Patent Publication Number: US-2023152505-A1

Title: Display device and optical element for use in the display device

Description:
TECHNICAL FIELD 
     The present invention relates to a display device and an optical element for use in the display device. 
     BACKGROUND ART 
     Recently, a display device called a “head-up display” has been proposed. This display device projects characters and/or an image (hereinafter collectively referred to as “image”) onto a screen to form a virtual image through virtual image optics including a combiner in front of the eyes of a pilot of an aircraft or a driver of a vehicle. In such a display device, generally, in order to compactly configure the device, luminous flux is formed by converting the light emitted from each point of an image display element with an expansion into collimated light by a collimator, and the luminous flux is introduced into a light guide to be enlarged. 
     As an example of such a device, Patent Document 1 describes a display device provided with a light guide. The light guide is provided, in a flat glass plate, with a reflective surface that reflects the light incident from the collimator in a predetermined direction and a plurality of beam splitters translucent to visible light. This display device is set such that the luminous flux introduced into the light guide is reflected at the interface between the flat plate and the outside in a direction satisfying the toral reflection condition. With this, the luminous flux propagates in a zigzag fashion while being repeatedly totally reflected in the light guide to be entered to one of the beam splitters. In the beam splitter, a part of the incident luminous flux is emitted to the outside of the flat plate from the interface between the flat plate and the outside by being reflected in a direction that does not cause the total reflection at the interface between the flat plate and the outside, and the remainder of the incident luminous flux transmits the beam splitter. The luminous flux that has transmitted the first beam splitter is incident on the second beam splitter, and the part of the incident luminous flux is reflected in the same direction as the luminous flux reflected by the first flat plate to be emitted to the outside of the flat plate, and the remainder of the luminous flux passes through the beam splitter. Such emission of the luminous flux from the flat plate occurs at the position of all of the beam splitters, and the luminous flux emitted from each point on the image becomes a larger luminous flux than when it was incident on the light guide and is emitted from the light guide. When the luminous flux is incident on the observer&#39;s eye, the visible range of the observer is enlarged. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: U.S. Pat. No. 10,133,067 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the display device described in Patent Document 1, not only the light of the image generated by the image forming unit but also the light incident on the interface between the flat plate and the outside from the outside of the light guide are incident on the light guide through the interface and then exit to the outside of the light guide again, resulting in stray light to be incident on the eyes of the observer. 
     An object to be solved by the present invention is to provide a display device capable of suppressing stray light to be incident on the eyes of an observer and an optical element for use in the display device. 
     Means for Solving the Problem 
     A display device according to the present invention made to solve the above-described problems is provided with: 
     an image display element configured to display an image to be displayed; 
     a collimator configured to convert light emitted from each point on the image displayed on the image display element into collimated light; 
     a first light guide formed of a flat plate made of a first material transparent to the light; 
     a second light guide formed of a plate member made of a second material transparent to the light, the second light guide having one surface and the other surface, the one surface of the second light guide being a flat surface in contact with one of surfaces of the flat plate of the first light guide, the other surface of the second light guide being a sawtooth surface; and 
     a beam splitter formed at an interface between the first light guide and the flat surface of the second light guide, the beam splitter being configured to transmit a part of the light and reflect the remainder of the light, 
     wherein the sawtooth surface of the second light guide is configured by alternately combining a first surface and a second surface, the first surface being non-parallel to a propagating direction of the light propagating through the second light guide after passing through the interface and being configured to transmit the light, and the second surface being substantially parallel to the propagating direction. 
     An optical element for use in a display device according to the present invention is an optical element for use in a display device configured to display an image in an enlarged manner based on light obtained by converting light emitted from each point on an image displayed on an image display element into collimated light by a collimator. 
     The optical element is provided with: 
     a first light guide formed of a flat plate made of a first material transparent to the light; 
     a second light guide formed of a plate member made of a second material transparent to the light, the second light guide having one surface and the other surface, the one surface of the second light guide being a flat surface in contact with one of surfaces of the flat plate of the first light guide, the other surface of the second light guide being a sawtooth surface; and 
     a beam splitter formed on an interface between the first light guide and the flat surface of the second light guide, the beam splitter being configured to transmit a part of the light and reflect the remainder of the light, 
     wherein the sawtooth surface of the second light guide is configured by alternately combining a first surface and a second surface, the first surface being non-parallel to a propagating direction of the light propagating through the second light guide after passing through the interface and being configured to transmit the light, the second surface being substantially parallel to the propagating direction. 
     Note that the second material may be the same as the first material. 
     Also, note that the term “substantially parallel to the propagating direction” regarding the light should not be construed as being strictly parallel to the propagating direction but may be construed as being inclined slightly (e.g., ±15°) with respect to the parallel direction. 
     Effects of the Invention 
     In the display device and the optical element for use in the display device according to the present invention, the light emitted from each point on the image displayed on the image display element and converted into collimated light by the collimator is incident on the beam splitter. A part of the light incident on the beam splitter passes through the beam splitter and is emitted from the first surface of the second light guide. The remainder of the light, i.e., the light reflected by the beam splitter without passing therethrough, is reflected by the surface of the flat plate of the first light guide on the opposite side of the beam splitter and is incident on another position of the beam splitter. A part of the incident light passes through the beam splitter and exits from a position different from the position of the first surface of the second light guide where the light first was emitted. In this way, the beam splitter repeatedly causes the partial transmission of the light and the reflection of the remainder of the light, and the light that has passed through the first surface of the second light guide is emitted. As a result, the light from each point on the image becomes larger than that when it was incident on the first light guide. This increases the range in which the observer can visually view the image. Note that the second surface of the second light guide is substantially parallel to the propagating direction of the light, and therefore, the light is prevented from being obstructed by the second surface. This prevents a partial shadow from occurring in the light that has passed through the first surface to be reached the eyes of the observer. 
     On the other hand, the light (external light) incident on the sawtooth surface of the second light guide from the outside of the display device is at least partially incident on the second surface or is incident on the second surface after being reflected by the first surface. On the second surface, absorption, reflection, diffusion, dimming, etc., of the light occur. Therefore, it is possible to suppress the incidence of the external light on the light guide. For this reason, it is possible to suppress stray light, which is external light to be emitted to the outside of the light guide again after being incident on the light guide, from being incident on the eyes of the observer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic configuration diagram showing a first embodiment of a display device according to the present invention. 
         FIG.  2    is a schematic configuration diagram showing a second embodiment of a display device according to the present invention. 
         FIG.  3    is a schematic configuration diagram showing a modification of the display device of the first embodiment. 
         FIG.  4    is a schematic configuration diagram showing another modification of the display device of the first embodiment. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     With reference to  FIGS.  1  to  4   , some embodiments of a display device and an optical element for use in the display device according to the present invention will be described. 
     (1) Configuration of Display Device and Optical Element for Use in the Display Device of First Embodiment 
       FIG.  1    shows a schematic configuration diagram of a display device  10  of a first embodiment. This display device  10  is provided with an image display element  11 , a collimator  12 , an optical element  13  for use in the display device, and a combiner  14 . The optical element  13  for use in the display device is provided with a first light guide  131 , a second light guide  132 , and a beam splitter  133 . Hereinafter, these constitutional elements of the display device  10  will be described below. 
     The image display element  11  is an element for displaying an image (hereinafter referred to as “original image”) prior to enlargement, which is a base of an image to be displayed on the display device  10  of the first embodiment. The image display element  11  may be a transmissive liquid crystal display element, a reflective liquid crystal display element, a digital micromirror device (DMD), an organic EL display element, a micro-LED display element, or the like. 
     The collimator  12  is an optical element for converting the light emitted with a spread from each point of the original image displayed on the image display element  11  into collimated light. The collimator  12  is configured by a lens, a mirror, and the like. 
     The first light guide  131  is configured by a rectangular flat plate for guiding the light converted by the collimator  12 . The first light guide  131  is configured by a transparent first material. As the first material, various kinds of optical glasses, resins, and the like, can be used. Within the first light guide  131  in the vicinity of one end  1311  in the rectangular long-side direction, a reflective surface  1351  inclined at a predetermined angle with respect to the flat plate is formed. The collimator  12  is arranged such that the light emitted from the collimator is incident on the reflective surface  1351 . The predetermined angle is set such that the light reflected by the reflective surface  1351  satisfies the total reflection condition at the interface between the flat plate and the outside (air). The reflective surface  1351  may be formed, for example, by cutting a flat plate into the same cut surface as the reflective surface  1351 , applying a coating of a light reflecting material to the cut surface, and then rejoining them. 
     The second light guide  132  is configured by a plate member made of a second material transparent to the light. The second light guide  132  has a flat surface  1323  and a sawtooth surface  1320 . The flat surface  1323  is in contact with a portion of the surface of the flat plate of the first light guide  131  on a side opposite to the end  1311  than the reflective surface  1351 . The sawtooth surface  1320  faces the flat surface  1323 . As the second material, optical glass, resin, or the like, can be used as in the case of the first material. The first material and the second material may be the same or different from each other. However, the first material and the second material are selected so as to have a refractive index satisfying a condition that total internal reflection does not occur on the flat surface  1323  when the light is incident on the interface between the first light guide  131  and the second light guide  132  at an angle satisfying the total reflection condition at the interface between the flat plate of the first light guide  131  and the outside (air). 
     At the interface in which the first light guide  131  and the second light guide  132  are in contact with each other, a beam splitter  133  that transmits a part of the light and reflects the remainder of the light is provided. In this embodiment, the interface has been subjected to optical coating to allow the transmission of the light at a predetermined transmission. Note that instead of subjecting the interface to optical coating as described above, a first light guide and a second light guide made of materials different in refractive index from each other may be combined to utilize the reflection due to the difference in the refractive index. In this way, by bonding the first light guide  131  and the second light guide  132 , it is possible to function the bonding surface as a beam splitter. 
     As described above, the first light guide  131 , the second light guide  132 , and the beam splitter  133  are provided. Thus, the light reflected by the reflective surface  1351  propagates toward the other end side while being totally reflected by both surfaces of the flat plate at the portion  1352  of the first light guide  131  not in contact with the second light guide  132  and is incident on the beam splitter  133 . The reflective surface  1351  and the portion  1352  of the first light guide  131  not in contact with the second light guide  132 , which correspond to an optics for causing the light to be incident on the beam splitter  133 , are collectively referred to as an “introductory optics  135 ” in the first embodiment. 
     The sawtooth surface  1320  has a first surface  1321  and a second surface  1322 . The first surface  1321  and the second surface  1322  are non-parallel to each other, and both of them are not parallel to the flat surface  1323 . The first surface  1321  and the second surface  1322  are alternately arranged in the long-side direction. The first surface  1321  is a surface non-parallel (in this embodiment, substantially perpendicular) to the light propagating through the second light guide  132  after having been passed through the beam splitter  133  at an angle satisfying the total reflection condition at the interface between the flat plate of the first light guide  131  and the outside (air). On the other hand, similarly, the second surface  1322  is a surface substantially parallel to the light propagating through the second light guide  132 . The first surface  1321  and the second surface  1322  are configured as described above. Thus, the light propagating through the second light guide  132  is emitted to the outside through only the first surface  1321 , almost without passing through (being incident on) the second surface  1322 . 
     In this embodiment, the second surface  1322  is coated with black paint that is a light absorber for absorbing visible light. With this, the external light incident on the second surface  1322  from the outside (indicated by a dashed line in  FIG.  1   ) is shielded by the second surface  1322 . In addition, the external light (indicated by a two-dot chain line) incident on the second surface  1322  after being reflected by the first surface  1321  is shielded by the second surface  1322 . That is, such external light is absorbed by the light absorber to be incident on the second light guide  132  without passing through the second surface  1322 . Note that no light absorber is applied to the first surface  1321 . 
     Although the sawtooth surface  1320  having the same shape and size is arranged at substantially equal intervals in this embodiment, it is also possible to use a sawtooth surface that differs in shape and/or size, or to arrange a sawtooth surface at unequal intervals. 
     Note that the second light guide  132  has a shape similar to a diffraction grating at first glance but does not function as a diffraction grating because the interval of the sawtooth surface  1320  is sufficiently longer (e.g., 10 times or more) than the wavelength (0.4 to 0.7 μm) of visible light. 
     The light reflected by the beam splitter  133  is incident on the surface  134  of the flat plate of the first light guide  131  on the opposite side of the beam splitter  133  but is totally reflected by the surface  134  because the total reflection condition is satisfied at the surface  134 . 
     The combiner  14  is a plate member that transmits a part of visible light and reflects the remainder of the visible light. The combiner  14  is arranged such that it faces the sawtooth surface  1320  and is non-parallel to the light on the optical path of the light emitted from the first surface  1321 . With this, a part of the light emitted from the first surface  1321  is reflected by the combiner  14  and is visually recognized by the observer viewing the combiner  14  from the direction (rightward in  FIG.  1   ) in which the light is reflected. At the same time, the observer can view the region on the opposite side of the combiner  14  through the combiner  14 . 
     (2) Operation of Display Device and Optical Element for Use in the Display Device of First Embodiment 
     The operations of the display device  10  and the optical element  13  for use in the display device of the first embodiment will be described. 
     In the image display element  11 , an original image is displayed, and light is emitted from each point on the image with a spread. The light is converted into collimated light at the collimator  12 . This light is introduced into the first light guide  131  and reflected by the reflective surface  1351 . Thereafter, the light propagates toward the side of the second light guide  132  while being totally reflected on both surfaces of the flat plate at the portion  1352  of the first light guide  131  not in contact with the second light guide  132  and is incident on the beam splitter  133 . 
     The beam splitter  133  transmits a part of the incident light and reflects the remainder of the light. The light reflected by the beam splitter  133  is totally reflected by the surface  134  and then is again incident on the beam splitter  133  on the side opposite to the reflective surface  1351  (right side in  FIG.  1   ) farther than the position where the light was first incident on the beam splitter  133 . A part of the incident light transmits the beam splitter  133 , and the remainder of the light is reflected by the beam splitter  133 . As described above, the beam splitter  133  causes repeated transmission and reflection of the light at a plurality of mutually different locations, but the light that has passed through at the respective positions are parallel to each other (see the dashed lines in  FIG.  1   ). 
     The light that has passed through the beam splitter  133  propagates in the second light guide  132  and passes through the first surface  1321  to be emitted to the outside. 
     The light emitted from the first surface  1321  is at least partially reflected by the combiner  14  and reaches the eyes of the observer looking at the combiner  14  from the direction (the right direction in  FIG.  1   ) in which the light was reflected. As described above, the light that has passed through the beam splitter  133  at different locations are parallel to each other, but are incident on the eyes of the observer while maintaining the parallel relation even after being reflected by the combiner  14 . With this, the light emitted from each point of the original image displayed on the image display element  11  and converted into collimated light at the collimator  12  is incident on the eyes of the observer as light larger than when it was emitted from the collimator  12 . Therefore, the range in which the observer can visually recognize the image is enlarged. 
     Note that the second surface  1322  of the second light guide  132  is substantially parallel to the propagating direction of the light, which suppresses the blocking of the light by the second surface  1322 . As a result, it is possible to suppress the generation of partial shadows in the light that has passed through the first surface  1321  and reached the eyes of the observer. 
     On the other hand, the at least external light (see the dashed line in  FIG.  1   ) directly incident on the second surface  1322  of the second light guide  132  and the external light (see the two-dot chain line in  FIG.  1   ) incident on the second surface  1322  after being reflected by the first surface  1321  out of the external light incident on the optical element  13  for use in the display device from the outside are absorbed by the light absorber of the second surface  1322 . Therefore, it is possible to prevent that the external light incident on the second surface  1322  is incident on the second light guide  132  (is further incident on the first light guide  131 ) and then is emitted to the outside of the second light guide  132  to be incident on the eyes of the observer. Therefore, it is possible to suppress stray light. 
     In the display device  10  and the optical element  13  for use in the display device of this embodiment, the following effects can be exerted in addition to the effect of suppressing stray light as described above. That is, it is enough to provide only one beam splitter  133  at the interface between the first light guide  131  and the second light guide  132 . For this reason, it is possible to more easily produce the display device and the optical element for use in the display device than that described in Patent Document 1 in which a plurality of beam splitters must be formed inside the light guide. 
     (3) Configuration of Display Device and Optical Element for Use in the Display Device of Second Embodiment 
       FIG.  2    shows a schematic configuration diagram of a display device  20  according to a second embodiment. The display device  20  is provided with an image display element  11 , a collimator  12 , and a combiner  14 , in the same manner as in the display device  10  of the first embodiment. It is further provided with an optical element  23  for use in the display device that differs from that of the first embodiment. The optical element  23  for use in the display device is provided with a first light guide  231 , a second light guide  232 , and a beam splitter  233 . Hereinafter, the configuration of the optical element  23  for use in the display device will be mainly described. 
     The first light guide  231  is configured by a rectangular flat plate made of a first material transparent to the light converted by the collimator  12 . The end face  2310  of at one end of the rectangular long-side direction (one short-side of the rectangle) has a shape formed by cutting obliquely with respect to the upper and lower surfaces of the flat plate. Note that in the display device  20  of the second embodiment, the same first light guide  131  as that of the first embodiment may be used. Alternatively, in the display device  10  of the first embodiment, the same first light guide  231  as that of the second embodiment may be used. 
     The second light guide  232  is a plate member made of a second material transparent to the light. The second light guide  232  is provided with a flat surface  2323 , a sawtooth surface  2320 , and a second flat surface  2324 . The flat surface  2323  is a surface in contact with a portion of the surface of the flat plate of the first light guide on the opposite side of the end face  2310  of the flat plate of the first light guide  231 . The sawtooth surface  2320  is a surface provided on a part of the surface facing the flat surface  2323  on a side opposite to the end face  2310 . The second flat surface  2324  is a surface provided in a region (hereinafter referred to as “second flat surface forming region  2325 ) of the surface facing the flat surface  2323  closer to the end face  2310  than the sawtooth surface  2320 . The sawtooth surface  2320  has a configuration in which a first surface  2321  and a second surface  2322  are alternately combined in the same manner as the sawtooth surface  1320  of the first embodiment. In the same manner as in the first embodiment, the second surface  2322  is coated with a light absorber for absorbing visible light. 
     As the first material forming the first light guide  231  and the second material forming the second light guide  232 , the same first material and the same second material as in the first embodiment are used, respectively. 
     The collimator  12  is arranged so as to face the end face  2310  of the first light guide  231 . The direction of the collimator  12  is set to satisfy both of the following conditions with respect to the optical element  23  for use in the display device. That is, these conditions include a condition that the light to be introduced into the first light guide  231  after being emitted from the collimator  12  is totally reflected at the interface between the first light guide  231  and the outside and the condition that the light is totally reflected at the interface between the second flat surface  2324  and the outside of the second light guide  232 . 
     The beam splitter  233  is made of an optical coating for allowing the light to pass therethrough at a predetermined transmission, the optical coating being applied to the interface at which the first light guide  231  and the second light guide  232  are in contact with each other. Note that the beam splitter  233  is provided not only at a position facing the sawtooth surface  2320  of the second light guide  232  but also on the second flat surface forming region  2325 . 
     (4) Operation of Display Device and Optical Element for Use in the Display Device of Second Embodiment 
     The operation of the display device  20  and the optical element  23  for use in the display device of the second embodiment will be described. The operation from the emission of the light from each point of the original image displayed on the image display element  11  with a spread to the conversion of the light into the collimated light at the collimator  12  is the same as that in the first embodiment. 
     This light is introduced from the end face  2310  to the first light guide  231  and reaches the second flat surface forming region  2325  while repeating the total reflection at the interface between the first light guide  231  and the outside. In the second flat surface forming region  2325 , a part of the light passes through the beam splitter  233  and is totally reflected by the second flat surface  2324 , and the remainder of the light is reflected by the beam splitter  233 . The light totally reflected by the second flat surface  2324  and the light reflected by the beam splitter  233  are parallel to each other and propagate along the optical paths offset in the longitudinal direction of the first light guide  231  (see the dashed lines in  FIG.  2   ). All of the light is totally reflected by the surface of the first light guide  231  on the opposite side of the beam splitter  233  and is again incident on the beam splitter  233 . A part of the light incident on the beam splitter  233  passes through the beam splitter  233  and is totally reflected by the second flat surface  2324 , and the remainder of the light is reflected by the beam splitter  233 . As described above, the light repeats the reflection at different positions, i.e., at the second flat surface  2324  and at the beam splitter  233 . Thereafter, the light passes through the beam splitter  233  to reach the first surface  2321  of the sawtooth surface  2320  and is emitted from the first surface  2321  toward the combiner  14 . The subsequent operations are the same as those of the display device  10  of the first embodiment. 
     According to the display device  20  and the optical element  23  for use in the display device of this embodiment, the light passes through a number of paths offset from each other in the longitudinal direction of the first light guide  231  to thereby spread in the direction. With this, it is possible to eliminate or reduce the region through which the light does not pass through the first surface  2321 , which enables the observer to visually recognize an image with less unevenness. 
     Further, according to the display device  20  and the optical element  23  for use in the display device of this embodiment, similarly to the first embodiment, it exerts the following effects. That is, at least the external light directly incident on the second surface  2322  of the second light guide  232  out of the external light incident on the optical element  23  for use in the display device from the outside is absorbed by the light absorber of the second surface  2322 . Further, at least the external light reflected by the first surface  2321  and then incident on the second surface  2322  out of the external light incident on the optical element  23  for use in the display device from the outside is absorbed by the light absorber of the second surface  2322 . As a result, stray light can be suppressed. 
     (5) Modified Embodiments 
     The present invention is not limited to the above-described embodiments, and various modifications can be made. 
     For example, in the above-described embodiments, it is configured such that the second surface  1322  ( 2322 ) absorbs visible light to attain the light-shielding. However, the second surface  1322  ( 2322 ) may be subjected to coating to reflect or diffuse visible light. Alternatively, an external mask for shielding visible light may be attached to the second surface  1322  ( 2322 ). 
     Further, the second surface  1322  ( 2322 ) may be subjected to coating, processing, or the like to absorb, reflect, or diffuse a part of external light to cause dimming. In such a case, it is possible to suppress the incidence of the external light on the second light guide  132  ( 232 ) to cause stray light, though not as effective as the light-shielding. 
     In the above-described embodiments, only one plate member constituting the combiner  14  is provided, but two or more plate members (for example, a first plate  141  and a second plate  142  as shown in  FIG.  3   ) may be provided so as to be substantially parallel to each other. With this, a part of the first incident light that has passed through the first plate  141  out of the light that has passed through the first surface  1321  is reflected by the second plate  142 . Thus, the light reflected by the first plate  141  and the light reflected by the second plate  142  are parallel and shifted in position with each other. For this reason, it is possible to further enlarge the area in which the observer can visually recognize an image. Note that although  FIG.  3    shows an example in which two plate members are used, and three or more plate members may be used. 
     Alternatively, as shown in  FIG.  4   , the combiner  14  may be omitted. In this instance, the observer can view the image by viewing the optical element  13  for use in the display device from the direction in which the light that has passed through the first surface  1321  propagates. 
     The configuration other than the combiner  14  have the same configuration as that of the display device  10  of the first embodiment shown in  FIGS.  3  and  4    but may be the same configuration as that of the display device  20  of the second embodiment or that of the other modification. 
     In the above-described embodiments, the light is incident on the first light guide  131  such that the incident angle of the light is an angle satisfying the total reflection condition at the interface between the flat plate of the first light guide  131  and the outside (air). However, it may be configured such that the surface of the first light guide  131  other than the portion in contact with the beam splitter  133  is provided with a coating that reflects light and that the light is incident on the first light guide  131  at an angle deeper than the angle satisfying the total reflection condition. Alternatively, the light may be incident on the first light guide  131  at an angle deeper than the angle satisfying the total reflection condition without subjecting it to such coating. Also in this case, at least a part of the light is reflected by the surface of the first light guide  131 , and therefore, it is possible to function as the display device and the optical element for use in the display device. 
     [Various Aspects] 
     It will be readily understood by those skilled in the art that the above-described embodiments and the modifications thereof are specific examples of the following aspects. 
     (Item 1) 
     A display device according to Item 1 is provided with: 
     an image display element configured to display an image to be displayed; 
     a collimator configured to convert light emitted from each point on the image displayed on the image display element into collimated light; 
     a first light guide formed of a flat plate made of a first material transparent to the light; 
     a second light guide formed of a plate member made of a second material transparent to the light, the second light guide having one surface and the other surface, the one surface of the second light guide being a flat surface in contact with one of surfaces of the flat plate of the first light guide, the other surface of the second light guide being a sawtooth surface; and 
     a beam splitter formed at an interface between the first light guide and the flat surface of the second light guide, the beam splitter being configured to transmit a part of the light and reflect the remainder of the light, 
     wherein the sawtooth surface of the second light guide is configured by alternately combining a first surface and a second surface, the first surface being non-parallel to a propagating direction of the light propagating through the second light guide after passing through the interface and being configured to transmit the light, and the second surface being substantially parallel to the propagating direction. 
     (Item 6) 
     An optical element for use in a display device according to Item 6 is an optical element for use in a display device configured to display an image in an enlarged manner based on light obtained by converting light emitted from various points on an image displayed on an image display element into collimated light by a collimator, 
     the optical element comprising: 
     a first light guide formed of a flat plate made of a first material transparent to the light; 
     a second light guide formed of a plate member made of a second material transparent to the light, the second light guide having one surface and the other surface, the one surface of the second light guide being a flat surface in contact with one of surfaces of the flat plate of the first light guide, the other surface of the second light guide being a sawtooth surface; and 
     a beam splitter formed on an interface between the first light guide and the flat surface of the second light guide, the beam splitter being configured to transmit a part of the light and reflect the remainder of the light, 
     wherein the sawtooth surface of the second light guide is configured by alternately combining a first surface and a second surface, the first surface being non-parallel to a propagating direction of the light propagating through the second light guide after passing through the interface and being configured to transmit the light, the second surface being substantially parallel to the propagating direction. 
     In the display device according to the above-described Item 1 and the optical element for use in the display device according to the above-described Item 6, the light emitted from each point on the image displayed on the image display element and converted into collimated light by the collimator is incident on the beam splitter. A part of the light incident on the beam splitter passes through the beam splitter and is emitted from the first surface of the second light guide. The remainder of the light reflected by the beam splitter without passing through the beam splitter is reflected by the surface opposite to the beam splitter and is incident on the other position of the beam splitter. A part of the light incident on the beam splitter passes through the beam splitter and is emitted from a position of the first surface of the second light guide that differs from the position where the light was first emitted. In this way, at the beam splitter, the transmission of the part of the light and the reflection of the remainder of the light are repeated, and the light that has passed through the beam splitter is emitted. For this reason, the light from each point on the image becomes larger than that when it was incident on the first light guide. This increases the range in which the observer can visually recognize the image. 
     On the other hand, the light incident on the sawtooth surface of the second light guide from the outside of the display device is at least partially incident on the second surface, or incident on the second surface after being reflected by the first surface. At the second surface, the light is absorbed, reflected, diffusion, dimmed, and the like. Therefore, it is possible to suppress stray light, which is the light emitted to the outside of the light guide again after the external light is incident on the light guide, from being incident on the eyes of the observer. 
     (Item 2) 
     According to the display device as recited in Item 2, in the display device as recited in the above-described Item 1, the second surface of the sawtooth surface is a surface that shields or dims visible light. 
     (Item 7) 
     According to the optical element for use in a display device as recited in the above-described Item 7, in the optical element for use in a display device as recited in the above-described Item 6, 
     the second surface of the sawtooth surface is a surface configured to shield or dim visible light. 
     According to the display device according to the above-described Item 2 and the optical element for use in the display device according to the above-described Item 7, the external light directly incident on the second surface of the sawtooth surface of the second light guide from the outside of the display device is shielded or dimmed by the second surface. Alternatively, the external light reflected by the first surface and then incident on the second surface is shaded or dimmed by the second surface. Therefore, the external light can be prevented from being incident on the light guide (in the case of shading) or suppressed (in the case of dimming). Therefore, it is possible to further suppress stray light from entering the eyes of the observer. 
     In this disclosure, the term “shielding visible light” means preventing the transmission of all the incident visible light and includes the meaning of absorbing all the incident visible light, reflecting all the incident visible light, and scattering all the incident visible light. Also, the term “dimming visible light” means preventing the transmission of a part of the incident visible light and includes the meaning of absorbing a part of the incident visible light, reflecting a part of the incident visible light, and scattering a part of the incident visible light. 
     (Item 3) 
     According to the display device as recited in Item 3, in the display device as recited in the above-described Item 1 or 2, 
     the second light guide is provided with a second flat surface at a position closer to a point where the light is introduced from the collimator than the sawtooth surface. 
     (Item 8) 
     According to the optical element for use in a display device as recited in the above-described Item 8, in the optical element for use in a display device as recited in the above-described Item 6 or 7, 
     the second light guide is provided with a second flat surface at a position closer to a point where the light is introduced from the collimator than the sawtooth surface. 
     According to the display device as recited in Item 3 and the optical element for use in the display device as recited in the above-described Item 8, the light that has passed through the beam splitter and reflected by the second flat surface and the light reflected by the beam splitter pass through paths shifted from each other and are emitted from the first surface to the outside of the optical element for use in the display device. Therefore, it is possible to eliminate or reduce the region through which the light does not pass through the first surface, and therefore, the observer can visually recognize the image with less unevenness. 
     (Item 4) 
     According to the display device as recited in Item 4, in the display device as recited in any one of the above-described Items 1 to 3, 
     the collimator and the first light guide are arranged such that the light is introduced from the collimator to the first light guide at an angle satisfying a total reflection condition at an interface with an outside in the first light guide. 
     According to the display device as recited in the above-described Item 4, it is possible to suppress the loss of the light in the first light guide. 
     (Item 5) 
     According to the display device as recited in Item 5, in the display device as recited in any one of the above-described Items 1 to 4, it is further provided with: 
     a combiner formed of a plate member arranged on an optical path of the light emitted from the first surface of the second light guide so as to be non-parallel to the light, the combiner being configured to transmit a part of visible light and reflect the remainder of the visible light. 
     According to the display device as recited in the above-described Item 5, a part of the light emitted from the first surface of the second light guide is reflected by the combiner and is incident on the eyes of the observer. Furthermore, the observer can visually view the other region of the combiner therethrough. Thus, for example, by arranging the combiner in front of the aircraft&#39;s cockpit, the observer sitting in the cockpit can view the image displayed on the image display element in an enlarged manner while observing the front (the opposite side of the combiner). 
     DESCRIPTION OF SYMBOLS 
     
         
           10 ,  20 : Display device 
           11 : Image display element 
           12 : Collimator 
           13 ,  23 : Optical element for use in a display device 
           131 ,  231 : First light guide 
           1311 : One end of the first light guide 
           132 ,  232 : Second light guide 
           1320 ,  2320 : Sawtooth surface 
           1321 ,  2321 : First surface 
           1322 ,  2322 : Second surface 
           1323 ,  2323 : Flat surface 
           133 ,  233 : Beam splitter 
           134 : Side of the first light guide opposite to the beam splitter of the flat plate 
           135 : Introductory optics 
           1351 : Reflective surface 
           1352 : Part of a first light guide not in contact with the second light guide 
           14 : Combiner 
           141 : First plate 
           142 : Second plate 
           2324 : Second flat surface 
           2325 : Second flat surface forming region