GB2213955A - Reflectors providing constant angle between input and output beams - Google Patents

Abstract

A reflector is arranged so that an output beam and an input beam remain at a constant angle to each other for at least a limited range of angles of incidence of the input beam. The device comprises a thin collimating lens (2) through which the beam passes, a pair of opposed reflectors (4, 8) positioned equal distances on opposite sides of the collimating lens and a tranverse reflector (6). The input beam passes through the lens and, after reflection at one of the opposed reflectors, is focussed at the surface of the transverse reflector and then passes out again through the lens after reflection at the other opposed reflector. The lens may be replaced by a concave mirror or other optical element capable of focussing the input beam and collimating the output beam. The reflectors may be constituted by the surfaces of a solid block of transparent material. <IMAGE>

Description

REFLECTORS This invention relates to reflectors and is particularly concerned with optical reflectors in which the angle between an input and an output ray remains substantially constant with change of the angle of incidence of the input ray, at least over a small range of angles of incidence.

A known form of reflector in which the output ray remains at a substantially constant angle relative to the input ray is a pentaprism. However, in pentaprisms, the output ray also shifts laterally as the angle of incidence of the input ray changes.

In one aspect, the invention provides a reflector in which this problem may be alleviated at least to some extent.

The invention is described by way of example with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic perspective view, with parts broken away, of a reflector according to a first embodiment of the invention; and Figs. 2A and B illustrate the operation of the embodiment of Fig. 1.

The reflector shown in Fig. 1 comprises a thin focussing lens 2 and three plane mirrors 4, 6 and 8.

The mirrors 4 and 8 extend parallel to each other and parallel to the axis 10 of the lens 2. The mirrors 4 and 8 are equispaced on opposite sides of the axis 10.

The mirror 6 is normal to the mirrors 4 and 8 and positioned at the opposite ends thereof relative to the lens 2.

The arrangement of the lens 2 and mirrors 4, 6 and 8 is such that a collimated beam of light 12 entering the lens 2 at the side thereof opposite to the mirrors and at an angle to the axis 10 is reflected from the mirror 4 and, after that reflection, focussed at the surface of mirror 6, from which the light is reflected to the mirror 8 at which it is again reflected and passes out of the device through the lens 2 to form an output collimated beam 14. The arrangement is such that the angle between the input and output collimated beams 12 and 14 remains substantially constant for a limited range of angles between the input beam 12 and the axis 10.

The way in which this is achieved will be understood from consideration of Figure 2. In Figure 2a, the input collimated beam 12 is shown at an angle A to the axis 10 such that, after reflection from the mirror 4, it is focussed to a point 16 on mirror 6 which is coincident with the axis 10 and as a consequence the angle B which the output beam 14 makes with the axis 10 is equal to the angle A. Reference 16a in Figure 2A indicates the virtual image of point 16 associated with the reflection of the beam from the mirror 8 and thus point 16a is spaced behind (to the left as seen in the drawings) mirror 8 by a distance equal to the distance by which the point 16 is spaced in front of the mirror 8. In considering Figure 2A, it should be noted that a ray 14a of the output beam which has passed through the centre of lens 2 does not change its direction upon passing through the lens 2.Thus, the direction of the output beam 14 may be determined by drawing a straight line between the centre of the lens 2 and the virtual image 16a.

In Figure 2b, the angle between the input beam 12 and the axis 10 is shown as having a value C larger than the angle A and as a consequence the point of focus 18 on mirror 6 is to the left of the axis 10 and therefore to the left of point 16 of Figure 2A. In other words, point 18 is closer to mirror 8 than point 16. It follows that the virtual image 18a of point 18 is closer to mirror 8 then virtual image 16a and determination of the direction of the output beam 14 by drawing a straight line between the virtual image 18a and the centre of the lens 2 will show that as the angle between the axis 10 and the input beam 12 is increased, the angle between the output beam 14 and the axis 10 has correspondingly decreased, this latter angle being shown as having a value D in Fig. 2B.

Thus, within limits, the angle of the incident beam 12 may change whilst maintaining a substantially constant angle between the input beam 12 and the output beam 14. This is achieved without the lateral shifting which takes place with a pentaprism and accordingly the invention is advantageous compared to pentaprisms in this respect. A further advantage possessed by the invention compared to pentaprisms is that the reflector according to the invention may be used for pupil imaging or relaying in an optical system, in view of the presence of the lens 2 or other optical element with power.

Various modifications are possible. For example, the mirrors may be constituted by the surfaces of a solid block of transparent material such as glass, with which the lens 2 may also if desired be integral.

Reflection at these surfaces may be achieved by total internal reflection and/or by the provision of silvering or the like on the surfaces of the block.

It is also possible to dispense with the lens 2 and use instead, for example, a concave mirror or other optical element or elements with power for focussing the input beam and collimating the output beam.

Further, although in the illustrated embodiments, the various reflective surfaces have been described as being plane, it is possible to provide, for example for correction of astigmatism, a limited profiling of one of the reflecting surfaces. For example, one of the mirrors 4 or 8 could be slightly toric, in particular slightly curved about an axis transverse to the axis 10 to achieve this correction.

Claims (6)

CLAIMS:

1. A reflective device for providing an output beam which maintains an approximately constant angle with an input beam, at least over a limited range of variations in the angle of incidence of the input beam, comprising first and second opposed reflective surfaces, a third reflective surface arranged transversely to said first and second reflective surfaces, and optical means with power arranged to focus an input beam at or near to said transverse reflective surface after reflection at one of said opposed surfaces and to provide said output beam after reflection at said other opposed reflective surface, the arrangement being such that the angle between the output and input beams remains substantially constant at least over a limited range of angles of incidence of said input beam.

2. A device according to claim 1, wherein said reflective surfaces are plane.

3. A device according to claim 1 or 2, wherein said optical means comprises a single optical element acting on both the input beam and the output beam.

4. Apparatus according to any of claims 1 to 3, wherein the optical means is a collimating lens.

5. A device according to claim 4, wherein said opposed surfaces are positioned at equal distances on opposite sides of the axis of said lens, said lens and said transverse reflective surfaces being at opposite ends of said opposed surfaces.

6. A reflective device substantially as herein described with reference to the accompanying drawings.