NIGHT VISION BINOCULARS

The present invention relates to night-vision binoculars, comprising:          a. a fixed lens assembly comprising a projection lens having an output axis referred to as the projection axis,     b. two eyepieces, each eyepiece having an output axis referred to as the vision axis, the vision axes of the two eyepieces being parallel and separated by an adjustable distance, called inter-pupillary distance, the vision axis of each eyepiece also being parallel to the projection axis of the lens assembly and having the same non-zero centre-to-centre distance from the projection axis of the lens assembly, each eyepiece being rotatably movable relative to the projection axis of the lens assembly so as to adjust the inter-pupillary distance.

This invention concerns night-vision binoculars.

More precisely, the invention concerns night-vision binoculars configured to capture and intensify images originating from a scene. The spectral domain of intensification is typically between 450 and 950 nm. When such binoculars are configured to project information originating from a screen on the intensified image, they are referred to as ‘connected’.

Connected night-vision binoculars comprising two capture paths, each associated with an eyepiece, are known from the prior art. However, only one of the two paths is associated with means of projection of information from a screen on the image captured. Thus, the information originating from the screen is only visible through one of the eyepieces, which is problematic when this eyepiece does not correspond to the dominant eye of the user.

This projection on only one of the two paths is explained by the technical and economic difficulties of providing a mechanism for adjusting the inter-pupillary distance (IPD) between the two eyepieces. In particular, in the night-vision binoculars available on the market, the IPD adjustment is either linear or rotary.

In the case of linear IPD adjustment, the two binocular bodies are guided mechanically by a slide link on a bracket. The bracket connects the two binocular bodies to the mechanical mount of the helmet or head harness. For unconnected night-vision binoculars (no screen), the power supply of the intensifier tube goes through flexible webs connecting the binocular bodies to the bracket or through electrical contacts on a track.

In the case of connected night-vision binoculars, the cables providing the power supply for the screen and the connection to video signals are added to those providing power to the intensifier tube, which makes the incorporation of such cables into the binocular body both technically and economically complex. The night-vision binoculars available on the market thus do not have an adjustable IPD and have a screen on a single path.

In the case of rotary IPD adjustment, the two binocular bodies are guided mechanically by a pivoting link on a bracket connecting the two binocular bodies to the mechanical support frame of the helmet or head harness. This type of IPD adjustment makes the passage of the cables easier than in the case of linear IPD adjustment solutions.

However, in the case of connected night-vision binoculars, this type of IPD adjustment induces rotation of the images from the screen. The images from the screen are thus perceived at an incline by the user. Solutions that incorporate a mechanical derotator are conceivable, but unsuitable from the microeconomic standpoint. Thus, for this reason, too, the night-vision binoculars available on the market do not have an adjustable IPD and have a screen on a single path.

There is thus a need for connected night-vision binoculars allowing for an intensified image of the scene comprising information from a screen to be visualized via both eyepieces, whilst also having an adjustable IPD.

To this end, this description concerns night-vision binoculars comprising:a. one or two fixed lens assemblies, the/each lens assembly comprising:i. a capture lens configured to capture an image of a scene,ii. a light intensification device configured to intensify the image captured in order to obtain an intensified image,iii. a screen suited to generate an additional image,iv. a projection lens configured to project the additional image on the intensified image such that the output beam of the projection lens (‘projection beam’) transports the resultant image, wherein the projection lens has an output axis (‘projection axis’),b. two eyepieces that receive either the same projection beam, when the binoculars comprise a single lens assembly, or different projection beams, when the binoculars comprise two lens assemblies,wherein each eyepiece has an output axis (‘vision axis’), the vision axes of the two eyepieces being parallel and separated by an adjustable distance (‘inter-pupillary distance’),wherein the vision axis of each eyepiece is further parallel to the projection axis of the corresponding lens assembly and has the same non-zero centre-to-centre distance as the projection axis of the corresponding lens assembly,wherein each eyepiece is rotatable relative to the projection axis of the corresponding lens assembly so as to adjust the inter-pupillary distance.

In particular embodiments, the binoculars comprise one or more of the following characteristics, alone or in any combination technically possible:the projection beam is a collimated or near collimated beam;the vision axis of each eyepiece is the optical axis of the eyepiece;the binoculars comprise two lens assemblies, the IPD being the sum of a nominal distance and an adjustment range, the value of the adjustment range being a function of the rotation of each eyepiece and being within a limited range centred on zero, the centre-to-centre distance between the vision axis of each eyepiece and the projection axis of the corresponding lens assembly being equal to one-half of the positive limit of the limited range;each eyepiece has an input axis that coincides with the projection axis of the corresponding lens assembly, wherein the vision axis of each eyepiece is offset from the input axis of the eyepiece by a layover formed by two dioptres, each dioptre having a flat optical surface parallel to the flat optical surface of the other dioptre, wherein the first dioptre is configured to reflect at least part of the projection beam output by the corresponding lens assembly in the direction of the second dioptre, wherein the second dioptre is configured to reflect the projection beam in the direction of the vision axis;the IPD is the sum of a nominal distance and an adjustment range that is a function of the rotation of each eyepiece, wherein the adjustment range is a function of the centre-to-centre distance between the projection axis and the vision axis of the eyepieces, a nominal orientation of the eyepieces, and a rotation angle of each eyepiece relative to the nominal orientation;the binoculars comprise two lens assemblies;the binoculars comprise a single lens assembly, such that the projection axis is the common rotation axis of the two eyepieces, wherein the flat optical surface of the first dioptre of one of the eyepieces (‘first eyepiece’) is partially reflective, so as to reflect part of the projection beam in the direction of the second dioptre of the first eyepiece and to transmit the other part in the direction of the other eyepiece (‘second eyepiece’);the or each lens assembly comprises an input axis that is offset from the projection axis AP) of the corresponding projection lens by a layover formed by two dioptres, wherein each dioptre has a flat optical surface parallel to the flat optical surface of the other dioptre, wherein the first dioptre is comprised within the capture lens, and wherein the second dioptre is comprised within the projection lens and is in the path of the beam reflected by the first dioptre;the additional image is an image of the scene in a spectral band different to the spectral band of the image captured by the capture lens.

Examples of night-vision binoculars10are shown schematically inFIGS.1and2.FIG.1corresponds to a first and a second embodiment of the invention.FIG.2corresponds to a third embodiment of the invention. As described below, the binoculars10are ‘connected’ binoculars because they comprise a screen and a projection lens.

This description first describes the elements common to the three embodiments. Each embodiment is thereafter described more specifically.

GENERIC EMBODIMENT

The binoculars10are, e.g., intended to be mounted on a helmet or head harness. Advantageously, the binoculars10are also intended to be affixed to a vertically (up-down) adjusted support, thus allowing the height of the binoculars10to be adjusted.

As shown inFIGS.1and2, the binoculars10comprise at least one lens assembly12and two eyepieces14A,14B.

When the binoculars10comprise two distinct lens assemblies12, as inFIG.1, each eyepiece14A,14B is associated with a respective lens assembly12, and thus does not receive beams of light originating from the other lens assembly12. When the binoculars10comprise a single lens assembly12, as inFIG.2, the lens assembly12is shared by the two eyepieces14A,14B.

The association of one eyepiece14A,14B with the corresponding lens assembly12forms an optical path, with the binoculars10thus comprising two optical paths.

Each lens assembly12is fixed, i.e. it cannot be translated or rotated.

Each lens assembly12comprises at least the following elements: a capture lens20, a light intensification device22, a screen24, and a projection lens26.

The capture lens20is configured to capture an image of a scene. For example, the capture lens20comprises an assembly of several lenses.

The light intensification device22is configured to intensify the image captured in order to obtain an intensified image. For example, the light intensification device22comprises one or more intensifier tubes.

The screen24is suited to generate an additional image. The additional image is intended to provide additional information to the user of the binoculars10.

For example, the additional image is an image of the scene in a spectral band different to the spectral band of the image captured by the capture lens20. For example, the different spectral band is within an infrared (near, far, or middle infrared) band, whilst the capture lens20is, e.g., suited to capture images in the visible band (380-780 nm) or extended (400-900 nm) band. The additional image is, e.g., obtained by an additional optical path present in the binoculars10.

In one variant, the additional image originates from data obtained by sensors or other sources of information.

The projection lens26is configured to project the additional image on the intensified image such that the output beam of the projection lens26(‘projection beam FP’) transports the resultant image (superimposition of the intensified image and the additional image). It should be noted that only one ray of the beam FP is shown in the relevant figures for the sake of simplicity.

Advantageously, in the intermediate space between the projection lens26and the corresponding eyepiece14A,14B, the projection beam FP is a collimated or near collimated beam. ‘Collimated’ means that the rays of the projection beam FP are parallel or nearly parallel in the intermediate space. ‘Near collimated’ means that the rays of the projection beam FP are nearly parallel locally, i.e. over a distance of less than or equal to a value that allows the optical axis of each eyepiece14A,14B to be made insensitive to eccentricities of the mechanical axis between the eyepieces14A,14B and the one or more corresponding lens assemblies12.

The projection lens26has an output axis (‘projection axis AP’), which is also the output axis of the lens assembly12.

For example, the projection lens26comprises an assembly of several lenses.

Each eyepiece14A,14B is an image-transporting lens, i.e. it is suited to transport the image resulting from the projection to the user's eye.

When the binoculars10comprise two distinct lens assemblies12, as shown inFIG.1, the two eyepieces14A,14B receive distinct projection beams FP and thus transport distinct resultant images. When the binoculars10comprise a single lens assembly12, as shown inFIG.2, the two eyepieces14A,14B receive the same projection beam FP and thus transport the same resultant image.

Each eyepiece14A,14B has an output axis (vision axis AV′) (see, inter alia,FIG.3-8, which will be described in detail below). The vision axes of the two eyepieces14A,14B are parallel and separated by an adjustable distance (‘inter-pupillary distance IPD’). The IPD is typically the sum of a nominal distance N (fixed) and an adjustment range R (variable).

The vision axis Av of each eyepiece14A,14B is parallel to the projection axis AP of the corresponding lens assembly12and is a non-zero centre-to-centre distance E from the projection axis A P of the corresponding lens assembly12. The centre-to-centre distance is the same for both eyepieces14A,14B.

Each eyepiece14A,14B is rotatable relative to the projection axis AP of the corresponding lens assembly12so as to change the adjustment range R and thus adjust the inter-pupillary distance IPD.

The aspects of the operation of the binoculars10that are common to the three embodiments will now be described.

To adjust the IPD between the eyepieces14A,14B of the binoculars10, the user turns each of the eyepieces14A,14B about the projection axis AP of the corresponding lens assembly12. In particular, for an average adjustment configuration in which the adjustment range R is nil, the user turns each of the eyepieces14A,14B outward in order to increase the IPD and inward in order to decrease the IPD.

Once the IPD has been adjusted, the image resulting from the projection of the additional image over the intensified image of the scene is visible to the user through each of the eyepieces14A,14B.

Thus, such connected night-vision binoculars10allow the image resulting from the projection of the additional image over the intensified image of the scene to be visualised via both eyepieces14A,14B. This makes it easier to visualise such an image than with the known-art devices, in which the resultant image can only be seen on one optical path.

In particular, compared to known-art binoculars that comprise linear IPD adjustment, the adjustment of the IPD via a rotation mechanism makes it easier to incorporate the power cables of the light intensification device22and the screen24into the binoculars10. Thus, the cables do not pass through the eyepieces14,14B and are independent of the IPD adjustment mechanism.

Moreover, unlike known-art binoculars that comprise rotary IPD adjustment, the IPD adjustment does not affect the resultant image of the scene. This is due to the fact that only the eyepieces14A,14B are mobile, whilst the one or more lens assemblies12comprising the screen24are fixed.

Thus, the connected night-vision binoculars10allow the resultant image to be visualised on each of the eyepieces14A,14B, whilst maintaining the adjustability of the IPD, by means of a dissymmetry between the projection axis APof the/each lens assembly12and the vision axis Av of the corresponding eyepieces14A,14B.

Such an architecture can be adapted both for binocular and binocular binoculars. In particular, it allows for night-vision binoculars with optical fusion (intensified path and infrared path) with a single infrared capture path that is redistributed over the two projection paths (right, left) and compatible with IPD adjustment on both paths.

Moreover, when the beam in the intermediate space is collimated or near collimated, the maintenance of parallelism between the right and left paths is facilitated after the IPD has been adjusted.

First Embodiment

Below, the specific aspects of the binoculars10according to the first embodiment are described by reference toFIGS.1,3, and4.

As noted above, the binoculars10according to the first embodiment comprise two lens assemblies12. Such lens assemblies12are advantageously identical. The optical axis of each lens assembly12advantageously coincides with the projection axis APof the projection lens26of the lens assembly12.

In the first embodiment, the vision axis Av of each eyepiece14A,14B coincides with the optical axis of the eyepiece14A,14B. Thus, the input axis of each eyepiece14A,14B coincides with the vision axis AV of that eyepiece14A,14B.

In the first embodiment, the adjustment range R has a value within a limited range [−X; +X] centred on zero. The maximum limit +X of the range is thus equal to the opposite of the minimum limit −X of the range. As shown inFIG.3, the centre-to-centre distance E between the vision axis AV of each eyepiece14A,14B and the projection axis AP of the corresponding lens assembly12is equal to half +X/2 the positive limit +X of the limited range (maximum limit).

FIG.4shows three configurations obtained by rotating each eyepiece14A,14B about the corresponding projection axis APso as to obtain different adjustments of the IPD. In particular, this figure also shows the exit pupils P1of the lens assemblies12and the exit pupils P2of the eyepieces14A,14B.

The medium configuration (medium adjustment) corresponds to an average distance for which the IPD is equal to the nominal distance N, where the adjustment range R is nil. In this configuration, the relative eccentricity of the eyepieces14A,14B is orientated vertically, and does not contribute to the IPD adjustment. In the configuration shown, the eccentricity is orientated downward. However, upward eccentricity is also possible. When the binoculars10are affixed to a vertically (up-down) adjusted support, the vertical eccentricity of the binoculars10can be compensated for by translation.

The configuration on the left corresponds to a minimal IPD, for which the adjustment range R is equal to the lower limit −X of the limited range [−X; +X]. In this case, the IPD is equal to N−X. Relative to the medium adjustment (medium configuration), the eccentricity of each eyepiece14A,14B relative to the corresponding lens assembly12is orientated inward.

The configuration on the right corresponds to a maximum IPD, for which the adjustment range R is equal to the upper limit +X of the limited range [−X; +X]. In this case, the IPD is equal to N+X. Relative to the medium adjustment (medium configuration), the eccentricity of each eyepiece14A,14B relative to the corresponding lens assembly12is orientated outward.

Thus, the binoculars10according to the first embodiment have a dissymmetry obtained by eccentricity between the projection lenses and the corresponding eyepieces14A,14B, which allows for the IPD adjustment and the other advantages described in relation to the generic embodiment.

Second Embodiment

Below, the specific aspects of the binoculars10according to the second embodiment are described by reference toFIGS.1,5, and6.

As noted above, the binoculars10according to the second embodiment comprise two lens assemblies12(one for each eyepiece14A,14B). Such lens assemblies12are advantageously identical.

In the specific example shown inFIG.5, the input axis of each lens assembly12does not coincide with the projection axis APof the projection lens26of the lens assembly12.

In particular, in this example, the input axis of each lens assembly12is offset from the projection axis APof the corresponding projection lens26by a layover formed by two dioptres L1, L2. In this example, each dioptre L1, L2has a flat optical surface that is parallel to the flat optical surface of the other dioptre. Such a layover is also referred to as a rhombohedral layover. The first dioptre L1is comprised in the capture lens20and is suited to reflect the captured and intensified image of the scene in the direction of the second dioptre L2. The second dioptre L2is comprised in the projection lens26, and is on the path of the beam reflected by the first dioptre L1.

In this example, the flat optical surface of the second dioptre L2is partially reflective, so as to reflect the beam originating from the first dioptre L1on the one hand, and, on the other, transmit the beam originating from the screen24, such that the two beams are superimposed in the direction of the projection axis AP exiting the second dioptre L2. For example, the first dioptre L1is a reflective mirror.

Persons skilled in the art will understand that the second embodiment is not limited to such a configuration of the lens assemblies12, and is functional no matter the configuration of the lens assemblies12. Thus, in one variant, the input axis of each lens assembly12coincides with the projection axis APof the projection lens26of the lens assembly12, as is the case in the first embodiment.

In the second embodiment, as shown inFIG.5, each eyepiece14A,14B has an input axis that coincides with the projection axis AP of the corresponding lens assembly12. The vision axis AV (output axis) of each eyepiece14A,14B is offset from the input axis of the eyepiece14A,14B by a layover formed by two dioptres L2′. In this example, each dioptre L1′, L2′ has a flat optical surface that is parallel to the flat optical surface of the other dioptre. Such a layover is also referred to as a rhombohedral layover. The input axis of the rhombohedron (and thus, of the eyepiece) is centred on the projection axis APof the corresponding projection lens26.

In particular, the first dioptre L1′ is positioned so as to be in the path of the projection beam FPas it exits the corresponding projection lens26and to reflect the projection beam FPin the direction of the second dioptre L2′. The second dioptre L2′ is positioned so as to reflect the projection beam FP in the direction of the vision axis Av. In one example, the first dioptre L1′ and the second dioptre L2′ are reflective mirrors.

In this second embodiment, the adjustment range R is a function of: The centre-to-centre distance E between the projection axis APand the vision axis AV of the corresponding eyepiece14A,14B, a nominal orientation β of the eyepieces14A,14B, and a rotation angle αp, αn of each eyepiece14A,14B relative to the nominal orientation β. The nominal orientation β is defined as the angle between the plane comprising the two projection axes AP(right and left) and the symmetry axis of the rhombohedron.

More precisely, for example, the nominal distance N is given by the following formula:

Where:D is the centre-to-centre distance between the projection axes of the two lens assemblies12(shown inFIG.6).

The adjustment range R is, e.g., given by the following formula:

Where:αpis the rotation angle of each eyepiece14A,14B relative to the nominal orientation β when moved apart, andαnis the rotation angle of each eyepiece14A,14B relative to the nominal orientation β when brought closer together.

Thus, the IPD is adjusted by rotating each of the eyepieces14A,14B about the corresponding projection axis AP.

In particular, in the medium adjustment position, the relative rotation of the plane of symmetry of the rhombohedra of the eyepieces14A,14B relative to the lens assemblies12is orientated on the nominal orientation β (αpand αnare nil). At maximum distance, this rotation is orientated outward by the angle β−αp. At minimum distance, this rotation is orientated inward by the angle β+αn.

Thus, the binoculars10according to the second embodiment have a dissymmetry obtained by a rhombohedral layover of the eyepieces14A,14B, which allows for the IPD adjustment and the other advantages described in relation to the generic embodiment.

The range of rotation of the eyepieces14A,14B for the IPD adjustment is reduced compared to the first embodiment. Moreover, aberrations are reduced because the overall optical system has rotational symmetry.

Moreover, in the second embodiment, the length of the binocular bodies10is reduced compared to conventional binocular inline optics. As such, the cantilever of the binoculars10mounted on a helmet or head harness is reduced.

Third Embodiment

Below, the specific aspects of the binoculars10according to the third embodiment are described by reference toFIGS.2,7, and8.

As noted above, the binoculars10according to the third embodiment comprise a single lens assembly12shared by both eyepieces14A,14B. In particular, the projection axis APis the shared rotation axis of the two eyepieces14A,14B. For example, the lens assembly12is a lens assembly according to any of the examples described for the first or third embodiment.

In the third embodiment, as shown inFIG.7, each eyepiece14A,14B has an input axis that coincides with the projection axis APof the same lens assembly12. The vision axis AV(output axis) of each eyepiece14A,14B is offset from the input axis of the eyepiece14A,14B by a layover formed by two dioptres: L1-A, L2-A for the eyepiece14A and L1-B, L2-B for the eyepiece14B. Each of the dioptres L1-A, L2-A has a flat optical surface that is parallel to the flat optical surface of the other dioptre L1-A, L2-A. Each of the dioptres L1-B, L2-B has a flat optical surface that is parallel to the flat optical surface of the other dioptre L1-B, L2-B. As with the third embodiment, such layovers are rhombohedral. The input axis of each rhombohedron is centred on the projection axis APof the projection lens26.

In particular, the flat optical surface of the first dioptre L1-A of the first eyepiece14A is partially reflective. The first dioptre L1-A of the first eyepiece14A is positioned so as to be in the path of the projection beam FPexiting the projection lens26so as to reflect part of the projection beam FPin the direction of the second dioptre L2-A of the first eyepiece14A and to transmit the other part in the direction of the first dioptre L1-B of the second eyepiece14B.

The second dioptre L2-A of the first eyepiece14A is positioned so as to reflect the projection beam FPoriginating from the first dioptre L1-A in the direction of the vision axis AVof the first eyepiece14A. For example, the second dioptre L2-A is a reflective mirror.

The first dioptre L1-B of the second eyepiece14B is positioned so as to receive and reflect the part of the beam transmitted by the first dioptre L1-A of the first eyepiece14A in the direction of the second dioptre L2-B of the second eyepiece14B. For example, the first dioptre L1-A is a reflective mirror.

Le second dioptre L2-B of the second eyepiece14B is positioned so as to reflect the projection beam FP originating from the first dioptre L1-B of the second eyepiece14B in the direction of the vision axis AVof the second eyepiece14B. For example, the first dioptre L1-B is a reflective mirror.

Advantageously, the centre-to-centre distance E between the projection axis AP and the vision axis AV of each eyepiece14A,14B (which also corresponds to the distance between the reflective surfaces de each rhombohedron) meets the following condition:

Where:D1is the distance, along the vision axis AVof the first eyepiece14A, between the first dioptre L1-A and the focal point PA of the first eyepiece14A,D2is the distance, along the projection axis AP, between the first dioptre L1-A of the first eyepiece14A and the first dioptre L1-B of the second eyepiece14B, andD3is the distance, along the vision axis AVof the second eyepiece14B, between the second dioptre L1-B and the focal point P B of the second eyepiece14B.

In this third embodiment, the adjustment range R is a function of: The centre-to-centre distance E between the projection axis APand the vision axis AVof the corresponding eyepiece14A,14B, a nominal orientation β of the eyepieces14A,14B, and a rotation angle αp, an of each eyepiece14A,14B relative to the nominal orientation β.

More precisely, for example, the nominal distance N is given by the following formula:

The adjustment range R is, e.g., given by the following formula:

Where:αpis the rotation angle of each eyepiece14A,14B relative to the nominal orientation β when moved apart, andαnis the rotation angle of each eyepiece14A,14B relative to the nominal orientation β when moved closer together.

Thus, the IPD is adjusted by rotating each of the eyepieces14A,14B about the corresponding projection axis AP.

In particular, in the medium adjustment position, the relative rotation of the plane of symmetry of the rhombohedra of the eyepieces14A,14B relative to the lens assemblies12is orientated on the nominal orientation β (αp and an are nil). At maximum distance, this rotation is orientated outward by the angle β−αp. At minimum distance, this rotation is orientated inward by the angle β+αn.

Thus, the binoculars10according to the third embodiment allow for a binocular vision device having dissymmetry with each eyepiece14A,14B, produced by a rhombohedral layover of the eyepieces14A,14B. This allows for IPD adjustment and the other advantages described in relation to the generic embodiment.

The range of rotation of the eyepieces14A,14B for the IPD adjustment is reduced compared to the first embodiment. Moreover, aberrations are reduced because the overall optical system has rotational symmetry.

Moreover, in the third embodiment, the length of the binocular bodies10is reduced compared to conventional biocular inline optics. As such, the cantilever of the binoculars10mounted on a helmet or head harness is reduced.

Persons skilled in the art will understand that the embodiments described above can be combined where compatible. In particular, the rhombohedral lens assembly12described in relation to the second embodiment is compatible with the first and third embodiment. Likewise, the inline lens assembly12described in relation to the first embodiment is compatible with the second and third embodiment.

Moreover, persons skilled in the art will understand that, for an optical system, the term ‘output axis’ corresponds to the optical axis of the optic at the output of the optical system, and that the term ‘input axis’ corresponds to the optical axis of the optic at the input of the optical system. Thus, where the optical system is centred, the output axis and the input axis both correspond to the optical axis of the optical system. In particular, in the embodiments described, the projection axis AP(output axis of the lens assembly) is parallel to the input axis of the lens assembly (optical axis of the capture lens20).

Lastly, persons skilled in the art will understand that, in the second and third embodiment (FIGS.5and6, on the one hand, andFIGS.7and8, on the other), the nominal orientation refers to an orientation taken as a reference. In particular, in the second embodiment (FIGS.5and6), the nominal orientation is the angle β between the plane comprising the two projection axes and the symmetry axis of the rhombohedron. The symmetry axis of the rhombohedron corresponds to the plane of symmetry of the rhombohedron, with this plane being the one containing the output axis of the projection lens and the output axis of the corresponding eyepiece. In the third embodiment (FIGS.7and8), because there is only a single projection axis, it is also possible to define the nominal orientation as the angle between the plane comprising the two output axes of the eyepieces (IPD adjustment in nominal position) and the axis of symmetry of the rhombohedron.