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Патент USA US2121765

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June 21, 1938.
Filed Nov. 25, 1935
4 Sheets-Sheet _l
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Po'lNT Sowecz AN
POLAR 'I/v-rE/vs/TY Cum/E
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' June 21, 193a.
Filed NOV. 25, 1935
4 Sheets-Sheet 2
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June 21, 1938'.
Filed NOV. 25, 1935
4 Sheets-Sheet 3
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‘ June 21, 1938.
Patented June 21, 1938
Clarence K. ltelman, Newton, and Joseph F.
(look, In, West Roxbury, Mass, assignors to
Birdseye Electric Company, a corporation of
Application November 25, 1935, Serial No. 51,364
9 Claims. (01. 176-34)
This invention is concerned with improvements ‘and illuminated by a single lamp and re?ector,
in the design and' construction of incandescent
electric lamps, and particularly with the type of
lamp in which some part of ‘the bulb surface is
coated to act as a highly e?icient re?ector for
the purpose of directing the light and conserving
it for useful purposes.
In one aspect the present invention consists
in an improved lamp of such construction as to
10 make possible the uniform illumination, on a
plane perpendicular to the main axis of the lamp, ‘
of the area served by the lamp. For example,
this consideration is of much importance.
In order to obtain'uniform illumination over _'
the entire area illuminated by a single re?ecting '
lamp, it is obvious that considerably more than
half the total light from the ?lament must be
' re?ected, and this major portion of the total
light so directed that it will fall on the ?oor near
the outskirts of the lighted area. That is, it may
be considered roughly that the. direct light will 10
account for most of the illumination directly
under the lamp, and since the area of the annular
with a single direct overhead lamp, designed to ~ space between this center. area and the periph
concentrate by means of re?ecting surfaces the ery oi the lighted circle is greater than the center
15 major part of the total light emitted from the area, more light must be re?ected into this an 15
?lament in a circle, say 30 feet in diameter, it is' nular space than falls directly into the center
possible withv the lamp herein disclosed to have area. Thus a re?ector to accomplish this re
the intensity of the illumination falling on the sult must completely ho'od the light source,
?oor on every unit area within this 30 foot circle, that is, the ?lament must be well up inside the re
20 substantially uniform. Not only_is this resultnovel ?ecting area, which is preferably a silver coating 20
and an important improvement in the lighting on the inside of the bulb itself, so that much
art, when accomplished by means of the lamp more than half the total light will be gathered
by the re?ector and directed into the annular
only. and without the use of any external re?ect
ing device, but as far as we know‘ it has never be- _
25 fore been fully-accomplished even by the use of
an external re?ector.
In designing a re?ector to produce uniform
illumination of the lighted area when this re
?eeting surface is to_ be part of the surface ]0f
In the past, uniform illumination on the work
the bulb of the lamp, there are many consider
ing ?oor of a factory, for example, has been at
ations that do not enter into the design of ex
tempted by arranging the placement of the over
ternal re?ectors. Thus the re?ector must ob-J so
80 head external re?ector lamps so that by over
viously be of limited size, and the shape must
lapping the areas lighted by each lamp, an ap
proximately uniform result was obtained.‘ The be such that it is‘ physically possible to incor- _
desired intensity level is approximately that porate this special shape as part of a lamp bulb
found directly under .one of the lamps, and the of rational contour. Because of these factors
intensity along the ?oor-in any direction from a and the fact that the re?ector is close to the
point directly under a single lamp gradually light source necessitating more accurate design
diminishes out torrthe edge .of the area lighted _ than is required for the larger and much more
by the‘ lamp in question. This effect may be distant external re?ector, the ?nal resulting re
shown graphically by means of a curve, the or- , ?ector is of di?erent shape from an external
40 dinates of which represent foot-candles, and the re?ector designed to attempt the same effect.
We have discovered that it is possible 8.0-’
absclssae the distance in feet along the floor.
The overlapping e?ect of the other lamps border ‘curately to design a series of re?ecting surfaces ing on this ?eld is generally shown in the same, , which when made contiguous. with part of the
manner, and then a ?nal curve representing the‘ surface of the bulb enclosing the ?lament, will
45 sum of the ordinates (intensity from all sources) 7 produce uniform illumination over the illumi
is plotted for each of the ?oor ‘points, and this nated area in a plane perpendicular to the major
may approximate a straight line. However, it axis of the bulb. The actual intensity of illumi
never is a straight line and beyond this error nation in lumens per square foot will depend
of approximation this method is, upset by any (a) upon the total light ?ux of the lamp, (1:)
50 pillar or column or part wall, or in fact any upon the distance from the plane illuminated, 50
object that stands on the ?oor which will throw and (0) upon the angle of cut-oif—that is, the
a shadow and eliminate the light from some one
' or more of the sources on a given area.
the unit area to be lighted in a factory is fre
55 quently a bay, bounded on the corners by pillars,
angle between the vertical line from the center ,
of the lamp to the plane and the outside of the
cone of light shining from the lamp to the plane.
However, the'distribution will, within reasonable
limits, be substantiially uniform regardless of
the magnitude of these factors. This relation
holds true except when a wide angle lamp is
moved very close to the plane-at which time
a noticeable excess of light falls on, the area
directly under the lamp, and at great distances
non-uniformity is also apparent. For working
distances of from 3 to 20 feet, a substantially
uniform illumination may be derived from a
lamp constructed in i 'Jrdance with our in
Other features and advantages of the invention
will be understood and appreciated from the fol
lowing description of a preferred embodiment
thereof. selected for purposes of illustration and
shown with explanatory diagrams in the accom
the combined sources of the direct and re?ected
The several re?ectors indicated by their half
traces in the various ?gures of the drawings are
designed for uniformly lighting a given area and
vary only in ‘their cut-off angles which determine
the size of the circle to be illuminated. We have
found that efficient lamps may be constructed 10
having a cut-off angle of any magnitude between
approximatelyv30° and 67°. For smaller angles
the length of the re?ector, and therefore the bulb,
becomes impractical for commercial construction,
and for larger angles the light re?ected from one 15
side of the re?ector strikes the lower edge of the
opposite side and interferes with the desired re
panying drawings, in which,—
Fig. 1 is a diagram showing the shape of re
?ecting surfaces which, under different indicated
conditions, produce uniform light distribution
and includes a polar ‘intensity diagram.
Fig. 2 is a polar intensity diagram of light
emanating from a compactly arranged ?lament
In Fig. '7 is depicted the data of a re?ecting
lamp having a cut-off angle of 45° and in this 20
figure is indicated a group of light rays starting
from the source 0, striking the surface of the re
?ector, and then passing down to the plane being
without any re?ector.
Figs. 3, 4 and 5 are diagrams showing the curve
illuminated. It will be noted that the light rays
striking the lowest part or skirt of the re?ector-—
of the re?ecting surfaces and polar intensity dia
grams of lamps having respectively 60°, 65° and
nearest the transmitting part of the bulb-are '
30° of cut-oil‘.
Fig. 6 is a view in elevation, partly in section, '
of a lamp embodying our invention, the neck por
tion being broken away, and
Fig. '7 is a, diagram showing the curve of the
re?ecting surfaces and polar intensity diagram of
hatched area. is proportional to the intensity of
the light sent in the direction of that radius from
thrown farthest away from the center of the
circle to be illuminated, while rays re?ected by
parts of the re?ector nearest its apex are re
?ected in a direction bringing them closer‘ and 30
closer to the center of the light circle. Rays from
the apex, in the case of a point source of light,
would of course be re?ected downwardly and per
a lamp having a 45° angle of cut-off.
pendicularly, although the ?lament and the
The diagrams above referred to show traces in
the plane of the paper of one half of the re?ector,
mount somewhat interfere with the efficiency of 35
producing uniform illumination, when a theo- >
covered by the hooding re?ector must be directed
the re?ection in this area since most of the rays
toward the more distant part of the circle to be
illuminated. In order‘to produce uniform illumi
nation, it is apparent that the skirts of the re 40
?ector must be designed with especial care.
approximately that shown in Fig. 2.
From the same reasoning it is apparent that, con
In Fig. 1 we have shown the traces of two sim
ilar re?ectors, the two curves overlapping versely, the curve of the re?ecting surface nearer
throughout much of their extent, and both being its apex is of less importance.
Referring again to the fact, shown in Fig. '7, 45
pp GI designed to produce uniform illumination when
the light source is centered at the point 0. One that the re?ected rays cross each other, those
from the base of the skirt of the reflector being
curve represents the theoretical case where the
light source is considered to be a point with directed to the circumference of the circle, we
spherical emission intensity and with a reflection wish to stress the point that this has been the >
case in every re?ector answering all the require 50
50 factor of 100%. ‘This curve is shown in dot and
dash lines and is marked “Re?ecting surface with ments of this problem of uniform illumination
point source. and 100% reflection”. The other that we have designed. It is so in each of the
retical light source is located at its center at the
point 0, and when the light source is a commer
60 cial ?lament having an emission intensity curve
curve is the practical case where a dimensional
speci?c cases shown in the drawings.
?lament having a non-uniform emission intensity
is employed with a re?ecting surface having a
re?ection factor of 70%. This curve is shown in
Fig. l in full lines and is marked “Re?ecting sur
face with dimensional source and 70% re?ection”.
It will be understood that the only important
We have examined the shape of the traces of
curves designed as re?ectors to satisfy the re 55
quirements of this problem of uniform illumina
tion, and in every case we have found the major
part of this trace, ‘and always including the skirts
difference between these curves occurs near the
of the re?ector and continuously to a point near
the apex of the re?ector, to be half of a true cate 60
apex or closed part and, as will be seen herein
nary, having a mathematical formula or equa
after, this part of the re?ector is less important
and less critical in the requirements of its design
than the side walls of the re?ector which actually
65 hood the light source.
In Fig. 1 and in the other diagrams of the
drawings there is shown a cross-hatched area
which represents the luminous intensity on polar
coordinates necessary'in order to satisfy the re
70 quirement of uniform illumination for the cut
o? angle shown, that is 30° in Fig. 1. Since the
bulb and re?ector are symmetrical about the
main axis, this diagram is the same in any plane
passing through the axis. The length of any
radius from the point 0 to the edge of the cross
where X and Y are the rectangular coordinates,
(e) the base of the Naperian logarithm, and (a)
the distance along the axis Y from the X-axis to
the point where the curve cuts the Y axis. The 70
part of the curve which is a true catenary ends, in
each case, at some point marked I 0 on the severalv
?gure-sof these traces shown in the drawings, and
does not extend to the apex of the re?ector it
self. The rest of the curve from this point III to 75
tempted unsuccessfully to determine its mathe
part of the bulb, although this does introduce a
measurable loss in efficiency.
A catenary re?ecting bulb for indirect lighting
matical equation, we have not been greatly in
may be designed in more than one way. For ex
the apex of the re?ector is some other not readily
determined curve, and although we have at
terested in this part of ' the curve, since its useful
ample, the same type of bulb used for overhead
ness and accuracy is of much less importance to a
proper solution of the problem of uniform illum
direct lighting may be used in the reverse posi- ‘
ination than the skirts of the curve, and for the
reasons indicated above. Thus the two half/
10 catenaries, when in proper position with respect
to the trace of the complete re?ector, appear to
be connected by a smooth curve which is of course
perpendicular‘ to the axis of re?ection where it
crosses it.
It is necessary to point out that the entire curve
of . the re?ector is not a catenary, nor is the entire
curve with the exception of the part near the
apex. Nor is the axis of the re?ector the axis of
tion, with the base and stem of the bulb pointing
downwards, and the lighting of the room obtained
indirectly by re?ection from the ceiling. Another
method is to suspend the bulb with the neck 10
pointing upwards, and in this case the catenary
re?ector would be formed around the ?lament
and on the surface of the globular part of the
bulb, a conical re?ector preferably being arranged
around the mount.
Having thus described our invention, what we
claim as new and desire to secure by Letters Pat
ent of the United States, is:
the catenary. The axis of the catenary forming
1. An electric lamp comprising a bulb and a . '
20 most of one, side of any of the re?ectors shown in.
symmetrical re?ecting surface associated there 20
with of such shape that any plane passed through
the axis of‘ the re?ecting surface will be cut by
the drawings is in all cases the line N—N, perpen
dicular to the trace of the re?ector at the point
where the catenary curve ends. The axis N--N it in a curve symmetrical with respect to the
forms an angle with the axis of the re?ector axis, the major portion of said curve being a
catenary located with its axis at an acute angle 25
25 marked A on the drawings. The catenary por
to the axis of the re?ecting surface.
tion of the traceforming the other half of the re
2. An electric lamp comprising a light source
?ector has a similar axis, and if these two half _
curves are moved in relation to each other, until and a bulb having- a re?ecting surface associated
these two axes coincide and so that the points therewith of such shape that any plane passed
30 where the two half catenaries cut their respective through the axis of the re?ecting surface will 30
axis also coincide, there will be formed a full ‘present the trace of half an inwardly concave
catenary. Thus the re?ecting surface of our in
catenary on each side of the axis described about
vention may be described as a surface of revo
an axis inclined to the axis of the bulb.
lution generated by revolving about the axis of
3. An electric lamp comprising a light source
35 the re?ector, a curve‘,'the major portion of which
and a bulb having associated therewith a re?ect-I 35
is a catenary, the re?ector however, not being a ing surface which is the locus generated by half
catenary of revolution, about its own axis, over of a catenary which is inwardly concave revolved
any part of its surface.
about an axis which intersects the axis‘ of the‘
In Fig. 6 we have illustrated a re?ecting electric catenary.
4. A re?ecting electric lamp comprising a bulb 40
40 lamp designed in accordance with the princi
ples above described and havinga re?ecting sur
having a portion formed as a surface of revolu- ‘
face for producing uniform illumination and a
tion by a catenary curve which is inwardly con
cut-off angle of 65°. The main body of the bulb
20 is a surface of revolution formed by revolving
45 a half catenary curve about a vertical axis in the
manner above described. The bulb is provided
with an internal silver re?ecting vcoating extend
ing from a line in the neck 22 and substantially
to the ,line of‘ maximum bulb diameter.. The
curved/ end of the bulb 2| may be of clear. glass
or it may be frosted. The ?lament mount 23 is of
the usual construction and supports a ?lament 25
by means of lead-in and supporting wires 24 in
such relation to the re?ecting surface as to de
55 termine the cut-offangle of the lamp at 65°.
cave about the axis of the re?ecting surface, said
axis intersecting the axis of the catenary curve,
a re?ecting medium applied to said surface of 45
revolution, and a concentrated light source.
' 5. A re?ecting electric lamp comprising a bulb
having reflecting side walls formed as a surface
of revolution by a catenary curve revolved about
the axis of a lamp and being concave with respect 50
to said axis, and a light source hooded by said
surface, the axis of the catenary intersecting the
axis of the lamp at a point in advance of the
light source.
The ?lament 25 is shown as'being of the coiled
type and is compactly disposed in a V-shaped de
6. In an electric lamp, a bulb having a re?ect 55
ing surface of revolution whose generatrix is one
branch of an inwardly concave catenary curve,
sign so that it serves as a concentrated light
source. A circular dished re?ector 26 is located
60 in theneck of the bulb in position to act as the
nary curve at a point within said surface of '
apex of the catenary vcurve, and its re?ecting
curvature is substantially continuous with that
of the bulb itself. The design and location of the
re?ecting disk 26 is not critical because as al
65 ready pointed out it occupies a less critical part
of the re?ecting area.
In order to avoid re?ected images and shad
ows of the ?lament on the illuminated area, we
prefer to design and construct these-bulbs so that
70 the re?ecting surfaces are also diffusing surfaces,
for example by ?rst frosting the surface of the
catenary area and then silvering over this frosted
area as described in application. Serial No. 47,581
of Birdseye and Deren. A diffused light may also
75 be obtained by frosting the clear non-re?ecting
and whose axis intersects the axis of said cate
'7. In an electric lamp, a bulb having a re?ect
ing surface of revolution whose generatrix is one
branch of a catenary. curve, and of which the
axis intersects the axis of said catenary curve
at a point not less than 1/5 nor more than ‘1/2 of 65
the distance from the vertex of said surface to
the open end thereof, said angle of intersection _
being not less than 10 nor more than 30 degrees,
said surface being closed at its smaller end by
continuing said catenary to meet the axis of said 70
surface in a smooth curve.
8. A source of illumination which includes a
bulb having a re?ecting surface of revolution
generated by the rotation about a line of one
branch of a catenary curve whose axis intersects 75.
said line in an angle of not less than 10 nor more
than 30 degrees, and at a point within said sur
of the bulb by revolving as the generatrix a com
pound curve of such shape that the ?lament is
said line in a smooth curve, and a source of light
positioned on said line between said intersection
completely hooded by the re?ecting surface gen
.erated, said curve in all portions being concave
toward the bulb axis, and the lower and the outer
and major portion being a half catenary disposed
and the vertex of said surface.
with its axis intersecting the bulb axis at an acute
face of revolution, the surface being closed at its
smaller end by continuing said catenary to meet
9. A re?ecting electric lamp comprising a ?la
' ment light source and a bulb having a re?ecting
10 surface upon a portion of its walls which is
formed as a surface of revolution about the axis
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