Incandescent lamp with ellipsoidal envelope and infrared reflector

An incandescent electric lamp having an envelope in the shape of an ellipse rotated about a center line and defining a circle of focal points, said envelope having thereon a coating which reflects infrared energy produced by the filament and at least a part of the coating transmitting all or a selected portion of the visible range light energy produced by a filament which is shaped and located so that at least a part of the filament lies on or adjacent to the focal circle so that infrared energy produced by the filament at one focal point on the circle will be reflected by the coating back to another focal point on the circle.

BACKGROUND OF THE INVENTION 
A variety of incandescent lamps exist for specialized purposes. Once such 
specialized purpose is a traffic signal lamp in which the lamp is mounted 
on a fixture which is generally located above the line of sight. 
Consequently, the filament of such a lamp is designed so that when it is 
placed in its fixture it will radiate light downward rather than upwardly, 
where it would be wasted. One such lamp uses a W-shaped filament with the 
bottom portion of the W located below the central medial plane of a 
spherical shaped envelope. For traffic purposes, the lamp can either be of 
clear glass with a filter, such as a colored glass filter, placed in front 
of it so that the appropriate color would be transmitted, i.e. red, green 
or yellow. In other types of lamps, the lamp itself is colored, generally 
with a painted organic pigment color over the lamp envelope. 
Work has also been done in connection with improving the efficiency of 
incandescent lamps by applying to the lamp envelope a visible 
transmissive-infrared reflective (heat mirror) coating. The envelope of 
such a lamp is often optically shaped and the coating placed therein will 
reflect back to the filament a substantial portion of the IR energy that 
is produced to raise its operating temperature. This in turn reduces the 
amount of power needed to heat the filament to its operating temperature, 
thereby increasing the efficiency of the lamp. 
The heat mirror coating also transmits a large portion of the visible range 
energy produced by the filament. One such type of lamp is shown, for 
example, in the Thorington, et al. U.S. Pat. No. 4,160,909, which is also 
assigned to the assignee of the subject application in which the coating 
is a composite of three discrete films of Ti0.sub.2 /Ag/Ti0.sub.2 which is 
capable of transmitting an average over the the visible range of about 60% 
and above of the visible range energy and reflect an average of about 60% 
and above of the IR range energy. Other types of such lamps also have been 
proposed using various other types of coatings than that disclosed in the 
Thorington, et al patent. Another incandescent lamp using a different type 
of coating is disclosed in application Ser. No. 45,645, filed June 5, 1979 
in the name of Peter Walsh, which is a continuation of application Ser. 
No. 863,155, filed Dec. 22, 1977, now abandoned, both of which are also 
assigned to the assignee of the subject application. In that application, 
the coating is an etalon of a dielectric film sandwiched between two films 
of silver. 
In lamps of the type using a heat mirror coating, theoretically a point 
source filament precisely located at the optical center of a spherical 
envelope, for example, would be ideal so that the maximum amount of IR 
energy reflected by the coating will impinge back onto the filament. 
However, a point source filament is not reliable and, instead a "compact" 
filament is used. The term "compact" is meant to mean an elongated 
filament in which the length to diameter ratio of the filament is made 
relatively small. Such filament is generally mounted vertically in the 
envelope with respect to the lamp base. 
The use of such a lamp with a heat mirror coating and "compact" filament in 
a specialized environment, such as a traffic signal lamp, would be 
somewhat inefficient. Although the overall efficiency of the lamp has been 
raised by the coating, the light emitted by the filament would not be 
preferentially directed downwardly. Also, from the point of view of 
operating life, in general service types of lamps as well as in traffic 
signal lamps, a compact filament is not as desirable as a C-shaped (or 
circular shaped) filament, which is the type of filament usually used in 
general service lamps. Such C-shaped filaments have three mounting 
supports, one at each end and the third in the center and is quite rugged. 
If a C-shaped or circular filament were used in a spherical-shaped envelope 
having an IR reflective coating, the lamp would be inefficient since all 
of the filament would be located far from the optical center of the 
envelope and the IR energy would not be optimally reflected back to the 
filament. 
Accordingly, the present invention is directed to a novel incandescent lamp 
having a visible transmissive-IR reflective (heat mirror) coating on the 
envelope in which a filament, such as a C-shaped or circular filament, is 
used. The envelope is uniquely shaped as an ellipse which is rotated about 
its center with the two foci of the ellipse forming an infinite number of 
foci lying in a circle called the focal circle. The filament is located on 
or near the focal circle so that the energy reflected by the heat mirror 
coating impinges on it. 
The lamp can be utilized with either a heat mirror coating which can 
transmit light over the entire visible range, or it can be used with a 
coating such as to produce a selective color. The latter improves doubly 
the efficiency of the lamp both from the point of view of the IR 
reflective coating increasing the energy efficiency and the selective 
color coating being more efficient than a pigment coating and thereby 
reducing the amount of energy needed to produce a given amount of light at 
the particular color. 
It is, therefore, an object of the present invention to provide an 
incandescent lamp utilizing an envelope in the shape of an ellipse of 
revolution with the filament being located on or near the focal circle of 
the envelope. 
A further object is to provide an incandescent lamp having a curved 
filament located on or near a focal circle of an envelope shaped as an 
ellipse of revolution, with the lamp having a coating thereon to reflect 
IR energy back to the filament. 
An additional object is to provide an incandescent lamp having an envelope 
in the shape of an ellipse of revolution with a curved filament located on 
or near a focal circle defined by the ellipse, with the coating also 
transmitting only a selected color portion of the visible light.

Referring to FIGS. 1A and 1B, there is shown an incandescent lamp 10 having 
an envelope 12 of lime glass, PYREX, or any other suitable glass material, 
the exact nature of which is not critical to the subject invention so long 
as it is capable of transmitting light in the portion of the visible range 
of concern. The shape of the envelope 12, as viewed in elevation, is an 
ellipse. As described below, in some cases, the drawings may be 
exaggerated as to the shape of the ellipse. The ellipse has two foci, 
designated f1 and f2. The envelope is rotated about a center line C--C 
midway between the two foci f1, f2 as shown in FIG. 1A to form an 
ellipsoid. The major axis of the ellipsoid in FIG. 1A is shown in the 
horizontal plane perpendicular to C--C. As the ellipse is rotated, the two 
foci f1, f2 describe a circle FC as shown in FIG. 1B. That is, FC is the 
circle of an infinite number of conjugate focal points. 
Located on all, or a substantial portion, of the wall of the envelope, 
either on the interior or exterior thereof, but preferably on the 
interior, is a coating 14 of a material which is reflective to IR energy, 
but transmissive to light energy over the complete visible range or over a 
selected portion thereof. Such material is called a heat mirror. Typical 
coatings are disclosed in the aforesaid patent to Thorington et al., which 
discloses a composite coating + formed of a film of metal sandwiched 
between two discrete films of an insulator material. In the coating of 
that patent, the metal is silver and the dielectric materials are titanium 
dioxide or magnesium fluoride. Such a coating has the capability of 
transmitting light over substantially all of the visible light range while 
reflecting IR energy. It is preferred that such coating have a high 
transmissivity (e.g. 60% and above on average) over the visible range and 
a high reflectivity (e.g. 60% and above on average) over the IR range. 
Another coating, a so-called etalon coating, is disclosed in the aforesaid 
application to Peter Walsh. In this application, an etalon heat mirror 
coating is disclosed in which a film of a dielectric material, such as 
titanium dioxide or magnesium fluoride, is sandwiched between two discrete 
films of metal, such as silver. Another suitable type of coating is 
described in application Ser. No. 174,711, in the name of Peter Walsh, 
filed Aug. 1, 1980 which is also assigned to the assignee of the subject 
application. The coating of that application is also an etalon coating, 
but it is designed to transmit light only in a selected portion of the 
spectrum, for example, red, blue, green, etc., or over a wider band to 
produce "white" light. 
While in FIG. 1 the coating 14 is located over the entire surface of the 
envelope, it should be understood that it need be used only in the area 
from which light is to be transmitted. In this case, the remainder of the 
wall of the envelope may be coated with a material, such as silver, gold 
or copper, which reflects both visible and infrared energy. 
The envelope 12 has an opening near the bottom therein in which a neck 
portion 18 is formed. Attached to the neck and extending upwardly into the 
lamp is the stem 20 containing the tubulation 22. A pair of lead wires 24 
and 26 extend upwardly from the stem and are attached to the ends of a 
filament 30, which is described in greater detail below. An insulated lead 
wire 28 also extends from the stem 20 and is used as a support for the 
filament. The filament 30 is curved, in a C or ring shape, and is mounted 
to the wires 24, 26, 28 with its ends electrically connected to lead wires 
24, 26. The filament can be of any conventional type, for example, coiled 
or coiled-coil, and of any suitable material, for example, plain or doped 
tungsten. 
The lead wires 24 and 26 exit through the stem, one making contact with a 
metal base member 32, shown illustratively as being screw-threaded and the 
other with a contact tip 34 at the bottom of the base. 
If desired, a reflector 36 can be located on the stem in conformity with 
the shape of the envelope to substantially complete the reflecting optical 
surface of the envelope so that light emitted by the filament will not go 
into the neck portion of the base and disappear. The reflector 36 need be 
only reflective to IR energy since visible light cannot pass out through 
the base. 
The interior of the lamp envelope is exhausted through the tubulation 22 
which is tipped off in the usual way before the base 32 is applied onto 
the neck. Before tipping the envelope off, the lamp can be filled with any 
desired and suitable fill gas, for example, argon, krypton, mixtures of 
various gases, etc. depending upon the characteristics of the lamp. 
While a particular more or less conventional base arrangement has been 
shown for the envelope 12, it should be understood that other types of 
base arrangements can be used. For example, a glass button base having the 
filament mounted thereon can be sealed directly into the opening in an 
envelope and contacts made to the glass button base. 
As indicated, the envelope 12 is an ellipse which has been revolved about a 
center line C. Considering first the ellipse showing the cross-sectional 
shape of the envelope, such an ellipse would have two foci, at the points 
f1 and f2 as shown in FIG. 1A. Any ray of light emitted from portion of a 
filament located on a focal point, e.g. f1, would be transmitted to a 
point on the envelope from which the visible light will exit. The IR 
portion of the energy of the ray is reflected from the coating 14 back to 
the opposite focal point f2. If another portion of the filament were 
located at the focal point f2, then the IR energy emitted from point f1 
would be reflected onto focal point f2 with the only loss being the loss 
in the coating 14. 
When the ellipse is rotated to form the overall ellipsoidal shaped 
envelope, an infinite number of focal points are produced, all of which 
lie on a circle FC whose center is the point C. A circular filament having 
the same diameter as the circle of focal points and located on such is 
effective in that energy radiated from any part of the filament which is 
on the focal circle is reflected by the coating and returned to the focal 
point which lies diametrically opposite on the circle from the point where 
the energy was radiated. 
It can be shown that in an envelope of the type under consideration, that 
the aberrational losses are relatively small. Consider the case of a 
coiled filament which is a circle and has a radius R. The radius of a turn 
of the coil is given as r and the filament lies on the focal circle with 
the focal circle being coaxial with the center of the coil of radius r. 
In the ellipsoidal envelope of FIG. 1, consider that S is the distance from 
the filament to the adjacent focal circle. By using some approximations it 
can be shown that the aberrational reflection factor f, that is, the 
fraction of those rays which are emitted from any point of the filament 
and which return to the filament on the first pass, that is, only one 
reflection from the envelope, is approximately equal to: 
##EQU1## 
when r is small. The aberrational reflection factor f is near unity when 
##EQU2## 
For S less than this value the simple approximations do not hold but f 
approaches closer to unity as S is reduced. Thus as a simple 
approximation, when 
##EQU3## 
only small abberrational losses will be encountered. 
If a circular envelope is used, the focal circle reduces to a point at the 
center of the circle. In that case S is large for normal C-shaped filament 
and f is small. Thus very large aberration losses are found when C-shaped 
filaments are used in a circular envelope with a reflective coating. These 
losses are overcome by using an elliptical envelope, as disclosed, so that 
the filament lies everywhere near the focal circle, allowing S to be 
small. 
FIG. 2 shows another embodiment of the invention. As indicated previously, 
in some specialized applications such as a traffic signal lamp, it is only 
necessary to direct the light outwardly and more or less downwardly when 
the lamp fixture is located fairly high above the ground. In FIG. 2 the 
filament 50 is in the shape of a semi-circle which would be oriented 
toward the bottom of the fixture when the lamp is inserted. Since light 
need not be transmitted out of the top half, or some other similar 
portion, of the envelope the coating on the portion which does not have to 
transmit light need not be light transmissive. Instead, another coating 52 
for example, a thick film of metal such as silver or other material which 
is highly reflective to IR is provided. Here, the IR portion of the rays 
from the filament must make two reflections from the envelope wall to 
return to the same point on the filament from which it left. It should be 
understood that since there is no upper half for the filament that a ray 
from a focal point f1 has no conjugate focal point f2 to land on and must 
be again reflected from the envelope to return to f1. Therefore, the use 
of a highly reflective metal 52 provides more efficient return of the rays 
toward the filament and to the lower half of the envelope which is coated 
with the IR reflective and light transmissive coating 14. The metallic 
film further reduces the cost of the envelope. The increased IR 
reflectivity of a metal only coating as compared to a coating of thin film 
further increases the efficiency of the lamp. 
If the curved filament is not wound in an exact circle or portion of a 
circle, the IR energy from a given light ray must make at least two 
reflections from the envelope coating before returning to the original 
point of production of the ray rather than to the focal point opposite 
itself on the circle. Deliberate off-centering of the filament can be 
somewhat advantageous from the point of view of eliminating certain of the 
manufacturing problems which are inherent with attempting to try to 
precisely center a filament on the focal circle. However, the efficiency 
of such a lamp would be reduced somewhat depending upon the degree of 
off-centering. An incandescent lamp with heat mirror coating and an 
off-centered filament is described in U.S. patent application Ser. No. 
952,267, filed Oct. 18, 1978, now U.S. Pat. No. 4,249,101 granted Feb. 3, 
1981, which is also assigned to the assignee. 
The portion of the envelope coated with a metal film 52 can alternatively 
be the upper portion of the lamp of FIG. 1, the lower portion of the lamp 
of FIG. 1, or the lower portion of the lamp of FIG. 2, when light is to 
exit from the top portion in this last arrangement. Where these 
arrangements are utilized, in which a portion of the envelope is coated 
with a metal rather than with the IR reflective-visible transmissive 
material, such portion has higher reflectivity to IR energy thereby 
tending to offset the reduction in efficiency due to off centering the 
filament. 
It should be understood that the filament need not be totally curved. FIG. 
3 shows a modified version of the filament 80 in which the focal circle FC 
is shown in dotted line and the filament is made generally W-shaped. As 
seen, a major portion of the filament 80 lies along the focal circle FC so 
that substantial gains are obtained. In addition, even those portions 
which lie off the focal circle still will receive some returned radiation, 
particularly if the distance off the focal circle is less than 
##EQU4## 
It should be understood that in a typical lamp, the envelope would appear 
to the eye to be more or less spherical and, therefore, the envelope 
shapes of the drawings may be considered to be somewhat exaggerated. For 
example, in a lamp in which the radius of a circular filament is 
approximately 18 mm and it was desired to locate such filament on the 
focal circle of an ellipsoidal envelope, the overall dimensions of the 
envelope would be approximately 59 mm along the line CC of FIG. 1 and 
approximately 62 mm on a line transverse to the line CC. Thus, the 
envelope of such a lamp would appear to the eye to be generally spherical. 
While the filaments of the lamps are shown as being horizontal with the 
base of this envelope in a downward direction, an envelope can be made in 
which the filament extends vertically with the base down. This is shown in 
FIG. 4. Here the filament 92 is generally C-shaped. The envelope has been 
rotated to a position where the plane of the focal circle lies in the 
paper.