Note: Descriptions are shown in the official language in which they were submitted.
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ELLIPSOIDAL ENVELOPE FOR INC~NDESCENT
1 _ INFRA-RED_ENERGY REFLECTING LAMP
~ackground of the Invention
The use of a visible transmit-ting infrared reflecting
coating on the envelope of an incandescent lamp to reflect
infrared energy back -to the filament to raise its operating
temperature and thereby reduce the amount of energy consumed by
the filament to a desired temperature is known. A typical
approach is to use optically precise spherical envelopes and a
compact filament which is located at the optical center of the
envelope. A lamp of this type is shown in United States Patent
4,160,929, granted July 10, 1979, which is assigned to the
: assignee of this invention. Using such an approach, energy
savings in excess of 50~ with the coating of the foregoing patent
in a spherical envelope is theoretically attainable. This
15 corresponds to an increase in luminous efficacy in excess of ~:~
about 2 times that attainable in a normal lamp operating at -the
same filament temperature.
From a practical point of view, it is not possible to
make either a point source or a spherical filament. In general, it
20 has been determined that an elongated filament, either coiled-coil
or triple coiled which is either horizontally or vertically mounted
with respect to a spherical envelope is the most practical embodi-
ment of lamp to be made with an infrared reflecting coating.
However, when an optically precise spherical reflector is used in
: 25 conjunction with a non spherical, such as an elongated, filament
some of the radiation returned from the reflector on the envelope
is lost due to aberra-tion a-t the ends of the filament. Once this
radiation is lost on one reflection, it is essentially lost for all
subse~uent reflections from the envelope reflective coating unless
30 some type of recovery mechanism is employed.
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The present irvention relates to an improved incandescent lamp
having a filament therein -in which the envelope is ellipsoidal in shape and
has a coating for reflecting infrared energy emitted by the filament back
toward the filament and for transmitting visible energy. The filament is
elongated and mounted along the major axis of the ellipsoid of the envelope.
The foci of the envelope are located on the axis of the filament and at
predetermined distances from each end of the filament to reduce side or end
aberrational losses By using this approach, the aberrational losses can
be reduced to-about half that of a sphere enclosing the same cylindrical
filament. Also, the use of an ellipsoidal element concentrates the returned
IR radiation at two points of predetermined distances from the ends of the
filament rather than at a single point. This makes the temperature gradient
more uniform.
It îs, therefore, an obiect of the present invention to provide
an incandescent lamp whose envelope has an infrared reflective coating means
in which the envelope is in the shape of an ellipsoid.
Another object is to provide an incandescent lamp with an
ellipsoidal envelope having its foci lying along the axis of an elongated
- filament.
A further object is to provide an incandescent lamp whose
envelope is in the shape of the ellipsoid, with the ellipsoid design such
that the two foci of the ellipsoid are located at a predetermined distance
from each end of the filament.
According to a broad aspect of the invention there is provided
an incandescent electric lamp comprising: an ellipsoidal shaped envelope
having a base portion, said envelope defining a major axis on which is a
pair of spaced focal points, an incandescent filament mounted within said
envelope, means for supplying electrical energy to said filament to cause it
to incandesce to produce energy in both the infrared and the visible range9
~0 means for cooperating with said envelope to return to the filament a sub-
stantial portion of the infrared energy produced by said filament and for
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transmitting therethrough a substantial portion of the visible range
energy produced by said filament said filament being elongated, generally
cylindrical and linear along its ma~or axis, at least as long as tlle dis-
tance between the two foci of the major axis of the envelope~ and being
mounted along the ma~or axis of the ellipsoidal envelope so that the two foci
of the ellipsoid are located on the filament at spaced points therealong.
Other ob~ects and advantages of the present invention will
become more apparent upon reIerence to the following specification and
annexed drawings in which:
Figure 1 is an elevational view in section o~ a prior art
incandescent lamp;
Figure 2 is a side vie~ of the lamp of Figure 1 illustrating
the aberrational effects;
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:~
~ ~ ~2a~
7 2
1 Fig. 3 is an elevational view of an ellipsoidal envelope
showing the principals of the invention; and
Fig. 4 is a cross-sectional view of the lamp of Fig. 3
showing the placement of the f ilament.
Fig. 1 shows a type of prior art incandescent lamp 10.
The lamp includes an envelope 11 which is preferably of a desired
optical shape, the illustrative shape being shown as being spheri-
cal except at the base portion. The lamp has a mechanism for
returning infrared (IR) energy produced by the filament upon in-
candescence to the filament. The lamp 10 has coated on the major
part o~ its spherical surface, either internally or externally, a
coating 12 which is highly transparent ~o visible wavelength energy
and highly reflective to IR wavelength energy. A suitable coating
is described in the aoresaid U.S. Patent 4,160,929, granted July
10, 1979, and which is assigned to the same assignee. Other
coatings can be used.
A filament 22 is mounted on a pair of lead-in wires 18,
20 held in an arbor or stem, 17. The lead-in wires 18,20 are
brought out through the arbor to electrical contacts 14,16 on a
base 13. Arbor 17 also has a tubulation (no~ shown) through
which the interior of the lamp envelope is exhausted and filled,
if desired, with a gas. Suitable gases are, for example, argon, a
mixture of argon-nitrogen, or a high molecular weight gas, such as
` krypton. The lamp also can be operated as a vacuum type.
When voltage is applied to the lamp, the filament 22
incandesces and produces energy in both the visible and the
IR range. The exact spectral distribution of the filament depends
upon the resistance of the filament. Typical f ilament operating
temperatures are in the range of from about 2650K to about 2900K,
although operation at a temperature as low as 2000K and as high
as 3050K can be used. As the filament operating temperature
increases, the spectral distribution shifts further to the red,
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1 i.e. it produces more infrared energyO
The coating 12, in combination with the optical shape of
the lamp, serves to reflect back to the filament a substantial, and
preferably as large a portion as possiblel i.e. about 85% or more,
of the IR energy produced by the filament. When the energy is
reflected back to the filament, it increases i~s operating tempera-
ture and thereby decreases the power (wattage) required to operate
the filament at this temperature.
Fig~ 2 shows how the aberrational effects from the ends
of the filament are produced. This shows the lamp cross~section
along the longitudinal axis of the envelope. For purposes of
explanation, it is considered that the envelope is a closed
sphere in this direction. Consider that ~he filament 22 has a
length ~ and that the center of this filament C is located at
the optical center of the spherical envelope. The filament also
is in the general form of a cylinder having a diameter D. Con-
sidering the rays which originate from one end O of the filament
at a point off the axis as it incandesces, these rays are produced
effectively at a variety of angles covering a spherical surface.
Two such rays Rl and R2 are shown which are given off at an
angle which is substantially acute with respect to the axis of
the filament. Two other rays Sl and S2 are shown which are
produced at angle which is somewhat obtuse with respect to the
longitudinal axis of the filamentO As shown, the image point for
the rays Rl and R2 will be near the image point I2, which
is outside of the filament, while the image points for the rays
Sl and S2 will be near the image point Il, which is at the
end of the filament. It can be shown that for all of the rays
originating from the end point O that the image points for many
of these rays lie in a region outside of the end of the filament
opposite the end O. The infrared energy which is not re-imaged
back onto the filament is lost unless recaptured.
7 2
1 A similar analysis can be made for the rays which are
emitted from the end of the filament opposite 0. An analysis is
presented below for calculating these end losses.
The total aberration losses arise from image distortion
associated with the sides, or outside surface of the filament, as
well as its ends. The losses from the sides occur because the
filament geometry does not precisely conform to the shape of the
envelope so that the wavefront of rays from the sides of the
filament also does not exactly correspond to the envelope and
there will be some aberration when they are reflected back to the
filament.
It can be shown that the aberrational losses Lc from
the sides of an elongated filament centered at the optical center
of an optically precise sphere of radius R is as follows:
~ L = _L ~ ~c~
c 31J~ C ~t~ePre )
Where:
Q is the filament length
R is the radius of the Eilament cylinder.
c is the surface area of the cylinder assumed
for the diameter of the filament.
AE is the end area of the filament.
` Ec is the emissivity of the cylinder.
Ee is the emissivity of the end of the filament.
For a filament 1300mm long in an 80mm G 25 spherical
glass enclosure for Ec equals 0.55 and an Ee is approxi-
mately 1, the side losses Lc can be calculated to be about
3.1%. That is, this amount of infrared energy will not re-image
onto the filament and will be lost.
Fig. 3 shows an ellipsoidal envelope made to reduce the
end losses while Fig. 4 is a schematic elevational view to
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7 2
1 that is, an ellipse is taken and rotated by 360 to produce the
ellipsoid. The envelope has a base 13 with stem and tubulation.
The incandescent filament 22, which is preferably coiled-coil or
triple coiled, is treated as a cylinder whose axis lies along the
major axis of the ellipser The envelope 42 is coated, either on
the interior or the exterior with the IR reflective and visible
transmissive coating 12.
Referring to Fig. 4, the envelope 42 is designed with
respect to the filament so that the location of the foci of the
ellipse are positioned to minimize the sum of end and side aberra-
tions. In an ellipsoid, rays emitted near one of the foci points
located along the length of the filament will be reflected by the
coating on the envelope wall back to companion points near the
foci points on the opposite side, considering the center C as a
dividing line, of the filament from which the ray is emitted.
Usually one internal reflection is required. However, the visible
energy passes out through the coating, by an amount determined by
the coating transmittance, on the first impingement of the coating.
Rays emitted from near the foci points have no aberraO
tion. However, there is still aberration at the ends of the
filament due to distortion.
In an IR reflecting envelope, whether spherical or
ellipsoidal, The image of a ray emitted from the end of the
filament at an angle ~ with respect to the filament axis, forms
at some distance S behind the opposite given by the e~uation
2(1/ )2 cos2 e
S(~ (2)
The distance from the end oE the filament to the center is~
Thus, at some range of angles of rays emitted by each end of the
filament, there will be a loss, that is, the rays will not
re-image on the opposite end of the filament after reflection
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1 from the coating. It can be shown that the rays within the solid
angle between 0 and 2 are lost to the filament. That isl
the rays from the left end of the filament hetween the two conical
angles are not intercepted by the filament.
The end loss LE can be calculated to be
LE = (cos ~1 ~ cos ~2) [ EeAe
where, the other quantities have been defined above.
For the 13mm long filament in an 80mm diameter G-25
envelope, ~1 and ~2 are about 10.8 and 64.3. The end
aberration loss LE is about 6~8% and the side is about 3.1~,
as previously discussed, giving a total of 9.9~. This would
be the loss in a spherical enclosure where the elongated filament
was precisely optically centered.
To determine dimensions of the ellipse to minimize the
aberration losses, it is considered that for an ellipse that is
not too eccentric, that the end aberration will depend upon the
distance from one of the foci in the same manner as the aberration
depends upon the distance of the end of the filament from the
center of the sphere. This spherical aberration depends on the
square of the distance from the center and it is assumed that
the elliptical aberration depends on the square of the distance
from the nearest of the two foci loca~ed at a distance from the
` ellipse cneter. The elliptical aberration loss L is then taken as
the sum of both the end and side losses.
~ ~ ) c L ~ (4)
; where,
Lc = the lo~s along the cylinder.
The terms ( ~Q _ X)2 and x2 in the brackets arise
from side aberration away from the ellipse center and toward the
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3 ~2~17~
1 ellipse center. When X = 0, the ellipse degenerates into a
sphere and:
L = LE + LCl as required.
The minimum aberration is given by setting d~ = o. Solving
this equation leads to:
X ~ ) (5)
~/~ E 2Lc
If LE = I X is half way to a filament end from the
` center. That is, the foci is located at X by a distance one-
fourth of the length of the filament from the end of the filament.
At this location of the foci, the elliptical aberration is one-
quarter of the aberration of a filament in a spherical envelope.
If Lc = ~ then X is at the filament end and there is
no elliptical aberration. Thus, the spherical aberration is
reduced by a factor one-fourth or less within the ellipse.
In a practical example~ X - 0.76 (Q/2) and the foci
are located about three-fourths of t:he distance from the center
! to an end. The total aberration loss is then about 2.4% compared
to about 9.9~ in a sphere.
As mentioned, the use of the ellipsoid having two foci
has further advantages in that the reflected IR radiation will be
~ concentrated about two points, the foci, rather than about a single
; point in a sphereical geometry. This makes the temperature more
evenly distributed along the length of the filament. Further, the
returned energy will be focussed at two points rather than one and
` there will be less defocusing in between. Since the luminous
efficacy of a ~ilament is a function of temperature, decreasing at
the cooler positionsf the more even temperature distribution will
produce a greater lumen output. Also, unequai temperature
gradients lead to shorter filament lives and the more even tempera- ;
ture gradient helps to eliminate this.
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