GB2194321A - Lens system - Google Patents

Abstract

A lens system is formed by an array of lens elements (16), each of which elements (16) has one lens surface on one side of the array (17) which is convex, and a smaller lens surface (19) on the other side of the array (17) which is preferably concave. These lens surfaces define light paths through the array. Louvres (20) are then placed on one side of the array (17) to prevent external light reaching the array (17), which would cause "phantom" illumination or "washout". This may also be assisted by projections (18) adjacent the smaller lens surfaces (19). The lens surfaces are asymmetric, and so can direct light downwardly. This finds use in a traffic signal, in which light from a source (11) is formed into a beam by a parabolic reflector (12), which beam is then incident on the array. Also disclosed are other louvre arrangements for blocking part of the beam through each lens element (16), or for reflecting or otherwise modifying part of it. <IMAGE>

Description

SPECIFICATION Lens system The present invention relates to a lens system for use, for example, in traffic signals.

In a lamp, such as those used in traffic signals, a beam of light is generated by having light from a suitable light source incident on a parabolic mirror, to form a generally parallel beam. Normally, the beam passes through a coloured filter which gives the light a suitable colour. A problem with such lamps, however, is that they suffer from "phantom" or "ghost" signals due to externally incident light being reflected internally back through the filter, which is then perceived by the observer as illumination of the lamp, and also from "washout", which is caused by light reflected from the front surface of the lamp adding to the normal illumination of the lamp. Thus, when there are high levels of external light, it is difficult to tell whether any particular lamp is illuminated by its own light source, or by reflected external light.

It is known from e.g. EP-A-0081361 to form a lens system for such a lamp in which the lens system has an array of lens elements, each with a larger curved surface and smaller curved surface on opposite sides of the array, to focus light passing through the lens system into a plurality of individual beams, one for each lens element. The lens system is then placed in a lamp with the smaller lens surfaces outermost, and masking material may be placed on the array around the smaller lens elements to reduce the light passing through or being reflected from that surface. However, such an arrangement doesnot prevent spurious reflection from curved surfaces of the lens elements themselves nor does it prevent external light entering through the lens surfaces themselves.

In order to overcome this problem, the present invention proposes that an array of louvres be provided adjacent an array of lens elements, with each louvre being outside the limit of the optical paths defined by those lens elements. In this way the louvres do not interfere with the light passing through the lens system, but block light incident on the lens system from the exterior. In practice, since direct light from the sun is the main cause of "phantom" illumination and "washout", by providing louvres which block in a particular direction, substantially all "phantom" illumination and "washout" can be prevented.

The louvres may be positioned such that they prevent direct sunlight falling onto the small lens surfaces (and the surfaces between them). This reduces the requirement to make the small lens surfaces very much smaller than their corresponding larger lens surface a condition which is otherwise necessary to achieve low levels of "phantom" and "washout". This relaxation reduces the critical requirements, on dimensional accuracy, to ensure that the signal light incident upon the larger convex face reaches the smaller lens surface and exits in the required directions.

In a further development the louvres extend around the adjacent lens surfaces, to increase the angles over which they block external light. To achieve this it is possible to use one array of louvres each of which louvres extends in one direction parallel to the array of lens elements, and a second array of louvres, each of which louvres extends in a perpendicular direction, so that the light path from each adjacent lens surface is enclosed within adjacent louvres. Alternatively, however, the louvres may be curved around the adjacent surfaces of the lens elements.

In many cases, such as in traffic signals, it is desired that the light be emitted preferentially in a particular direction, e.g. inclined downwardly. To achieve this it is desirable for the lens elements to have their lens surfaces shaped to cause the downward focusing of the light, and to achieve this the lens elements preferably each to have a larger lens surface formed by a convex part and a planar part extending generally perpendicularly to the array, and a smaller surface either convex or concave. By providing a directed light beam in this way, the optical paths of the beam are further confined, and this enables the louvres to be made larger, to achieve the maximum blockage of external light.

The idea of guiding the light from the lens system into a restricted beam leads to another aspect of the present invention, namely the provision of an array of louvres which extend into the optical path and modify the light incident thereon. These louvres may also block external light. One type of louvres for modifying the light have reflective surfaces which cause light which is incident on them to be re-directed. There are existing systems for blocking part of the beam, in order to direct it, but the result of the known systems is that the total output of the lamp is reduced. By using a reflective arrangement, this can be avoided. Such reflective louvres may be used in conjunction with the louvres outside the optical path, but also may be used on their own, and are therefore an independent aspect of the present invention.

Another type of louvre for modifying the light has at least a part which is translucent and coloured. The effect of this is that there may be a range of viewing angles for which the light is seen directly from the lens surfaces (i.e. uncoloured), and another range of angles for which the light is seen through the translucent louvres, and thus is coloured. This enables the lamp to give a visual indication of the observer's position relative to the lamp.

As was mentioned earlier, when a lens system according to the present invention is used in a traffic signal, it will be necessary to provide a filter to give the beam an appropriate colour. This could be done by providing a continuous filter covering the entire array, but it would also be possible, however for the lens array itself to be coloured.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows optical paths through a first example of a lens array suitable for use in the present invention; Figure 2 shows a second example of a lens array suitable for use in the present invention; Figure 3 shows part of a second embodiment of the present invention; Figure 4 shows part of a second embodiment of the present invention; Figure 5 shows part of a third embodiment of the present invention; Figures 6 and 7 show views at right angles to the lens array viewed from the side having larger-lens surfaces and the side having the smaller lens surfaces, when the lenses are shaped to produce a downwardly directed beam; and Figure 8 shows a sectional side view of a lamp incorporating an embodiment of the present invention.

Figs. 1 and 2 show two different lens arrangements of a lens array 2 for use in the present invention, each having a plurality of elements having a convex lens surface 1 on one side of the element and a corresponding lens surface 3 on the other side of the element, which is smaller than the convex lens surface 1. In Fig. 1, the smaller lens surfaces 3 are concave, whilst in Fig. 2, they are convex. Although it would be possible to use lens surfaces which are symmetrically curved, the lens surfaces shown in Figs. 1 and 2 are each curved on only one side of the optical axis 0,0', and the convex curved surfaces 1 join with a plane surface la which extends perpendicularly to the array. The convex lens surface 1 converges light falling onto it towards the optical axis 0,0'.The curvature of that lens surface 1, together with the refractive index of the material of the army 2, is such as to converge the light rays onto the other lens surface 3. The amount by which that surface may be smaller than the convex surface 1 is determined by the convergence of the light rays. The size of the lens surface 3, therefore, is to a large extent determined by the thickness of the array 2, i.e. the separation between the faces of the lens elements. However, the use of a concave surface 3, as in Fig. 1, enables the thickness of the array 2 to be reduced, for a given angle of the output light beam, and is therefore preferred.

For traffic signals it is only necessary that light emerge downwardly from the lens array, and it can be seen from Figs. 1 and 2 that the use of asymmetric lens surfaces achieves this. A parallel beam falling on the convex lens surfaces 1 is formed into a downwardly directed beam on emergence from the other lens surfaces 3. When parallel light falls upon the concave lens surface 1, that light which falls upon the part of the surface furthest from the plane surface 1 a is refracted the most, and emerges from the array at the largest angle 0 to the optical axis 0,0'. The maximum angle of the exiting light can be controlled by the size of the lens surface 1. This control applies in any plane parallel to and intersecting the optical axis 0,0' of the lens surface 1.

Therefore, the upper and lower vertical maximum output light beam angles, the horizontal light beam output angles, and the maximum output beam angles in other directions can be controlled by the size of the lens surface 1 in the relevant plane.

When parallel, or near parallel, light is incident onto the array 2, then the output light beam patterns from each lens element, formed by a corresponding pair of lens surfaces 1,3, will be similar. The intensity of the light output from the lens surface 3 will vary in proportion to the intensity of the incident light.

At normal viewing distances, the intensity distribution pattern of the array will be the sum of the intensity from the lens surfaces 3.

Light from a source which deviates slightly from parallel and axial (with respect to the optical axis of the lenses) can be used but requires a modified lens design to ensure that the light is directed through the lens surfaces 3, and that the lens surfaces 1,3, give the required angles for the output beam.

Referring now to Fig. 3, louvres 4 are provided adjacent the array 2, to block external incident light. By suitable positioning of the louvres 4, each louvre being associated with a corresponding one of the lens elements, it can be ensured that they are outside the limits of the optical paths of light through the lens elements of the array 2. Thus, using the array of Fig. 1, it can be seen that the downwardly directed light beams have gaps therebetween, into which the louvres 4 may be positioned.

To ensure maximum light blocking capability, the planes of the louvres 4 are inclined to the optical axis 0,0', with an angle of inclination close to, but slightly greater than, the angle to the optical axis of the light beam from the part of the lens surface 1 furthest from the plane surface 1a. Therefore, externally incident light 5 is blocked by the louvres 4, and therefore cannot reach the array 2 to cause "phantom" illumination or "washout" It would also be possible, however, for the plane of each louvre to be parallel to the array rather than inclined, or even have louvres which are curved around the smaller lens surfaces.

It can be appreciated that by use of the lens array of Fig. 1, rather than that of Fig. 2, the size of the gaps between the limits of the optical paths of light which has passed through the lens elements are larger, permitting the louvres to be larger and therefore increase their light blocking effect. Indeed, as shown in Fig. 3, the louvres 4 may partially cover the lens surfaces 3, when viewed perpendicularly to the lens array 2 but still allow light to pass along, and below, the optical axis 0-0'.

In a further development of the present invention, it is required to modify the angular distribution of the light output, e.g. to restrict the angles from which the light is visible. The use of louvres to block light is known, but existing arrangements normally use long louvres with a relatively small number of spaces therebetween. These long louvres substantially reduce the light output at many viewing angles. It has been found that by using the lens arrays discussed above, it is possible to provide louvres which block only the unwanted part of the light beam, each louvre being associated with a corresponding lens element. Of course, the exact position of the louvres may be selected in dependence on the desired geometry, and Fig. 4 shows four examples (at 6,7,8 and 9) of louvres which may be used.These louvres are placed in an array to intercept the part of the light beam output between the angles O and 0 for each lens element. If 0 is the maximum unrestricted output angle of the beam, then the louvres restrict the output beam to the angle 0 adjacent the optical axis 0,0'. One design of louvre, that shown at 8, which has a louvre extending perpendicularly to the lens array, may have each louvre extended back towards the array 2, so that the louvres may be mounted directly on that array 2. Of course, the size and positions of each louvre associated with a given lens element must be chosen to avoid interference with the light from the adjacent lens element. The louvres shown at 9 inevitably achieve this, because they are on the signal light incident side of the array 2.

The louvres are normally opaque, but can be translucent, either clear or coloured, as desired so that a visual indication is given of the observer's position.

Fig. 5 shows a further development of the light blocking louvres, in which one surface of each louvre 10 is made reflective. The louvres 10 may otherwise be similar to the louvres 8 in Fig. 4. It can be seen that by making the louvre 10 reflective, the light between angles 6 and 0, which in the arrangement of Fig. 4 was blocked, is instead reflected in the required direction, thereby limiting the light beam without reducing its intensity significantly.

Although the louvres of Fig. 3, and the louvres of Figs. 4 and 5 have been described separately, they may be used in conjunction.

Figs. 6 and 7 show views of the array 2 of lens elements, from either side of the array.

As shown, the array consists of a plurality of rows A,B,C,D of lens elements, each element having a convex lens surface 1 on one side of the array, and another lens surface 3 on the other side of the array. For maximum light output, it can be seen that the lens surfaces 1 are shaped so as to fill as much as possible of the surface of the array, while retaining the asymmetric shape desired above, whereas the lens surfaces 3 are much smaller.

Fig. 8 shows a lamp, e.g. for use in a traffic signal, using a lens system according to the present invention. A light source 11 is fixed at or near the focus of a parabolic mirror 12, so that a substantially parallel light beam is formed which is then incident on a plurality of lens elements 16 formed into an array 17.

The lens elements 16 of the array 17 are generally as shown in Fig. 3. However, as can be seen, there are projections 18 adjacent each of the smaller lens surfaces 19, which give a further blocking to external light. This effect may be increased by covering the projections 18 with an opaque material. However, the main blocking is achieved by the louvres 20 which are positioned outside the limits of the optical axes of the light beam from the smaller lens surfaces 19, as has already been described with reference to Fig. 3. An outer protective screen 21 may be provided in front of the array 17.

The light source 11 is preferably a tungsten halogen light.

In a further development of the present invention, it is possible for the lamp to illuminate a suitable symbol, defined by a mask.

When secrecy of the symbol, when non-illuminated, is required, the mask is preferably fitted between the lens array and the louvres or possibly close to the lens array on the side adjacent to the mirror 12. If, on the other hand, secrecy is not required, then the mask may be fitted in front of the louvres, or incorporated into the protective screen. The output beam may be coloured, produced by a dye preferably contained within the array 17 or alternatively within the screen 21.

The present invention, as discussed above, has been concerned particularly with traffic signals, but the invention is not restricted to such a use. The invention can be used in other applications where it is required to have a defined light beam pattern of illumination, and where the perceived signal output is required not to be affected by externally incident light.

Claims (18)

1. A lens system comprising: a plurality of lens elements extending in an array, each lens element having a pair of lens surfaces on opposite sides of the lens element one having a smaller area than the other, each lens element defining an envelope of optical paths through and beyond that lens element, the limits of that envelope being defined by the shape of the lens surfaces and the material of the lens element between those surfaces; and an array of louvres extending generally parallel to the array of lens elements and being closer to the smaller lens surface than the larger, with each louvre being outside the limits of the envelope of optical paths defined by the lens elements.

2. A lens system according to claim 1, wherein the planes of each louvre are inclined to the perpendicular from the lens array.

3. A lens system according to claim 1 or claim 2, having a further array of louvres extending generally parallel to the array of lens elements, each louvre having at least a part within the limits of the optical paths defined by corresponding lens elements.

4. A lens system according to claim 3, wherein the further array of louvres is closer to the larger lens surfaces of the lens elements than the smaller lens surfaces.

5. A lens system according to claim 3, wherein the further array is closer to the smaller lens surfaces than the larger lens surfaces.

6. A lens system according to claim 5, wherein the plane of each louvre of the further array is generally perpendicular to the lens array.

7. A lens system according to claim 6, wherein at least one surface of the louvres is reflective.

8. A lens system according to claim 5, wherein the plane of each louvre of the further array of louvres is generally parallel to the array of lens elements.

9. A lens system according to any one of claims 4 to 6 or 8, wherein the louvres of the further array are translucent.

10. A lens system comprising: a plurality of lens elements extending in an array, each lens element having a pair of lens surfaces on opposite sides of the lens element one having a smaller area than the other, each lens element defining an envelope of optical paths through and beyond that lens element, the limits of that envelope being defined by the shape of the lens surfaces and the material of the lens element between those surfaces; and an array of louvres extending generally parallel to the array of lens elements, each louvre having at least a part which extends into the limits of the optical paths through corresponding lens elements, and each louvre having at least a part adapted to modify light incident thereon.

11. A lens system according to claim 10, wherein the part of each louvre adapted to modify the light is reflective.

12. A lens system according to claim 10, wherein the part of each louvre adapted to modify the light is translucent and coloured.

13. A lens system according to any one of the preceding claims, wherein the larger lens surface of each lens element has a convex part and a planar part, the planar part being substantially perpendicular to the array of lens elements.

14. A lens system according to any one of the preceding claims, wherein the smaller lens surface of each lens element is concave.

15. A lens system according to any one of the preceding claims having projections on the array of lens elements adjacent the smaller lens surfaces.

16. A lens system substantially as herein described with reference to and as illustrated in Figs. 3,4,5 as modified by Figs. 1,2,6 and 7 of the accompanying drawings.

17. A lamp having a light source positioned at or near the focus of a parabolic reflector, and a lens system according to any one of the preceding claims positioned to receive light from the parabolic reflector.

18. A lamp substantially as herein described with reference to and as illustrated in Fig. 8 of the accompanying drawings.