determined from
geometrical optics that the dimension of region 40
depends on the filament diameter, while the
dimension of region 42 depends on filament length.
Relatively large changes in the shape of reflector
produce only minor changes in the central part of
the beam shown in FIG. 4. Instead, the boundary of
region 42 in the low intensity part of the beam
changes. The outer part of the light pattern is
well beyond the main, and commonly and most useful,
part of the beam. The beam angle is typically
defined as the angle which includes a region where
the light intensity is greater than 50% of the
maximum intensity. In general, a beam pattern of
the type shown in FIG. 4 is undesirable because it
has a central region of high intensity and a
relatively large surrounding area of low but
significant intensity. A more desirable beam
pattern has a relatively uniform intensity within a
desired beam angle and smoothly falls off to an
insignificant intensity outside the beam angle. A
preferred beam pattern is shown in FIG. 5.
In accordance with the present invention, a
faceted reflecting surface is used to provide a
desired light pattern with a long, small diameter
filament. The pattern is generally of the form
shown in FIG. 5 and can have a desired beam width in
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- 12 -
tae range of about 7 to 65 degrees, even ~~rhen lcng,
small 3:ameter filaments are utilized, The fac2~s
are esed to control the beam patterr_ from the
reflector.
A preferred gene=aI form of the reflector is
shown in FIGS. &A and sH. A reflecti~g surface 60
includes a multiplicity c-_' reflecting facets 62.
Each f acet 62 has a ref letting surf ace of a def fined
size and curvature as discussed below. .n this
ambcdirnent, the facets are arranged in rings 66, the
rings centered ora a reflector axis o4 and lying ir~
planes perpendicular to :ef_ector axis 6~.- The
facets 62 are arranged iz columns 68, as :nest shown
in FIG. 6A, so that the facets in a coiui>1n ara
aligned in an ax:.al direction. In ~he embodiment of
FIGS. fiA and 68, each of the r:.ngs 66 has the same
number of facets. Mare gene=ally in accordance with
t~,e present invention, tse reflecting surface 60
includes one or more groups of facet rings. The
zacet rings within each group have she same number
of facets. Thus, for 2xa.~ple~ a reflecting suryace
may include a group of 12 rings eac:~ having 50
facets and a group of 7 =ings each having 25 facets.
The size and shape of a light beam reflected
from a single facet 62 is a .unction of three
parameters: 1) the beam sFread that would occur in
the limit as the area of the facet shrinks to taro
(this is the spread discussed above in. conjunction
with FIG. 1), 2) the area~af the facet and 3) the
surface curvature of the face. The area and the
surf aca curvature are interactive. The spread in
S~1BS'~1~'t~'~~ S~~ST
WO 92/17733 PCT/US91/07373
- 13 -
any plane containing the central normal to the facet
is a function of the product of the linear dimension
of the facet in that plane and the curvature of the
intersection of the facet with the plane.
Referring again to FIG. 1, a smooth, nonfaceted
reflector with a long, small diameter filament
produces a light distribution as shown in FIGS. 3
and 4. The objective is to make a light pattern of
the general form shown in FIG. 5 where the beam
angle 8 shown in FIG. 5 is significantly greater
than the beam angle shown in FIG. 4. To assist in
understanding the light pattern produced by a facet,
meridional and sagittal planes are defined. The
meridional plane for a facet is that plane
containing the reflector axis of revolution and
passing through the center of the facet. The
sagittal plane is that plane containing the central
ray of the reflected solid angle pencil of rays and
normal to the meridional plane. Spread added to the
local beam in the meridian plane increases the
elemental beam 34 shown in FIG. 2 in the Y-Y
direction. Commonly, the spread in the Y-Y
direction in the absence of a facet is larger than
necessary to obtain a desired lamp performance.
Sagittal plane spread increases the spread of
elemental beam 34 in the X-X direction. When the
multiple elements over the reflector surface spread
in the X-X direction, the required beam spread of
FIG. 5 is obtained.
If no spread is provided in the Y-Y direction,
the coils of an axially located incandescent
WO 92/17733 PCT/US91/07373
~1.~~~~f~~
- 14 -
filament produce cosmetically objectionable
ring-type striations about the center of light
beam. Therefore a very small spread is introduced
by the facet 62 in the meridian plane (direction Y-Y
of FIG. 2). The angular spread to be added in the
meridional plane is approximately that of the
angular separation of striations when the beam is
viewed using a smooth reflector.
The spread produced by facets in different areas
of the reflector is different because different
facets have different orientations and spacings
relative to the filament. In accordance with the
present invention, the facets over at least 50% of
the reflecting surface of reflector 60 have
dimensions and curvature selected to produce
filament image 34 (FIG. 2), wherein a ratio of
filament width spread in the X-X direction to
filament length spread in the Y-Y direction produced
by the facets is at least 2:1 and is preferably 3:1
or greater. The reflector configuration of the
invention is advantageously used with filaments
having a length to diameter ratio of 6:1 or greater
and is preferably utilized with filaments having a
length to diameter ratio of 12:1 or greater.
Typical filaments rated above 120 volts and below
150 watts have a length in a range of about 10 mm to
25 mm and a diameter in a range of about 0.2 mm to 2
mm.
The design of a reflector lamp assembly starts
with the filament required to meet the desired
WO 92/ 17733 PCT/US91 /07373
- 15 -
voltage rating. The desired life rating of the lamp
determines the filament temperature. The luminous
flux to form the required intensity distribution
determines the filament power, or wattage. From the
voltage, temperature and power of the filament, the
size of a practical, stable and manufacturable
filament can be determined by standard, well-known
filament design techniques.
Next, the elemental beams from various parts of
a smooth reflector of the desired size are
determined either by experiment or by analysis using
standard ray tracing techniques of geometric optics
as they apply to reflective surfaces. For example,
see "Mirror and Prism Systems", R. Hopkins, in
Applied Optics and Optical Engineering, Vol. III, R.
Kingslake Editor, Academic Press, 1965. pages 269 to
308, which is hereby incorporated by reference.
Then, sagittal spread and spread in the meridional
plane are added until the desired beam pattern is
obtained. The general facet pattern is shown in
FIGS. 6A and 68.
Since the added spread in a given direction due
to the facets is proportional to the product of
facet width and curvature, a tradeoff of these two
parameters can be made to obtain a practical
reflector design. If the facets are too small,
tooling is expensive. Also, uncontrolled scattering
due to the facet intersections increases as the
number of facet intersections increases. If the
facets are too large, not enough elemental patterns
WO 92/17733 PCT/US91/07373
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overlap to form the desired beam quality. Also,
excessively wide facets may introduce more spread
than is desired and more spread than can be
compensated for by any practical curvature. If the
curvature of the facets is too large (radius of
curvature is too small), local glass thickness
changes are undesirably large for molded glass
reflectors. In metal reflectors, the metal would be
subject to excessive local deformation during
forming. Also, large local changes in a reflector
surface increase the difficulty of virtually every
manufacturing process, whether it be molding of
glass or plastic, or drawing of metal.
Since the width and curvature of a facet cannot
be determined by a closed form equation, these
parameters are determined by ray traces using
standard techniques of geometrical optics. Rays are
traced from the perimeter of the projected area of
the filament as seen from a facet location, as
illustrated by solid angles 22 and 28 in FIG. 1.
The reflected_rays are first determined about the
normal to the base reflector at the location of the
facet. This represents the inherent spread due to
the filament size and orientation and the location
of the filament relative to the facet. This is the
inherent spread for the point that cannot be reduced
by local modification of the reflector. Next, the
change in direction of the normal in the sagittal
plane is determined for the required total beam
spread in the sagittal plane (X-X direction spread
WO 92/17733 PCT/US91/07373
j~. ~~~t,
- 17 -
in FIG. 2). Next, the change in direction of the
normal in the meridional plane is determined for the
required total meridional spread of the beam (Y-Y
direction spread in FIG. 2). These changes in
direction of local normals are achieved by a
combination of offset and curvature of the facets
from the initial normal position.
The details of reflection from a single facet
are shown in FIG. 8. A facet 76 has a width W and a
radius of curvature R. The radius of curvature can
be convex or concave with respect to the reflecting
surface. In the example of FIG. 8, the curvature of
facet 76 is convex. An incident perimeter ray 78
from the filament reflects about a normal 80 to the
base reflector curve as a reflected ray 82. The
facet is essentially centered about normal 80. The
facet 76 has normals 84 and 86 at the edges defining
width W. A ray 88 from the same initial perimeter
point as ray 78 is at a small acute angle « from
ray 78 and is incident on the facet edge at normal
84. A reflected ray 90 deviates from the original
reflected ray 82 by an angle B. By examining the
perimeter rays in relation to the desired spread,
the required angular separation of the edge normals
84 and 86 can be determined. A given separation can
be achieved by various combinations of width W and
radius of curvature R.
Although the facets have been described as
symmetrical about a normal to the base curve near
their center as shown in FIG. 8, the facets can be
WO 92/17733 PCT/US91/07373
r~~~~ ~~~~
- 18 -
asymmetric about that normal, particularly in
sagittal plane. This configuration directs light
more to one side of the meridian plane through the
facet than to the other side. Also, the facets at
various circumferential locations about the
reflector are not required to be the same. Such
variations can be used to produce asymmetric light
patterns with a reflector of nominal revolution.
An example of the present invention will now be
described with reference to FIGS. 7 and 9. A
paraboloidal reflector 70 for a lamp of 95 mm
overall diameter and having an aperture diameter of
82 millimeters and a focal length of 13 mm was
constructed. The filament for the present example,
designed to operate at 225 volts, had a length of
19.3 mm and a diameter of 0.71 mm. The filament was
located on the X axis of the reflector shown in FIG.
7. The nominal reflecting surface of revolution was
described by Y2 - 52X, where the X and Y
directions are shown in FIG. 7. The reflecting
surface included facets arranged in 19
circumferential rings as specified in Table 1
below. The facet rings are numbered consecutively
starting with the front surface of the reflector and
proceeding toward the base of the reflector. Rings
1, 2 and 3 are shown by way of example in FIG. 7.
For each of the 19 rings, Table 1 indicates the
number of facets in the ring, the mean diameter Y of
the ring, the width W of each facet in the ring, the
radius of curvature R of each cylindrical facet in
WO 92/17733 PCT/US91/07373
- 19 -
the ring and the height H of the ring. The axis of
the cylinder is in a meridional plane through the
center of the facet. The dimensions in Table 1 are
in millimeters. The parameters mean diameter Y and
H are shown in FIG. 7. The parameters W and R are
shown in FIG. 8. Each ring of facets was displaced
by 1° with respect to adjacent rings. Offset of
facet rings is described below.
TABLE 1
Ring Number of Mean W R H
Number Facets Y
1 50 40.31 5.07 24.0 2.5
2 5p 38.90 4.89 23.0 2.5
3 50 37.47 4.71 22.0 2.5
4 50 36.01 4.53 21.5 2.5
50 34.51 4.34 20.5 2.5
6 50 32,98 4.14 19.5 2.5
7 50 31.41 3.95 18.5 2.5
g 50 29.79 3.74 17.5 2.5
5p 28.31 3.54 16.5 2.5
50 26.42 3.32 15.5 2.5
11 50 24.66 3.10 14.5 2.5
12 50 22.84 2.87 13.0 2.5
13 25 21.14 5.31 35.0 2.0
14 25 19.58 4.92 31.0 2.0
25 17.97 4.52 27.0 2.0
16 25 16.30 4.10 23.0 2.0
17 25 14.57 3.66 19.0 2.0
18 25 12.76 3.21 15.0 2.0
19 25 10,88 2.73 12.0 2.0
WO 92/17733 PCT/US91/07373
~~~~~ ~(
- 20 -
The beam pattern produced by the above example
of the present invention is shown in FIG. 9 as curve
94. The beam pattern has a central region of more
or less uniform intensity and a beam width of about
30°. The intensity decreases rapidly outside the
beam pattern.
The central part of the light beam produced by
the reflector 60 shown in FIGS. 6A and 6B is
composed of overlapping beams from many or all of
the reflector facets. Due to the overlapping of
many components of different spread and orientation,
the central part of the beam is relatively uniform.
Near the outer periphery of the beam pattern,
however, only a few facets contribute light at any
given point. The light pattern of each facet is
long radially and narrow circumferentially. The
angular separation of facet columns in the
circumferential direction appears in the light
pattern as circumferential variations in light
intensity. If the angular separation of the few
beams that overlap in the outer region is
sufficient, radial streaks or striations are
observed near the edge of the beam.
Decreasing the facet width circumferentially so
as to increase the number of facet columns will
decrease the angular separation of the elemental
beams from the facets and will make the outer
regions of the beam more uniform. However, facet
width is one of the parameters that determines the
overall beam angle of the reflector lamp and cannot
WO 92/17733 PCT/US91/07373
- 21 -
be freely changed. Furthermore, an increase in the
number of facets increases scattered light and
increases the cost of tooling as discussed above.
In accordance with another aspect of the
invention, the total beam from a column of facets is
spread circumferentially without changing the spread
from any individual facet by centering each facet in
the column on a different meridian plane or at least
distributing the facet centers among several
meridian planes. This technique can be described in
terms of circumferential facet rings. Each facet
ring is advanced, or offset, by a small angle with
respect to the adjacent ring.
As shown in FIGS. l0A and lOB, a reflector 100
includes a facet ring 102 that is offset in a
circumferential direction from an adjacent facet
ring 104. Similarly, facet ring 104 is offset in a
circumferential direction from an adjacent facet
ring 106. In general, facet rings are successively
offset in a circumferential direction. The
reflector 100 is the same as the faceted reflectors
described above, except that the facet rings are
offset in a circumferential direction. A general
description of the reflector appearance is that the
facet columns spiral. The size of the angular
offset for each ring depends on the striations that
must be concealed and their sharpness. However, it
is not necessary that each pair of adjacent rings
have a full offset. Large angular offsets between
adjacent rings can cause significant discontinuities
WO 92/17733 PCT/US91/07373
~~~~~f~~f_ 22 _
between elements of the reflecting surface. Such
discontinuities make production of the reflector
more difficult and increase light scattering and
loss. Preferably, small offsets are provided
between adjacent rings such that the cumulative
offset provides the necessary smoothing.
As an example, a reflector and filament having
facets produced a 1o° beam angle. The inherent
spread of the reflector without facets was about
3.5°, and the spread added by the facets was about
6.5°. Little light was transferred to the outer
part of the beam by this small spread, and radial
striations were obvious at the beam edge when the
facet rings were not offset. The reflector had 44
facet columns so that each of the 44 facets in a
ring subtended an angle of about 8.18° about the
reflector centerline. There were 24 facets in each
column between the face and the vertex region of the
reflector. One degree of angular offset was added
successively to each facet ring such that the facet
column spiraled a total of 24° about the reflector
centerline. This offset configuration eliminated
the radial striations that had a period of about
8.18°.
The period of striations near the outer
boundaries of the beam pattern matches the facets in
a ring. The minimum total angle of spiral for a
column of facets to provide satisfactory smoothing
generally falls between one-half of the facet angle
and the full facet angle. Preferably, the offset
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between adjacent facet rings is not greater than
about 40% of the angle subtended by one of the
facets. The offset between adjacent facet rings is
typically about 30% or less of the angle subtended
by one of the facets. Since the offset of facet
rings within a column of facets contributes to
smoothing in general for a variety of
nonuniformities, the offset angle can be increased
to a value greater than that required for
controlling radial striations at the edge of the
beam when it is advantageous to do so. Furthermore,
successively offset facet rings can be utilized in
any reflector lamp to reduce or eliminate
circumferential nonuniformities in the light
pattern.
The techniques describes herein are typically
utilized with open faced lamps or lamps having a
clear cover. However, the disclosed techniques can
be utilized with a lens to further vary the light
distribution. Although the reflector and lamp
combination described herein is typically a PAR
lamp, the invention can be applied to reflectors in
luminaires using separate lamps.
Examples of lamps in accordance with the present
invention are shown in FIGS. 11 and 12. A lamp 120
shown in FIG. 11 includes a reflector 122 having
offset facet rings, a base 124 with an electrical
connector 126, a cover 128 and a lamp (not shown).
A lamp 130 shown in FIG. 12 includes a reflector 132
with offset facet rings, a base with a bipin
WO 92/17733 PCT/US91/07373
- 24 -
connector 134, a cover 136 and a lamp (not shown).
While there have been shown and described what
are at present considered the preferred embodiments
of the present invention, it will be obvious to
those skilled in the art that various changes and
modifications may be made therein without departing
from the scope of the invention as defined by the
appended claims.
