GB2211923A

GB2211923A - Vehicle headlamp

Info

Publication number: GB2211923A
Application number: GB8709187A
Authority: GB
Inventors: Stephen Durkin; Frank Evans
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-04-16
Filing date: 1987-04-16
Publication date: 1989-07-12
Also published as: GB8709187D0

Abstract

A vehicle headlamp is provided with colour control means comprising a voltage controlled liquid crystal cell in the path of light from a light source, the colour control means being adapted to pass substantially white light therethrough when the liquid crystal is in one state and to selectively absorb red and blue light and pass substantially only yellow light when the liquid crystal cell is in the opposite state. This allows a normal headlamp to be converted into a yellow lamp, suitable for driving in fog by the control of a liquid crystal cell.

Description

DESCRIPTION IMPROVEMENTS IN VEHICLE HEADLAMPS.

The present invention relates to vehicle headlamps and is concerned in particular, but not exclusively, with vehicle fog lamps.

Fog lamps are the most specialised of car lamps and are designed to help the driver see in conditions of poor visibility. Ideally, the beam pattern is fanshaped, spreading out through 160 degrees horizontally and being seen tb end horizontally in line with the driver's eyes. The fan-shaped bean "slices" through a fog bank, rather than piercing it linearly. The problem with normal headlamps in fog or mist is that light is reflected back into a driver's eyes by the minute particles of water forming a haze in front of the car, obscuring vision. In conventional fog lamps, by eliminating light above the level of the lamps, these reflections are minimised, allowing the beam to penetrate the fog and illuminate front and sides.

Yellow headlamps also help to eliminate this effect because scattering of light by small particles, e.g. water droplets of fog, is proportional to the inverse of the fourth power of the wavelength of the light. Thus, if yellow light is used, the light towards the blue end of the spectrum, which is scattered most, is no longer present, and hence less scattering cf the light occurs.

Furthermore, yellow light is seen more easily in foggy conditions due to the fact that the human eye is more sensitive in the yellow and green range of the frequency spectrum.

It is also known that the use of polarised light is helpful in at least partially eliminating the scattering effect.

It is further known that the beam of a good fog lamp should be most intensive immediately below the cut-o point (i.e. the upper extremity of the beam) decreasing closer to the ground, thus producing a carpet of light on the road providing good illumination of road markings and reflecting road studs.

Many cars do not possess fog lamps and use of standard headlamps in fog is one reason for the many accidents that occur in such conditions.

It is one object of the present invention to provide a headlamp which may be reversibly converted into a fog lamp when needed. Inter alia, this will have the further advantage over separate fog lamps, which are usually bolted onto the vehicle bumper, that the beam is fixed more reliably relative to the vehicle body.

Yellow light beams are also less dazzling on main beam headlights, a problem which is particularly prevalent with halogen headlamps. It is thus a further object of the invention to provide a means of reve::slbly converting the colour of a headlight beam, as desired.

In accordance with the present invention, a vehicle hesGlazp is provided with colour control means comprising a voltage controlled liquid crystal cell in the path of light from a light source, the colour control means being adapted to pass substantially white light therethrough when the liquid crystal is in one state and to selectively absorb red and blue light and pass substantially only yellow light when the liquid crystal cell is in the opposite state.

This allows a headlamp of normal colour to be converted into a yellow lamp, suitable for driving in fog, by the control of a liquid crystal cell.

The colour control means may be an integral part of the headlamp or may be fitted onto an existing headlamp.

In one e:nbodiment, the liquid crystal cell is disposed between a pair of transparent polarising layers in the path of the light from the light source, the polarising layer furthest from the light source containing an isotropically absorbtive dye which passes white light therethrough when the incident light thereon is polarised in one direction but which selectIvely sorbs red and blue light and passes substantially only yellow light when the incident light is polarised in the opposite direction.

The effect of this arrangement is that when the voltage controlled liquid crystal cell is not activated the relative polarisations are such that white light from the.light source, polarised.by the polarising layer nearest to the light source, is passed through the second polarising layer containing the isotropically absorbtive dye substantially without alternation of its spectral content. However, when a voltage is applied to the liquid crystal cell the resulting polarisation of the light reaching the second polarising layer containing the isotropically absorbtive dye is such that the red and blue ends of the spectrum are preferentially absorbed and substantially only yellow is transmitted.

This may be achieved by positioning a twisted nematic liquid crystal between the two polarising layers whose optical axes are orthogonal to each other. Thus, when the cell is unactivated, the cell twists the plane of polarisation of the light through 900, parallel to the optical axis of the isotropically absorptive polariser. When the cell is activated, it no longer twists the plane of polarisation of the light, which is thus orthogonal to the optical axis of the isotropically absorptive polariser, thereby producing substantially only yellow light.

Advantageously, a further polarising layer is disposed beyond the second polarising layer so as to lie in the top part of the emergent beam, the further polarising layer being dimensioned and orientated such that when the liquid crystal cell is not activated it has no effect on the white light emerging from the second polarising layer, but when the polarisation of the emergent beam has been rotated by activation of the liquid crystal cell, the further polarising layer blocks the passage of light therethrough whereby to alter the beam configuration to that suitable for a fog lamp.

Preferably, the above-described components are all mounted within the glass body of the lamp, in front of the lamp lens.

Alternatively, the colour control means may be in the form of a Heilmeir cell or a White-Taylor effect liquid crystal cell. In each case, a yellow anthraquinone dye may advantageously be used to produce the required colour change.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a highly diagrammatic exploded view of a first embodiment of a headlamp in accordance with the present invention which is selectively convertible nto a fog lamp; and Figs. 2 and 3 are highly diagrammatic exploded views of second and third embodiments respectively of headlamp in accordance with the present invention, which ae selectively convertible into a fog lamp.

The first embodiment of headlamp, in Fig. 1, comprises a first polarising layer 10, i.e. a layer which is substantially transparent to light but which polarises that light in a particular direction. Light from an incandescent source 11, rendered plane polarlsed on passage through the polarising layer 10, passe through an ultra-violet filter 13 and is incident on a voltage-controlled liquid crystal cell 12 consisting of a liquid crystal 14 sandwiched between respective transparent electrodes 16,18. The ultra-violet filter 13 reduces the amount of U.V.

light incident on the liquid crystal cell, which would otherwise cause the liquid crystal cell to degrade.

The emergent light from the liquid crystal cell 12 then passes through a second polarising (analyser) layer 20 whose optical axis is orthogonal to that of the first polariser 10 and which contains an iso tropically absorbtive dye which selectively absorbs light at the red and blue ends of the spectrum when that light is polarised in a direction orthogonal to the optical axis of the analyser 20 but which passes white light polarised in a direction parallel to the latter optical axis.

The liquid crystal comprises a twisted nematic liquid crystal cell which, whenin the unenergised state, twists the plane of polarisation of the light from the first polariser 18 through an angle of 900.

This is done by treating the insides of the plates containing the liquid crystal to induce the liquid crystal molecules to lie parallel to the plates. By having the induced direction on one plate at 900 to that of the other direction, the liquid crystal layer has a 900 twisted structure. Thus, upon emergence from the liquid crystal cell, the plane of polarisation of the light is parallel to the optical axis of the second polarising layer 20. Since the plane of polarisation of the light incident upon the second polarising layer 20 is aligned. with the optical axis of the polarising layer, the light will pass through substantially unimpeded, and substantially white light thus emerges from the headlamp.

When the liquid crystal cell is activated, by applying an a.c. voltage (typically 1.5 to 3.0 volts) across the electrodes, the cell no longer twists the plane of polarisation of the incident light through 900, but instead allows transmission of the light without affecting the plane of polarisation, by aligning the molecules of the liquid crystal in the direction of the applied field. Thus, the plane of polarisation of the light emerging from the liquid crystal cell and incident upon the second polariser 20 is in the direction of the optical axis of the polariser 10, i.e. orthogonal to the optical axis of the analyser. Consequently, as explained previously, the isotropically absorptive analyser absorbs light from the red and blue ends of the spectrum and transmits substantially yellow light.

Thus, by the simple operation of an electrical switch (not shown), the lamp can be caused to provide either a white light beam or a yellow light beam.

Approximately 40-50% of the light from the incandescent source is lost by attenuation in traversing the layers 10 to 20 and a brighter bulb than usual must therefore be used in the present lamp, e.g. halogen 120 watt main beam and 100 watt dipped beam. This results in a beam of the same brightness as a conventional headlamp but polarised in one direction. iMen the liquid crystal device is actuated, the colour of the emitted beam changes to yellow but there is no further appreciable reduction in the emergent light intensity.

It is also possible to include a further strip 24 of polarising material similar to that of the first polarising layer 10, whose optical axis is parallel to that of the second polarising layer 20 and which is positioned so as to intersect the upper part of the light beam from the source 11. When the liquid crystal device 12 is nbt activated, the relative polarisations of the layers 10 and 24 allow the light to pass through the layer 24 without attenuation, since the plane of polarisation of the light is twisted through 900 by the liquid crystal cell.

However, when the liquid crystal device is activated so that the liquid crystal cell no longer twists the plane of polarisation of the light through 900, the resulting polarisation is such that the upper part of the light beam is blocked by the layer 24, which is effective for light of all wavelengths, unlike the second polar ser 20, and only the lower part emerges through the front lens 22 (as yellow light, as previously described). The layer 24 thus acts as a selectively activatable hood which has no effect when the headlamp is used in its normal mode but which modifies the shape of the emergent beam when the headlamp is in its fog-lamp mode, i.e. when the liquid crystal cell is activated.

Although shown simply as rectangular in the drawing, in practice the layer 24 is configured in accordance with the shape of the light beam which is desired for the fog-light mode.

A second embodiment of headlamp in accordance with the present invention is illustrated in Fig. 2 and comprises a Heilmeir Cell in the form of a first polariser 20, as in the first embodiment, and a Guest Host liquid crystal 28 comprising a liquid crystal 30 sandwiched between two transprent electrodes 32, 34.

The liquid crystal produces a homogenous-homeotropic phase change and comprises a nematic liquid crystal material having positive dielectric anisotropy in which is dissolved a light-stable yellow anthraquinone dye. Suitable dy-e solutions are D80E201 and D80E63 manufactured by BDH Chemicals Ltd., of Broom Road, Poole, England. An ultra-violet filter 13 is disposed between the cell and the polariser 10, and the device is enclosed by a front lens 22.

In the unactivated state, the liquid crystal 28 allows substantially white light to pass through, whereas when the liquid crystal 28 is activated by application of an a.c. voltage across the electrodes, the "guest" molecules of the dye are aligned, by movement of "host" molecules of the liquid crystals, in the direction of the applied field, and the light emerging from the device is yellow.

A further liquid crystal cell 36 may also be employed to alter the shape of the beam, as in the first embodiment. The cell 36 comprises a first polariser whose optical axis is aligned with that of the polariser 20, and a second polariser 40 whose optical axis is orthogonal to the polariser 38, with'a twisted nematic liquid crystal 42 situated therebetween. When te cell 36 is unactivated, the cell 36 merely twist the plane of polarisation of the incoming light through 900, and thus substantially all the light incident on the polariser 38 is transmitted.

Upon activation of the cell 36, the liquid crystal 42 no longed twists the plane of polarisation of the light emerging from the polariser 38 and thus the second polariser 40 does not transmit the light incident upon it, since its optical axis is orthogonal to the plane of polarisation of the incident light.

The circuit controlling the cell 36 may be connected to the circuit controlling the main cell 28, such that production of yellow light by the cell 28 automatically results in altering the shape of the bear produced. As in the first embodiment, the hood may be configured in accordance with the shape of the light beam which is desired in the fog-light mode.

A third embodiment is illustrated in Fig. 3, and comprises a White-Taylor effect liquid crystal cell in the form of two transparent electrodes 46, 48 with a Guest-Host liquid crystal material 50 therebetween.

The polariser 10 is dispensed with, but a U.V. filter is provided to protect the liquid crystal. The liquid crystal produces a cholesteric-nematic phase change and, as before, comprises a nematic material having positive dielectric anisotropy in which a yellow anthraquinone dye is dissolved.

VLlen unactivated, the cell allows white light to pass through, but when activated the "guest" dye molecules are aligned in the direction of the applied field by movement of the "host" liquid crystal molecules, and yellow light emerges from the cell. As before, a twisted nematic liquid crystal hood 36, identical to that of the second embodiment, is arranged to alter the shape of the beam produced.

As indicated in the drawing, the components of the lamps are preferably mounted within the glass body of the lamp, between the incandescent light bulb 11 and the front lens 22. It is conceivable, however, that these components could be formed as a unit which is mounted outside the lamp body on the front of the lens so as to convert an existing lamp into one which can be selectively converted into a fog-lamp. In the event that the components are mounted within the lamp body, it is necessary either to build into the lamp a means for converting the 12 volts d.c. of the normal vehicle supply into the required 1.5 to 3.0 volts a.c.

or to provide an a.c. generating means outside the lamp and also to provide a means of connecting same to the electrodes 16, 18, 30, 32 and 46, 48. This can be achieved in any suitable manner and the invention is not limited to any particular format.

The device in accordance with the present invention may form an integral part of the headlamp or may be adapted to be secured to the exterior of an existing headlamp.

Claims

CLAIM 1

A vehicle headlamp provided with colour control means comprising a voltage controlled liquid crystal cell in the path of light from a light source.The colour control means being adapted to pass substantially white light therethrough when the liquid crystal is in one state and to selectively absorb red and blue light and pass substantially only yellow light when the liquid crystal cell is in the opposite state.

CLAIM 2 The type of liquid crystal display used would be of the twisted nematic type.The display would be placed between two polarising layers whose optical axes are orthogonal to each other.When the cell is activated the cell twists the plane of polarisation of the light through 90' parallel to the optical axis of the isotropically absorptive polariser.

When the cell is unactivated it no longer twists the plane of polarisation of the light,which is thus orthogonal to the optical axis of the isotropically absorptive polari ser. thereby producing substantially only yellow light suitable for driving in fog.