Electric lamp and discharge devices – Incandescent lamps
Reexamination Certificate
1999-10-19
2001-03-20
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Incandescent lamps
C313S578000, C313S624000, C313S569000, C313S573000
Reexamination Certificate
active
06204598
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an electric incandescent lamp comprising:
a shaped glass plate which is connected in a gastight manner to current conductors and to a metal tube which extend through said plate;
an incandescent body occupying a predetermined position relative to the shaped plate and connected to the current conductors;
a glass bulb around the incandescent body, connected in a gastight manner to the shaped plate by means of enamel;
a filling gas having a pressure of at least 1 bar inside the bulb,
said metal tube having a gastight seal outside the bulb.
Such an electric incandescent lamp is known from FR-B-913,579.
The known lamp has a molded or pressed glass plate with a circular edge with a locating stud thereon. The lamp is designed for being passed with its bulb in front through an opening in a reflector and for being pressed home with the edge of the plate against a boundary of the opening. The reflector with the lamp may be used as a motor vehicle headlamp for generating a passing beam and a driving beam.
It is a disadvantage of the known electric lamp that the shaped glass plate can only be manufactured with wide dimensional tolerances. This means that the position of the incandescent body is also badly defined.
It is desirable in other applications of incandescent lamps, for example at the rear of vehicles such as motor cars, to have available electric incandescent lamps which can be moved backwards, i.e. with a lamp cap or base in front, against a carrier, and whose incandescent body then occupies an accurately defined position relative to this carrier with the bulb facing away from the carrier. Such lamps may be used for the stoplight, the taillight, the reversing light, the rear fog lamp, the indicator lights, etc.
Known electric incandescent lamps for these functions are provided with lamp caps. The incandescent body thereof has a position which is defined within no more than wide limits relative to the lamp cap, and the lamps have a comparatively short life. In addition, there is a comparatively high early failure rate owing to lamp leaks.
SUMMARY OF THE INVENTION
It is an object of the invention to render possible an accurate, predetermined position of the incandescent body relative to the shaped glass plate as well as a compact construction.
According to the invention, this object is achieved in that the shaped glass plate is a sintered body made of a first glass and the bulb is made of a second glass, the first glass and the second glass have coefficients of linear thermal expansion which correspond to each other.
The sintered glass plate can be manufactured to a high dimensional accuracy. The current conductors and the metal tube may be present right from the start and may be passed through the glass in a gastight manner in that case. The sintered glass plate has the further advantage that it has a light color, for example white or a pale grey, so that it reflects incident light. It is prevented thereby that the light incident thereon is lost to the light beam which may be formed by means of a reflector.
After the sintered glass plate and the glass bulb are joined, they cool down to a temperature below which they are liable to build up permanent stresses. This temperature is generally known as the strain point, e.g. for a soft glass with a coefficient of linear thermal expansion of about 10*10
−6
K
−1
a normal strain point is about 500° C. The correspondence in coefficients of linear thermal expansion of the first and the second glass, meaning a relatively small difference in these coefficients at least in a temperature range below the strain points of the respective glasses, avoids the built up of high permanent stresses. A large difference would unavoidably lead to high permanent stresses in both the sintered glass plate and the glass bulb due to a different absolute shrinking of these two joined glass parts. Such high permanent stresses enhance the risk of crack formation in the glass and a subsequent early failure of the lamp. Therefore a generally accepted limit in the difference in the coefficients of linear thermal expansion by men skilled in the art is 0.7*10
−6
K
−1
. Preferably the difference is less than 0.5*10
−6
K
−1
.
For the ease of manufacturing of the lamp the two glasses should have correspondent softening points, the temperature at which the glass deforms under its own weight. The softening points should be high enough so that both the sintered glass plate and the glass bulb should be able to retain their shape when they are connected by means of the enamel.
The lamp has a comparatively long life thanks to the filling gas pressure above 1 bar. It is favorable to choose the pressure of the filling gas at room temperature to lie between 2 and 15, in general between 2 and 8 bar, for example between 3 and 5 bar. It is furthermore favorable for lamp life and also for counteracting a decrease in the luminous flux of the lamp caused by blackening of the bulb owing to deposits of tungsten evaporated from the incandescent body when the filling gas comprises Xe, Kr, or a mixture thereof, for example in the ratio in which they are present in the air, i.e. approximately 6% Xe by volume. The evaporation of tungsten is strongly hampered by the high molecular weight of these gases and by their pressure, so that it is possible to use a comparatively small bulb while achieving a high luminous maintenance. This renders it possible to give the lamp a very small constructional height, so that luminaires in which the lamp will be accommodated may be comparatively flat. The lamp may have, for example, a dimension of less than 2 cm from the outside of the sintered glass plate to the top of the bulb.
It may be favorable for the filling gas to comprise a few % of N
2
by volume for preventing breakdown, for example in the case of a comparatively high burning voltage, for example of 24 V or more. It may also be favorable to add a halogen or halogen compound to the filling gas so as to prevent blackening of the bulb.
In a favorable embodiment, a water vapor getter is present in the bulb. The water vapor getter may be provided on the sintered glass plate or against a current conductor, but a particularly convenient getter is formed by a coating on at least one of the current conductors. The water vapor getter renders it possible to heat the bulb and the sintered plate to a comparatively high temperature and thus to dissipate comparatively high powers in a comparatively small bulb. The lamp is accordingly capable of dissipating powers of up to approximately 25 W at said small dimensions. Water vapor released from the glass is bound by the getter, so that it is prevented that a water vapor cycle will arise in the lamp, transporting tungsten from the incandescent body to the sintered plate and to the bulb wall.
It is attractive if the closure of the metal tube is a solidified molten metal. It is possible in that case to seal off lamps in a clean atmosphere, for example in a gastight chamber, for example by means of a laser. Another possibility is that a drop of metal is deposited, for example of tungsten, whereby, for example, a TIG weld is obtained. These methods of providing gastight seals have the advantage that they can be realized quickly and are very reliable. A metal tube has the advantage that it and its seal are not very vulnerable, and that the tube can be sealed off while strain therein is avoided.
In a favorable embodiment, the incandescent body is connected to the current conductors by means of a solidified molten metal. This embodiment not only has the advantage over alternative embodiments, such as pinches or resistance welds, that it can be quickly realized and is reliable, but also that it is accurate. The incandescent body may be brought into an accurate, previously defined position relative to the sintered plate in that case, preferably relative to an outer surface thereof facing away from the bulb, ends of the incandescent body being moved to adjacent the current conductors, an
Bruggemann Ursula
Geboers Jacques M.J.
Kohlmann Wilfried L.
Van Hees Antonius J.M.
Day Michael H.
Faller F. Brice
U.S. Philips Corporation
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