FR2549244A1 - Beam-splitter with optically symmetrical structure - Google Patents

Abstract

A semitransparent film of light reflective material is sandwiched between two optical plates of transparent materials. The plates have respective thicknesses and indices of refraction suitable for providing equivalent optical paths on either side of the film. The beamsplitter is located symmetrically between two image sources such that the light rays emanating from one source and reflected by the beamsplitter together with corresponding light rays emanating from the other source and transmitted by the beamsplitter combine and travel common paths from the film. An observer's eye or any similar focussing system can be positioned over a relatively large range of viewing angles to receive the combined light rays and produce a composite image.

Description

 The present invention relates generally to optical devices, and more particularly relates to an optical device comprising a plate-shaped beam splitter which has an optically symmetrical structure.

 An optical beam splitter may include an optical plate made of a transparent material and serving as a substrate for supporting, on one of its flat surfaces, a transparent film of a light-reflecting material. When used to separate a light beam or a light image, the beam splitter is placed in an intercept position in the path of the beam or image and is oriented at a certain angle relative to this path. As a result, a partial component of the beam or image is transmitted through the film and continues to propagate in the initial direction of propagation. Furthermore, a partial component of the beam or image is reflected by the angularly oriented film of the beam splitter and propagates in a direction at an angle to the initial direction of propagation.

 When used to combine two light beams or two light images propagating along intersecting paths, the beam splitter is arranged inside the two paths and makes an angle with each of these paths. As a result, one of the beams or image is transmitted through the film of the beam splitter and continues to propagate in the initial direction of propagation. The other beam or image is reflected by the angularly oriented film of the beam splitter and propagates in the same direction as the beam or image transmitted through the film. Therefore, if the film of the beam splitter constitutes a plane of symmetry between two sources arranged angularly, the transmitted image and the reflected image propagating through the beam splitter towards the eye of an observer seem to be superimposed on one another to form a composite image.

The prior art has therefore described a number of optical systems, such as those used for example, in photographic rangefinders and measuring microscopes in which beam splitters are arranged to superimpose an image of a reticle or the like on an image of 'an object observed. In addition, the prior art presents a number of color image projectors using beam splitters to superimpose different color images of the same object to obtain a composite color image. For example, a state patent application
United States of America no 350,469 filed on February 19, 1982 by the applicant describes a color image display apparatus comprising an optical system coupled with a flat output screen of a display tube to combine two symmetrical images of the same object produced in different colors, for example red and green, on respective halves of the output screen.

 The optical device described in this patent application comprises a beam splitter in the form of a common type plate aligned with the half producing green light of the output screen, and oriented at an angle relative to the plane of the screen. . This beam splitter has a transparent optical plate supporting a film of a material which partially transmits and which partially reflects green light. Therefore, the green light image from the aligned half of the output screen has a partial component transmitted through the film in the direction of its initial propagation and a partial component reflected by the film in a direction perpendicular to the initial direction of propagation. The reflected component is again reflected by a first mirror arranged perpendicularly to the beam splitter where it is transmitted by the film of the splitter to the eye of an observer.

 In order to recover the component of the image of green light transmitted by the beam splitter, it is possible to have a second mirror, perpendicular to the first, beyond the separator and parallel to the output screen. The second mirror reflects the component of 11 image of green light transmitted by the beam splitter to the film of this splitter, where it is reflected in the same direction as the reflected component of the image of green light leaving the splitter and propagating towards the eye of the observer. As a result, the components of the image of green light, transmitted and reflected by the beam splitter appear superimposed on the eye of the observer and form a composite image of green light provided that the direction of observation is approximately parallel to the optical axis.

 However, it has become apparent that if the direction of observation changes so that it is no longer really parallel to the optical axis, there may be a notable separation of the components of the image of green light transmitted and reflected by the beam splitter. , causing the appearance of a double green image to the eye of the observer. For a given angle of observation, the optimal position of the second mirror depends on the refractive index and on the thickness of the substrate plate of the beam splitter as well as on the surface of the plate which supports the film. Unfortunately, the optimal position of the second mirror is also a function of the viewing angle.

 To overcome this problem, it can be proposed that the beam splitter is of the prism type comprising two prisms at right angles to the semi-transparent film of light-reflecting material interposed between the hypotenuse surfaces of the two prisms. Also, the first and second mirrors may include silver exterior surfaces of the two prisms. However, for the projection of large images, the prisms must be bulky and therefore excessively heavy. It is therefore difficult and expensive to obtain an adequate optical quality for the projection of images in large prisms. Furthermore, the size resulting from these prisms requires a cumbersome mechanism for moving or rotating the beam splitter relative to the optical axis in a system that requires this maneuver.

 The invention therefore relates to an optical device with a plate-type beam splitter having an optically symmetrical structure which, compared to an equivalent beam splitter of the prism type, is less bulky and can be moved or rotated much more easily. This new beam splitter comprises a semi-transparent film of a light-reflecting material interposed between two optical plates of transparent materials having respective thicknesses and respective refractive indices which are suitable for obtaining substantially equivalent optical paths of the images transmitted through film and images reflected by film. Thus, for optical systems producing large composite images, such as an image projection device for example, it is much easier and less expensive to provide the plates of this new beam splitter with the large lateral dimension necessary while maintaining the desired optical quality of the materials of the plate, only if it were necessary to provide prisms with the large surfaces necessary in an equivalent beam splitter of the prism type.

 ~ The invention also relates to an optical device in which the beam splitter is aligned with an image source and has an adjacent surface oriented at an oblique angle relative to the source.

Therefore, the light rays from the image source in the direction of the beam splitter passes through the adjacent transparent plate and meets the film of the splitter at an angle to the normal of the film. As a result, a partial component of each ray is reflected by the film at an angle equal to the angle of incidence; another partial component of the ray is transmitted through the film to penetrate the other transparent plate of the separator at an angle having a value equal to the angle of incidence. Consequently, the reflected and transmitted ray components pass through the transparent materials of the respective plates along equivalent optical paths and exit the respective opposite surfaces of the beam splitter.

 A first and a second mirror which are oriented symmetrically with respect to the film of the beam splitter are arranged in the paths of the reflected component and of the transmitted component. As a result, these two mirrors direct the components of the respective rays along paths which intersect at a surface of symmetry at which the beam splitter is arranged as an image combiner. Consequently, the components of each ray initially reflected and transmitted, after reflection by the first and second mirrors, cross equivalent optical paths by the respective paths of the separator and are combined again at the level of the film of the separator, on which the combined rays are again separated into two components , one retransmitted towards the image source and the other reflected towards the observer. Consequently, the image reflected by the first mirror, transmitted by the film, and the image reflected by the second mirror, reflected by the film, appear superimposed on each other to the eye of the observer as a recombined image. .

 If the direction of observation is changed by an appreciable angle with respect to the axial direction, there is no significant separation of the two images forming the recombined image because the optically symmetrical structure of this beam splitter produces equivalent paths for the respective components reflected and transmitted light rays from the image source.

 The invention also relates to an optical device in which the beam splitter comprises a film interposed as a surface of symmetry between two image sources arranged angularly, seen from the splitter. Therefore, light rays from a discrete region of an image source are incident on the plate surface adjacent to the beam splitter and in the splitter, and they propagate along an optical path through the plate near the separator, the separator film and the other plate to exit the opposite flat surface and then propagate in a direction parallel to the optical axis, for example towards the eye of an observer. '' a discrete region, corresponding to the other image source, is incident on the opposite surface of the separator and in the separator, propagating along an equivalent optical path, passing through the plate adjacent to the separator, to be reflected by the film, and to propagate by the neighboring plate along the part of the path followed by the light rays of the first optical source. As a result, light rays from corresponding discrete regions of the respective image sources exit the opposite plate surface of the separator and propagate in the same direction towards the eye of the observer. Thus, for the eye of the observer, the two images of the respective sources seem to be superimposed one on the other and form a recombined image.

Due to the symmetrical structure of this beam splitter, the two images still appear to be superimposed on each other even if the direction of observation clearly deviates from a parallel to the axial direction.

 Other characteristics and advantages of the invention will emerge during the description which follows.

In the accompanying drawings, given solely by way of examples, in no way limiting and on which
Fig. 1 is a schematic cross-sectional view of a color image display apparatus according to the invention,
Fig. 2 is a plan view of the outlet end part of the tube of FIG. l, taken along line 2-2 and looking in the direction of the arrows,
Fig. 3 is an elevational view of the outlet end part of the optical system of the
Fig. 1, seen from the position of the observer's eye,
Fig. 4 schematically represents an inverter scanning circuit of the beam deflection device of FIG. 1,
Fig. 5 is a time diagram of the electrical signals produced in the use of the circuit of the
Fig. 4,
Fig. 6 is a partial schematic section on a large scale of an outlet part of the viewing tube and of an aligned part of the optical device comprising the beam splitter of FIG. 1,
Fig. 7 is a partial schematic view similar to that of FIG. 6 but with a standard beam splitter of the plate type placed instead of the plate beam splitter of the invention,
Fig. 8 is a schematic view of another optical device comprising a conventional beam splitter of the plate type arranged symmetrically between two image sources arranged angularly; and,
Fig. 9 schematically represents another optical device similar to that of FIG. 8 but with the new beam splitter according to the invention positioned symmetrically between the two image sources arranged angularly.

 Among the figures, in which the same references designate identical elements, FIG. 1 shows an apparatus 10 for displaying a color image comprising a tube 12 of the cathode ray type.

The tube 12 comprises a casing 14 in the form of a funnel made of a suitable dielectric material, such as glass, with an axis 15 passing through the neck 16 of the casing 14. The neck 16 ends at one end of the 'enve
loppe 14 by a queuot 18 sealed in a leaktight manner along its periphery, that pins 19 arranged in a circle and spaced through
smells tightly. Pins 19 are a means
to support and electrically connect the elements
respective of an electron gun 20 which is disposed
axially in the neck 16.

 The electron gun 20 has a heating filament 22 disposed axially near the socket 18 dt in an inverted cathode cup 26 which has a closed end carrying an outer coating (not shown) of electronic emission material, sensitive to heat. The cathode cup 26 is disposed axially and at a certain distance in a first grid 28 in the shape of an inverted cup comprising a closed end but with a central opening aligned with closed end close to the cathode cup 26. The closed end of the first grid in the form of an inverted cup 28 is arranged in close juxtaposition with a closed end provided with a central opening of a second grid 32 in the form of a vertical cup. An elongated focusing cup 36 with a closed end comprising a central opening is arranged at a distance in the opposite open end of the second grid 32.

 The opposite open end of the focusing cup 36 forms the exit end part of the barrel 20 from which an electron beam 40 is directed axially towards the opposite end part of the casing 14. The electrodes respective beam forming 28, 32 and 36 of the barrel 20 are fixed by being isolated from each other, for example as a sub-assembly which is mounted on several dielectric rods 42 arranged axially and angularly spaced around the barrel 20. In addition , the subset of the beam-forming electrodes 28, 32 and 36 can be held on the axis 15 by several axially spaced collars 46 which surround the electrodes of the barrel 20 and which bear against the interior surface of the neck 16.

 The neck 16 came in the material from an opposite end part 50, with a larger diameter of the envelope, by means of an interposed part 52, flared outwards. The part 50 of larger diameter ends in a front plate 54 sealed along its periphery, made of a transparent material, for example glass. An output luminescent screen 60 comprising respective halves 56 and 58 is deposited by any usual means on the inner surface of the front plate 54. The half 56 is made of a luminescent material, for example of yttrium oxide activated by lleuropium which emits red light locally under the effect of the penetration of electrons from the beam 40. The half 58 is made of a different luminescent material, such as zinc silicate activated by manganese, for example, which locally emits green light under the effect of the penetration of the electrons of the beam 40. Thus, the respective halves 56 and 58 of the screen 60 are arranged in the same plane and have contiguous edges lying along the axis 15 of the envelope 12.

 An anode coating 62 of an electrically conductive material, such as aluminum, reflecting visible light, is deposited on the interior surface of the screen 60. The coating 62 covers not only the entire interior surface of the part of larger diameter 50 but also extends axially as well as annularly on the flared part 52 of the casing 12. The anode coating 62 is electrically connected to an anode socket 64 passing hermetically through the wall of the flared portion 52 for the purpose of establishing an electrical connection with the anode electrode of the tube 12. The anode plug 64 and the anode coating 62 are electrically connected to another anode coating 66 which extends from the socket 64 towards the neck 14 of the casing 12. The covering 66 is made of an appropriate electrically conductive material, for example carbon, arranged axially and annularly along the interior surface inclined of the flared part 52 and in the neck portion 16. Inside the neck 16, the anode coating 66 ends at a distance and surrounding the exit end of the barrel 20 from which the axially directed electron beam 40 exits. Thus, the anode coatings 66 and 62 form an anode electrode in the form of an inverted cup in which a space practically free of field is established.

 In operation, and as shown schematically in FIG. 1, the cathode 26 of the barrel 20 can be electrically connected by a conductor 68 to a cathode voltage terminal of a polarized voltage source 70. The electrode of the first grid 28 of the barrel 20 can be electrically connected by a conductor 72 has a voltage terminal of a source 70 which is electrically negative with respect to the cathode voltage terminal for the purpose of controlling the flow of electrons in the beam 40. The electrode of the second grid 32 of the barrel 20 can be electrically connected by a respective conductor 74 to an associated voltage terminal of the source 70 which is more positive with respect to the cathode voltage terminal; and the focusing electrode 36 of the barrel 20- can be electrically connected by a respective conductor 76 to an associated voltage terminal of the source 70 which is even more positive compared to the cathode voltage terminal of the source 70. The anode terminal 64 can be electrically connected by a conductor 78 to an anode voltage terminal of source 70 which is strongly electrically positive with respect to the cathode voltage terminal of source 70. Thus, the forming electrodes 28, 32, 36 of the barrel 20 and the cup-shaped anode electrode of the tube 12 are maintained at suitable electrical potentials relative to the potential of the cathode 26 to concentrate the electrons of the beam 40 on a small point of the luminescent screen 60, thereby producing a localized emission of visible light by the penetration of the luminescent material.

A beam deflection device 80 comprises, surrounding the outer surface of the neck 16, adjacent
at the flared part 52 of 11 envelope 14, an electromagnetic deflection block 82 through which the electron beam 40 from the barrel 20 passes. The deflection block 82 comprises two horizon deflection coils
opposite tale not shown, interconnected which are electrically connected by an amplifier 83 of horizontal deflection to a horizontal scanning generator 84 of current type.

The block 82 also includes two opposite and interconnected vertical deflection coils which are electrically connected by a vertical deflection amplifier 85 to an inverter scanning circuit 86. The inverter scanning circuit 86 receives scanning signals from a scanning generator. vertical 87 of current type and drive signals from a synchronization device 88. The synchronization device 88 comprises an element of a control signal source 81 which also transmits the drive signals to the sweep generator horizontal 84. The inverter scanning circuit 86 also receives signals from a video signal generator 89 comprising another element of the source 81 and it applies output signals from a current type video amplifier 134 to the electrode. of the first grid 28 of the tube 12.

As shown in Fig. 4, the inverter scanning circuit 86 comprises a double operational amplifier 90 of commercial type, such as Op Amp MC 1747 distributed by Motorola Semi-conductor Products of Pheonix,
Arizona, for example. The device 90 comprises an operational amplifier 92 whose non-inverting input (+) is connected by resistance to the output of the vertical scanning generator 87 and also by resistance to the cursor of a potentiometer 94 for DC offset. The non-inverting input (+) and the inverting input (-) of the operational amplifier 92 are also electrically connected to the electrical ground by respective resistors. The device 90 also includes an operational amplifier 96, including an inverting input. (-) is connected by resistance to the output of the vertical scanning generator 87 and also by resis tax to the cursor of a potentiometer 98 for DC offset.

The non-inverting input (+) of the operational amplifier 96 is connected to the electrical ground by a resistor.

The outputs of the operational amplifiers 92 and 96 are connected by resistance to their respective inverting inputs (-) and are also electrically connected to the respective inputs of one half of a switching device 100 with integrated circuit, one output of which is electrically connected. to the vertical deviation amplifier 85. The device 100 can be of a commercially available type, such as a Dual-In-Line Package
DG 201 BP, Quad SPST, CMOS Analog Switch, broadcast by Siliconix of Santa Clara, California, for example. The other half of switching device 100 has input terminals electrically connected to potentiometer sliders 102 and 104 for video gain and an output electrically connected to a conductor 106. The conductor 106 is electrically connected by the video amplifier 134 and the conductor 72 to the first gate electrode 28 (FIG. 1). Each half of the switching device 100 has control terminals connected to an output 108 of a bistable circuit 110 and other control terminals connected to an output 112 of the device 110. The device 110 may be of a type available in trade, like a model SN 7473,
Dual-JK With Clear, manufactured by Motorola Semiconductor
Division of Pheonix, Arizona, for example. The device 110 has a clock or synchronization input 114 electrically connected to the synchronization circuit 88.

 The synchronization circuit 88 therefore applies to the input 114 of the bistable circuit 110 a drive voltage signal which, as shown in FIG. 5, can be represented by the waveform 116 comprising pulses 118 regularly spaced. On receipt of a pulse 118, the device 110 switches from one to the other of two operating states, thus changing the output voltages applied to its respective output terminals 108 and 112. Thus, as shown in the rectangular waveforms 120 and 122, a pulse 118 at the input 114 can cause the device 110 to apply a positive retention pulse to its output 108 for a predetermined time interval while a zero voltage pulse is applied to the output terminal 112. On reception of the next driving pulse 118 at its input 114, the device 110 changes its operating mode Rour apply a positive voltage pulse to its output 112 during a predetermined time interval equal to the previous while a zero voltage pulse is applied to its output terminal 108.

 As a result, the switching device 100 is controlled during the predetermined time interval to connect the output of the operational amplifier 92 to the vertical deflection amplifier 85 so that it receives a voltage signal represented by the positive sawtooth waveform 124.

Then, the switching device 100 is controlled for an equal time interval to connect the output of the operational amplifier -96 to the vertical deflection amplifier 85, so that it receives a voltage signal represented by the shape of tooth waves. negative saw 126. Consequently, the vertical deflection amplifier 85 receives a composite voltage signal represented by the waveform 128 comprising a negative sawtooth pulse 130 which begins at a value A, established by the potentiometer cursor 94, corresponding to the contacting edges of the screen portions 56 and 58, and ending at the value B. The waveform 128 comprises another alternating positive sawtooth pulse 132 which begins at a value C adjusted by the cursor potentiometer 98 above the central value 131 and ending at the value D.

 The deflection amplifier 85 is of a current type which converts the voltage signal 128 received into an equivalent electric current which circulates in the deflection block 82 to develop a magnetic field which deflects the electron beam 40. Consequently, the electron beam 40 (Fig. 1) is deflected line by line to scan a screen of the luminescent screen part 56 from the axis 15 of the tube 12 towards the periphery of the front plate 54 in the direction indicated by the arrow 133.Then the electron beam is deflected vertically to a point below the axis 15 of the tube 12 and it is deflected laterally line by line to sweep an inverted symmetrical frame of the part of lunnescent screen 58 from the point below the axis 15 of the tube 12 towards the periphery of the front plate 54. Thus, the inverter scanning circuit 86, with the generators 84 and 87, respectively of vertical and horizontal scanning comprise a symmetric frame generator risk which symmetrically scans inverted frames by the beam 40 on the display parts of the screen 56 and 58 respectively.

 As shown in Fig. 1, the function of the video signal circuit 89 is, through the inverter scanning circuit 86 and the video amplifier 184, to instantly modify the potential of the first gate electrode 28 relative to the potential of the cathode 26 thereby producing instantaneous variations in the electron current of the beam 40 when it scans each of the symmetrical frames of the screen parts 56 and 58.

 Going back to Fig. 4, it appears that when the switching device 100 is controlled to connect the output of the operational amplifier 92 to the vertical deflection amplifier 85, it also connects the cursor of the potentiometer 102 by the output conductor 106, the amplifier video 134 and the conductor 72 to the first grid electrode 28 of the tube 12. Similarly, when the switching device 100 is controlled to connect the output of the operational amplifier 96 to the deflection amplifier 85, it also connects the potentiometer cursor 104 to the grid electrode 28 of tube 12. Consequently, when the beam 40 scans reverse frames on the two halves 56 and 58 of the luminescent screen 60, the video signal coming from of the video signal circuit 89 applied to the first grid electrode 28 can be adjusted to give the desired brightness produced by the respective luminescent substances constituting the display screen parts sation 56 and 58.

 Thus, and as shown in FIG. 2, the screen parts 56 and 58 produce symmetrical images 136 and and 138 of different colors of the same subject, such as a flag 139 directed outwards from a pole 137 for example. The image 136 is produced by a localized emission of red light by the luminescent material of the screen part 56 and the image 138 is produced by the localized emission of green light by another luminescent material of the screen part 58.But it is important to note that the cushion distortion of the two images 136 and 138 has been exaggerated to show the symmetry of these distortions themselves. Thus, due to the physical symmetry obtained by the described operation of the tube 12, even imperfections which appear in the image 136 when the beam 40 scans the red part 56 of the screen 60 on one side of the axis of the tube 12 are symmetrically repeated in the image 138 guano the beam 40 scans the green part 58 of the screen 60 on the other side of the axis of the tube 12. Thus, if the symmetrical images 136 and 138 are folded l 'one on the other along the respective edges of the axis 15, not only the straight poles 137 are aligned with each other but also the curvatures of the corner parts of the symmetrical flags 139 are in alignment without the need to compensate or correct distortions.

 Fig. 1 also shows, optically coupled with the luminescent screen 60, an optical device 140 which can be supported in the current manner in a preferred relative position relative to the plane of the screen parts 56 and 58. The device 140 comprises a dichroic mirror 144 which transmits red light and reflects green light, and it is arranged in the same plane as the axis of the tube 12 and the neighboring edges of the screen parts 56 and 58.

A second mirror 142 which reflects the red light emitted by the screen part 56 is arranged in alignment with the screen part 56 and oriented at an angle of about 45 degrees relative to the plane of the screen part 56 and in the plane of the vertical mirror 144. A plate-shaped beam splitter 146 which transmits the red light and which partially reflects and partially transmits the green light emitted by the screen part 58 is arranged in alignment with the part d screen 58 and oriented at an angle of about 45 degrees relative to the plane of screen portion 58 and to the plane of the dichrolic mirror 144.

The beam splitter 146 has an optically symmetrical structure which will be described more particularly with reference to FIG. 6.

 In operation, the red light coming from the image 136 produced on the screen part 56 is reflected by the mirror 142 and passes through the mirror 144 towards the beam splitter 146 in which a substantial part of the red light is transmitted by the splitter 146 towards the eye 148. Furthermore, the green light of the image 138 produced on the screen part 58 is partially reflected by the beam splitter 146 towards the dichroic mirror 144 so as to be reflected again towards the splitter of beam 146. A substantial part of the green light reflected by the mirror 144 passes through the separator 146 towards the eye 148.

It should be noted that the images 136 and 138 are symmetrical images which can be folded optically in correspondence one over the other. Ega1eni ,, the optical elements of the device 140 are arranged so as to provide equal optical path lengths for the red and green rays propagating from corresponding discrete regions of these images towards the eye. Furthermore, the red and green light rays coming from corresponding discrete regions of the respective images 136 and 138 arrive at the eye 148 in identical directions. Consequently, and as shown in FIG. 3, corresponding discrete regions of images 136 and 138 are seen by the eye 148 in alignment with one another. I1 follows that it seems to the eye 148 that the images I36 and 138 are superimposed one on the other in a virtual image plane.
150 located behind the mirror 144.

The optical distance between the image plane
virtual 150 and eye 148 is the same as the distance
optics between images 136 and 138 and the eye 148, and
it is possible to imagine that they were created in
straightening the paths of light rays at the surfaces
reflective. The color of each region
discrete from the color image, arriving at the eye 148
is determined by the relative intensities of the lights
red and green in the corresponding discrete regions of images 136 and 138.The intensity of red in the image
136 and the intensity of the green in image 138 can be ordered by them. amplitudes of the video signals applied between
grid 28 and catnode 26 during the intervals
time ot-beam 40 respectively sweeps the part
red 56 and the green part 58 of the luminescent screen 60.

As already explained, the green light
from image 138 on screen part 58 is
partially reflected by beam splitter 146
towards the dichroic mirror 144. But the green light
from image 148 is also partially trans
put by beam splitter 146 to keep going
propagate in the initial direction of propagation at
from screen part 58. The beam splitter
146 partially transmits green light, allowing
thus the direct passage through the separator 146 of the image
of green light reflected by the dichroic mirror 144.

Therefore, to recover the green light from the image
138 transmitted by the beam splitter 146, a mirror
152 highly reflective is arranged in the inter position
ception in the path of the transmitted green light and
it is oriented roughly parallel to the plane of the screen part 58 and is almost perpdiculalrerrent to the dichroic mirror 14. By
Consequently, the recovery mirror 152 is produced and
positioned to reflect a green light image
transmitted by the beam splitter 146, back to the latter so that it is reflected by this splitter 146 towards the eye 148.

 As shown in Fig. 7, it is possible to have, instead of the beam splitter 146 according to the invention, a conventional beam splitter 162 of the plate type comprising an optical transparent plate 164 carrying, on its surface planeadjaoente to the mirror 152, a semi-film transparent 166 of a material reflecting light. It has been found that if the conventional beam splitter 162 is used and the eye 148 is arranged to observe the image in a direction roughly parallel to the axial direction represented by the line 163, the recovery mirror 152 can be positioned with respect to the beam splitter 162 so that the image of green light transmitted by the beam splitter, and which is reflected by the mirror 152, seems to the eye 148 superimposed on the image of reflected green light by the beam splitter and which is reflected by the dichroic mirror 144. The reason for this apparent superimposition of the images of green light is that the film 166 of the conventional beam splitter 162 constitutes an optical plane of symmetry between the respective mirrors 144 and 152 when the eye 148 is positioned to observe the image in a direction roughly parallel to the axial direction 163.

Thus, a light ray 165 emanating from a point 58P of the screen part 58 perpendicular or roughly perpendicular to the plane of this screen part 58 is
refracted to the surface of the beam splitter plate 164 and meets at a predetermined angle a point such as point 166P, for example, of the film 166 of the beam splitter. As a result, a component ray of light 167P is transmitted by the point 166P of the film 166 and propagates in a direction roughly parallel to the initial direction of propagation of the light ray 165.

In addition, a component ray of light 169P is reflected by the point 166P of the film 166 and, after refraction on the surface of the substrate plate 164, it propagates in a direction almost perpetual to the initial direction of propagation of the ray. 165. The beam of transmitted component 167P is reflected by the recovery mirror 152 as a light ray 167R; and the reflected component ray 169P is reflected by the dichroic mirror 144 as a light ray 169R. These reflected light rays 167R and 169R are incident on the film 166 at points sufficiently close to one another so that they can be represented as a single point 166R. Thus, the light ray 167R which is reflected by the point 166R of the film 166 and the light ray 169R which is transmitted by the point 166R of the film 166 propagate in roughly the same direction towards the eye of the observer 148 which is located in the small angle 1 by relative to the axial direction 163. Consequently, for the eye 148 positioned in the small angle 91 the two rays 167R and 169R seem to form a single combined ray 168.

 But in practice, the eye 148 can move to observe the point 58P of the screen part 58, in a direction making an appreciable angle 02, for example greater than ten degrees, relative to the axial direction 163. As a result, there may appear to be a significant separation between the image of green light transmitted by the beam splitter and the image of green light reflected by the beam splitter because the film 166 no longer constitutes an optical plane of symmetry between the mirrors 144 and 152. Consequently, each discrete surface of the film 166 can be located at an optical path length from the associated surface of the mirror 144 greater or less than the length of the optical path from the same surface of the film 166 to the corresponding surface of the mirror 152.

 Thus, a light ray 165T emanating from point 58P of the screen part 58 and leaving the front plate 54 at an angle 62 relative to the normal to the plane of this screen part is refracted on the surface of the plate 164 of the beam splitter, current 162 and it is incident on the film 166 at a point such as 166T, for example. As a result, a component ray of light 167T is transmitted by point 166T of the semitransparent film 166 and propagates in a direction almost parallel to the initial direction of propagation of the 165T ray. of light 169T of radius 165T is reflected by the point 166T of the film 166 and, after refraction on the surface of the substrate plate 164, it propagates in a path which is not perpendicular to the initial direction of propagation of the radius 165T. The traveling beam 167T is reflected by the recovery mirror 152 as a light ray 167U which is incident on the film 166 at point 166V. Furthermore, the reflected beam i69T reflected is reflected by the dichroic mirror 144 in the form of the light beam 169U which, after refraction on the surface of the substrate plate 164, meets the film 166 at a point 166U clearly offset from the point 166V of film 166.

Consequently, the component radius 167T transmitted by the beam splitter and the component ray 169T reflected by the beam splitter approximates the eye 148 of the respective observer along paths 167W and 169W aecales lateralem .ent. Thus, the image of green light transmitted by the beam splitter and the image of green light reflected by the beam splitter appear to the eye 148 as a double image of green light.

 It appeared that for a given angle of observation, the optimal position of the recovery mirror 152 relative to the beam splitter 162 depends on the refractive index and on the thickness of the material of the substrate of the current beam splitter. , as well as the flat surface of the substrate which supports the semi-transparent film of light reflecting material. Unfortunately, this optimal position of the recovery mirrors 152 is also a function of the angle of observation.

 Therefore, to eliminate the separation of green light images, the optical system 140 includes the new beam splitter 146 having an optically symmetrical structure for both the transmitted green light image and the reflected green light image. As shown in Fig. 6, the beam splitter 146 comprises a semi-transparent film 154 interposed between two optical plates 156 and 158 made of a transparent material, for example glass.

Film 154 is made of a material which, under the correct thickness, partially reflects and partially refracts green light. The plates 156 and 158 may have approximately equal thicknesses and may consist of materials which have approximately equal indices of refraction
The film 154 can be deposited on a flat surface of one of the transparent plates by any suitable means, for example by evaporation or spraying.

Then, the surface of the plate supporting the film 154 can be glued, for example by an optically transparent adhesive, on a flat surface of the other of the transparent plates 156 and 158.

 Thus, a ray 160 of green light coming from point 58P of the screen portion 58 can emerge from the front plate 54 at a point A and propagate in an appropriate interface medium, such as air, for example the along an AB route. The path AB can make an arbitrary angle O 2 with respect to the normal to the front plate 54. Consequently, the ray 160 is incident on the adjacent planar surface, arranged an obscurely from the beam splitter plate 156 at a point B and under a predetermined angle of incidence. The ray 160 is refracted on the surface of the plate 156 to propagate along a path BC as a function of the refractive index of the material of the plate and it is incident on the surface adjacent to the film 154 at a point C at a predetermined angle of incidence. As a result, a reflected ray 160R is produced, propagating through the material of the plate 156 along a path CD at an angle with respect to the normal of the film 154 equal to the angle of incidence of the radius 160 at point C.

 At point C is also produced a transmitted ray 160T which propagates through the material of the plate 158 along a path CI. The path CI makes with respect to the normal to the film 154 an angle of the same value as the angle of incidence of the radius 160 at point C because the materials of the plates 156 and 158 have approximately basal indices of refraction. The transmitted ray 160T leaves the surface remote from the plate 158 and propagates through an appropriate interface medium, such as airJ for example, along a path IJ. Also, the reflected ray 160R leaves the neighboring surface of the plate 156 and propagates in the air along a path DE. The respective paths DE and IJ make. angles approximately equal to normal to the film 154 because the materials of the plates 156 and 158 not only have refractive indices of approximately equal, but almost. also opposite surfaces are approximately parallel to the film 154. Likewise, the medium adjacent to the opposite exposed surfaces of planes 156 and 158 are of the same material or have approximately equal indices of refraction.

Thus, if the mirrors 152 and 144 are arranged symmetrically with respect to the film 154, the reflected ray 160R and the transmitted ray 160T are incident on equidistant points E and J on the surfaces adjacent to the mirrors 144 and 152 on angles d 'incidence approximately equal to the respective normals.

 The reflected ray 160R and the transmitted ray 160T are therefore reflected by the mirrors 144 and 152 under angles of reflection which are practically equal and they propagate along paths and the same length, respectively EF and JK.

Thus, the reflected ray 160R and the transmitted ray 160T are incident on the opposite planar surfaces, arranged angularly, of the beam splitter 146 at angles of incidence respectively equal even compared to the normal of the film 154 and are refracted respectively to the surfaces plates 156 and 158. As a result, the reflected ray 160R and the transmitted ray 160T are incident at approximately equal angles 3. on the opposite surfaces of the film 154 which is arranged in a plane of symmetry between the respective mirrors 144 and 152. A part of the reflected ray 160R is transmitted by the film 154 at the point G and propagates along a path GH at an angle roughly equal to its angle of incidence relative to the normal to the film 154. Part of the transmitted ray 160T is reflected by the film 154 at point G and also propagates along a path identical to the path GE followed by the ray 160R. Therefore, parts of the reflected ray 160R and the transmitted ray 160T are combined and emerge from the exposed surface of the plate 158 at point H. The combined rays 160R and 160T are refracted at point H and propagate as a combined ray 160U by the interface air medium to the eye 148 which can be positioned with an offset from the axis equal to the angle a 2.

 Since the recombination of the reflected ray 160R and the transmitted ray 160T has been represented for an arbitrary angle e2, it can be shown that it is valid for all points like 58P of the output screen part 58 for example and viewing angles e2 of the eye 148 relative to the optical axis of the device 140. Thus, the symmetrical optical structure of the beam splitter 146 provides equivalent optical paths for the reflected ray 160R and the transmitted ray 16QT, so that the combined rays can be viewed at a wide angle without separation occurring. It also results therefrom that a set of points emitting light, - even exile example point 5BP,. And constituting the image produced on the output screen part 58 emits light rays which follow transmission paths and reflection in the beam splitter as described to be recombined correspondingly.

 Fig. 8 shows another embodiment which consists of an optical system 180 comprising first and second image sources 181 and 182 respectively, arranged perpendicularly to each other to direct the respective images along intersecting paths . The image source 181 can constitute, for example, a specimen-- and the image source 182 can consist, for example, of a reticle.

An image combiner is arranged angularly between the respective sources 181 and 182 and may comprise a beam splitter 162 of current type comprising the transparent substrate plate 164 supporting on one of its flat surfaces the semi-transparent film 166 of material reflecting light. The beam splitter 162 can therefore be located so that its film 166 constitutes an optical plane of symmetry with respect to the sources 181 and 182 when the eye 148 of an observer is positioned to observe images of the two sources in a direction approximately near parallel to the axial direction represented by line 163.

 A beam 183X of light rays coming from a point 181X of the source 181, for example the center of a specimen, and perpendicular or almost perpendicular to the plane of the source 181 is refracted at the surface of the beam splitter plate 164 and it is transmitted by a discrete surface 166X of the beam splitter film 166. Also, a beam 184X of light rays emanating from a point 182X of the source 182, as the center of a reticle, and perpendicular or almost perpendicular to the plane of the source 182, is reflected by the discrete surface 166X of the beam splitter film 166. As a result, the respective beams 183X and 184X of light rays propagate along a common path 185X from the discrete surface 166X from film 166 to the eye of the observer 148.

Consequently, the observer's eye 148 sees the images of points 181X and 182X in coincidence with one another at point 186 in the virtual image plane 187.

 But in practice, the eye 148 of the observer can move to observe the point 181 X of the specimen 181, in a direction making a notable angle 82, for example greater than ten degrees, relative to the axial direction 163. For this angle e2, the beam splitter film 166 no longer constitutes a plane of optical symmetry between the sources 181 and 182. A beam 183Y of light rays emanating from the point 181X of the source 181 now propagates along a path of a different ion in the beam splitter 164 and it is transmitted by a discrete surface 166Y. of the film 166 to propagate along a path 185Y towards the eye 148, at the angle e2 with respect to the axial direction 163. The eye 148 therefore sees the discrete surface 181X of the source 181 at a point 181Y in a new image plane 188 located at a different distance from the beam splitter 162.

 In addition, the eye 148 sees in alignment with the point 181Y a different point 182Y in the virtual image plane 187 of the source 182. This point 182Y corresponds to a point 189 of the image source 188 from which a beam 184Y of light rays, reflected along the path 185Y towards the eye of the observer. It thus appears that an appreciable angular movement of the eye 148 with respect to the axial direction 163 causes a lateral movement of the point 181X with respect to the point 182X which can be, for example, the center of a measurement reticle.

This apparent lateral movement can be eliminated by replacing the conventional beam splitter 162 with the beam splitter 146, according to the invention, like the mor, be the
Fig. 9 to constitute an optical system 190 which is similar to the optical system 180 shown in FIG. 8, with the exception of the incorporated beam splitter. The optical device 190 can therefore comprise first and second image sources 181 and 182 which are arranged perpendicularly to each other to direct respective images along paths which intersect. The beam splitter 146 according to the invention, arranged angularly between the respective sources 181 and 182 comprises the semi-transparent film 154 of light-reflecting material interposed between the transparent plates 156 and 158. The beam splitter 146 is therefore located between the sources 181 and 182 so that the separator film 194 constitutes a plane of optical symmetry with respect to the sources 181 and 182, independently of the fact that the eye 148 of the observer can be positioned to observe the images of the respective sources in a direction roughly parallel or not to the indicated axial direction by line 163.

 Also, a beam 191X of light rays emanating from a point 181X of the source 181, for example the center of a specimen, and perpendicular or almost perpendicular to the plane of the source 181 is refracted at the surface close to the plate 156 of the beam splitter. As a result, the beam 191X of the refracted light rays follows an optical path which ccanFrena the transmission by an aligned part 156X of the separator plate 156, a discrete aligned region 154X of the film 154 and an aligned part 158X of the separator plate 158. Likewise, a beam 192X of light rays emanating from a point 182X of the source 182, like the center of a measurement reticle, is refracted at the surface close to the separator plate 158. Thus, the beam 192X refracted light rays follow an optical path comprising the transmission by an aligned part 193X of the plate 158, the reflection by the discrete aligned surface 156X of the film 156 and the transmission by the part 158X of the plate 158, followed by the beam 191X of light rays. As a result, the beams of light rays 191X and 192X emerge from the exposed flat surface of the plate 158, at about the same place and propagate along a common path 194X towards the eye 148 of the observer. Consequently, the eye 148 sees the images of the respective points 181X and 182X in coincidence with each other at point 186 of the virtual image plane 197.

 But in this case, when the eye 148 of the observer moves to observe the point 181X of the source 181 in a direction making an appreciable angle 92 with the axial direction 163, the film 154 of the beam splitter constitutes another plane of optical symmetry between the respective sources 181 and 182 due to the symmetrical structure of the separator 146. Thus, a l91Y beam of light rays emanating from the point 181X of the source 181 is now refracted on the surface of the separator plate 156 to follow a longer or shorter optical path in the beam splitter.

146, comprising transmission by an fS6Y part of the separator plate 156, an aligned discrete region 154Y of the film 154 and an aligned part 158Y of the plate 158. Also, a beam 192Y of light rays coming from point 182X is now refracted to the surface of the beam splitter plate 158 to follow a similarly longer or shorter optical path in the beam splitter. beam 146, including transmission through an aligned part 195Y of plate 158, reflection by the discrete region 156Y of film 156 and transmission by part 158Y of plate 158, followed by beam l91Y of light rays. As a result, the respective beams l91Y and 192Y of light rays emerge again from the exposed surface of the plate 158, in about the same place and propagate along a common path 196Y even if the eye 148 of the the observer is positioned at an angle 02 with respect to the axial direction 163.

Consequently, the virtual image of point 181X and the virtual image of point 182X coincide with point 187 even if point 187 is in a position different from point 186 established previously in the case of observation along the direction axial 163.

 It is obvious that the above considerations, described with regard to the eye of the observer apply in the same way to other focusing systems, for example lenses and focusing mirrors used in conjunction with microscopes, telescopes, cameras and image projectors.

 Of course, numerous modifications can be made to the embodiments described and illustrated by way of nonlimiting examples without departing from the scope or the spirit of the invention.

Claims (20)

 1. Symmetrical beam splitter, characterized in that it comprises in orientation, a device (154) for transmitting and reflecting a radiation beam, said transmission and reflection device comprising a first and a second opposite surface, and a device with radiation transmission plates (156, 158) coupled with said first and said second opposed surfaces to form near said surfaces, radiation transmission media having equivalent radiation transmission properties.  2. Separator according to claim 1, characterized in that said transmission and reflection device (154) comprises a semi-transparent layer of reflective material.  3. Separator according to claim 1, characterized in that said radiation transmission plates test a first and a second plate (156, 158) made of radiation transmission materials and superimposed respectively on said first and said second surfaces.  4. Separator according to claim 3, characterized in that said radiation transmission materials have substantially equal refractive indices.  5. Separator according to claim 3, characterized in that said first and said second plates 1156, 158) have substantially equal thicknesses measured respectively from said first and said second surfaces.  6. Symmetrical beam splitter, characterized in that it comprises a semi-transparent piece (154) of reflective material having first and second surfaces, and a device with transmission plates (156, 158) arranged in intimate contact with said first and said second surfaces to form equivalent transmission media against said surfaces.  7. Separator according to claim 6, characterized in that said part (154) nomprend a layer having first and second opposite surfaces.  8. Separator according to claim 6, characterized in that the transmission plates (156, 158) comprise an optical plate of transparent material.  9. Separator according to claim 6, characterized in that said transmission plate device comprises first and second optical plates (156, 158) disposed respectively in contact with said first and said second surfaces.  10. A separator according to claim 9, characterized in that said first and said second plates (156, 158) are made of transparent materials having substantially equal refractive indices.  11. Optical device, characterized in that it comprises, in combination, an optical device (140) arranged to direct light along a predetermined path and a device (146) for transmitting and reflecting light arranged in said path, this device comprising a semi-transparent layer (154) of reflective material with first and second opposite surfaces and a device with transmission plates (156, 158) arranged in intimate contact with said first and said second surfaces to form in proximity to said surfaces of equivalent transmission media.  12. Device according to claim 11, characterized in that said optical device comprises light sources (181, 182) intended to direct light along a first path towards said light transmission and reflection device.  13. Device according to claim 12, characterized in that said semi-transparent layer (154) of reflective material is disposed in said first path and is oriented at an angle with respect to the latter - to transmit part of said light coming from said source along a second path aligned with said first path, and for reflecting another portion of said light from said source along a third path disposed at an angle to said first and said second aligned paths.  14. Device according to claim 13, characterized in that said optical device comprises a first reflecting device (144) arranged in said second path for reflecting said part of the light transmitted to said device and a second reflecting device (152) arranged in said third path for reflecting said part of the light reflected towards said device.  15. Device according to claim 14, characterized in that said light source comprises an image production device (58) for directing an image of visible light along said path to said device.  16. Optical device, characterized in that it comprises, in combination, light sources (181, 182) polishing directing light along a first and a second respective path which meet at an intersection, and a device (154 ) of light transmission and reflection positioned at said intersection and comprising opposite surfaces arranged at an oblique angle with said first and said second path for receiving said light directed along said paths, the light transmission and reflection device comprising a reflecting device semi-transparent (154) for transmitting light directed along the first path in its direction of propagation and for reflecting light directed along said second path in said direction of propagation.  17. Device according to claim 16, characterized in that said light source comprises first and second image sources (181, 182) arranged to direct first and second light images along said first and second paths.  18. Device according to claim 17, characterized in that said semi-transparent reflecting device comprises a semi-transparent layer (154) of light reflecting material arranged in a plane of symmetry between said first and said second image sources.  19. Device according to claim 18, characterized in that said first and said second light images are of the same object and have corresponding discrete surfaces in alignment with one another in said plane of symmetry.  20. Device according to claim 19, characterized in that said first and said second light images of the same object have different colors.