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(11) | EP 0 104 798 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Lamp unit |
| (57) A reflector 20 has a reflective surface 21 which is composed of a plurality of portions
22 all lying substantially on a first paraboloidal surface having a focus 23, and
a plurality of portions 24 all lying substantially on a second paraboloidal surface
whose focus 25 is displaced from the focus 23. The portions 22, 24 are arranged radially
with respect to the foci 23, 25 and alternate with one another in the circumferential
direction. A twin filament light bulb 28 is received by a bulbholder 27a so that its
filaments 29, 30 are located on the two foci 23, 25 respectively. |
[0001] This invention relates to a lamp unit, particularly though not exclusively for use on a road vehicle.
[0002] Our U.K. Patent No. 1591013 describes a lamp unit which is generally of the type shown in schematic cross-section in Figure 1 of the accompanying drawings. In this lamp unit, light emitted by a light bulb 10 is reflected by a reflector 11 towards a lens element 12 which overlies the light bulb. The lens element 12 includes a multiplicity of individual lenses 13 each of which focusses a respective pencil of light rays through a respective light-transmitting interstice 14 of an otherwise opaque baffle 15 which is disposed on the opposite side of the lens element 12 to the light bulb 10. Each of the interstices 14 is located on the optical axis of the respective lens 13 and is limited in its extent to that necessary to transmit substantially the whole of the pencil of light rays focussed by that lens 13. The divergent light emerging from the baffle 15 is re-directed and/or re-distributed by lensing 16 on a light-transmitting cover 17 which forms an external surface of the lamp unit.
[0003] The above-described construction has two main advantageous features. Firstly, the amount of externally incident light which passes through the cover 17 and the baffle 15 to be reflected by the reflector 11 and re-emitted is small, due to the small size of the interstices 14 as compared with the baffle 15 as a whole. Accordingly, the risk of so-called "phantom" signals is substantially reduced. Secondly, the presence of the baffle 15 obscures the internal components of the lamp unit (i.e. the light bulb 10, the reflector 11 and the lens unit 12) whilst not substantially reducing the intensity of the light transmitted from the lamp unit by the light bulb.
[0004] Accordingly, the aesthetic appearance of the lamp unit is improved, and it is possible to make the lamp unit of the so-called "indescernible" type, i.e. where the lamp unit normally has the appearance of a decorative panel and only becomes visible when the light bulb is energised.
[0005] In order that the baffle 15 does not significantly reduce the intensity of the light emitted from the bulb 10, it is important that the light rays reaching the lens element 12 are substantially mutually parallel so that they may easily be focussed through the baffle interstices 14. Accordingly, it is the usual practice to make the reflector 11 of parabolic or paraboloidal form and to mount the light bulb 10 such that an incandescent filament 18 thereof is positioned on the focal point of the parabola or paraboloid. Whilst this arrangement is perfectly satisfactory for light bulbs having only a single incandescent filament, problems arise where for example a twin-filament bulb is employed since only one of these filaments can be positioned exactly on the focal point. Therefore the light rays emitted by the other filament will not necessarily be mutually parallel when they reach the lens element 12 after reflection by the reflector 11, and may not be focussed properly through the baffle interstices 14.
[0007] According to the present invention, a lamp unit includes a reflector whose reflective surface is composed of a plurality of first portions which all lie substantially on a geometrical surface having a first focus and a plurality of second portions which all lie substantially on a further geometrical surface having a second focus which is displaced from the first focus, the first and second portions alternating with each other over the area of the reflector in at least one given direction, and a bulbholder which in use receives a twin incandescent filament assembly such that said filaments are positioned on the first and second foci respectively
[0008] Preferably, the lamp unit also includes a lens element overlying the reflector and having a multiplicity of individual lenses, and a baffle disposed on the opposite side of the lens element to the bulbholder and including a plurality of light-transmitting portions each of which is located on the optical axis of a respective one of the lenses, the remainder of the baffle being light-absorbing or opaque, each lens being arranged to focus a pencil of light rays through the respective light-transmitting portion of the baffle and the light-transmitting portion being limited in its extent to that necessary to transmit substantially the whole of the pencil of light rays. In this case, the first and second geometrical surfaces should both be parabolic or paraboloidal to ensure that the light rays reaching the lens element after reflection by the reflector are substantially mutually parallel. The lamp unit may further include a light-transmitting cover which is disposed on the opposite side of the baffle to the lens element and which is provided with lensing to re-direct and/or redistribute the light emerging from the baffle.
[0009] In a particular arrangement, the first and second portions are arranged generally radially with respect to the first and second foci such that they alternate with each other in a circumferential direction.
[0010] Where the lamp unit is a combined stop and tail light unit for use on a road vehicle, the total area of those portions of the reflector having the filament for the stop function at their focus is greater than the total area of those portions having the filament for the tail light function at their focus.
[0011] The invention will now be further described, by way of example only, with additional reference to the remaining figures of the accompanying drawings, of which:-
Figure 2 is a head-on view of a reflector which forms part of a lamp unit according to the present invention;
Figure 3 is a section taken along the line A-A in Figure 2;
Figure 4 is a section taken along the line B-B in Figure 2;
Figure 5 is a sectional on an enlarged scale taken along the line C-C in Figure 2; and
Figure 6 is a diagram illustrating the manner in which the reflector shown in Figure 2 can be applied to the lamp unit of Figure 1.
[0012] Figures 2 to 4 show a reflector body 20 which is generally of dished configuration and which is of rectangular shape when viewed head-on (although the invention is equally applicable to other shapes of reflector bodies). A reflecting surface 21 of the body 20 is composed of a plurality of first portions 22 which all lie on a first paraboloidal surface whose focus is indicated at 23, and a plurality of second portions 24 which all lie on a second paraboloidal surface whose focus is indicated at 25. The focus 25 is displaced from the focus 23 by a distance a in a direction perpendicular to the major axes of the two paraboloids and by a distance b in a direction parallel to those axes (see Figure 3). The portions 22 and 24 are disposed radially of the paraboloid axes such that they alternate with each other in a circumferential direction (i.e. circumferentially around the paraboloid axes). For the sake of convenience, the portions 22 are indicated by hatching in Figure 2.
[0013] The reflecting surface 21 is punctuated at a generally central position by a circular opening 26 defined by a tubular spigot 27. Received in the interior of the spigot 27 is a bulbholder 27a which is capable of micromanipulation and which receives in use a twin-filament light bulb 28. When finally adjusted, the light bulb is mounted such that one of its incandescent filaments (referenced 29) is positioned on the focus 23 while the other filament 30 is positioned on the focus 25. When the light bulb is energised, light emitted by the filament 29 and falling on the portions 22 of the reflecting surface 21 will be reflected by the latter as mutually parallel rays, as will light emitted by the filament 30 and falling on the portions 24. Thus, mutually parallel light rays will be obtained irrespective of which of the filaments 29 and 30 is energised at any particular time.
[0014] In general, the major axes of the two paraboloidal surfaces will be mutually parallel, so that the light rays emitted by the filament 29 and reflected by the portions 22 will be parallel to the light rays emitted by the filament 30 and reflected by the portions 24. This is of particular advantage when, as depicted in Figure 6, the reflector body 29 is incorporated into the type of lamp unit shown in Figure 1, since then the light rays will be properly focussed through the baffle interstices irrespective of which filament 29 or 30 is energised. In Figure 6, those parts in common with the lamp unit of Figure 1 are designated by the same reference numerals but with 100 added.
[0015] In a particular embodiment of the invention, the reflector body 20 forms part of a combined stop and tail light unit for a road vehicle. For the purpose of the ensuing explanation, it will be assumed that the filament 29 of the light bulb 28 performs the "stop" function and that the filament 30 provides the tail light function. In order to obtain the best balance between these two functions, taking into account their wattage and specification requirements, the ratio of the total area of the portions 22 to the total area of the portions 24 is chosen to be 1.4:1. Thus, the reflecting surface 21 is divided into thirty portions 22 each having an angular extent ( , Figure 2) of 7° and thirty portions 24 each having an angular extent 2 of 5°. the dimensions of the reflecting surface 21 as viewed head-on are 65mm width by 100mm breadth, and the focal length of each paraboloidal surface is 18mm, giving light collection over a solid angle of 2.4 steradians for the filament 29 and over a solid angle of 2.4 steradians for the filament 30. The focus 25 is displaced from the focus 23 such that a=b=2.8mm, corresponding to the spacing between the filaments 29 and 30 in a commercially available twin-filament light bulb.
[0016] Although the total area of the portions 22 or the total area of the portions 24 is rather less than the overall area of the whole reflecting surface 21, any loss in light-gathering power can easily be compensated by increasing the reflection efficiency of the portions 22 and 24. For example, these portions can be metallised in aluminium.
[0017] In the illustrated example, the portions 22 and 24 lie on identical paraboloidal surfaces whose foci are spaced from one another, and in a region indicated generally at 31 in Figure 2 these surfaces intersect one another. In order to simplify manufacture of the reflector body 20, the portions 22 and 24 may merge into a common surface in this region.
[0018] The portions 22 and 24 have been described above as alternating circumferentially of the paraboloid axes, so that each portion is oriented generally radially of those axes. This particular arrangement has the advantage that side walls 32 (see Figure 5) which link each portion 22 with the adjacent portions 24 can be made near-perpendicular to a plane at right-angles to the paraboloidal axes, to facilitate production of the body 20 by moulding. Other arrangements can however be employed in which the portions 22 and 24 alternate with each other in a direction other than a circumferential one, although in this case the body 20 will probably have to be produced using more complicated manufacturing techniques.
[0019] Although the reflector body 20 has been described as being suitable for incorporation into a lamp unit of the type shown in Figure 1, it will be appreciated that the invention is much more widely applicable than this, and that the reflector body can be included in any suitable type of lamp unit which employs a twin-filament light bulb.
1. A lamp unit including a reflector (20) and a bulbholder (27a) which receives a
twin incandescent filament assembly (28), characterised in that a reflective surface
(21) of the reflector (20) is composed of a plurality of first portions (22) which
all lie substantially on a geometrical surface having a first focus (23) and a plurality
of second portions (24) which all lie substantially on a further geometrical surface
having a second focus (25) which-is displaced from the first focus (23), the first
and second portions (22, 24) alternate with each other over the area of the reflector
(20) in at least pne given direction, and the filaments ((29,30) of the filament assembly
(28) are positioned on the first and second foci (23, 25) respectively.
2. A lamp unit according to Claim 1, characterised in that a lens element (112) overlies
the reflector (20) and has a multiplicity of individual lenses (113), and a baffle
(115) is disposed on the opposite side of the lens element (112) to the bulbholder
(27a) and includes a plurality of light-transmitting portions (114) each of which
is located on the optical axis of a respective one of the lenses (113), the remainder
of the baffle being light-absorbing or opaque, each lens (113) being arranged to focus
a pencil of light rays through the respective light-transmitting portion (114) of
the baffle (115) and the light-transmitting portion (114) being limited in its extent
to that necessary to transmit substantially the whole of the pencil of light rays.
3. A lamp unit according to Claim 2, characterised in that a light-transmitting cover
(117) is disposed on the opposite side of the baffle (115) to the lens element (112)
and is provided with lensing (116) to re-direct and/or re-distribute the light emerging
from the baffle (115).
4. A lamp unit according to Claim 1, 2 or 3, characterised in that the first and second
portions (22, 24) are arranged generally radially with respect to the first and second
foci (23,25) such that they alternate with each other in a circumferential direction.
5. A lamp unit according to any preceding claim, characterised in that each of said
geometrical surfaces is a paraboloidal surface.
6. A lamp unit according to any preceding claim, characterised in that it is in the
form of a combined stop and tail light unit for use on a road vehicle, the total area
of those portions (22) of the reflector (20) having the filament (29) for the stop
function at their focus (23) being greater than the total area of those portions (24)
having the filament (30) for the tail light function at their focus (25).
7. A lamp unit according to Claim 6, wherein the ratio between the total areas is
1.4:1 or substantially so.