ELECTRIC INCANDESCENT LAMP

Abstract: The electric incandescent lamp according to the invention has an annular mirror coating (10) on a hemi­ spherical part (7) of its lamp vessel (1) and a filament (5) arranged zigzagwise between supporting points (12,13), which are located on the surface of an imaginary cone (14). The filament (5) is situated outside the part of the lamp vessel provided with the mirror coating (10). The lamp is particularly suitable for use as a traffic signal lamp and produces together with an external paraboloidal reflector a light beam of high luminous intensity at the centre and in directions enclosing a small angle with the axis of the beam, as a result of which the lamp may be designed for a comparatively low power.

Description: The invention relates to an electric incan­ descent lamp provided with a glass lamp vessel, which is sealed in a vacuum-tight manner, to which a lamp cap provided with contacts is secured and in which a filament is arranged, which is electrically connected to contacts of the lamp cap, the lamp vessel having opposite to the lamp cap a sub­ stantially hemispherical part, which has an axis of sym­ metry and has a reflective coating except in a region near and around this axis, the filament being arranged between a plurality of sup­ porting points, around the axis of symmetry near the widest boundary of the reflective coating. Such a lamp is known from GB 2,109,990 (Thorn EMI PLC, 8 June 1983). The known lamp has a filament, which is ar­ ranged in a flat plane which is at right angles to the axis of symmetry. The filament is situated within the hemispherical part of the lamp vessel. The lamp of the kind mentioned in the opening paragraph is intended to be used in, for example, an ex­ ternal paraboloidal reflector. Light thrown by the fila­ ment onto the reflective coating of the lamp vessel is reflected to the external reflector and is concentrated by the latter to a light beam together with light thrown directly onto the external reflector by the filament. In the hemispherical wall portion of the lamp vessel, the known lamp has a region not provided with a mirror-coating, which has for its object to keep the temperature of the lamp cap as low as possible. If this region should also be provided with a mirror coating, thermal and luminous radiation thrown onto this coating would in fact be reflected to the lamp cap. The known lamp is suitable to be used at areas at which by a high luminous intensity the attention should be drawn to an object. However, the known lamp has proved to be not particularly suitable for applications in which stringent requirements are imposed on the shape of the light beam formed by the lamp together with an external reflector. Traffic signals form such an application. It has in fact been found that in the known lamp the position occupied by the filament with respect to the reflector is particularly critical. A forward or backward displacement of the filament of a few tenths of a millimetre with res­ pect to the focus of the external reflector is already inadmissible in this lamp when used as a traffic signal lamp, unless the lamp consumes a higher power than is necessary in case of a correct positioning of the fila­ ment. The particularly small tolerance in the position of the filament with respect to the external reflector requires, when used as a traffic signal lamp, an extreme­ ly accurate mounting of the filament in the lamp vessel. However, it is thus not guaranteed that the described re­ sult is obtained. Fatal deviations from the correct po­ sition of the filament with respect to the external re­ flector may still be obtained due to the fact that the lamp is screwed more or less firmly into the lamp holder. However, also with the use of a Swan lamp cap and Swan lamp holder, such deviations may be obtained in case of a wrong positioning of the lamp holder with respect to the reflector. Moreover, it has been found that the known lamp has a short life. The invention has for its object to provide a lamp of the kind mentioned in the opening paragraph, which is particularly suitable to be used as a traffic signal lamp, more particularly a lamp of which the po­ sition of the filament is not particularly critical. Fur­ thermore, the invention has for its object to provide a lamp which, when used in an external reflector, produces a light beam having a high centre value and a satisfac­ tory beam width, which nevertheless consumes a compara­ tively low power and which has a comparatively long life. According to the invention, this object is achieved in that the filament is situated at least sub­ stantially outside the part of the lamp vessel provided with a reflective coating and is arranged zigzagwise be­ tween supporting points, which are located at least sub­ stantially on the surface of an imaginary cone, which is at least substantially coaxial with the hemispherical part of the lamp vessel. Due to the location of the supporting points and the zigzagwise arrangement of the filament between these supporting points, several effects are obtained. The filament extends over a certain distance along the axis of symmetry. Thus, it is achieved that the position of the filament with respect to the external reflector with which the lamp has to cooperate is not particularly critical. A small displacement of the filament in for­ ward or backward direction substantially does not in­ fluence the beam produced. Always parts of the filament will be situated in the focal plane of the external re­ flector (the plane through the focus at right angles to the axis of the reflector) and other parts will be situated immediately before and behind this plane, res­ pectively. Another effect of the arrangement of the fila­ ment is that the filament has a small extent so that the parts of the filament are close to each other, as a re­ sult of which the emitted light can be satisfactorily concentrated to a beam. At the centre of the beam pro­ duced, a high luminous intensity is thus attained, as a result of which the beam has a large range of action. On the other hand, when the lamp is used as a traffic signal lamp, it has to be achieved that the traffic be­ ing near the signal and therefore generally laterally of the centre line of the bean can observe the signal. The arrangement of the filament, which ensures that parts the filament are situated closer to the axis of symmetry than other parts, is then of great importance. It has proved to be favourable if the imaginary cone is orien­ tated so that its base is situated near the lamp cap and its tip is remote from the lamp cap. The window in the reflective coating has the favourable consequence that the luminous intensity at the centre of the beam produced is higher than in the absence of this window. It has proved to be favourable if the dimensions of the window transverse to the axis of symmetry is at least as large as the largest trans­ verse dimension of the filament. According as a beam of larger width is required, the window can be chosen to be larger. In order to maintain a high luminous intensity at the centre of the beam, the window will generally not be chosen to be wider than 2 times, more particularly 1.75 times the largest transverse dimension of the fila­ ment. The reflective coating may consist of a, for example, vapour-deposited gold, silver or aluminium layer, for example at the inner surface or the filament. Due to the fact that the filament is situated at least substantially outside the part of the lamp ves­ sel provided with the mirror coating, it is prevented that parts of the filament or parts of its support are strongly heated by radiation reflected by the mirror coating. Thus, the filament is prevented from breaking prematurely, which would result in the end of the life of the lamp. Although in the construction according to the invention the support of the filament is heated by radiation, local excessive heating, which occurs if a filament or part thereof arranged within the mirror-­ coated part of the lamp vessel is imaged by the mirror coating on the support or on the filament, is avoided. An embodiment of a lamp according to the in­ vention is shown in the drawing. In the drawings: Fig. 1 shows a lamp in side elevation, Fig. 2 shows the filament of the lamp shown in Fig. 1 with its supporting points in front elevation. In Fig. 1, the electric incandescent lamp com­ prises a glass lamp vessel 1, which is sealed in a vacuum-­ tight manner, to which a lamp cap 2 is secured having con­ tacts 3 and 4, and in which a filament 5 is arranged, which is electrically connected through current supply conductors 6 to the contacts 3,4 of the lamp cap 2. Opposite to the lamp cap 2, the lamp vessel has a hemispherical part 7, which has an axis of symmetry 8 and which has a mirror coating 10 except in a region 9 near and around this axis 8. In the lamp shown, the lamp vessel 1 is transparent, except at the area of the mirror coating 10, which in the embodiment shown is an internal vapour-deposited aluminium layer. A transparent lamp ves­ sel is advantageous because the light rays can then ema­ nate without being scattered. Near the widest boundary 11 of the mirror coat­ ing 10, the filament 5 is arranged between a number of supporting points 12,13 around the axis of symmetry 8. The Figure shows that the filament 5 is situated at least substantially outside the part of the lamp vessel provided with the mirror coating 10 and is arranged zigzagwise between supporting points 12,13, which are located at least substantially on the surface of an imaginary cone 14, which is at least substantially coaxial with the hemispherical part 7 of the lamp vessel. The tip 15 of the cone 14 is remote from the lamp cap 2, while the base 16 is near the lamp cap 2. Figures 1 and 2 together illustrate that the filament 5 is arranged zigzagwise between a first series of supporting points 12 forming a wide circle and a se­ cond series of supporting points 13 forming a small circle. The filament 5 has a certain height, that is a certain dimension in the direction of the axis 8 (about 11 mm), as a result of which a large tolerance is obtain­ ed for the position of the filament 5 with respect to the focus of an external reflector, which focus must be lo­ cated according to the design of the lamp at the point 17. Also with a non-ideal positioning of the filament 5 with respect to the said focus, parts of the filament 5 are situated in and on either side of the plane through the said focus and at right angles to the axis 8. As a result, a non-ideal positioning does not or substantial­ ly does not influence the light beam. In the embodiment shown, the largest transverse dimension of the filament is about 42 mm, while the dia­ meter of the region 9 not provided with a mirror-coating is about 38 mm. It has been found that also under practical conditions in which it is frequently switched on and off and is subjected to vibrations, the lamp shown has a long life. The lamp produces together with an outer reflector an excellent light beam, as a result of which the lamp need consume a comparatively low power. The lamp according to the invention was operated in a traffic light with a red lens and was test­ ed with respect to the Netherlands Standard NEN 3322. The lamp was compared on the one hand with lamps having a filament of the same shape, but having a lamp vessel not provided with a mirror coating, and on the other hand with a lamp having an annular mirror on the lamp vessel, as in the lamp according to the invention, but having a flat filament stretched in a plane at right angles to the axis of the lamp. The results are indicated in Table 1. It appears from this table that the clear lamp 1 of 75 W does not satisfy the standard. The lamp 2 of 100 W amply satisfies this standard, but the standard is also amply attained by the lamp of 75 W (lamp 3) ac­ cording to the invention. It further appears that with a mirror-coated lamp (4) having a flat filament and consuming 70 W the standard is not reached. However, if the same filament is arranged in accordance with the invention (lamp 5), this standard is largely exceeded. Since traffic signal lamps mostly operate a large number of hours each day, substantial savings in energy consumption can be attained with the lamp accord­ ing to the invention. It should be noted that the values of the lu­ minous intensity of lamp 3 cannot immediately be compared with those of lamp 5 because of greatly different dimen­ sions of the filament due to the different operating voltages.

Claims: 1. An electric incandescent lamp provided with a glass lamp vessel which is sealed in a vacuum-tight manner, to which a lamp cap provided with contacts is secured and in which a filament is arranged, which is electrically connected to contacts of the lamp cap, - the lamp vessel having opposite to the lamp cap an at least substantially hemispherical part having an axis of symmetry and having a reflective coating except in a region near and around this axis, - the filament being arranged between a plurality of supporting points, around the axis of symmetry, near the widest boundary of the reflective coating, characterized in that the filament is situated at least substantially outside the part of the lamp vessel provid­ ed with the reflective coating and is arranged zigzagwise between supporting points, which are located at least substantially on the surface of an imaginary cone, which is at least substantially coaxial with the hemispherical part of the lamp vessel. 2. An electric incandescent lamp as claimed in Claim 1, characterized in that the base of the imaginary cone is located near the lamp cap, while its tip is re­ mote from the lamp cap.

Inventors: HUYBERS, ANTONIUS JOHANNES, INGEVELD, JOHANNES WILHELMUS
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: H01K1
IPC codes: H01K1