Patent Publication Number: US-2016223818-A1

Title: Image display device

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a transmission type image display device to be mounted in front of the face or on the head of an observer to show an image to the observer. 
     2. Description of the Related Art 
     Unexamined Japanese Patent Publication No. 2003-279883 discloses a display device provided with a display element and a transmission-type concave mirror. The display element, with a display surface facing the front side, is hung at a position which is below a bill of a cap and is in front of the forehead of an observer and above a position of the pupil of the observer. The transmission-type concave mirror, with a reflection surface facing toward the pupil of the observer, is hung on a lower surface of the bill at a position ahead of the display element. According to the conventional display device, the observer may simultaneously observe image light of the display element reflected from the concave mirror and external environment light transmitted through the transmission-type concave mirror. 
     SUMMARY 
     An image display device of the present disclosure includes a display element configured to project image light based on an input image signal, a concave mirror configured to transmit external environment light incident from the outside and to reflect the image light from the display element, and a housing configured to retain the display element and the concave mirror in a predetermined optical position relation, and having an opening portion for allowing the external environment light and the image light to pass therethrough. The concave mirror and the display element are mounted on the housing in a predetermined position relation in which the concave mirror guides the reflected light of the image light in a fixed direction toward the opening portion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a concept of a configuration of an image display device according to a first exemplary embodiment; 
         FIG. 2  is a diagram illustrating a wavelength characteristic of an LED light source included in a display element; 
         FIG. 3  is a diagram illustrating a reflectance characteristic relative to a wavelength, of a wavelength selective film; 
         FIG. 4  is a diagram illustrating angular dependence of reflectance on an incident angle, of the wavelength selective film; 
         FIG. 5A  is a perspective view of a concrete example of the image display device viewed from an oblique front side; 
         FIG. 5B  is a perspective view of the concrete example of the image display device viewed from an oblique rear side; 
         FIG. 6A  is a top view of the image display device; 
         FIG. 6B  is a front view of the image display device; 
         FIG. 7  is a diagram illustrating an internal configuration of the image display device illustrated in  FIGS. 5A to 6B ; 
         FIG. 8  is a diagram describing a spectacle frame type mounting tool mounted with the image display device; 
         FIG. 9A  is a diagram of a neck band type mounting tool mounted with the image display device, viewed from an oblique front side; 
         FIG. 9B  is a diagram of the neck band type mounting tool in  FIG. 9A , viewed from an oblique rear side; and 
         FIG. 10  is a diagram describing a helmet mounted with the image display device. 
     
    
    
     DETAILED DESCRIPTION 
     According to a conventional image display device, an inappropriate state of a reflection angle of a concave mirror reduces reflected light from a display element and therefore lightens a displayed image (brightness of the displayed image becomes low). Additionally, when the reflection angle deviates from a sweet spot, distortion caused by the concave mirror becomes large. Accordingly, it is necessary to align the concave mirror at an optimum position in consideration of the brightness of the image and the distortion. 
     In the conventional image display device, in order to perform the alignment, a user needs to adjust a tilt angle of a rotary mechanism disposed in the concave mirror so as to orient the reflected light of the display element toward his or her pupil. Additionally, since the concave mirror has a focal length, it is necessary to appropriately adjust a distance between the display element and the concave mirror. The tilt angle and the distance in this case are linked with each other. Hence, when one of the tilt angle and the distance is moved, the other is also moved in a linked manner. Therefore, the user needs to find the optimum position by adjusting the tilt angle and the distance in turns. 
     However, it is extremely complicated and troublesome for the user to simultaneously perform those two adjustments. In particular, it is extremely difficult to align the concave mirror at the optimum position, because a position of the displayed image largely depends on a rotation angle of the concave mirror. 
     The present disclosure provides an image display device requiring no positional adjustment of an optical element (display element) by a user. 
     Hereinafter, exemplary embodiments will be described with reference to accompanying drawings as needed. However, unnecessarily detailed description may be omitted. For example, detailed description of already well-known matters and repetition of substantially the same configuration may be omitted. All of such omissions are intended to facilitate understanding by those skilled in the art by preventing the following description from becoming unnecessarily redundant. 
     Moreover, the inventors provide the accompanying drawings and the following description for those skilled in the art to fully understand the present disclosure, and do not intend to limit the subject matter described in the claims by the accompanying drawings and the following description. 
     First Exemplary Embodiment 
     Hereinafter, a first exemplary embodiment is described with reference to the accompanying drawings. 
     [1-1. Configuration] 
       FIG. 1  is a schematic diagram illustrating a concept of a configuration of a transmission type image display device according to the first exemplary embodiment. Image display device  100  receives an image signal from an external image signal generating device and displays an image according to the input image signal. Image display device  100  includes display element  12 , drive circuit  14 , transmission-type concave mirror  20 , and housing  16 . 
     Display element  12  is a display device configured to generate image light, and is configured with, for example, a liquid crystal display element. Drive circuit  14  is a circuit configured to drive display element  12  based on the image signal from the external image signal generating device (not illustrated). 
     Transmission-type concave mirror (hereinafter, referred to as a “concave mirror”)  20  has optical characteristics which transmit part of external environment light R 3  incident from the outside of image display device  100 , and reflect part of external environment light R 3 . Concave mirror  20  has a lens shape having a concave surface and a convex surface. A concave surface side of the mirror is applied with wavelength selective film  21  having high reflectance to a specific wavelength, and a convex surface side of the mirror is applied with anti-reflection coating film  22 . Concave mirror  20  transmits external environment light R 3  incident from the convex surface side, and reflects image light R 1  sent from display element  12  and incident from the concave surface side. Concave mirror  20  is subjected to aspheric surface processing so as not to cause astigmatism in horizontal and vertical directions. 
     Housing  16  is a member for accommodating display element  12 , drive circuit  14 , and concave mirror  20  therein. Display element  12  and drive circuit  14  enclosed in an enclosure (not illustrated in  FIG. 1 ) are mounted on housing  16  via the enclosure. Concave mirror  20  is mounted on a front opening portion of housing  16 . In the present exemplary embodiment, housing  16  is formed of an opaque material. However, housing  16  may be formed of a transparent material such as acryl resin. 
     Moreover, optical window  25  is disposed on a rear surface of housing  16 . Optical window  25  is a window through which a user visually recognizes an image from display element  12  together with a scene of external environment. To configure optical window  25 , a member which transmits light is mounted on opening portion disposed on the rear surface of housing  16  (not illustrated in  FIG. 1 ). However, optical window  25  may be configured with only the opening portion. Optical window  25  is preferably aligned at a position spaced apart from concave mirror  20  by a focal length of concave mirror  20 . 
     Housing  16  configured as described above retains concave mirror  20  and display element  12  at a predetermined position for obtaining a desired magnification factor and suitable brightness (details will be described later). The user may visually recognize external environment light R 3  transmitted through concave mirror  20  from the convex surface side of concave mirror  20  and image light R 2  which is sent from display element  12  and is reflected by a surface of a concave surface side of concave mirror  20  at the same time, through optical window  25  disposed on the rear surface of housing  16 . Here, the image which is sent from display element  12  and is visually recognized by the user is magnified by concave mirror  20 . 
     [1-2. Operation] 
     An operation of image display device  100  configured as described above will be described below. 
     Image display device  100  receives the image signal from the external image signal generating device (not illustrated) through a HDMI cable and the like. The received image signal is sent to drive circuit  14 . Drive circuit  14  drives display element  12  based on the received image signal. Display element  12  generates image light R 1  composed of R (red), G (green), and B (blue) based on control of drive circuit  14 , and projects the generated image light to concave mirror  20 . Concave mirror  20  magnifies image light R 1  of display element  12  with a magnification determined by a curvature of concave mirror  20 , and sends image light R 2  of display element  12  to the eye of the user through optical window  25 . 
     At the same time, external environment light R 3  incident from the convex surface side of concave mirror  20  is transmitted through concave mirror  20  to be sent to the eye of the user through optical window  25 . 
     Next, an optical function of concave mirror  20  will be described. 
       FIG. 2  is a diagram illustrating a wavelength characteristic of an LED light source included in the display element.  FIG. 3  is a diagram illustrating a reflectance characteristic relative to a wavelength, of the wavelength selective film. In  FIG. 2 , a horizontal axis represents a wavelength (nm), and a vertical axis represents relative energy. In  FIG. 3 , a horizontal axis represents a wavelength (nm), and a vertical axis represents reflectance (%). 
     The concave surface side of concave mirror  20  is applied with wavelength selective film  21 . Wavelength selective film  21  has a characteristic of reflecting only image light R 1  of display element  12 . As the wavelength selective film, a film having multilayer structure in which metal and the like is deposited on a base material of lens is typically known. In such a wavelength selective film, a reflected wave by a certain layer and a reflected wave by another layer, which configure a composite wave, are mutually strengthened or weakened according to a condition of film formation. The composite wave may enhance reflectance of a specific wavelength. A known method for enhancing the reflectance of the specific wavelength is, for example, to control film thickness (contributes to optical path length) and a refractive index (contributes to refraction across film layers). 
     Display element  12  of the present exemplary embodiment includes the LED light source configured to emit each color light of RGB. Wavelength selective film  21  applied on concave mirror  20  has high reflectance for each wavelength of RGB of the LED light source. For example, in the case where each wavelength region corresponding to each color light of RGB has a characteristic illustrated in  FIG. 2 , wavelength selective film  21  having the reflectance characteristic illustrated in  FIG. 3  is applied on concave mirror  20 . In the example in  FIG. 2 , the wavelength regions of RGB are respectively a region having a peak at 660 nm, a region having a peak at 525 nm, and a region having a peak at 470 nm. In the example in  FIG. 3 , wavelength selective film  21  has high reflectance (100%) for each wavelength region of each color light of RGB and has low reflectance (0%) for the other wavelength regions. Moreover, a reflection characteristic of wavelength selective film  21  may be controlled corresponding to a predetermined incident angle, in order to enhance the reflectance by means of the composition of the reflected waves by respective film layers. 
     More specifically, the reflectance of wavelength selective film  21  varies corresponding to an incident angle of incident light.  FIG. 4  is a diagram illustrating angular dependence of reflectance on an incident angle, of wavelength selective film  21  of the present exemplary embodiment. In  FIG. 4 , a horizontal axis represents an incident angle (degree), and a vertical axis represents reflectance. In  FIG. 4 , a horizontal axis represents an incident angle (degree), and a vertical axis represents reflectance (%). As illustrated in  FIG. 4 , the reflectance of wavelength selective film  21  varies in a mountain shape while having a peak at 22.5 degrees. Accordingly, display element  12  and concave mirror  20  are disposed such that image light R 1  from display element  12  enters concave mirror  20  at an incident angle which produces the peak of the reflectance (in the example in  FIG. 4 , 22.5 degrees), or at an incident angle in the vicinity thereof (for example,22.5 degrees±10 degrees). With this configuration, image light R 1  from display element  12  may be efficiently reflected on concave mirror  20 . Image display device  100  of the present exemplary embodiment fixes (retains) display element  12  and concave mirror  20  at a predetermined position in housing  16  such that image light R 1  from display element  12  enters concave mirror  20  at the aforementioned angle. 
     Next, housing  16  will be described. As illustrated in  FIG. 1 , the following description will be made on a state where housing  16  is disposed such that upper surface part  16   a  of housing  16  is equal to a level. That is, a normal direction of upper surface part  16   a  of housing  16  is equal to a vertical direction. 
     Concave mirror  20  is fixed to housing  16  in an inclined manner such that an angle that a plane passing through center (pole) P of concave mirror  20  and contacting with a convex surface side of concave mirror  20  forms with a vertical line becomes θ. 
     Similarly, display element  12  is fixed to housing  16  in an inclined manner such that an angle that a light-emitting surface of display element  12  forms with a vertical line becomes 2θ. With this configuration, display element  12  is disposed such that normal line L 1  passing through a center of the light-emitting surface of display element  12  intersects concave mirror  20  at center P thereof. 
     By placing concave mirror  20  and display element  12  in this manner, image light R 1  projected from the light-emitting surface of display element  12  enters concave mirror  20  at an incident angle θ, and is reflected on concave mirror  20  at an angle of reflection θ in a direction toward optical window  25 . Reflected image light R 2  advances in a horizontal direction (fixed direction) toward optical window  25 , in housing  16 . With this configuration, the user perceives image light R 2  as light incident from the front. The user may obtain a clear and in-focus image by observing at a substantially focal point of concave mirror  20 . 
     Here, a condition of optical window  25  for preventing display element  12  from interrupting a field of vision of the user will be described with reference to  FIG. 1 . 
     By placing concave mirror  20  and display element  12  in the aforementioned manner, both of an angle at which normal line L 1  passing through the center of the light-emitting surface of display element  12  intersects mirror axis L 2  of concave mirror  20  and an angle that a plane including center P of concave mirror  20  and taking mirror axis L 2  of concave mirror  20  as a normal line of the plane forms with a vertical line become θ. Here, in image display device  100 , a distance from upper end Ea of optical window  25  to horizontal line L 0  passing through center P of concave mirror  20  is denoted by H. Horizontal line L 0  is, in other words, a horizontal line which coincides with a light path of light R 2  emitted from the center of the light-emitting surface of display element  12  and reflected at center P of concave mirror  20 . A distance between a horizontal position of center P of concave mirror  20  and a horizontal position of lowermost end Eb of display element  12  is denoted by D. In this case, distance H is set so as to satisfy the following formula. 
         H ≦D ·tan θ  (1)
 
     Distance H affects optical window  25  in size and in position of the upper end thereof. A configuration of optical window  25 , setting distance H so as to satisfy the condition of the above formula (1), allows a part of display element  12  to prevent from appearing in the field of vision of the user. 
     Due to the reflectance characteristic of wavelength selective film  21 , the brightness of reflected light R 2  of the image light from display element  12  varies according to the incident angle θ. Furthermore, a focusing state and the image magnification factor exerted by concave mirror  20  vary according to distance D. That is, the incident angle is determined so as to improve the brightness of the image which is visually recognized by the user at optical window  25  as much as possible. In addition, distance D is determined such that the image from display element  12  is focused in the vicinity of optical window  25 , and such that a desired magnification factor is obtained. Each of concave mirror  20  and display element  12  is fixed to housing  16  at a predetermined position with a predetermined orientation so as to realize the angle θ and distance D, which are determined in the above manner. By fixing concave mirror  20  and display element  12  to housing  16  in such a manner, alignment of concave mirror  20  and display element  12  by the user becomes unnecessary. 
     In this example, since a whole body of housing  16  is formed of the opaque material, distance H is defined as a distance from horizontal line L 0  to upper end Ea of optical window  25 . However, when housing  16  is formed of the transparent material, distance H may be a distance from horizontal line L 0  to lowermost end Eb of display element  12 . 
     As described above, wavelength selective film  21  applied on concave surface side of concave mirror  20  has the characteristic depending on the incident angle. Then, by matching or substantially matching the angle which produces the peak of the reflectance of wavelength selective film  21  and the incident angle θ, it becomes possible for the user to observe external environment light R 3  and image light R 1  (R 2 ) at the same time, in the most suitable state. 
     Moreover, anti-reflection coating film  22  is applied on the convex surface side of concave mirror  20  to prevent reflection of image light R 1  sent from display element  12  on a rear side of the convex surface inside concave mirror  20 . With this configuration, part of image light R 1  which is transmitted through the concave surface of concave mirror  20  is not reflected on the rear side of the convex surface. This prevents visual recognition of unpleasant double images by the user. 
     A reflection characteristic of anti-reflection coating film  22  also has angular dependence. Anti-reflection coating film  22  is also adjusted to have a characteristic in which reflectance thereof becomes lowest at an incident angle θ or an angle in the vicinity of the incident angle θ. This characteristic decreases reflection on the convex surface and therefore allows the user to visually recognize only suitable image light, even when a position of the pupil of the user is somewhat deviated. 
     In the configuration of the present exemplary embodiment, a range of the field of vision in which the user may observe image light R 1  without distortion is a range about±10 degrees from a center of a pupil. Therefore, applying anti-reflection coating film  22  having a good anti-reflection characteristic in the range of θ±10 degrees (the reflectance is low in the range) on concave mirror  20  may reduce the double images in practical use. The range of angle to suppress reflection, which is required for anti-reflection coating film  22 , depends on an angle of view of image light R 1 . 
     [1-3. Concrete Examples] 
       FIGS. 5A to 6B  are diagrams each illustrating an appearance of a concrete example of image display device  100  to which the thought of image display device  100  of the present exemplary embodiment is applied.  FIG. 5A  is a perspective view of image display device  100  viewed from an oblique front side.  FIG. 5B  is a perspective view of image display device  100  viewed from an oblique rear side.  FIG. 6A  is a top view of image display device  100  and  FIG. 6B  is a front view of image display device  100 . In housing  16  of image display device  100  illustrated in  FIGS. 5A to 6B , only upper surface part  16   a  is formed of the opaque material and the other parts are formed of the transparent material. Only upper surface part  16   a  formed of the opaque material may intercept sunlight incident from above. Parts other than upper surface part  16   a  formed of the transparent material may prevent housing  16  from interrupting the field of vision of the user, resulting in that visibility may be improved. At this time, as described above, distance H is defined as the distance from horizontal line L 0  to lowermost end Eb of display element  12 . Alternatively, a whole body of the housing may be formed of the transparent material with a focus on ensuring the field of vision. 
       FIG. 7  is a diagram illustrating an internal configuration of image display device  100  illustrated in  FIGS. 5A to 6B . Enclosure  31  enclosing display element  12  and drive circuit  14  therein is mounted on upper surface part  16   a  of housing  16 . In this structure, display element  12  and concave mirror  20  are also disposed in housing  16  such that display element  12  and concave mirror  20  satisfy the predetermined optical position relation. Settings of the configuration in  FIG. 7  are, for example, θ=20 degrees, D=33 mm, and H=12 mm. 
     [1-4. Mounting Tools of Image Display Device] 
       FIGS. 8 to 10  are diagrams describing configurations for mounting aforementioned image display device  100  on the user.  FIG. 8  is a diagram describing spectacle frame  200  mounted with image display device  100 . Image display device  100  is mounted on a lens portion for one eye in spectacle frame  200 .  FIG. 9A  is a diagram of a neck band type mounting tool  300  mounted with image display device  100  viewed from an oblique front side.  FIG. 9B  is a diagram of neck band type mounting tool  300  in  FIG. 9A  viewed from an oblique rear side. As illustrated in  FIGS. 9A and 9B , neck band type mounting tool  300  is supported by both ears of the user and is used while being mounted so as to surround the back of the head of the user.  FIG. 10  is a diagram describing helmet  400  mounted with image display device  100 . Image display device  100  is mounted on helmet  400  via fixture  410  such that image display device  100  is located in front of one eye of the user when helmet  400  is mounted on the user. 
     [1-5. Effects, etc.] 
     As described above, in image display device  100  of the present exemplary embodiment, concave mirror  20  and display element  12  are fixed to housing  16  at the predetermined positions, such that the emitted light from image display device  100  has the fixed direction. Thereby the direction of the emitted light from image display device  100  is fixed. Thus a user may suitably observe the external environment light and the image light by only placing his or her pupil in front of optical window  25  of image display device  100 , without adjusting the optical arrangement of optical elements such as concave mirror  20  and display element  12 . 
     Specifically, image display device  100  includes display element  12  which projects image light based on the input image signal, concave mirror  20  which transmits external environment light R 3  and reflects image light R 1  from display element  12 , and housing  16  which retains display element  12  and concave mirror  20  in the predetermined position relation and has optical window  25  for allowing external environment light R 3  and image light R 2  to pass therethrough. Concave mirror  20  and display element  12  are mounted on housing  16  in the predetermined position relation in which concave mirror  20  guides reflected light R 2  of the image light to the fixed direction (horizontal direction) toward optical window  25 . The predetermined position relation is set according to the reflectance characteristic of the reflection surface of concave mirror  20 , for example. Since display element  12  and concave mirror  20  are fixed at the suitably arranged positions by housing  16 , a user may observe the in-focus and bright image (the image improved in visibility) in the easy and secure manner, without requiring complicated operations for the user to adjust focus and brightness of the image. 
     Moreover, a configuration satisfying the above condition (H≦D·tan θ) may prevent the display element from appearing in the field of vision of the user. 
     Moreover, respective surfaces of concave mirror  20  may be applied with wavelength selective film  21  and anti-reflection coating film  22  respectively. Image light R 2  from display element  12  may be efficiently sent to the pupil of the user by using wavelength selective film  21 , and further the reflection of the image light inside concave mirror  20  may be reduced by using anti-reflection coating film  22 . As a result, the visibility may be improved. 
     Moreover, in the present exemplary embodiment, the angle difference between the angle producing the highest reflectance in the reflectance characteristic of wavelength selective film  21  (incident angle) and the incident angle of the image light from display element  12  relative to concave mirror  20  is set so as to be in the predetermined range (for example, the range of±10 degrees). Thereby the bright reflected light of the image light may be obtained at the observation position of the user. 
     The image display device of the present exemplary embodiment allows the user to observe image light R 1  from display element  12  and external environment light R 3  in a bright state, at the same time. 
     Other Exemplary Embodiments 
     It is known that a diameter of the human pupil is generally 11 mm to 15 mm. Since it is assumed that a center of the pupil of the user is located on horizontal line L 0 , distance H may be set so as to satisfy the following condition. 
         H&gt; 15 mm/2=7.5 mm   (2)
 
     With respect to the formula of distance H related to optical window  25 , distance H may be determined so as to satisfy the following formula, when a radius of curvature of concave mirror  20  is denoted by r. 
         H ≦( r/ 2)·tan θ  (1b)
 
     Distance H determined in this manner may also prevent display element  12  or housing  16  from appearing in the field of vision of the user. As a result, a suitable field of vision may be secured. 
     In the above exemplary embodiment, concave mirror  20  and display element  12  are arranged so as to allow reflected light R 2  of image light R 1  to advance horizontally, to position the image at the center of the field of vision of the user. However, it is needless to say that the image may be positioned, without degrading image quality, at upper/lower side in the field of vision according to a use, by slightly increasing or decreasing the angles (θ, 2θ) of concave mirror  20  and display element  12 . 
     Thus, the exemplary embodiments are described as examples of the technique in the present disclosure. The accompanying drawings and the detailed description are provided for that purpose. 
     Therefore, the structural elements shown in the accompanying drawings and described in the detailed description may include not only structural elements that are essential for solving the problem but also other structural elements that are not essential for solving the problem in order to exemplify the aforementioned technique. Hence, these non-essential structural elements should not be immediately recognized as being essential based only on the fact that they are shown in the accompanying drawings and described in the detailed description. 
     Furthermore, since the purpose of the aforementioned exemplary embodiments is to give an example of the technique in the present disclosure, various modifications, substitutions, additions, omissions, and the like may be implemented within a scope of the claims and equivalents thereto. 
     The present disclosure is applicable to a head-mounted type image display device. Specifically, the present disclosure is applicable to a head-mounted type display, a head-up display, and the like.