GB2149933A - Optical light beam splitter/combiner - Google Patents

Abstract

A light beam splitter/combiner comprises a prism member (1) having front and back faces (2,3) and an inclined partially transmitting/partially reflecting interface (4). A further face (5) forms an extension to the interface (4) so that light from a source (9) is internally reflected from the further face (5) and the front face (2) before reflection at the interface (4) to combine with other light travelling along an optical axis (0) through the prism member (9) may be a light emitting or display source such as a graticule. <IMAGE>

Description

SPECIFICATION Improvements in or relating to optical apparatus This invention concerns improvements in or relating to optical apparatus.

In some circumstances there is a requirement for effectively combining beams of light in an optical system and in others for effectively splitting a beam of light into respective components. As particular examples, in a low light level or night vision system it is sometimes required to inject a graticule image into an observer's view of an image intensifier phosphor; in a head-up display a stand-by display is sometimes required to be superimposed on the main display; in a colour television camera there can be a requirement for optical channel combining or splitting.Combining, or where appropriate splitting, of light beams in such systeins can be effected by use of a prism block arrangement having a partially reflecting/partially transmitting interface, prior examples of which are described in UK Patent Specifications 1,261,884, 1,303,503, GB 2,079,971A and GB 2,090,012A. There is a continuing requirement for minimising the prism block thickness relative to the effective height of the interface, principally so as to enable the block to fit into a small space and so as to reduce its weight.

Broadly according to the present invention there is provided optical apparatus for combining light to travel, or for splitting light travelling, generally along an optical axis, comprising a prism member having a partially transmitting/partially reflecting interface inclined to the optical axis and at which the light is combined or split, and a front face at which light is internally reflected as it travels to, or from, the interface, in which the prism member has a further face which extends from the end of the interface remote from the front face, and is inclined relatively to the front face and to the optical axis, at which further face light is internally reflected as it travels to, or from, said front face, and the apparatus has off-axis means for introducing light into, or receiving light from, the prism member for, or after, internal reflection at said further face.

Said faces and interface are preferably planar. Said front face is preferably substantially normal to the optical axis and the prism member preferably has a back face parallel to said front face. Said further face extending from the interface is preferably coplanar with the interface so as to form an extension thereof later referred to as an extension face.

Said partially transmitting/partially reflecting interface and part of said further face may lie between prism components, which are preferably gas, and conveniently air, spaced.

More particularly the present invention provides optical apparatus for combining light to travel, or for splitting light travelling, generally along an optical axis, comprising a prism member having planar front and back faces substantially normal to the optical axis and a partially transmitting/partially reflecting planar interface extending part way between said front and back faces and inclined relatively thereto and to the optical axis, the prism member having a further planar face extending from the end of the partially transmitting/partially reflecting interface nearest said back face and coplanar with that interface so as to constitute an internally reflecting extension face lying partly between prism components, which are preferably gas, and conveniently air, spaced, and the apparatus including off-axis means for introducing light into, or receiving light from, the prism member such that the light path between said off-axis means and travel generally along the axis involves internal reflection at said extension face and at said front face and reflection at said interface.

The invention further provides optical apparatus for combining light to travel, or for splitting light travelling, generally along an optical axis, comprising a prism member having planar front and back faces substantially normal to the optical axis and a partially transmitting/partially reflecting planar interface extending between said front and back faces and inclined relatively thereto and to the optical axis, the prism member having a further planar face extending from the end of the interface at said back face and coplanar with the interface so as to constitute an extension face, and the apparatus including off-axis means for introducing light into, or receiving light from, the prism member such that the light path between said off-axis means and travel generally along the axis involves internal reflection at said extension face and at said front face and reflection at said interface.

The prism member may have an entry, or exit, face substantially parallel to the optical axis, and preferably planar, through which light is transmitted from or to, said off-axis means.

Said interface, and said further or extension face, may be inclined relatively steeply to the optical axis, and may for example be at an angle in the range of 20 to 23 degrees to a plane normal to the optical axis. Said interface may have wavelength selective transmission/reflection characteristics and is preferably dichroic. The internal reflection at said further or extension face and/or at said front face is preferably simple total internal reflection.

Apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of one embodiment, Figure 2 is another view of the apparatus of Fig. 1, Figure 3 is a schematic representation of the apparatus of Fig. 1 incorporated in a magnifier, and Figure 4 is a schematic representation of another embodiment.

Fig. 1 shows a linear optical axis 0 along which light travels. A composite prism member, generally indicated as 1, is disposed on the optical axis and has parallel planar front and back faces 2 and 3 respectively which are normal to the optical axis, i.e. to which the axis 0 runs perpendicularly. The prism member 1 has an internal planar interface 4 which is inclined relatively to the axis 0 and to the front and back faces 2 and 3 between which it extends. The interface 4 is partially transmitting and partially reflecting to light preferably on a wavelength selective basis, and in particular may be dichroic so as to reflect light over a desired wavelength band but transmit light of other wavelengths.A coplanar extension of the interface 4 provides a planar external further or extension face 5 extending from the end of the interface 4 at the back face 3, i.e. the end remote from the front face 2 The prism member 1 further has a planar light entry face 6 at the top which lies parallel to the axis 0. As viewed in Fig. 1 the prism member 1 also has a bottom planar face 7 parallel to the axis 0 and an inclined planar face 8 between the top face 6 and the front face 2, but, as explained later, light does not strike these faces 7 and 8 in use of the prism member.

Fig. 1 also indicates a light emitting or display source 9, such as a graticule, at an off-axis location adjacent the top face 6. In use, light from the source 9 enters the prism member 1 through the top face 6, is totally internally reflected at the extension face 5, is then totally internally reflected at the front face 2, and is then reflected at the interface 4 to emerge from the prism member through the front face 2. Such light can therefore effectively be combined with other light travelling along the optical axis 0 and transmitted through the back face 3, the interface 4, and the front face 2 of the prism member. The front and back faces 2 and 3 are such as to permit transmission of light at and near normal incidence but the front face 2 can effect total internal reflection of light from the source 9. These faces may therefore be untreated with any coating or the like.Likewise, the top face 6 through which light from the source 9 is transmitted at normal or near normal incidence, may also be untreated. The extension face 5 may similarly be untreated, the angle of incidence of light from the source 9 being such that total internal reflection occurs, but the extension face 5 could have a reflective coating, and possibly a dichroic coating as at the interface 4, if desired since there is no requirement for transmission of light through this face. A dichroic interface 4 can suit a narrow wavelength band source 9 so that substantially all the light from the source 9 incident on the interface is reflected thereby.The light travelling along the axis 0 and transmitted through the prism member 1 may come from a display source, such as the phosphor of an image intensifier tube or a cathode ray tube, and may be over a narrow or a broad wavelength band or bands transmitted by the interface. As shown in Fig.

1, the light reflected at the interface 4 and emerging from the front face 2 travels along the axis 0 in parallel with the light transmitted through the prism member 1. In this context, it will be understood that the term 'axis' is used in the sense of indicating a general direction for light travel, and references to travel along the axis are to be construed accordingly.

The prism member 1 can be assembled in a manner essentially known per se. As can be seen from Fig. 1, it is formed from two prism components of the same optical glass, namely a triangular prism 10 having the back face 3, the bottom face 7 and a hypotenuse face at the interface 4, and a quadrilateral prism 11 having the front face 2, the inclined face 8, the top face 6 and a long face at the interface 4 and providing the extension face 5. The coating which imparts to the interface 4 its required reflection/transmission characteristics is applied to the hypotenuse face of the prism 10 and/or the appropriate part of, or possibly the entire, long face of the prism 11, and the two prisms are then cemented together.

It will be understood that the inclined face 8 is effectively a chamfer on a basically triangular prism component 11, the chamfer being made primarily to prevent ghost imaging. The components 10 and 11 forming the prism member 1 may further be shaped and effectively chamferred in view of weight and/or space saving considerations. Thus, the prism member 1 may have a transverse shape as shown in Fig. 2, which represents it when viewed in the direction of arrow A in Fig. 1.

The interface 4 and its extension face 5 can be inclined relatively steeply to the optical axis 0, thus permitting the prism member 1 to be relatively thin while providing a good interface size. In particular the extent of the interface 4 may be such that its height (when viewed along a horizontal axis 0) is greater than the main field of view so that the main field of view by way of light transmitted through the prism member 1 is not interrupted by 'lines' at the respective ends of the interface. The interface 4 may, for example, be at an angle of approximately 21 degrees to the front face 2 (which is in a plane normal to the optical axis 0) providing an interface height of about 2.6 times the prism member thickness (i.e.

the distance between the front and back surfaces 2 and 3). It will be appreciated that a relatively thin prisms member can have space and weight advantages and this compares favourably with prior arrangements in which an interface inclined at 45 degrees to the optical axis provided an interface height equal to the thickness of the prism block, or in which an interface inclined at 60 degrees to the optical axis (i.e. at 30 degrees to a front face normal to the axis and from which light is internally reflected to the interface) provided an interface height of about 1.7 times the prism block thickness. Further, in some circumstances the provision of an entry face 6 parallel, or substantially parallel, to the optical axis 0 can be of advantage, e.g. in avoiding protrusion of a graticule adjustment plate or the like.Still further, the arrangement is not generally such as to give rise to disturbing ghost images.

A prism member or block as shown in Fig.

1 may, for example, be incorporated in the magnifier of a low light level or night vision device. This is schematically illustrated in Fig.

3 which shows the phosphor end of an inverting image intensifier tube 1 2 in a night vision sight with a lens element 1 3 suitably profiled to fit the tube. The magnifier comprises further lens elements 14 and 1 5 through which an observer views along axis 0 to see a magnified version of the phosphor image. The prism block 1 is located on the axis 0 between the lens elements 1 3 and 14 so that light from a graticule can be injected as described above to superimpose on the observer's view an image of the graticule.

Another example of equipment in which apparatus essentially as shown in Fig. 1 could be employed is a head-up display in which a stand-by display, which may comprise a graticule, can be injected so as to be superimposed on the observer's view of a main display, e.g. on a cathode ray tube, which is seen by way of light transmitted through the prism block.

It will be appreciated that, if desired, a pair of prism members as shown in and described with reference to Fig. 1 may be disposed in adjacent bottom end-to-end relationship so that light from respective sources 9, e.g.

respective half graticules, can be injected from opposite sides of the axis 0. The prism members could be located with their faces 7 adjacent one another at the axis, or the respective prism components 10 may be combined into one (thus eliminating the faces 7) to provide an overall three component prism member straddling the axis 0 and into each end of which a half graticule, for example, can be injected to provide a combined full graticule injection.

A further example of equipment in which apparatus essentially as shown in Fig. 1 could be used is a colour television camera.

It will be appreciated that, although described above in terms of injecting light into the prism member 1 for combination with light travelling along the optical axis and transmitted through the prism member 1, the apparatus could be used in converse mode to split a light beam incident on the prism member 1 so as to extract some light. Thus, the Fig. 1 arrangement is described above as receiving a first beam of light travelling along the axis 0 from the right and effectively injecting a further beam of light from the source 9 so that the two beams combine at the interface 4 to travel together along the axis 0 to the left.The arrangement could alternatively be used to receive a beam of light travelling along the axis 0 from the left and effectively to split that beam at the interface 4 into a first beam consisting of light that is transmitted by the interface 4 to continue travel along the axis 0 to the right, and a second beam consisting of light that is reflected by the interface 4 and emerges, after total internal reflection at the front face 2 and at the extension face 5, through the top face 6. In this case the top face 6 is, of course, an exit face rather than an entry face, and the source 9 is replaced by some form of receiver, such as a detector, for the second beam. The beam splitting can be on a wavelength selective basis by use of an interface 4 having wavelength selective reflection/transmission characteristics, and in particular a dichroic interface.

It will further be appreciatedthat although the further or extension face 5 is conveniently made coplanar with the interface 4 as described above, the face 5 could be inclined relatively to the interface 4. Thus the face 5 could be a further planar face extending from the end of the interface 4 remote from the front face 2, i.e. at the junction of the interface with the back face 3 but inclined relatively to the interface and relatively to the back face.The further face 5 is, of course, also inclined relatively to the axis 0 and to the front face 2 and the relative angles and the angle at which light is injected into (or received from) the prism member 1 are so arranged that the light is internally reflected at the further face 5 and the front face 2 as it travels to the interface 4 where it is reflected to travel along the axis 0 (or is, after diversion from travel along the axis 0 by reflection at the interface 4, internally reflected at the front face 2 and then at the further face 5 to emerge through the axis face 6). The entry (or exit) face 6 may also be inclined, rather than parallel, relatively to the optical axis 0, but it is preferably, although not necessarily, within + 10 degrees relatively to the optical axis 0.

It will further be understood that the angle of the interface 4 and extension face 5 relative to the prism front face 2, i.e. to a plane normal to the optical axis, could be outside the indicated preferred range of 20 degrees to 23 degrees which is given only by way of example as appropriate to convenient pupil sizes and glass refractive index where a member, such as a graticule adjustment plate, is to be kept at least approximately parallel to the optical axis for compactness.

Further, although the front and back planar surfaces 2 and 3 are preferably and usually disposed normal to the optical axis 0, some departure from such normality may be tolerable or even desirable in some circumstances, e.g. to effect a bending of the light path. It will be understood that the main optical effects of the present invention, namely internal reflection at the faces 5 and 2 and at the interface 4 whilst also permitting transmission through the interface 4, occur in the prism component 11 and therefore the prism component 10 may be considered as inessential to the invention in its broadest aspect. This prism component 10 serves to avoid unwanted refraction of the transmitted light at the interface 4 and to equalise optical path lengths, as well as to protect the interface coating, and could be omitted if these factors are not of concern.The material of the prism components 11 and 10 is preferably optical glass but other materials might conceivably be employed in some circumstances. Although planar faces and a planar interface for the prism member 1 are generally convenient and highly desirable, one or more at least slightly curved faces or interface might be used in some circumstances, e.g. if some degree of optical power is to be imparted.

Those skilled in the art may perceive other possible modifications, adaptions and uses which can be applied, without departing from the scope of the invention, to the apparatus specifically described above by way of illustration and example.

Another embodiment is shown in Fig. 4 which is basically similar to that of Fig. 1 and the same references are used to indicate like parts. Thus, the Fig. 4 embodiment comprises a prism member 1 having planar front and back faces 2 and 3 substantially normal to an optical axis 0, and a planar partially transmitting/partially reflecting dichroic interface 4 between prism components 10 and 11. The prism member 1 has a planar further face 5 coplanar with, and forming an extension face to, the interface 4. In this embodiment, however, the dichroic interface 4 extends only part way between the front and back faces 2 and 3.In particular the dichroic interface 4 extends from the front face 2 to a position 1 6 which is spaced from the back face 3, and the prism components 10 and 11 are air-spaced to provide an air gap 1 7 therebetween so that the long face of the prism component 11 is rendered totally internally reflecting from the end 1 6 of the dichroic interface 4. In other words, the internally reflecting further or extension face 5 now extends from the position 16 (i.e. the end of the dichroic interface 4 nearest the back face 3) past the back face 3 and to the top of the prism member, and part of the further or extension face 5 therefore lies between the prism components 10 and 11, which are air-spaced by a suitable spacer 18 in a manner known per se.

The effect of lengthening the further or extension face 5 in this way is to increase the size or range of movement of a graticule 9, or the like, which can be accommodated by the apparatus (with a given thickness of prism member). Light from the graticule 9 is transmitted through a planar entry face 6 at the top of the prism member 1 and lying parallel to the axis 0. The introduced light is totally internally reflected at the face 5, then at the front face 2, and is then reflected at the dichroic interface 4 to travel generally along the axis 0 combined with other light transmitted through the prism member 1 and the interface 4. The greater effective length of the totally internally reflecting extension face 5 permits a larger, or bigger range of movement for, the graticule 9.

It will be appreciated that the space between the prism components 10 and 11 is conveniently an air gap, but could be occupied by some other gas if desired. It will further be appreciated that other modifications to the particular embodiment shown in and described with reference to Fig. 4 may be made, in particular as suggested above in relation to the Fig. 1 embodiment, including the possibility of use of the apparatus for splitting light travelling generally along the optical axis, instead of combining light as specifically described above.

Claims (14)

1. Optical apparatus for combining light to travel, or for splitting light travelling, generally along an optical axis, comprising a prism member having a partially transmitting/partially reflecting interface inclined to the optical axis and at which light is combined, or split, and a front face at which light is internally reflected as it travels to, or from, the interface, in which the prism member has a further face which extends from the end of the interface remote from the front face, and is inclined relatively to the front face and to the optical axis, at which further face light is internally reflected as it travels to, or from, said front face, and the apparatus has off-axis means for introducing light into, or receiving light from, the prism member for, or after, internal reflection at said further face.

2. Apparatus according to Claim 1 in which said faces and interface are planar.

3. Apparatus according to Claim 1 or Claim 2 in which said front face is substantially normal to the optical axis.

4. Apparatus according to any preceding claim in which the prism member has a back face parallel to said front face.

5. Apparatus according to any preceding claim in which said further face extending from the interface is coplanar with the interface.

6. Apparatus according to any preceding claim in which said partially transmitting/partially reflecting interface and part of said further face lie between prism components.

7. Optical apparatus for combining light to travel, or for splitting light travelling, generally along an optical axis, comprising a prism member having planar front and back faces substantially normal to the optical axis and a partially transmitting/partially reflecting planar interface extending part way between said front and back faces and inclined relatively thereto and to the optical axis, the prism member having a further planar face extending from the end of the partially transmitting/partially reflecting interface nearest said back face and coplanar with that interface so as to constitute an internally reflecting extension face lying partly between prism components, and the apparatus including off-axis means for introducing light into, or receiving light from, the prism member such that the light path between said off-axis means and travel generally along the axis involves internal reflection at said extension face and at said front face and reflection at said interface.

8. Apparatus according to Claim 6 or Claim 7 in which said prism components are gas spaced.

9. Optical apparatus for combining light to travel, or for splitting light travelling, generally along an optical axis, comprising a prism member having planar front and back faces substantially normal to the optical axis and a partially transmitting/partially reflecting planar interface extending between said front and back faces and inclined relatively thereto and to the optical axis, the prism member having a further planar face extending from the end of the interface at said back face and copolanar with the interface so as to constitute an extension face, and the apparatus including off-axis means for introducing light into, or receiving light from, the prism member such that the light path between said off-axis means and travel generally along the axis involves internal reflection at said extension face and at said front face and reflection at said interface.

10. Apparatus according to any preceding claim in which the prism member has an entry, or exit, face substantially parallel to the optical axis through which light is transmitted from, or to, said off-axis means.

11. Apparatus according to Claim 10 in which said entry, or exit, face is planar.

12. Apparatus according to any preceding claim in which said interface and said further face are at an angle in the range of 20 to 23 degrees to a plane normal to the optical axis.

1 3. Apparatus according to any preceding claim in which said interface has wavelength selective transmission/reflection characteristics.

14. Apparatus according to Claim 1 3 in which said interface is dichroic.

1 5. Apparatus according to any preceding claim in which the internal reflection at said further face is simple total internal reflection.

1 6. Apparatus according to any preceding claim in which the internal reflection at said front face is simple total internal reflection.

1 7. Optical apparatus substantially as described herein with reference to any of the figures of the accompanying drawings.