GB2170922A - Light beam switching prism having mirrored faces - Google Patents

Abstract

A light-switching pentaprism designed to switch light from a fixed source into one or another direction, has a first position in which the light leaves the pentaprism at 90 DEG to its input direction, and a second position in which an additional input face 10 is directed towards the light source, the light entering that face 10 then effectively passing straight through the prism and emerging from an additional output face 11 in its original direction. Faces 10,11 may be curved or planar. Faces 6,7 are mirrored. <IMAGE>

Description

SPECIFICATION Light switching device This invention relates to devices for switching light and more particularly to the use of pentaprisms for switching light.

Pentaprisms are well known and generally have a first planar input face and a first planar output face at 90" to each other and two mirrored planar reflecting faces at 45" to each other, the line of intersection of any two of the face being parallel to any other line of intersection of any two of the faces, the faces being arranged so that radiation entering the prism via the first input face is reflected from both the mirrored faces, in turn, and leaves the prism via the first output face at 90" to its input direction, such pentaprisms being hereinafter referred to as being of the type specified.

Pentaprisms of the type specified will only deflect incoming light by 90 , however, and it is not possible to choose to either deflect the light, or not, as desired, except by moving the pentaprisms into and out of the path of the incoming radiation.

Moving pentaprisms in this way may be difficult and it is undesirable for fast, accurate switching devices. It also takes a considerable time which is also undesirable when fast switching times are important.

It is thus an object of the present invention to provide a pentaprism which can be used to switch light simply by rotating the prism.

Accordingly, the invention provides a pentaprism of the type specified, further comprising a second input face between said first input and output faces and a second output face between said two reflecting faces said second input and output faces being opposite each other, all the lines of intersection between all the faces being parallel, so that light entering via the second input face passes substantially straight through the prism and leaves via the second output face.

The said second input and output faces may be planar, but they are preferably arcs forming part of a cylinder whose axis is parallel to the lines of intersection of the faces.

According to a second aspect of the invention, there is provided a device for switching light incorporating a pentaprism of the type specified, the pentaprism further comprising a second input face between said first input and output faces and a second output face between said two reflecting faces said second input and output faces being opposite each other so that radiation entering via the second input faces passes substantially straight through the prism and leaves via the second output face, the device further comprising means for rotating said pentaprism, in use, so that incoming radiation impinges on either the first or second input face, as desired, and thus is either deflected by 90" or passes straight through the prism.

In a preferred embodiment, the second input and output faces are arcs forming part of a cylinder and the pentaprism is rotatable about the axis of the cylinder.

The pentaprism may be rotated by electromagnetic or non-electromagnetic means, such non-electromagnetic means including the use of hydraulic, pneumatic or manual means.

Preferably, said means for rotating the pentaprism includes a permanent magnet fixed to the pentaprism and two pairs of electromagnets are arranged around the pentaprism to attract or repulse the permanent magnet and so rotate the pentaprism into either of the desired positions.

Alternatively, said means for rotating the pentaprism again includes a permanent magnet fixed to the pentaprism, but this time, blocks of magnetic material are provided to attract the permanent magnet into the two desired positions and the pentaprism is rotated by forcing it to move from one position to the other, for example by being coupled to an arm coupled to a second magnet linearly movable within a solenoid.

The invention will now be more fully described by way of example with reference to the drawings of which: Figure 1 shows diagrammatically a conventional pentaprism; Figure 2 shows diagrammatically a pentaprism according to the invention; Figure 3 shows diagrammaticaly one method of rotating the pentaprism of Fig. 2 so that it may be used for switching light; and Figure 4 shows diagrammatically an alternative method to that shown in Fig. 3.

Thus, as shown in Fig. 1, a conventional pentaprism is a five-sided prism with all the faces normal to the same transverse planes.

Two of the faces 1 and 2 are mirrored (as shown by the hatched lines) so that radiation is internally reflected off these faces. The mirrored faces 1 and 2 are at an angle A of 45" to each other although the corner is usually cut off by face 3 to make the prism a more manageable shape.

The shape of the prism is completed by an input face 4 and an output face 5 at 90 to each other as shown. A beam of radiation striking the input face 4 of the prism will then pass into the prism, be reflected from the two mirrored faces 1 and 2 and leave the prism via output face 5, as shown. The direction of the beam on leaving the prism will always be at 90" to the input direction, provided the angle A is 45 , and minor angular displacements of the prism will not affect the output direction.

Turning now to the modified pentaprism according to the invention as shown in Fig. 2, this has mirrored faces 6 and 7 at 45" to each other similar to faces 1 and 2 in Fig. 1.

The modified pentaprism also has first input face 8 and a first output face 9 similar to faces 4 and 5 of Fig. 1. Faces 8 and 9 are also at 90" to each other but the corner has been rounded to form a second input face 10 in the shape of an arc of a cylinder centred on the optical centre of the prisms. A second output face 11 then forms a diagrammatically opposite arc of the same cylinder and cuts off the corner between the faces 6 and 7. Thus, the lines of intersection between all the faces and the axis of the cylinder are parallel.

It will be apparent that when a beam of radiation strikes the first input face 8, it will be reflected from both the faces 6 and 7 and leave via face 9, in a similar manner to that described above with reference to Fig. 1.

However when a beam of radiation strikes the second, curved, input face 10 it will pass straight though the prism and leave via the second output face 11.

It will further be apparent that the second input and output faces 10 and 11 need not be curved but could be planar. In this case, the light will leave the prism at exactly the same angle as it enters but it will be displaced slightly due to the refraction at the input and output faces. This means that any relative angular displacement of the beam from the normal to the input face will be passed on to the output beam. By making the second input and output faces curved to form part of a cylinder, however, the prism acts as a convex lens and tends to focus the beam, so that any relative angular displacement of the input beam to the normal input face will tend to be decreased by the focussing effect of the prism.

In order to use a prism having the construction described above with reference to Fig. 2 as a light switching device, it is required that the prism be rotated by 45" so as to present either the first input face 8 or the second input face 10 to the incoming light beam. By positioning light detectors at both the straight through position and the 90'' deflected position, the prism can then be used to switch the light from one detector to the other mereley by rotating it through 45".

Clearly, any method may be used to effect the rotation and three such methods will now be described, the second and third of which refer to Fig. 3 and 4. In all three cases the prism 12 is pivoted about the axis of the cylinder of which the second input and output faces form a part. In the first, non-electromagnetic, case (not shown) the prism is related about its axis by hydraulic, pneumatic or manual means. In the second and third, electro magnetic, cases there is also fixed to a end face of the prism a permanent bar magnet 13 aligned with the diameter of the cylinder join ing the second input and output faces.

In the embodiment shown in Fig. 3, two pairs of electromagnets 14 and 15 are arranged in positions around the prism as shown such that when the permanent magnet 13 is aligned with the first pair 14, the prism is in the position for a light beam travelling in the direction of arrow 16 to hit the first input face and be deflected by 90 , and when the magnet 13 is aligned with the secondpair of electromagnets 15, the prism presents the second input face to the light beam coming from the same direction and this time is allowed to pass straight through the prism.

The prism may be constrained so that it cannot be rotated beyond the desired positions and by providing the electromagnets with magnetically soft cores, the electromagnets can be arranged to either attract or repulse the bar magnet 13 as desired by altering the direction of current flow through the coils. When the prism is in a desired position, the currents can, of course, be turned off and the prism will remain stable in this position since the cores will remain magnetised in the given polarity until the current is once again switched on and reversed.

Fig. 4 shows an alternative means of rotating the prism 12 which is shown in full lines in the same position as shown in Fig. 3 for 90" deflection of the light being produced by an emitter 16 and detected by a detector 17, and in chain lines in the alternative position for no deflection where a detector 18 is used. In this embodiment the bar magnet 13 is attracted to the two stable positions by suitable permanent magnets 19 and is constrained from moving too far by suitably positioned stops 20.

In order to change the position of the prism from one state to the other the bar magnet 13 is coupled by an arm 21 to a second magnet 22 which is longitudinally movable inside a solenoid 23. By passing a current though the solenoid 23 in one direction, the magnet 22 is either attracted into the solenoid 23 and thus the prism 12 is moved into the position shown in chain lines in Fig. 4, or, by reversing the current, the magnet 22 is repulsed by the solenoid and the prism is moved into the position shown in full lines in Fig. 4. In each position, the prism is kept stable by the attraction between the bar magnet 13 and the magnets 19 and the position of the stops 20, the current to the solenoid being off.

It will be appreciated that any means for rotating a pentaprism according to the invention may be used and that an array of such rotatable pentaprisms can be used to make an optical switching network.

Claims (12)

1. A pentaprism of the type specified, further comprises a second input face between said first input and output faces and a second output face between said two reflecting faces, said second input and output faces being op posite each other, all the lines of intersection between all the faces being parallel, so that light entering via the second input face passes substantially straight through the prism and leaves via the second output face.

2. A pentaprism according to Claim 1, wherein the second input and output faces are planar.

3. A pentaprism according to Claim 1, wherein the second input and output faces are arcs forming part of a cylinder whose axis is parallel to the lines of intersection of the faces.

4. A device for switching light incorporating a pentaprism of the type specified, the pentaprism further comprises a second input face between said first input and output faces and a second output face between said two reflecting faces said second input and output faces being opposite each other so that radiation entering via the second input face passes substantially straight through the prism and leaves via the second output face, the device further comprising means for rotating said pentaprism, in use, so that incoming radiation impinges on either the first or second input face, as desired, and thus is either deflected by 90" or passes straight through the prism.

5. A device for switching light according to Claim 4, wherein the second input and output faces are arcs forming part of a cylinder and the pentaprism is rotatable about the axis of the cylinder.

6. A device for switching light according to Claim 5, wherein the pentaprism is rotated by non-electromagnetic means.

7. A device for switching light according to Claim 6, wherein the non-electromagnetic means are hydraulic, pneumatic or manual.

8. A device for switching light according to Claim 5 wherein the means for rotating the pentaprism includes a permanent magnet fixed to the pentaprism and two pairs of electromagnets arranged around the pentaprism to attract or repulse the permanent magnet and so rotate the pentaprism into either of the two desired positions.

9. A device for switching light according to Claim 5, including a permanent magnet fixed to the pentaprism, and blocks of magnetic material provided to attract and hold the permanent magnet into either one or other of the two desired positions, the means for rotating the pentaprism comprising an actuator capable of overcoming the magnetic attraction so as to force the pentaprism to move from one position to the other.

10. A device for switching light according to Claim 9, wherein the actuator comprises an arm coupled to the pentaprism and also to a second magnetic linearly movable within a solenoid.

11. A pentaprism as hereinbefore described with reference to Figs. 1 to 2 of the accompanying drawings.

12. A device for switching light as hereinbefore described with reference to Figs. 3 to 4 of the accompanying drawing.