Patent Publication Number: US-2018039065-A1

Title: Linearly disposed eyepiece video display

Description:
TECHNICAL FIELD 
     The present invention relates to an eyepiece video display mounted on a head mounted display (HMD) or the like. To be specific, the video display according to the present invention is an optical device that is installed in front of an observer&#39;s eye and causes the observer to visually recognize an image by guiding image light generated using a reflective liquid crystal display (reflective LCD) to an observer&#39;s pupil. 
     BACKGROUND ART 
     In recent years, a demand for a wearable device, which can be used in the state of being attached to a body of a user, for example, an HMD used in the state of being mounted on a head, has increased. In addition, for example, video displays such as computers, various sensor devices, and LCDs have been also downsized to such an extent of being mountable to wearable devices, and development of wearable devices mounting such devices has rapidly progressed. Such an HMD generally includes a display optical system that emits image light and an eyepiece optical system that guides the image light emitted from the display optical system to the observer&#39;s pupil. 
     Meanwhile, it is known that a transmissive type and a reflective type are used as a liquid crystal display that displays an image, in an image display optical system. The transmissive liquid crystal display is configured such that a light source is provided on a back side of a liquid crystal element, and image light is generated as output light from the light source is transmitted through the liquid crystal element. On the other hand, the reflective liquid crystal display is configured such that a reflection plate is provided on a back side of a liquid crystal element, light is made incident from a front side of the liquid crystal element, and image light is generated as the light transmitted through the liquid crystal element is reflected by the reflection plate. The transmissive liquid crystal display has a demerit that accuracy of an image deteriorates when external light is incident, and is considered to be unsuitable to be mounted to a video display used outdoors such as the HMD. For this reason, the reflective type has recently attracted attention as the liquid crystal display amounted to the HMD (Patent Literature 1 and the like). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2012-168239 A 
     SUMMARY OF INVENTION 
     Technical Problem 
       FIG. 5  is a schematic diagram illustrating a configuration of a conventional HMD using a reflective liquid crystal display as disclosed in Patent Literature 1, for example. As illustrated in  FIG. 5 , the conventional HMD is designed such that a main optical axis direction of light output from a light source and a main optical axis direction of light incident on a prism forming an eyepiece optical system are orthogonal to each other. More specifically, the conventional HMD includes a polarizing beam splitter (PBS), and light including a P-polarized component and an S-polarized component is made incident on the PBS from the light source. Output light from the light source is collected by a condenser lens, and the S-polarized component transmitted through a polarizing plate is reflected by a polarization separation surface of the PBS to progress in an orthogonal direction, and is guided to a reflective liquid crystal (for example, liquid crystal on silicon (LCOS) (registered trademark)). The reflective liquid crystal is controlled by a control circuit (not illustrated), modulates light of the S-polarized component incident from the PBS to generate predetermined image light, and reflects the image light toward the PBS. This image light includes the P-polarized component and the S-polarized component. Thus, when the image light is introduced into the PBS the light of the S-polarized component among the image light is reflected by the PBS, and light of the P-polarized component is transmitted through the PBS. The light of the P-polarized component that has been transmitted through the PBS is guided to the prism forming the eyepiece optical system arranged opposite to the reflective liquid crystal. Accordingly, the image light emitted from a display optical system including the PBS is configured to be guided to an observer&#39;s pupil by the eyepiece optical system including the prism. 
     Meanwhile, since the HMD is worn on the head of the observer and the eyepiece optical system is positioned in front of the observer&#39;s eye, it is necessary to make a configuration of an eyepiece video display slim as a whole. In the eyepiece video display using the reflective liquid crystal, however, the main optical axis direction of the light output from the light source forming the display optical system and the main optical axis direction of the light incident on the prism forming the eyepiece optical system are orthogonal to each other, as illustrated in  FIG. 5 . In such a configuration, it is necessary to arrange the light source and the prism in an orthogonal manner, and thus, there is a problem that it is difficult to make the configuration of the eyepiece video display slim while decreasing a degree of freedom in design of the HMD. 
     Thus, at present, there is a demand for a technique that is capable of configuring the eyepiece video display using a reflective image element (reflective liquid crystal or the like) to be compact and capable of enhancing the degree of freedom in design thereof. 
     Solution to Problem 
     The inventor of the present invention has obtained findings that it is possible to arrange a light source and an eyepiece optical system (prism) on a straight line by reflecting output light from the light source by a polarization separation element to be guided to a reflection section configured of a mirror or the like, introducing the light reflected by the reflection section into a reflective image element to generate image light, and reflecting the image light again by the polarization separation element, as a result of intensive studies on a solution to the problem of the related art. Further, the present inventor has conceived that it is possible to configure an eyepiece video display to be compact using the reflective image element by arranging the light source and the eyepiece optical system on a straight line, and completed the present invention. To be specific, the present invention has the following configurations. 
     A first aspect of the present invention relates to an eyepiece video display mounted on an HMD or the like. 
     The eyepiece video display of the present invention includes a display optical system  1  that emits image light and an eyepiece optical system  2  that guides the image light emitted from the display optical system  1  to an observer&#39;s pupil. 
     Here, the display optical system  1  includes a polarization separation element  10 , a light source  20 , a reflection section  30 , and a reflective image element  40 . 
     The polarization separation element  10  reflects first polarized component light as linearly polarized light and transmits second polarized component light as linearly polarized light having a different polarization plane from the first polarized component light. 
     The light source  20  outputs light to the polarization separation element  10 . 
     The reflection section  30  converts the first polarized component light included in output light from the light source  20  that has been reflected by the polarization separation element  10  into the second polarized component light. In addition, the reflection section  30  reflects this output light to be incident on the polarization separation element  10 . 
     The reflective image element  40  reflects reflection light from the reflection section  30  that has been transmitted through the polarization separation element  10 . In addition, at the same time, the reflective image element  40  converts the reflection light into image light including at least the first polarized component light, and causes this image light to be incident on the polarization separation element  10 . 
     Accordingly, the eyepiece video display of the present invention is configured such that the first polarized component light included in the image light reflected by the polarization separation element  10  is incident on the eyepiece optical system  2 . 
     With the above-described configuration, it is possible to align the eyepiece optical system  2 , the polarization separation element  10 , and the light source  20  on a straight line in the eyepiece video display of the present invention. That is, the eyepiece optical system  2  is positioned in a main optical axis direction of the output light from the light source  20 . Therefore, it is possible to realize a slim configuration in which the eyepiece optical system  2 , the polarization separation element  10 , and the light source  20  are aligned on a straight line, and to enhance a degree of freedom in design of the eyepiece video display and the HMD including the same according to the present invention. 
     In the present invention, it is preferable that the eyepiece optical system  2  further include one or a plurality of polarizing plates  21 . The polarizing plate  21  may be a first polarizing plate  21   a  arranged between the light source  20  and the polarization separation element  10  or may be a second polarizing plate  21   b  arranged between the polarization separation element  10  and the eyepiece optical system  2 . In addition, the eyepiece optical system  2  may include both the first polarizing plate  21   a  and the second polarizing plate  21   b . Further, each of the polarizing plates  21  has a function of transmitting the first polarized component light included in the output light from the light source  20  and blocking the second polarized component light. 
     When the polarizing plate  21  is arranged between the light source  20  and the polarization separation element  10  as in the above-described configuration, the unnecessary second polarized component light that is not reflected by the polarization separation element  10  is removed, and thus, it is possible to prevent unnecessary light from being incident on the eyepiece optical system  2 . 
     In the present invention, it is preferable that the reflection section  30  include a quarter wave plate  31  and a mirror  32 . 
     The quarter wave plate  31  converts the first polarized component light included in the output light from the light source  20 , which has been reflected by the polarization separation element  10 , into circularly polarized light and causes the circularly polarized light to be incident on the mirror  32 . 
     The mirror  32  reflects the circularly polarized light that has passed through the quarter wave plate  31 . 
     Thereafter, the quarter wave plate  31  converts the circularly polarized light reflected by the mirror  32  into the second polarized component light and causes the second polarized component light to be incident on the polarization separation element  10 . 
     When the quarter wave plate  31  and the mirror  32  are used as in the above-described configuration, it is possible to efficiently convert the first polarized component light reflected by the polarization separation element  10  into the second polarized component light that can be transmitted through the polarization separation element  10 . Thus, it is possible to cause the clear image light to be incident on the eyepiece optical system  2 . 
     In the present invention, it is preferable that the mirror  32  be a retroreflective mirror. 
     The retroreflective mirror means a mirror that is capable of reflecting (retroreflection) incident light in an incident direction thereof. The retroreflective mirror is capable of reflecting the incident light directly in the incident direction, which is different from reflection using a typical mirror in which the incident angle and a reflection angle are equal. When the typical mirror is adopted in the configuration of the eyepiece video display according to the present invention, there are a problem that an optical path length in the device becomes long and is hardly downsized and a problem that it is necessary to increase the intensity of the light output from the light source  20  so that a burden is imposed on an illumination system. In contrast, when the retroreflective mirror is adopted as the mirror provided in the reflection section  30  as in the above-described configuration, it is possible to shorten the optical path length in the device as a whole. Accordingly, the burden on the illumination system can be reduced, and thus, it is possible to extend service life of a battery or the like to drive the eyepiece video display. 
     A second aspect of the present invention relates to a head mounted display (HMD) including the eyepiece video display according to the first aspect. Except for the configuration of the eyepiece video display described above, known configurations can be appropriately adopted regarding the other configurations of the head mounted display. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to configure the eyepiece video display using the reflective image element to be compact and to enhance the degree of freedom in design thereof. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an overview of a configuration of an eyepiece video display according to the present invention. 
         FIG. 2  is a block diagram illustrating a polarization state and a progressing direction of light in the eyepiece video display according to the present invention. 
         FIG. 3  is a view obtained by modeling an optical path in the eyepiece video display according to the present invention, and illustrates an example using a typical mirror. 
         FIG. 4  is a view obtained by modeling an optical path in the eyepiece video display according to the present invention, and illustrates an example using a retroreflective mirror. 
         FIG. 5  is a block diagram illustrating an overview of a conventional eyepiece video display mounting a reflective liquid crystal. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The present invention is not limited to the embodiments described below, but includes changes thereto made appropriately by those skilled in the art to the extent obvious. 
       FIG. 1  schematically illustrates a configuration of an eyepiece video display  100  according to an embodiment of the present invention. In addition,  FIG. 2  schematically illustrates a polarization state of light and a progressing direction thereof in the eyepiece video display  100  according to the embodiment of the present invention. As illustrated in  FIGS. 1 and 2 , the eyepiece video display  100  includes a display optical system  1  and an eyepiece optical system  2 . The display optical system  1  includes a light source and an image element such as a liquid crystal display, and generates desired image light to be emitted toward the eyepiece optical system  2 . In addition, the eyepiece optical system  2  includes an optical element such as a prism and guides the image light emitted from the display optical system  1  to a pupil E of an observer. Thus, the eyepiece optical system  2  is arranged in the vicinity of the pupil E of the observer. Accordingly, the observer can visually recognize a virtual image of an image displayed by the display optical system  1 . 
     As illustrated in  FIG. 1 , the display optical system  1  includes a polarization separation element  10 , a light source  20 , a polarizing plate  21  (a first polarizing plate  21   a  and/or a second polarizing plate  21   b ), a condenser lens  22 , a uniformizing element  23 , a reflection section  30 , and a reflective image element  40 . 
     The polarization separation element  10  is an optical element that reflects first polarized component light as linearly polarized light and transmits second polarized component light as linearly polarized light having a different polarization plane from the first polarized component light. In the example illustrated in  FIG. 1 , a polarizing beam splitter (PBS) is used as the polarization separation element  10 . However, a known polarizing element for light separation, such as a wire grid polarizer, can also be used as the polarization separation element  10 . The polarization separation element  10  (PBS) has a structure in which two right angle prisms are bonded to each other, and a bonding face between the right angle prisms is coated with a dielectric multilayer film, a metal thin film, or the like. Therefore, this bonding face functions as a polarization separation surface  11  that transmits or separates light according to its polarization state. In addition, in the example illustrated in  FIG. 1 , the polarization separation surface  11  reflects S-polarized component light at a substantially right angle when the S-polarized component light is incident on this surface, and transmits P-polarized component light when the P-polarized component light is incident thereon as illustrated in  FIG. 2 . However, it is also possible to use a material that reflects the P-polarized component light and transmits the S-polarized component light as the polarization separation surface  11 . Hereinafter, a description will be given by exemplifying a case where the S-polarized component light is a light component (first polarized component light) that is reflected by the polarization separation surface  11 , and the P-polarized component light is a light component (second polarized component light) that is transmitted through the polarization separation surface  11 . 
     The light source  20  outputs light to the polarization separation element  10 . The light source  20  is connected to a control circuit and a power supply (not illustrated), and outputs light according to control of the control circuit. A known light emitting diode (LED) or the like can be used as the light source  20 . The output light from the light source  20  includes at least the S-polarized component light (first polarized component light), and may further include the P-polarized component light (second polarized component light). 
     As illustrated in  FIG. 1 , the first polarizing plate  21   a , the condenser lens  22 , and the uniformizing element  23  are arranged between the polarization separation element  10  and the light source  20 . The output light from the light source  20  is uniformized in illumination or the like by the uniformizing element  23 , and then, guided to the polarization separation element  10  by the condenser lens  22  such as a telecentric lens. In addition, the first polarizing plate  21   a  is arranged between the condenser lens  22  and the polarization separation element  10 . The first polarizing plate  21   a  transmits the S-polarized component light included in the output light from the light source  20  and blocks the P-polarized component light. Accordingly, only the S-polarized component light among the output light from the light source  20  is introduced into the polarization separation element  10 . In addition, the second polarizing plate  21   b  can also be provided between the polarization separation element  10  and the eyepiece optical system  2  as illustrated in  FIG. 1 . The second polarizing plate  21   b  transmits the S-polarized component light and blocks the P-polarized component light, which is similar to the first polarizing plate  21   a . It is possible to prevent unnecessary light from being incident to the display optical system  1  by providing both or any one of the first polarizing plate  21   a  and the second polarizing plate  21   b  in this manner. Known optical elements can be appropriately used as the polarizing plate  21  (the first polarizing plate  21   a  and/or the second polarizing plate  21   b ), the condenser lens  22 , and the uniformizing element  23 . 
     The reflection section  30  has a function of converting the polarization state of the incident light and a function of reflecting the incident light. The reflection section  30  is arranged at a position on which the output light (S-polarized component light) from the light source  20  that has been reflected by the polarization separation surface  11  of the polarization separation element  10  is incident. As illustrated in  FIG. 1 , the reflection section  30  is configured of a quarter wave plate  31  and a mirror  32  in the present embodiment. The quarter wave plate  31  converts the polarization state of the incident light from linearly polarized light into circularly polarized light or from circularly polarized light into linearly polarized light. The quarter wave plate  31  is arranged between the polarization separation element  10  and the mirror  32 . Thus, the quarter wave plate  31  converts the polarization state of the S-polarized component light reflected from the polarization separation element  10  from linearly polarized light into circularly polarized light and converts the circularly polarized light reflected from the mirror  32  into linearly polarized light again. In addition, the quarter wave plate  31  shifts a phase of light to be transmitted again by 90 degrees with respect to a phase of the incident light and outputs the phase-shifted light at the time of re-converting the circularly polarized light reflected from the mirror  32  into the linearly polarized light. That is, when the light incident on the quarter wave plate  31  is the S-polarized component light, the light reflected by the mirror  32  and re-transmitted through the quarter wave plate  31  becomes the P-polarized component light. In this manner, the reflection section  30  configured of the quarter wave plate  31  and the mirror  32  has the function of converting the S-polarized component light (first polarized component light) into the P-polarized component light (second polarized component light). In addition, it is preferable to adopt a retroreflective mirror, which is capable of reflecting (retroreflection) incident light in an incident direction thereof, as the mirror  32 . However, it is also possible to adopt a typical mirror in which an incident angle and a reflection angle are equal as the mirror  32 . A merit of adopting the retroreflective mirror will be described later in detail. 
     The reflective image element  40  is an optical member that reflects incident light and performs predetermined modulation to this incident light (reflected light) to generate image light to enable the observer to visually recognize the light. For example, a known reflective liquid crystal display can be used as the reflective image element  40 . The reflective image element  40  is arranged at a position opposing the reflection section  30  (particularly, the mirror  32 ) with the polarization separation element  10  interposed therebetween. Thus, the light (P-polarized component light), which has been transmitted through the polarization separation element  10  among the reflection light reflected by the reflection section  30 , is incident on the reflective image element  40 . The reflective image element  40  modulates the P-polarized component light to generate the image light including at least the S-polarized component light and reflects this image light toward the polarization separation element  10 . Incidentally, it is enough if the reflective image element  40  includes at least the S-polarized component light (first polarized component light), and the P-polarized component light (second polarized component light) may be included in addition to the S-polarized component light. 
     The image light generated by the reflective image element  40  is incident on the polarization separation element  10 , and the S-polarized component light (first polarized component light) included in the image light is reflected at a substantially right angle at the polarization separation surface  11 , and the P-polarized component light (second polarized component light) is transmitted. The image light of the S-polarized component light reflected by the polarization separation element  10  progresses straight in the air and is incident on the eyepiece optical system  2 . 
     The eyepiece optical system  2  includes a prism  50 . The prism  50  is a light guide member (optical crystal) that guides the image light internally. The prism  50  has, for example, a shape including an entrance surface  51 , a reflective surface  52 , and an exit surface  53  of the image light. Incidentally, the prism  50  may be configured using a single prism or may be configured by combining a plurality of prisms. The entrance surface  51  of the prism  50  is provided in a direction perpendicularly intersecting an optical axis of the image light. In addition, the exit surface  53  is provided so as to oppose the observer&#39;s pupil E. The reflective surface  52  has, for example, a rectangular shape (oblong shape), and functions as a unit to fold the optical path of the image light at a right angle. Specifically, the reflective surface  52  reflects the image light incident on the inside of the prism via the entrance surface  51  at a substantially right angle to be emitted from the exit surface  53 . Accordingly, the image light guided inside the prism  50  of the eyepiece optical system  2  is incident on the observer&#39;s pupil E. 
     Next, an operation of the eyepiece video display  100  according to the present invention will be described with reference to  FIG. 2 . 
     As illustrated in  FIG. 2 , the light output from the light source  20  is incident on the first polarizing plate  21   a  via the uniformizing element  23  and the condenser lens  22 . The first polarizing plate  21   a  transmits only the S-polarized component light (first polarized component light) among the output light from the light source  20  and blocks the P-polarized component light (second polarized component light). The S-polarized component light transmitted through the first polarizing plate  21   a  is incident on the polarization separation element  10 , reflected at a substantially right angle at the polarization separation surface  11 , and guided to the reflection section  30 . In the reflection section  30 , the S-polarized component light is converted into the circularly polarized light at the time of passing through the quarter wave plate  31 , is reflected by the mirror  32  in the same direction as the incident direction thereof, and passes through the quarter wave plate  31  again. At this time, the circularly polarized light reflected by the mirror  32  is converted into the P-polarized component light. The P-polarized component light emitted from the reflection section  30  in this manner passes through the polarization separation element  10  and is incident on the reflective image element  40 . The reflective image element  40  modulates the P-polarized component light to generate the image light including at least the S-polarized component light, and further, reflects this image light toward the polarization separation element  10 . The image light including the S-polarized component light is reflected at a substantially right angle by the polarization separation surface  11  of the polarization separation element  10 , propagates in the air, and is guided to the prism  50  forming the eyepiece optical system  2 . Incidentally, the second polarizing plate  21   b  may be provided between the polarization separation element  10  and the prism  50  instead of the first polarizing plate  21   a  or together with the first polarizing plate  21   a . The P-polarized component light transmitted through the polarization separation element  10  may be blocked by the second polarizing plate  21   b . Further, the prism  50  guides the incident image light to the observer&#39;s pupil E. Accordingly, it is possible to generate the image light by modulating the light output from the light source  20  using the reflective image element  40  and to allow the observer to visually recognize this image light. 
     As illustrated in  FIGS. 1 and 2 , it is possible to arrange the light source  20 , the polarization separation element  10 , and the prism  50  on a straight line in the eyepiece video display  100  of the present invention. That is, the polarization separation element  10  and the prism  50  are positioned in a main optical axis direction of the light output from the light source  20 . Therefore, it is possible to realize a slim configuration in which the light source  20 , the polarization separation element  10 , and the prism  50  are arranged on a straight line, and to enhance a degree of freedom in design of the eyepiece video display  100  and the HMD including the same according to the present invention. 
     Next, the merit of using the retroreflective mirror as the above-described mirror  32  will be described. 
     First,  FIG. 3  illustrates a view obtained by modeling an optical path in the eyepiece video display  100 , and illustrates an example of using the typical mirror. In a typical mirror  32 , an incident angle and a reflection angle of light are equal. When a dispersion width of light propagating inside the device around the reflective image element  40  is given as illustrated in  FIG. 3  in the case of using the typical mirror  32 , the dispersion width of light is widened and light progressing to the outside from the inside of the device also appears. Thus, when the typical mirror  32  is used, the amount of light guided to the eyepiece optical system  2  decreases among the light output from the light source  20 , and there is a problem that an image to be visually recognized by the observer becomes dark. Therefore, it is necessary to increase the intensity of the light output from the light source  20  in order to set brightness of the image, visually recognized by the observer, to be a certain value or more, which imposes a burden on the illumination system. In addition, it is necessary to increase an optical path length of light or to increase the number of lenses for collection of light in order to prevent the light inside the device from leaking to the outside. Then, there is a problem that the number of components of the device increases or the configuration of the device is increased in size in the case of using the typical mirror  32 . 
       FIG. 4  illustrates a model view when the retroreflective mirror is used as the mirror  32 . The retroreflective mirror can reflect (retroreflection) the incident light in the incident direction thereof. As illustrated in  FIG. 4 , the dispersion width of the light propagating inside the device is narrowed to an extent of being fit inside the device in the case of using the retroreflective mirror  32  as compared to the case of using the typical mirror illustrated in  FIG. 3 . That is, it is possible to prevent the light from leaking from the inside of the device to the outside by employing the retroreflective mirror  32 . Accordingly, it is possible to guide substantially the whole amount of the light output from the light source  20  to the reflective image element  40 , and further, it is also possible to guide substantially the whole amount of the image light generated by the reflective image element  40  to the eyepiece optical system  2 . Therefore, when the retroreflective mirror  32  is used, it is possible to set the intensity of the light output from the light source  20  to be lower than that in the case of adopting the typical mirror. Accordingly, it is possible to reduce the burden on the illumination system and to save a battery to drive the eyepiece video display. In addition, it is possible to suppress the dispersion of light by employing the retroreflective mirror  32 , and thus, it is possible to shorten the optical path length of light. In addition, an optical component such as an unnecessary lens becomes unnecessary, and it is possible to simplify the entire configuration of the device. Therefore, it is possible to realize the inexpensive and compact eyepiece video display  100  by employing the retroreflective mirror  32 . 
     The eyepiece video display  100  of the present invention is preferably used as a video display which is mounted on the HMD. Specifically, the HMD has a structure in which the eyepiece optical system  2  of the eyepiece video display  100  is arranged in front of one eye or both eyes of a user in the state of being worn around the user&#39;s head or neck. In addition, various sensors such as a camera, a microphone, a gyro sensor, and an optical sensor can be mounted to the HMD. A known configuration may be appropriately adopted as the configuration of the HMD. For example, it is possible to adopt a configuration of an HMD disclosed in Japanese Patent Application No. 5420793 and Japanese Patent Application No. 5593429. 
     The embodiment of the present invention has been described as above with reference to drawings in the specifications of the present application in order to express the content of the present invention. However, the present invention is not limited to the embodiment described hereinbefore, and encompasses obvious modifications and improvements made by those skilled in the art based on the matters described in the specifications of the present application. 
     INDUSTRIAL APPLICABILITY 
     The present invention relates to the eyepiece video display mounted to the HMD or the like. Thus, the present invention can be suitably used in a wearable device manufacturing industry. 
     REFERENCE SIGNS LIST 
     
         
           1  Display optical system 
           2  Eyepiece optical system 
           10  Polarization separation element 
           11  Polarization separation surface 
           20  Light source 
           21  Polarizing plate 
           22  Condenser lens 
           23  Uniformizing element 
           30  Reflection section 
           31  Quarter wave plate 
           32  Mirror 
           40  Reflective image element 
           50  Prism 
           51  Entrance surface 
           52  Reflective surface 
           53  Exit surface 
           100  Eyepiece video display