WO2001007966A1

WO2001007966A1 - Viewing system with automatic setting

Info

Publication number: WO2001007966A1
Application number: PCT/FR2000/002064
Authority: WO
Inventors: Eric Simon; Gérard Illiaquer; François Micheron
Original assignee: Thales
Priority date: 1999-07-23
Filing date: 2000-07-18
Publication date: 2001-02-01
Also published as: FR2796729B1; FR2796729A1

Abstract

The invention concerns the field of viewing systems comprising an eyepiece. It concerns a method for setting a viewing system comprising an image plane (1) and an eyepiece (3), the method comprising a step which consists in automatically controlling the relative position of the eyepiece (3) with respect to the image plane (1) on variations, based on the variations of the optical path between the image plane (1) and the eyepiece (3), of a given light signal which has traversed the eyepiece (3) after having been retro-reflected by the user's eye (2). The invention is particularly applicable to binoculars, cameras, telescopes, viewfinders, microscopes.

Description

AUTOMATIC FOCUSING DISPLAY SYSTEM

The invention relates to the field of display systems comprising an eyepiece. The eyepiece is an optical element which projects an image on the eye of the user of the viewing system. The image to be projected on the eye of the user is located in an image plane in the broad sense, that is to say in an image surface, preferably plane but not necessarily. The image may for example be located in an identified intermediate focal plane and have been formed by a lens from the observed scene. The image can also be located in the plane of a display. The generated image can then for example come from photoelectric sensors observing a scene themselves. Visualization systems include pairs of binoculars, cameras, telescopes, viewfinders, microscopes.

A visualization system requires adjustment. Conventional focusing devices, automatic or not, relate to the development of an objective situated between an object and an image plane, the objective focusing the image of the object on the image plane. The problem dealt with by the invention is different. This involves focusing on an eyepiece, that is to say an optical element imaging the image plane on the retina of the user's eye. When the development is good, the image plane and the retina of ^the user's eye are optically conjugated.

According to the prior art, the eyepiece of a viewing system must be developed for each user observing a given scene using the viewing system. Each user then performs manual focusing, the focusing being deemed to be carried out when the image appears sharp to the user.

This manual focusing has a drawback. When several users simultaneously use the same viewing system, as it is not always possible to use the viewing system while keeping their corrective glasses, each user must make a personalized adjustment of the eyepiece corresponding to his own view. This is tedious and a waste of time. In addition, some Visualization systems, called binoculars, require several consecutive adjustments of the eyepiece, which is all the more tedious.

To eliminate the above drawbacks, the invention provides a focusing method and a display system comprising a display device implementing the focusing method. The focusing process is automatic. It is based on the analysis of variations in a given light signal, said to be in focus, retroreflected by the user's eye, which varies when the optical path between the image plane and the eyepiece varies. The focus position is controlled by these variations in the light signal reflected by the user's eye. In certain preferred configurations, the position of good focusing is then reached for an extremum of the intensity of this light signal.

According to the invention, there is provided a method for developing a display system which comprises an image plane and an eyepiece, characterized in that the method comprises a step of automatic servo-control of the relative position of the eyepiece with respect to the image plane on the variations, as a function of the variations in the optical path between the image plane and the eyepiece, of a given light signal having passed through the eyepiece before and after having been retroreflected by the eye of a user . According to the invention, there is also provided a display system having an optical axis and comprising an image plane, an eyepiece, and a focusing device, characterized in that the focusing device implements this method of focusing and in that the focusing device comprises a source emitting the light signal, a detector capable of detecting variations in the light signal, and means for varying the optical path between the image plane and the eyepiece.

The invention will be better understood and other features and advantages will become apparent from the following description and the attached drawings, given by way of examples, in which:

- Figure 1 schematically shows a first embodiment of a display system according to the invention;

- Figure 2 schematically shows a second embodiment of a display system according to the invention; - Figure 3 schematically shows a third embodiment of a display system according to the invention;

- Figure 4 schematically shows a fourth embodiment of a display system according to the invention; - Figure 5 schematically shows a fifth embodiment of a display system according to the invention;

- Figures 6 and 7 schematically represent a first form of pattern used in a display system according to the invention;

- Figure 8 schematically shows a second form of pattern used in a display system according to the invention;

- Figure 9 schematically shows an example of the light signal variations as a function of the relative position of the eyepiece with respect to the image plane.

In a first type of display system according to the invention, the system comprises a pattern for masking the light signal, the pattern being arranged so that, on the one hand, the path of the light signal coming from the source passes successively through the pattern, the eyepiece, the user's eye, the eyepiece, and the pattern before arriving at the detector, and on the other hand the optical path between the image plane and the eyepiece and the optical path between the pattern and the eyepiece remain approximately equal to each other during focusing. A preferred example of this first type of display system is described in a first embodiment.

FIG. 1 schematically represents the first embodiment of a display system according to the invention. The dashed line represents the bent optical axis of the display system. The display system comprises an image plane 1. The image plane 1 is preferably an identified intermediate focal plane, that is to say either materialized or fixed. The image plane can also be a display generating an image to be displayed by the user, either directly or for example from the electrical signal coming from sensors located upstream in the display system. An automatic focusing device then proves to be particularly advantageous, since the image plane 1 being identified, in the event of a significant change in the characteristics of the eye of the user, for example during a change of user, the system of visualization becomes automatically defocused, requiring a new focusing operation.

The viewing system also includes an eyepiece 3 which produces the image of the image plane 1 on the retina of the user's eye 2 when the focus is good. The retina of eye 2 and the image plane 1 are then optically conjugated. Light or light rays from the image plane 1, in which an observed scene is for example represented, pass through the eyepiece 3 before being focused on the retina of the eye 2.

The display system also includes a separating plate 7 located between the image plane 1 and the eyepiece 3. The separating plate 7 is traversed by part of the light arriving on its surface while it reflects the other part. The separating plate 7 is preferably a 50/50 plate, that is to say that half of the quantity of light arriving on its surface at an incidence of 45 degrees is transmitted while the other half is reflected.

The display system also includes a light source 6, a separating blade 8 and a pattern 20. The source 6 generates a light signal which successively passes through the separating blade 8 and the pattern 20 before arriving on the separating blade 7 to be reflected. towards the eye 2 passing through the eyepiece 3. Part of the light coming from the separating plate 7 and arriving on the separating plate 8 is reflected by the latter towards a detector 10 which also includes the display system. If the distance between the pattern 20 and the separating plate 8 is large, the display system may include a light collimator between the pattern 20 and the separating plate 8.

The display system also includes means 4 and 1 1 for varying the optical path between the image plane 1 and the eyepiece 3. When the eyepiece 3 consists of several optical elements, for example several lenses, the optical path considered is then the optical path between the image plane 1 and the last optical element of the eyepiece 3. These means consist for example of electronic control means 11 and of a device 4 for translating the eyepiece 3 according to the optical axis of the display system, but they may consist of any other equivalent means making it possible to vary the optical path between the image plane 1 and the eyepiece 3. In the case where the eyepiece 3 consists of several optical elements, a relative displacement between some of these optical elements may lead to the same result. In certain embodiments such as those of FIGS. 4 or 5 described below, the translation, along the optical axis of the display system, of the image plane then materialized by a display, is also suitable. The translation device 4 is controlled by electronic control means 1 which are themselves controlled by the detector 10, more precisely by the output signal from the detector 10.

The image of a scene observed by the user is in the image plane 1. This image plane 1 is preferably fixed. Part of the light from the image plane 1 first crosses the separating plate 7 then the eyepiece 3 before arriving in the eye 2. The scene, observed at a given instant, fixes at this instant the diameter of the pupil of eye 2 and therefore the amount of light that can enter the eye 2. If we consider what happens in a very short time, the image located in the plane image 1 and the eye 2 of the user can be considered immobile. We can then consider that the eye, over a fairly short period of time, retroreflects identically over time a constant light signal. Therefore, the amount of retroreflected light signal varies if the light signal entering the eye varies. It is therefore preferable for this that the operation of varying the optical path between the image plane 1 and the eyepiece 3, in order to determine the relative position of the eyepiece 3 relative to the image plane 1 corresponding to good focusing. point for the viewing system, is carried out in a sufficiently short focusing time so that the image to be displayed as well as the user's eye remain stationary during this focusing time.

The light source 6 emits a light signal which successively passes through the separating plate 8 and the masking pattern 20. The masking pattern 20 is a mask of a particular shape which transforms the light signal into a determined light pattern or into a figure of several determined light patterns. A light pattern is determined if we know some of its characteristics. The pattern 20 is for example a screen comprising a circular hole. The light signal, after passing through the pattern 20, is in the form of a circular light pattern having substantially the same diameter as the circular hole in the frame of the approximation of geometric optics. However, other forms of pattern are possible. Part of the light signal having passed through the pattern 20 is reflected by the separating plate 7 in order to pass through the eyepiece 3 and enter the eye 2. The optical path between the image plane 1 and the retina of the eye 2 and the path between the pattern 20 and the retina of the eye 2, are or remain, preferably substantially equal to each other during focusing. As the optical path between the eyepiece 3 and the retina of the eye 2 is the same, as well for the light coming from the image plane 1 as for the light signal coming from the source 6 whether it is before or after retroreflection by the retina of eye 2, the preceding preferential condition is equivalent to this: the optical path between the image plane 1 and the eyepiece 3 and the optical path between the pattern 20 and the eyepiece 3 must remain substantially equal to each other for development. When the eyepiece 3 is made up of several optical elements, for example several lenses, the optical path considered is then the optical path between the image plane 1 and the last optical element of the eyepiece 3.

The margin of error in this equality corresponds to the defocusing authorized so that the user's eye 2 keeps a clear vision of the scene observed. If the optical paths are not strictly equal, and in absolute terms this is always the case, the difference between the optical paths must be sufficiently small for the relative position of the eyepiece 3 relative to the image plane 1, to which the enslavement step will have resulted, gives the retina of the eye 2 an image sufficiently well focused to appear clearly to the user. These optical paths can be chosen to have a substantially different value, but a correction calculation step at the level of the electronic control means 11 is to be provided, which makes the overall system substantially more complex. After entering eye 2, the signal is more or less well focused on the retina of eye 2, then is retroreflected in the direction of eyepiece 3. Part of this light signal is again reflected on the slide separator 7 and crosses the pattern 20 in the direction of the separating blade 8. If the position of the eyepiece 3 corresponds to a position of good focus, the light signal has been focused precisely on the retina of the eye 2 and the retroreflected signal follows, at the aberrations of the eye and other optical elements of the near visualization system, the principle of reverse light return. At the level of the pattern 20, the light signal substantially covers the shape of the pattern 20, which essentially allows it to cross the pattern 20 before being partially reflected by the separating plate 8 and arriving at the surface of the detector 10 to be detected there, as explained in more detail in FIG. 6. While if the focus is poor, that is to say if the display system is defocused, the light signal should be focused either upstream or downstream of the retina of the eye 2. Consequently, at the level of the retina of the eye 2, each point is imaged in a spot which, when it is retroreflected by the eye 2, brings l 'the whole of the light signal to overflow from the trace of the incident light signal coming from the source 6. Always considering as carried out during the development the previous condition of equality of the optical paths between on the one hand the image plane 1 and l eyepiece 3 and secondly the moti f 20 and the eyepiece 3, the light signal covers at the level of the pattern 20 a larger surface than the shape of the pattern 20, which stops a substantial part of the light signal, as explained in more detail in the level of FIG. 7. The part of the light signal having crossed the pattern 20 is less than in the case of good focusing. The part of the light signal detected by the detector 10 after reflection on the separating plate 8 is also lower than in the case of good focusing.

The case of good focusing corresponds for the display system shown in FIG. 1, to a maximum of signal detected by the detector 10. A good focusing corresponds either to the best focusing or to a focusing point close enough to the best focus so that the image to be viewed appears sharp to the user's eye 2. The automatic servoing step then consists in varying the optical path between the image plane 1 and the eyepiece 3, for example by translating the eyepiece 3 along the optical axis of the display system, and in servoing the relative position of the eyepiece 3 with respect to the image plane 1 on the maximum light signal detected at the detector 10. This maximum corresponds to the position of good development which will be all the closer to the position of best development that this maximum will be determined with greater precision. The arrows represented in FIG. 1 symbolize the direction of control, from the detector 10 towards the electronic control means 1 1 on the one hand, and electronic control means 1 1 towards the device 4 for translating the eyepiece 3 on the other hand.

In a second type of display system according to the invention, the system comprises at least two identical patterns for masking the light signal, the patterns being arranged so that, on the one hand, the path of the light signal coming from the source passes successively through a first pattern, the eyepiece, the user's eye, the eyepiece, and a second pattern identical to the first, before arriving at the detector, and on the other hand the optical path between the plane image and the eyepiece and the optical paths between each of the two patterns and the eyepiece remain substantially equal to each other during focusing. A preferred example of this second type of display system is described in a second embodiment.

FIG. 2 schematically represents the second embodiment of a display system according to the invention. The difference between FIGS. 1 and 2 is that the masking pattern 20 located between the separating blades 7 and 8 has been replaced by two masking patterns 5 and 9, the first pattern 5 being located between the separating blade 8 and the source 6 , the second pattern 9 being located between the separating blade 8 and the detector 10. When the two patterns 5 and 9 are identical, the relative position of the eyepiece 3 relative to the image plane 1 is controlled over a maximum intensity of the light signal detected by the detector 10, as for the first type of display system. The optical path between the image plane 1 and the eyepiece 3, the optical path between the first pattern 5 and the eyepiece 3, the optical path between the second pattern 9 and the eyepiece 3 all remain substantially equal to each other during the setting. on point.

In a third type of display system according to the invention, the system comprises at least two complementary patterns for masking the light signal, the patterns being arranged so that, on the one hand, the path of the light signal coming from the source past successively by a first pattern, the eyepiece, the user's eye, the eyepiece, and a second pattern complementary to the first, before arriving at the detector, and on the other hand the optical path between the image plane and the eyepiece and the optical paths between each of the two patterns and the eyepiece remain substantially equal to each other during focusing. A preferred example of this third type of display system is also described in the second embodiment represented in FIG. 2. The difference between the second and third types of display system consists in the fact that the patterns 5 and 9 are respectively complementary instead of being identical. For example, if the first pattern 5 is a non-transparent screen having a transparent circular hole, the second pattern 9 will be a circular non-transparent screen of the same diameter as the circular hole of the first pattern 5. The relative position of the eyepiece 3 by compared to the image plane 1 is then controlled on a minimum intensity of the light signal detected by the detector 10. But this type of visualization system with minimum intensity control is less reliable than the first and second types of display system with servo on maximum intensity, because the minimum of intensity is relative and not absolute, this is why a very large defocusing at the beginning of the development can risk to diverge the servo. Furthermore, it is possible to envisage using patterns 5 and 9, the shapes of which are neither identical nor complementary, but an additional calculation calculation step must be provided at the level of the electronic control means 11, which will make the noticeably more complex overall system.

The detector 10 is preferably a simple detector, for example a photoelectric photodiode type, making it possible to detect the overall intensity of the light signal arriving on its surface after passing through the second masking pattern 9. In any case where the display system comprises two masking patterns, that is to say in any display system of the second or third type, the detection can also be carried out for example a matrix detector integrating the second masking pattern 9 and the detector 10. Electronic processing downstream of this matrix detector would then make it possible to electronically reconstruct the masking function of the second pattern 9. The focusing device that comprises the display system according to the invention can operate according to different types of operating modes. One type of operating mode consists, for example, of a specific "debugging mode" which is activated on the initiative of the user. This mode can be selected by pressing a corresponding button. Another type of operating mode consists, for example, of an almost permanent focusing mode. This mode is permanently activated on condition of the presence of an eye 2 behind the eyepiece 3. This presence can be detected by considering a threshold of light intensity to be received by the detector 10. In fact, the presence of an eye 2 in principle ensures retroreflexion of a minimum of light intensity. A display system has either one of the preceding operating modes, or yet another type of operating mode.

In the second or third types of display system according to the invention, the system can advantageously include a display 16 in transmission, preferably matrix, integrating the source 6 and the first masking pattern 5. Preferably then, the matrix display 16 is located in the image plane 1 and alternately displays the light signal and an image to be viewed by the user. In fact, the image plane 1, the source 6 and the first masking pattern 5 are then combined into a single element which is the matrix display 16 in transmission. The successive and periodic display of the image to be displayed and of the light signal used for focusing is preferably carried out, as regards the light signal, with a sufficiently short period before the retinal persistence of the eye 2 so that the user is not inconvenienced. A preferred example of this second or third type of display system is described in a third embodiment.

FIG. 3 schematically represents the third embodiment of a display system according to the invention. The relative arrangement of the second masking pattern 9, of the detector 10, of the eyepiece 3, of the means 4 and 1 1 for varying the optical path between the image plane 1 and the eyepiece 3, of the eye 2 and of the separating blade 7 remains similar to that of FIG. 2. The pian image 1 is replaced by a matrix display 16 in emission also playing the role of the assembly constituted by the source 6 and the first masking pattern 5 in FIG. 2 The light signal retroreflected by the eye 2 and reflected by the separating plate 7 arrives directly on the second masking pattern 9. Consider the example of a color display 16 in which the image to be displayed is produced by a temporal sequencing of three images respectively red, green, and blue. The light signal can for example be inserted between the blue image and the red image at each refresh period of the image to be displayed. The light signal will preferably be in the near infrared range so as to be invisible so as not to disturb the user's eye 2 at all. The matrix display 16 in transmission makes it possible to directly generate a light signal for focusing having a particular section, for example approximately circular.

In the first type of display system according to the invention, the masking pattern can be a transmission display located in the image plane. The transmission display, illuminated by a source, preferably alternately generates the image to be displayed and the focusing light signal, as in the case of FIG. 3. A preferred example of this first type of display system is described in a fourth embodiment.

FIG. 4 schematically represents a fourth embodiment of a display system according to the invention. The display system then comprises a light source 14 and a display 15 illuminated in transmission by the light source 14. The image to be displayed, like the focus light signal, is determined by the different transmittance values at different points. of the surface of the display 15 in transmission. The display 15 in transmission is a spatial light modulator. The image located in the image plane at the level of the display 15 in transmission is focused on the retina of the user's eye 2 in the event of good focusing. The display system includes an eyepiece 3 performing this focusing. Between the source 14 and the display 15 is located a separating plate 12. The light signal retroreflected by the eye 2 having passed through the eyepiece 3 is reflected on the separating plate 12 to reach the surface of the detector 10. In addition to the separating plate 12 and the detector 10, the display system also includes means 4 and 1 1 for varying the optical path between the image plane and the eyepiece 3, the image plane here being materialized by the display 15 in transmission. These means 4 and 1 1 are controlled by the detector 10. Between the separating blade 12 and the display 15 in transmission is advantageously located a light collimator 13. This collimator 13 makes it possible not to impose a too powerful light source 14 or a detector 10 of too large surface, and this all the more so as the distance between the separating blade 12 and the display 15 in transmission is important. Instead of the display 15 located between the collimator 13 and the eyepiece 3, it is also possible to place two displays 15 in transmission respectively on the one hand between the source 14 and the separating blade 12 and on the other hand between the separating plate 12 and the detector 10. The image to be displayed and the focusing light signal are preferably distinct as in FIG. 3, the display 15 displaying for example alternately the image to be displayed and the light signal of focus. The image to be displayed and the focusing light signal can however also be confused in the case where the image to be displayed does not change substantially during the time necessary to effect a focusing. This is only possible because the display 15 plays the role both of the image plane, of the pattern crossed by the light signal before retroreflection by the eye 2, and of the pattern crossed by the light signal after retroreflection by the eye 2.

In the first type of display system according to the invention, the masking pattern can be a display in reflection located in the image plane. As in the case of FIG. 4, the reflection display, illuminated by a source, preferably alternately generates the image to be displayed and the focusing light signal. A preferred example of this first type of display system is described in a fifth embodiment.

FIG. 5 schematically represents the fifth embodiment of a display system according to the invention. The display system comprises a source 14 and a display 17 in reflection illuminated by the source 14. The display system also includes a light collimator 13, a separating plate 18, another separating plate 19, sequentially placed between the source 14 and the display 17 in reflection. Part of the light reflected by the display 17 in reflection is focused on the eye 2 of the user by an eyepiece 3 that includes the viewing. Part of the focusing light signal is retroreflected by the eye 2, passes through the eyepiece 3, is reflected by the separating plate 19, is reflected by the display 17 in reflection, passes through the separating plate 19, is reflected by the separating blade 18 to arrive on the surface of the detector 10 which also includes the display system. The display system also includes means 4 and 1 1 for varying the optical path between the image plane, that is to say here the display 17 in reflection, and the eyepiece 3. These means 4 and 1 1 are controlled by the detector 10. The means for varying the optical path between the image plane and the eyepiece preferably comprise a device 4 for translating the eyepiece along the optical axis of the display system, the device 4 being controlled by electronic control means 1 1. Other types of means can be used, in particular in the display systems represented in FIGS. 4 and 5. The device 4 could for example be a device for translating the display, whether this is a display in transmission or in reflection.

In all the embodiments shown in FIGS. 1 to 5, the focusing device uses a given light signal for focusing. This light signal from a source is advantageously transformed by one or more masking patterns according to the embodiment. Preferably, the masking pattern, when it is unique or the first masking pattern otherwise, transforms the light signal into one or more light patterns whose size is small enough so that substantial defocusing of the display system results in level of the pattern after retroreflection by the eye of a user, a difference in overlap between the shape of the masking pattern and the light patterns which is sensitive to the detector. Otherwise, the focusing device generally loses its sensitivity unless it uses a very sensitive detector which will then be expensive. Furthermore, the size of the light pattern (s) preferably remains greater than the resolution of the eye of a user, in order to avoid an overly sensitive control dominated by aberrations of the eye of the user and thus risking lead to a position of focus which is not optimal. In the extreme, a much too small pattern size would decrease the received signal by the detector 10 after retroreflection by the eye 2, thus degrading the signal to noise ratio and therefore the precision of the servo-control. In the case of FIGS. 3 to 5 comprising displays, the size of the pixels of the displays generally corresponds already to a size of patterns which is adequate, that is to say that with a few pixels, an array of patterns of adequate size, or if necessary, with one or more pixels, a light pattern of adequate size can be obtained.

Figures 6 and 7 schematically represent a first preferred form of masking pattern used in a display system according to the invention. The masking pattern is a screen 50 comprising a circular area 51, preferably centered on the optical axis, which determines a light pattern of circular section. This masking pattern has the advantage of simplicity. The screen 50 is advantageously opaque outside of the zone 51, it could also be reflective provided that it is arranged so that the light which it reflects does not disturb the display system. When the light signal passes through the pattern as in FIGS. 1, 2 and 4, the zone will for example be a hole or in transparent material, and when the signal is reflected by the pattern as in FIG. 5, the zone will for example be a mirror. FIGS. 6 and 7 show in hatched lines the trace of the light beam, that is to say the light pattern, after retroreflection by the eye of the user, on the screen 50, respectively in the case of good focusing. point corresponding to trace 52 and in the case of incorrect focusing corresponding to trace 53. For the first and second types of display system, the relative position of the eyepiece with respect to the image plane is controlled over a maximum of intensity of the light signal detected by the detector. In the case of good development of FIG. 6, the trace 52 of the light beam covers the hole 51 by slightly overflowing, as a result of which a maximum of energy crosses the screen 50 to then be detected by the detector. In the case of incorrect focusing of FIG. 6, the trace 53 of the light beam covers the hole 51, extending widely, as a result of which a proportion of energy significantly lower than the maximum, and all the more so as the system display is defocused, crosses the screen 50 to then be detected by the detector. FIG. 8 schematically represents a second preferred form of masking pattern used in a display system according to the invention. The masking pattern is a screen 50 comprising a network of circular zones 54, each circular zone 54 being similar to the circular zone 51 shown in FIGS. 6 and 7. This masking pattern is more complex than that shown in FIGS. 6 and 7, but its functioning remains similar. If, as in FIG. 8, the network of circular zones 54 comprises nxn, here 6 × 6, circular zones 54, the intensity of the focusing light signal received by the detector will be n ² , here 36, times greater than in the case of the pattern represented in FIGS. 6 and 7. For the same luminance of the source illuminating the pattern or of the display if applicable, as well as for the same detector sensitivity, the efficiency of the focusing device is substantially improved.

The detector measures the variations in the intensity of the focusing light signal after retroreflection by the user's eye, when the means for varying the optical path between the image plane and the eyepiece cause this optical path to vary. Variations in the intensity of the light signal have the property of being rapid when the optical path considered deviates from that corresponding to good focusing. This is called the "cat eye" effect. The rapidity of these variations allows effective and precise control. Indeed, as soon as the optical path between the image plane and the eyepiece is slightly different from the optical path of good focusing, that is to say as soon as the relative position of the eyepiece with respect to the image plane becomes slightly different from the position corresponding to an optimal focusing, the variations become sensitive for a moderately sensitive detector. In the case of a form of masking pattern in a network of zones, this has the advantage of better distributing the energy received by the eye over the retina of the user's eye than a form of single area pattern. Consequently, for a given and respected eye safety threshold, the energy received by the detector will be greater in the case of the pattern of networked zones.

FIG. 9 represents, by means of a curve C, an example of variations of the focusing light signal as a function of the relative position of the eyepiece with respect to the image plane. The scales represented in figure 9 are expressed in arbitrary units ua, figure lό

9 schematically representing the general shape of the curve C. The case studied corresponds to the display system of FIG. 2 of the second type, that is to say the one with two identical masking patterns. On the abscissa axis, the difference E is indicated between on the one hand the relative position considered of the eyepiece with respect to the image plane and on the other hand said position in the case of an optimal focusing, c that is to say the position of best focusing. On the ordinate axis, the intensity I of the light signal received by the detector is indicated after retroreflection by the user's eye. The relative position of the eyepiece with respect to the image plane is controlled by the maximum of the intensity of this light signal. Around the position of best focus, the curve C has a parabolic shape, the difference between the intensity I and the intensity of the maximum l _max (equal to 1 in ua) being proportional to the square of the difference E at the position of better focus. The sensitivity of the focusing device varies as an increasing function of the concavity of curve C. The concavity of this curve C depends on several parameters. Among these are the digital openness of the viewing system, the shape and size of the masking pattern, the quality of the user's eye. The concavity of this curve C varies as an increasing function of the digital aperture of the display system. However, it should be noted that the development of an open viewing system is less critical than that of a more open viewing system.

Preferably, the means for varying the optical path between the image plane and the eyepiece comprise a device for periodic modulation of the optical path between the image plane and the eyepiece, around a nominal distance d ₀ , at a constant frequency f ₀ , the detector demodulating at this frequency f ₀ the light signal so as to carry out a synchronous detection. The periodic modulation can be carried out by vibration of an optical element of the display system, the vibration taking place in translation along the optical axis of the display system. This optical element will advantageously be the eyepiece. In certain configurations, such as those of FIGS. 4 or 5, the vibrating optical element can also be the pattern, that is to say the display. In other configurations such as that of FIG. 5, it is even possible to carry out this periodic modulation by vibration in translation along the optical axis, of the blade separator 19. In the configurations where the pattern is not confused with the image plane, that is to say in FIGS. 1, 2 and 3 which do not have a display, the periodic modulation can also be achieved by vibration of the pattern or of a separating plate, but compliance with the condition "the optical path between the image plane and the eyepiece and the optical path between the pattern and the eyepiece remain substantially equal to each other during focusing" makes the system significantly more complex.

More particularly, the vibration at a substantially constant frequency f ₀ , causes the detector to detect a signal having an amplitude s and a frequency f ₀ . The detected signal is then demodulated at the frequency f ₀ , thus allowing the extraction of the variable s. The variable s corresponds to the slope of the curve C in FIG. 9. The variable s represents the derivative of the intensity of the light signal received by the detector after retroreflection by the user's eye, as a function of the relative position of the eyepiece with respect to the image plane. When this intensity passes through an extremum, its derivative is canceled. Synchronous detection makes it possible to replace the detection of a maximum or a minimum of signal, which can be sometimes delicate, by the detection of zero of an error signal, which is more convenient. The frequency f ₀ used in the synchronous detection should preferably be chosen to be large enough to prevent external disturbances, such as parasitic movements of the user's eye or the change of the image to be viewed, from having a significant effect. on the synchronous detection operation. In any case, this type of detection remains less sensitive to parasitic movements of the user's eye. Indeed, these disturbances act on the level of the maximum focusing light signal detected by the detector (represented on the ordinate axis in FIG. 9) and not on the position of this maximum (represented on the axis of the abscissa in FIG. 9), position which corresponds to the position of correct focusing of the display system. The preferential spectral range of application, as regards the focusing light signal, extends from the near ultraviolet to the near infrared, including the visible. Other portions of the light spectrum remain possible, provided however that the focusing power of the lens of the eye on the one hand and the retroreflexion power of the retina of the eye on the other hand are sufficiently high to remain compatible with the I o

power of the source and the sensitivity of the detector which are envisaged in the display system considered. A light signal focusing in the visible range has the advantage of simplicity. A light signal outside the visible range has the advantage of not disturbing the user's vision. As such, the near infrared domain is preferable to that of the near ultraviolet because the retroreflexion of the retina of the eye is higher there.

In an optional and advantageous embodiment of the invention, after focusing, the focusing device implements a step of automatic servo complementary to the relative position of the eyepiece with respect to the pian image so as to make coincide the image plane and the plane of the punctum remotum of a user's eye. Indeed, an inexperienced user looking at the image to be viewed, tends to over-accommodate to see the clearest possible image, which causes him to feel very tired during prolonged observation. When the step of automatic control of the relative position of the eyepiece with respect to the image plane is completed, very often the image plane coincides with the plane of the punctum proximum of the eye of the user. In a complementary automatic servoing step, the image plane is brought to coincide with the plane of the punctum remotum of the eye of the user, which gives the latter good visual comfort. To do this, from a “punctum proximum” position, the optical path between the image plane and the eyepiece is varied. The accommodation of the eye begins by following this variation at first, then drops out for a given threshold of defocusing of the visualization system. This drop-out results in a sudden drop in the intensity of the detected light signal at the detector. Coming back just before this stall, you are in the "punctum remotum" position.

Claims

1. A method for developing a display system which comprises an image plane (1) and an eyepiece (3), characterized in that the method comprises a step of automatic control of the relative position of the eyepiece ( 3) relative to the image plane (1) on the variations, as a function of the variations of the optical path between the image plane (1) and the eyepiece (3), of a given light signal having passed through the eyepiece (3) before and after being retroreflected by the eye (2) of a user.

2. Display system having an optical axis and comprising an image plane (1), an eyepiece (3), and a focusing device, characterized in that the focusing device implements the focusing method point according to claim 1 and in that the focusing device comprises a source (6, 16, 14) emitting the light signal, a detector (10) capable of detecting variations in the light signal, and means (4 and 11) to vary the optical path between the image plane (1) and the eyepiece (3).

3. Display system according to claim 2, characterized in that the system comprises a pattern (20, 15, 17) for masking the light signal, the pattern (20, 15, 17) being arranged so that, d 'firstly the path of the light signal from the source (6, 14) passes successively through the pattern (20, 15, 17), the eyepiece (3), the eye (2) of the user, the eyepiece (3), and the pattern (20, 15, 17) before arriving at the detector (10), and on the other hand the optical path between the image plane (1) and the eyepiece (3) and the optical path between the pattern (20, 15, 17) and the eyepiece (3) remain substantially equal to each other during focusing.

4. Display system according to claim 3, characterized in that the pattern (20, 15, 17) is a transmission display (15) located in the image plane (1).

5. Display system according to claim 3, characterized in that the pattern (20, 15, 17) is a reflection display (17) located in the image plane (1).

6. Display system according to claim 2, characterized in that the system comprises at least two identical patterns (5 and 9, 16 and 9) for masking the light signal, the patterns (5 and 9, 16 and 9) being arranged so that, on the one hand the path of the light signal from the source (6, 16) passes successively through a first pattern (5, 16), the eyepiece (3), the eye (2) of the user, the eyepiece (3), and a second pattern (9) identical to the first, before arriving at the detector (10), and on the other hand the optical path between the image plane (1) and the eyepiece (3) and the optical paths between each of the two patterns (5 and 9, 16 and 9) and the eyepiece (3) remain substantially equal to each other during focusing.

7. Display system according to claim 2, characterized in that the system comprises at least two complementary patterns (5 and 9, 16 and 9) for masking the light signal, the patterns (5 and 9, 16 and 9) being arranged so that, on the one hand the path of the light signal from the source (6, 16) passes successively through a first pattern (5, 16), the eyepiece (3), the eye (2) of the user, the eyepiece (3), and a second pattern (9) complementary to the first, before arriving at the detector (10), and on the other hand the optical path between the image plane (1) and the eyepiece (3) and the optical paths between each of the two patterns (5 and 9, 16 and 9) and the eyepiece (3) remain substantially equal to each other during focusing.

8. Display system according to any one of claims 6 to 7, characterized in that the system comprises a matrix display (16) in transmission integrating the source and the first pattern.

9. Display system according to claim 8, characterized in that the matrix display (16) is located in the image plane (1) and alternately displays the light signal and an image to be displayed by the user.

10. Display system according to any one of claims 6 to 9, characterized in that the system comprises a matrix detector integrating the second pattern (9) and the detector (10).

11. Display system according to any one of claims 3 to 10, characterized in that the (20, 15, 17) or the first (5, 16) masking pattern transforms the light signal into one or more light patterns including the size is small enough that a substantial defocusing of the display system causes, at the level of the pattern (20, 15, 17, 5, 16) after retroreflection by the eye (2) of the user, a difference in coverage between the shape of the masking pattern (20, 15, 17, 5, 16) and the light patterns which is sensitive for the detector (10).

12. Display system according to claim 1 1, characterized in that the size of the light pattern or patterns remains greater than the resolution of the eye (2) of the user.

13. Display system according to any one of claims 1 1 to 12, characterized in that the pattern (20, 15, 17, 5, 16) for masking is a screen (50) comprising a circular area (51) determining the size of the light pattern.

14. Display system according to any one of claims 1 1 to 12, characterized in that the pattern (20, 15, 17, 5, 16) for masking is a screen (50) comprising a network of zones (54) circulars determining the sizes of the light patterns.

15. Display system according to any one of claims 2 to 14, characterized in that the means (4 and 11) for varying the optical path between the image plane (1) and the eyepiece (3) comprise means (1 1) electronic control and a device (4) for translating the eyepiece (3) along the optical axis of the display system, the device (4) for translation being controlled by the electronic means (11) d servo themselves controlled by the detector (10).

16. Display system according to any one of claims 2 to 15, characterized in that the means (4 and 1 1) for varying the optical path between the image plane (1) and the eyepiece (3) comprise a device for periodically modulating the optical path between the image plane (1) and the eyepiece (3), around a nominal distance d ₀ , at a constant frequency f ₀ , and in that the detector (10) demodulates at this frequency f ₀ the light signal so as to perform synchronous detection.

17. Display system according to claim 16, characterized in that the periodic modulation is carried out by vibration in translation of the eyepiece (3).

18. Display system according to claims 3 to 5 and claim 16, characterized in that the periodic modulation is carried out by vibration in translation of the pattern (15, 17).

19. Display system according to claim 16, characterized in that the display system comprises a separating blade (19) killed between the image plane (1) and the eyepiece (3) and in that the periodic modulation is produced by translational vibration of the separating blade (19).

20. Display system according to any one of claims 2 to 19, characterized in that the spectral range of the light signal is the visible or the near infrared.

21. Display system according to any one of claims 2 to 20, characterized in that after the focusing, the focusing device implements an automatic control step complementary to the relative position of the ocular (3) with respect to the image plane (1) by making the image plane (1) coincide with the plane of the punctum remotum of the eye (2) of the user.