GB1602171A

GB1602171A - Headlamps for vehicles

Info

Publication number: GB1602171A
Application number: GB16656/78A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1977-12-10
Filing date: 1978-04-27
Publication date: 1981-11-11
Also published as: DE2755200C2; FR2411359B1; FR2411359A1; DE2755200A1

Description

(54) HEADLAMPS FOR VEHICLES (71) We, ROBERT BOSCH GMBH, a German Company of Postfach 50, 7 Stuttgart 1, Federal Republic of Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns a headlamp for a vehicle.

Plastics reflectors have the advantage that they do not corrode and have a sufficiently smooth surface even without lacquering. On the other hand, a disadvantage is that they can withstand high temperatures to a lesser extent than lacquered metal reflectors, which can lead to distortion of the parabolic reflection portions of the plastics reflector. The maximum temperatures which reflectors of this type may reach are even higher than in the case of metal reflectors, since the thermal conductivity of plastics material is poorer than that of metal. Particularly high temperatures ensue when the reflectors are fitted in housings and convective dissipation of heat is thereby largely suppressed.

In accordance with the invention there is provided a headlamp for vehicles, comprising a reflector made from plastics material, a lamp holder mounted in the reflector, an incandescent lamp detachably mounted in the lamp holder, and a housing enclosing the rear of the reflector. the incandescent lamp and/or the lamp holder having regions which extend so as to overlite the rear of the reflection to increase heat radiation, and the surface of the regions of the lamp and/or lamp holder which lie opposite the housing being provided with a layer which increases the thermal radiation thereof.

The invention is based on the analysis of the energy flows, according to which a large quantity of heat is transferred from the region of the incandescent lamp to the reflector by heat conduction from the metal elements. Apart from the incandescent lamp itself, the fastening element for securing the lamp and the lamp holder is the hottest member of the headlamp. Thus, cooling of this element by transmission of heat to external members, such as the housing, is the most effective means of reducing the temperature of the hotter members.

The dissipation of heat, that is, the transfer of heat, can be effected by convection, conduction and radiation. However. convective transmission of heat is largely excluded owing to the confined spatial conditions between the reflector, the fastening element and the lamp socket on the one hand. and the headlamp housing on the other hand.

Exhaustive tests have shown that it is possible to transfer heat from. the fastening element to the adjacent parts of the rear wall of the housing in a less expensive but very effective manner by radiation. For this purpose, it is necessary that the emissivity of the fastening element and of the lamp flange, as well as the absorptivity (which is identical to the emissivity) of the interior of the housing, should be high, that is substantially 1.0.

The Stefan-Boltzmann law applies to the thermal energy dissipated per second: A F (eF 8 Td - 80 8 TG4) At (TF, T0 = temperature of the lamp holder and of the housing, respectively; 5 = 5.67. 10'2 watts/cm2 K4; F = radiating area). When 8F = ec, t 1, AW/At -F3(T" - Tc,4).

With a measured temperature of the fastening element of 500 Kelvm and a measured temperature of the housing of 350 Kelvin, and an area F = cm2 available, at present, on the fastening element for the purpose of radiation, the following equation ensues: AW At = 5 5.67 . 10.12 (0.0625 . 10 12 - 0.015 . 10l2) = 1.35 watts The radiation of normal fastening elements made from metal is low, since the emissivity of metal is relatively low at approximately 0.1 to 0.3. Also, housings having a polished metal innerside absorb only a small amount of radiation and reflect the latter.

In order to obtain the high thermal flow of 1.35 Watts theoretically to be anticipated, the areas available for radiation are preferably blackened, and consequently the emissivity is increased. The lacquer used for this purpose, or some other coating increasing the emissivity, must be sufficiently heat-resistant and, furthermore, the lacquer or the coating must have a thickness which is sufficient to ensure an emissivity of substantially 1.0. In accordance with a further development of the invention, clear lacquers having a layer thickness of at least 25m are required for this purpose. Furthermore, it is proposed to provide pigmented lacquer having a layer thickness of at least 101lem, wherein soot can be used as the pigment.

When the rear wall of the housing of the headlamp is made from metal, it is advantageous to coat at least the region of the inside of the rear wall, located opposite the incandescent lamp and the lamp flange, with the lacquer increasing the thermal radiation.

The advantages particularly obtained by means of the invention reside substantially in the fact that the transfer of heat can be increased at least tenfold compared with conventional headlamps, and thus the maximum temperature of the headlamp of approximately 210 Celsius can be reduced to approximately 1700 Celsius and thus to temperatures which are less critical for plastics material.

The invention is described further by way of example, with reference to the accompanying drawings, wherein: Figure 1 is an axial section through a headlamp, and Figure 2 shows a portion of the rear view of the reflector without housing.

The headlamp 9 illustrated in Figures 1 and 2 essentially comprises a reflector 10 made from plastics material, a diffusing lens 11 mounted in front of the reflector, and a housing 12, made from metal or plastics material, sealing the headlamp 9. A metal lamp holder 13 is inserted into an opening in the reflector 10 and receives an incandescent lamp 14, wherein fastening elements 15 abutting against the incandescent lamp and engaging the reflector 10 secure the incandescent lamp 14 and the lamp holder 13 in their operating positions.

The lamp holder 13 heated by the incandescent lamp 14 has a large area and its rear regions 16 extend so as to overlie to rear of the reflector 10 and have a surface 18 which increases the thermal radiation. The inside 17 of the housing 12 and the outside of the lamp socket 21 and of the lamp flange thereon are also provided with respective surfaces 19 and 20 of this type which increase the thermal radiation.

These surfaces are ';blackened" for the purpose of the invention, that is, the thermal radiation is high in the infrared range. The surfaces 18 to 20 may be a phosphate layer or, alternatively, a heat-resistant clear lacquer or a lacquer pigmented with, for example, soot.

The hot regions of the lamp flange, the inside of the housing and, if required, the incandescent lamp 14, are consequently covered by the surfaces 18, 19 and 20 which increase the transfer of heat between the coated hot and colder regions of the individual surfaces.

WHAT WE CLAIM IS: 1. A headlamp for vehicles, comprising a reflector made from plastics material, a lamp holder mounted in the reflector, an incandescent lamp detachably mounted in the lamp holder, and a housing enclosing the rear of the reflector, the incandescent lamp and/or the lamp holder having regions which extend so as to overlie the rear of the reflector to increase heat radiation, and the surface of the regions of the lamp and/or lamp holder which lie opposite the housing being provided with a layer which increases the thermal radiation thereof.

2. A headlamp as claimed in claim 1, in which the lamp holder is made from metal.

3. A headlamp as claimed in claim 1 or 2, in which the housing is made from metal or plastics material, and at least that region of the inside of the housing which is located opposite the incandescent lamp and the lamp holder is provided with a layer which increases the absorption of the thermal radiation.

4. A headlamp as claimed in claim 3, in which the surface of the rear of the lamp holder and/or of the inside of the housing is a heat-resistant coating of lacquer or some other heat-resistant coating having a high emissivity, which coating constitutes said layer.

5. A headlamp as claimed in claim 4, in which the lacquer forming the coating of lacquer is a clear lacquer, the thickness of the coating of lacquer being at least 25clam.

6. A headlamp as claimed in claim 4, in which the lacquer forming the coating of lacquer is a pigmented lacquer, the thickness of the coating of lacquer being at least 10,us.

7. A headlamp as claimed in claim 6, in which the lacquer is pigmented with soot.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. AW At = 5 5.67 . 10.12 (0.0625 . 10 12 - 0.015 . 10l2) = 1.35 watts The radiation of normal fastening elements made from metal is low, since the emissivity of metal is relatively low at approximately 0.1 to 0.3. Also, housings having a polished metal innerside absorb only a small amount of radiation and reflect the latter. In order to obtain the high thermal flow of 1.35 Watts theoretically to be anticipated, the areas available for radiation are preferably blackened, and consequently the emissivity is increased. The lacquer used for this purpose, or some other coating increasing the emissivity, must be sufficiently heat-resistant and, furthermore, the lacquer or the coating must have a thickness which is sufficient to ensure an emissivity of substantially 1.0. In accordance with a further development of the invention, clear lacquers having a layer thickness of at least 25m are required for this purpose. Furthermore, it is proposed to provide pigmented lacquer having a layer thickness of at least 101lem, wherein soot can be used as the pigment. When the rear wall of the housing of the headlamp is made from metal, it is advantageous to coat at least the region of the inside of the rear wall, located opposite the incandescent lamp and the lamp flange, with the lacquer increasing the thermal radiation. The advantages particularly obtained by means of the invention reside substantially in the fact that the transfer of heat can be increased at least tenfold compared with conventional headlamps, and thus the maximum temperature of the headlamp of approximately 210 Celsius can be reduced to approximately 1700 Celsius and thus to temperatures which are less critical for plastics material. The invention is described further by way of example, with reference to the accompanying drawings, wherein: Figure 1 is an axial section through a headlamp, and Figure 2 shows a portion of the rear view of the reflector without housing. The headlamp 9 illustrated in Figures 1 and 2 essentially comprises a reflector 10 made from plastics material, a diffusing lens 11 mounted in front of the reflector, and a housing 12, made from metal or plastics material, sealing the headlamp 9. A metal lamp holder 13 is inserted into an opening in the reflector 10 and receives an incandescent lamp 14, wherein fastening elements 15 abutting against the incandescent lamp and engaging the reflector 10 secure the incandescent lamp 14 and the lamp holder 13 in their operating positions. The lamp holder 13 heated by the incandescent lamp 14 has a large area and its rear regions 16 extend so as to overlie to rear of the reflector 10 and have a surface 18 which increases the thermal radiation. The inside 17 of the housing 12 and the outside of the lamp socket 21 and of the lamp flange thereon are also provided with respective surfaces 19 and 20 of this type which increase the thermal radiation. These surfaces are ';blackened" for the purpose of the invention, that is, the thermal radiation is high in the infrared range. The surfaces 18 to 20 may be a phosphate layer or, alternatively, a heat-resistant clear lacquer or a lacquer pigmented with, for example, soot. The hot regions of the lamp flange, the inside of the housing and, if required, the incandescent lamp 14, are consequently covered by the surfaces 18, 19 and 20 which increase the transfer of heat between the coated hot and colder regions of the individual surfaces. WHAT WE CLAIM IS:

1. A headlamp for vehicles, comprising a reflector made from plastics material, a lamp holder mounted in the reflector, an incandescent lamp detachably mounted in the lamp holder, and a housing enclosing the rear of the reflector, the incandescent lamp and/or the lamp holder having regions which extend so as to overlie the rear of the reflector to increase heat radiation, and the surface of the regions of the lamp and/or lamp holder which lie opposite the housing being provided with a layer which increases the thermal radiation thereof.

8. A headlamp as claimed in claim 4, in which the heat-resistant surfaces have a high

emissivity in the infrared range.

9. A headlamp constructed substantially as herein particularly described with reference to and as illustrated in the accompanying drawings.