Patent Description:
The invention addresses the problem of condensation of water from air moisture in the interior of vehicle lighting devices, especially bedewing of the inner surface of the cover lens. The problem nowadays occurs to an increasing degree in the course of using light-emitting diodes (LED) as primary light sources for vehicle lighting devices. Due to the low degree of infrared radiation and waste heat emitted by LEDs, there is a significantly reduced degree of heating of the surrounding components compared to formerly used light sources, e.g. halogen bulbs. Therefore, the cover lens, which is furthest spaced apart from the light source and exposed to cooling airstreams during vehicle operation, is especially prone to bedewing.

The cover lens represents the foremost component of the lighting device with respect to the designated mounting position in a vehicle. It is typically injection-moulded e.g. from a transparent polycarbonate and features a free-form geometry dedicated to the particular lighting device. The central portion of the cover lens typically comprises the transparent visual section serving as the light exit aperture of the lighting device. The remaining portion of the cover lens, the blank section, is designated to be covered by the housing of the lighting device and/or it is designated to screen portions of the interior of the lighting device for an observer. To the latter purpose, the blank section is opaque and the cover lens is manufactured e.g. by a two-step injection moulding process using transparent and black-coloured moulding compounds.

Condensation of water on the inner surface of the cover lens, i.e. on the surface facing the interior of the lighting device, possibly interferes with the lighting function of the lighting device and might also represent a flaw based on the aesthetic perception of an observer. Especially, there is a corresponding bedewing test within the Federal Motor Vehicle Safety Standard <NUM> (FMVSS <NUM>), which regulates all automotive lighting, signalling and reflective devices in the United States. Similar standards apply to further countries, e.g. Canada.

There are several different approaches known in the prior art to address the issue of cover lens bedewing. The documents <CIT> and <CIT> for instance disclose cover lenses with integrated heating devices in the form of metallic layers serving as resistance heaters. The documents <CIT>,.

<CIT> and <CIT> teach the implementation of ventilation systems by a combination of fan devices and air in/outlet openings in the lighting device. The documents <CIT> and <CIT> disclose the integration of drying means in order to decrease the degree of air humidity in the interior of the lighting device. Finally, the <CIT> proposes to integrate a Peltier cooling element into the lighting device acting as local condensation element.

The document <CIT> discloses a cover lens for a vehicle lamp, the lens comprising an appearance portion and a concealed portion, wherein a condensation trap feature is formed along an inner surface of the concealed portion of the lens. The condensation trap feature provides increased surface area or increased airflow in order to increase the condensation rate.

The document <CIT> discloses a cover lens for a vehicle lighting device with multiple light sources of different power. In order to prevent inhomogeneous bedewing, the material thickness of the cover lens varies locally, so that an even temperature distribution along the cover lens is established during operation of the light sources.

The document <CIT> discloses a cover for covering a display unit i.e. hole-matrix display, the cover comprising a condensing zone provided outside a first section serving as main field of view on the display unit, wherein the material thickness of the condensing zone is smaller than that of the first section.

It is an object of the present invention to provide a new approach for the reduction of the bedewing of a cover lens in vehicle lighting device.

This object is achieved by a cover lens as taught by claim <NUM> of the present invention. Advantageous embodiments of the invention are defined in the subclaims.

The invention discloses the technical teaching that the blank section of the cover lens comprises at least one condensation area, wherein the wall thickness of the condensation area is lower than the wall thickness of the visual section, and wherein the condensation area features a surface structure dedicated to increase the wettability with water.

The core of the invention lies in the creation of a passive condensation trap in form of the thin-walled condensation area. Due to its lower wall thickness, there is a faster heat transmission from the interior of the lighting device to the exterior through the condensation area compared to the visual section, i.e. the cooling rate at the inner surface of the condensation area is higher than at the inner surface of the visual section. Consequently, during a cooling phase, the temperature drops below the critical dew point for a given humidity level at the inner surface of the condensation area at first, i.e. the bedewing process is initiated at the condensation area. By appropriate sizing of the condensation area, the amount of condensed water trapped at the condensation area represents a significant portion of the entire moisture contained in the interior of the lighting device, so that the local humidity at the inner surface of the visual section is lowered by such a degree, which is sufficient to prevent any condensation of water. Therefore, the inventive cover lens restricts the bedewing to an area, which is invisible from the exterior of the lighting device, while the visual section as the light exit area remains free of condensed water. In contrast to the approaches known in the prior art, the invention achieves this functionality by purely constructive means, instead of using powered auxiliary devices like fans or heating elements.

According to the invention the condensation area features a surface structure dedicated to increase the wettability with water. An increased wettability improves the capacity of the condensation area in terms of bonding and thus storage of condensed water. Appropriate surface structures for instance comprise dimple pattern or an adjusted roughness level.

In a preferred embodiment of the inventive cover lens, the condensation area is arranged below the visual section with respect to a designated mounting position of the lighting device. Such an arrangement prevents that condensed water trapped at the condensation area flows or trickles towards the visual section under the influence of gravity and/or vibrations during vehicle operation.

Advantageously, the wall thickness of the condensation area amounts to <NUM>% to <NUM>% of the wall thickness of the visual section. The thinner the cover lens in the condensation area compared to the visual section, the higher the difference in the local cooling rates at the inner surfaces. Exemplary thickness values amount to <NUM> for the visual section and <NUM> for the condensation area. Anyways, the detailed choice of the wall thickness ratio also depends on considerations concerning the mechanical stability of the cover lens. The lateral sizing of the condensation area represents a further important factor determining the amount of trapped condensed water.

According to another preferred embodiment of the inventive cover lens, the blank section comprises several condensation areas, which are arranged peripherally around the visual section. Such an arrangement yields a spatially homogeneous humidity removal from the air around the visual section. For example, as an additional degree of constructive freedom, the wall thickness may vary among the different condensation areas.

Advantageously, the condensation area features a hexagonal contour. In the technical field of light weight construction, hexagonal structures or similar appropriate pattern are well-known means to enhance the stiffness of thin-wall components. For the inventive cover lens, it was found that a proper arrangement of several thin-walled condensation areas even enhances the stiffness of the blank section compared to the case of a reference cover lens with all homogeneous wall thickness.

Furthermore, the invention concerns a vehicle lighting device comprising a cover lens according to one of the previous embodiments. The lighting device may especially be configured as a vehicle head light.

Additional details, characteristics and advantages of the object of the invention are disclosed in the following description of the respective figures - which in an exemplary fashion - show preferred embodiments of the cover lens according to the invention.

<FIG> shows the dew point curve of water in air at sea level pressure, i.e. the saturation vapour pressure ps as a function of the temperature T and the water vapour partial pressure p. The dew point curve separates the liquid water phase I from the water vapour phase v.

In the following, the bedewing properties of the inventive cover lens shall be discussed exemplarily in the context of the bedewing test for vehicle head lights according to the FMVSS <NUM> standard. In this test, the lighting device is subject to a conditioning stage, after which it is prepared in the initial condition s0 as depicted in <FIG>, i.e. featuring a temperature of T=<NUM> and humidity of about <NUM>% inside the lighting device corresponding to a water vapour partial pressure of p=<NUM> kPa. In the following, the lighting device under test is put into a wind tunnel and subject to a constant air stream with a temperature of T=<NUM>. Starting from the same initial condition s0, the local air volumes at the inner surfaces of the visual section and the condensation area evolve along the different phase space trajectories x1 and x20, respectively. Due to the higher cooling rates at the thin-walled condensation area, the local temperature there reaches the dew point around T=<NUM> at first, and condensation of water from the supersaturated water vapour sets in. In the following, the cooling rates at the condensation area are for instance high enough to pin the related trajectory x20 right to the dew point curve, thus permanently bedewing the inner surface of the condensation area and in turn lowering the humidity of the remaining air volume in the interior of the lighting device. At the inner surface of the visual section, the air temperature remains higher during the (early) cooling stage compared to the condensation area and the temperature of T=<NUM>, corresponding to the initial dew point temperature, is only reached after condensation has already set in at the condensation area, i.e. at a point in time, in which the humidity of the air is already significantly lowered. Consequently, water partial pressure remains at a subcritical level at the inner surface of the visual section, and with the condensation area constantly trapping more water from the ambient, phase space trajectory x1 of the air volume at the inner surface of the visual section runs through the vapour phase v during the entire test procedure. Finally, all condensed water is confined to the condensation section, which is invisible from the exterior of the lighting device, and the visual section is free of any bedewing.

<FIG> shows a sketch in front view of an inventive cover lens <NUM> for a vehicle lighting device comprising a visual section <NUM>, which is transparent and designated to form the light exit aperture of the lighting device, and a blank section <NUM>, which is opaque and/or which is designated to be invisible from the exterior of the lighting device.

Especially, the blank section <NUM> may be covered by the housing of the lighting device or by a portion of the vehicle body. The cover lens <NUM> may be injection moulded as one piece of equal material or the visual section <NUM> and the blank section <NUM> are manufactured and joint in a multiple-step process, especially from transparent and opaque materials, respectively. The blank section <NUM> comprises the two condensation areas <NUM> below and above the visual section <NUM>, wherein the wall thickness of the condensation areas, i.e. the thickness along the direction perpendicular to the plane of the sketch, is lower than the wall thickness of the visual section <NUM>.

<FIG> shows cross-sections of alternative embodiments of the schematic cover lens <NUM> of <FIG>, wherein the cross-sections correspond to the line AA in <FIG>. The embodiment on the left-hand side features condensation areas <NUM> with a wall thickness t20 corresponding to <NUM>% of the wall thickness t1 of the visual section <NUM>. The embodiment on the right-hand side features condensation areas <NUM> with a wall thickness t20 corresponding to <NUM>% of the wall thickness t1 of the visual section <NUM>, and furthermore, the condensation areas <NUM> protrude over the visual section <NUM>. Through such protuberance, the condensation areas <NUM> are possibly more exposed to cooling air streams during vehicle operation or test procedure, so that the bedewing condition is further improved.

<FIG> show sketches in front view of further preferred embodiments of the inventive cover lens <NUM> dedicated for a vehicle head light. The cover lens <NUM> features a free-form geometry with a foremost wedge-shaped visual section <NUM> and a rearwardly curved blank section <NUM>.

In <FIG> the four condensation sections <NUM> are arranged below the visual section <NUM> in order to prevent condensed water from running towards the visual section <NUM>. The cover lens <NUM> of <FIG> also features additional condensation sections <NUM> above the visual section <NUM> to yield a peripheral arrangement of condensation sections <NUM>. The lateral size and contour shape vary among the different condensation sections <NUM> and can be adjusted e.g. according to the geometry and dimensions of the corresponding lighting device. A stiffening effect results especially from the hexagonal shapes of the two condensation sections <NUM> on the bottom portion of the blank section <NUM> in <FIG>.

Claim 1:
Cover lens (<NUM>) for a vehicle lighting device comprising a visual section (<NUM>), which is transparent and designated to form the light exit aperture of the lighting device, and a blank section (<NUM>), which is opaque and/or which is designated to be invisible from the exterior of the lighting device, when the cover lens is mounted to the lighting device or when cover lens is mounted to the lighting device and the lighting device is mounted to the vehicle, wherein the blank section (<NUM>) comprises at least one condensation area (<NUM>), wherein the wall thickness (t20) of the condensation area (<NUM>) is lower than the wall thickness (t1) of the visual section (<NUM>), and wherein the condensation area (<NUM>) features a surface structure dedicated to increase the wettability with water.