Patent ID: 12235468

DETAILED DESCRIPTION

There has been proposed a virtual image display device that is configured to display a virtual image which is formed by a display light through reflection of the display light at a translucent member and is visually recognizable. This device includes a polarization adjustment member that adjusts polarization of at least one of the display light and an external light which travel along an optical path of an imaging optical system used for displaying the virtual image. In the polarization adjustment member, a polarizing film, which serves as a polarizing plate, is held by a translucent substrate which serves as a support plate.

The polarizing plate is often manufactured through a manufacturing process including a stretching treatment or the like to add a polarization property to the polarizing plate. Therefore, the polarizing plate is installed in the optical path of the imaging optical system used for displaying the virtual image while the polarizing plate is in the form of the polarization adjustment member bonded to the support plate in a state where a residual stress is left in the polarizing plate.

An external light, such as a sunlight transmitted through the translucent member may enter the polarization adjustment member when the external light travels along the optical path. When the polarization adjustment member gets hot due to the incident of the external light, the polarization adjustment member may possibly be deformed unintentionally due to the effect of relieving the residual stress from the polarization adjustment member in response to the temperature change. When deformation, such as warp, occurs in the polarization adjustment member, there is a concern that an expected optical action, which is supposed to occur in the non-deformed state of the polarization adjustment member, cannot be exhibited.

According to one aspect of the present disclosure, there is provided a virtual image display device configured to display a virtual image that is formed by a display light through reflection of the display light at a translucent member and is visually recognizable, the virtual image display device comprising:a projector that is configured to project the display light; anda polarization adjustment member that is located along an optical path, which extends from the projector to the translucent member, while the polarization adjustment member is configured to adjust polarization of at least one of the display light and an external light which travel along the optical path, wherein:the polarization adjustment member includes:a polarizing plate that is shaped in a thin plate form and has a property of blocking a light which is oscillated in an axial direction of a block axis of the polarizing plate, wherein the polarizing plate is in a state where a residual stress, a degree of which is maximized in a first residual stress direction along a plane of the polarizing plate, is left in the polarizing plate; anda support plate that is shaped in a plate form and supports the polarizing plate, wherein the support plate is in a state where a residual stress, a degree of which is maximized in a second residual stress direction along a plane of the support plate, is left in the support plate; andthe polarizing plate and the support plate are bonded together such that the first residual stress direction is along the second residual stress direction.

Furthermore, according to another aspect of the present disclosure, there is provided a polarization adjustment member configured to be placed along an optical path of an imaging optical system used for displaying a virtual image, the polarization adjustment member comprising:

a polarizing plate that is shaped in a thin plate form and has a property of blocking a light which is oscillated in an axial direction of a block axis of the polarizing plate, wherein the polarizing plate is in a state where a residual stress, a degree of which is maximized in a first residual stress direction along a plane of the polarizing plate, is left in the polarizing plate; and

a support plate that is shaped in a plate form and supports the polarizing plate, wherein the support plate is in a state where a residual stress, a degree of which is maximized in a second residual stress direction along a plane of the support plate, is left in the support plate wherein:

the polarizing plate and the support plate are bonded together such that the first residual stress direction is along the second residual stress direction.

According to these aspects, at the polarizing plate and the support plate, which are bonded together, the first residual stress direction of the polarizing plate is along the second residual stress direction of the support plate. That is, the direction, in which the degree of the residual stress left in the polarizing plate is maximized, and the direction, in which the degree of the residual stress left in the support plate is maximized, coincide with each other. Thus, even if the residual stress of the polarizing plate and the residual stress of the support plate are relieved in response to the temperature change of the polarization adjustment member due to the incident of the external light on the polarization adjustment member, it is possible to limit an occurrence of deformation of the polarizing plate and deformation of the support plate in different directions, respectively. In other words, the occurrence of the deformation, such as unintentional warpage, is limited.

For example, since the complicated deformation of the polarization adjustment member is limited, an occurrence of local unevenness of the optical action of the polarization adjustment member and also an occurrence of local unevenness (for example, luminance unevenness) in the display quality of the virtual image can be limited. Furthermore, for example, the undeformed state of the polarization adjustment member can be easily maintained if the minimum deformation countermeasure corresponding to the directions, which coincide with each other, is taken. As described above, it is possible to stably exert the optical action in the display of the virtual image.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. By assigning the same reference signs to the corresponding components in each embodiment, redundant description thereof may be omitted. When only a part of a structure is described in the following respective embodiments, the description of the preceding embodiment(s) may be applied to the rest of the structure. Further, besides a combination(s) of components specified in the description of each embodiment, components of the embodiments may be partially combined even if such a combination is not specified as long as there is no problem with respect to such a combination.

First Embodiment

As shown inFIG.1, a virtual image display device according to a first embodiment of the present disclosure is configured to be mounted on an automobile1(serving as a vehicle) and is formed as a head-up display (hereinafter HUD)10received in an instrument panel2of the automobile1. Here, the vehicle is broadly understood to include any one of various vehicles, such as automobiles, railroad vehicles, aircrafts, ships, and non-moving housings for a game.

The HUD10provides a display light of an image that is projected toward a windshield3of the automobile1. As a result, the HUD10displays the image as a virtual image VRI that is visually recognizable by an occupant (e.g., a driver) serving as a viewer. That is, when the display light, which is reflected by the windshield3, reaches a viewable range EB set in a vehicle cabin, the occupant whose eye point EP is located in the viewable range EB perceives the light of the image. The occupant can recognize various information through a display content that is superimposed on the scenery outside the vehicle.

Examples of the display contents may include contents for displaying the state of the automobile1such as a vehicle speed and the remaining fuel amount, contents for displaying navigation information, such as visibility assist information and road information, and the like.

In the following, unless otherwise specified, the directions indicated by front (a front side Ze inFIG.1), rear (a rear side Go inFIG.1), up (an upper side Ue inFIG.1), down (a lower side Si inFIG.1), left and right are indicated with reference to the automobile1placed on the horizontal plane HP.

The windshield3is a translucent member that is shaped in a plate form having translucency and is made of, for example, glass or synthetic resin. The windshield3is located on the upper side Ue of the instrument panel2. The windshield3is tilted such that the windshield3is further spaced away from the instrument panel2toward the upper side Ue as a location along the windshield3shifts from the front side Ze toward the rear side Go. The windshield3has a reflecting surface3athat reflects the display light and is shaped in a smooth concave or planar form. The reflecting surface3aof the present embodiment is configured to reflect the display light by surface reflection.

It should be noted that the windshield3may have a reflective holographic optical element to reflect the display light toward the viewable range EB by diffraction reflection induced by interference fringes instead of the surface reflection. Furthermore, instead of the windshield3, for example, a combiner, which is formed separately from the automobile1, may be installed in the vehicle cabin such that the combiner reflects the display light toward the viewable range EB.

The viewable range EB is a spatial range in which the virtual image VRI displayed by the HUD10can be visually recognized so as to satisfy a predetermined standard (for example, to implement the entire virtual image VRI having a predetermined brightness or higher) and is also referred to as an eyebox. The viewable range EB is typically set so as to overlap with the eyelips set in the automobile1. The eyelips is set for each of the eyes and is set as an ellipsoidal virtual range based on an eye range that statistically represents a spatial distribution of the occupant's eye point EP.

A specific structure of such an HUD10will be described below. The HUD10includes a housing11, a light guide unit21and a projector unit30.

The housing11is made of, for example, a synthetic resin or metal and is shaped in a hollow form to receive the light guide unit21and the projector unit30. The housing11is placed at an inside of the instrument panel2. The housing11has a window portion12that optically opens at an upper surface portion of the housing11which is opposed to the windshield3in a top-to-bottom direction. The window portion12may be physically opened or may be covered with a dustproof sheet13which can transmit the display light therethrough.

The light guide unit21is an optical system that guides the display light, which is outputted from the projector unit30, toward the windshield3. The light guide unit21may have any one of various configurations, such as a configuration, which includes a single concave mirror, a configuration, which includes a combination of a single plane mirror and a single concave mirror, or a configuration of a single convex mirror and a single concave mirror. Here, it is preferable that the light guide unit21has a function of magnifying the virtual image VRI visually recognized by the occupant with respect to the image formed by the projector unit30.

As shown inFIGS.2and3, the projector unit30is a unit in which a projector31and a polarization adjustment member41are integrated by using a common case51. The projector31projects the display light of the image that can be imaged as the virtual image VRI in the air. The projector31of the present embodiment is a liquid crystal projector that includes a liquid crystal panel32, which serves as an image display panel, and a backlight36.

The liquid crystal panel32is a transmissive TFT liquid crystal panel of an active-matrix type that uses a thin film transistor (TFT) and forms a plurality of pixels arranged in, for example, a two-dimensional array. The liquid crystal panel32is shaped in a plate form that has a rectangular contour with a longitudinal direction LD and a transverse direction SD. A flexible cable33for electrically connecting the liquid crystal panel32to a drive circuit is connected to a long side of the liquid crystal panel32. An illumination subject surface32aof the liquid crystal panel32is illuminated by the backlight36. The drive circuit controls a transmittance of an illuminating light of each of the pixels of the liquid crystal panel32such that the liquid crystal panel32displays a real image on a display screen32bthat is on a side opposite to the illumination subject surface32a. The liquid crystal panel32forms a rectangular frame32c, which surrounds an entire perimeter of the display screen32band the illumination subject surface32a, so that a size of the liquid crystal panel32is one size larger than the display screen32band the illumination subject surface32a.

The liquid crystal panel32has a pair of polarizing plates and a liquid crystal layer which are stacked such that the liquid crystal layer is held between the polarizing plates. Each of the polarizing plates has a transmission axis and a block axis which are perpendicular to each other to implement a property of transmitting a light oscillated in the axial direction of the transmission axis and absorbing a light oscillated in an axial direction of the block axis. The pair of polarizing plates are arranged such that the transmission axes of the pair of polarizing plates are perpendicular to each other. By applying a voltage for each of the liquid crystal pixels, the liquid crystal layer can rotate the polarization direction of the light which is transmitted through the liquid crystal layer according to the applied voltage. In this way, the liquid crystal panel32can change a ratio of the light transmitted through the polarizing plate placed on the display screen side, i.e., the transmittance, for each of the liquid crystal pixels by rotating the polarization direction. Color filters of different colors (e.g., red, green and blue) are provided to corresponding adjacent ones of the liquid crystal pixels, and various colors can be reproduced by combining these.

By setting the transmission axis TAd of the polarizing plate placed on the display screen32bside such that the transmission axis TAd extends along the longitudinal direction LD or the transverse direction SD of the liquid crystal panel32, the polarization direction of the display light outputted from the liquid crystal panel32is set to be along the longitudinal direction LD or the transverse direction SD of the liquid crystal panel32. Particularly, in the present embodiment, the polarization direction of the display light is set to be along the longitudinal direction LD of the liquid crystal panel32.

The backlight36shown inFIG.2is configured to illuminate an entire area of the illumination subject surface32aof the liquid crystal panel32. The backlight36includes a light source unit37and a light collecting unit38. The light source unit37includes a plurality of light source elements37cwhich are arranged on a surface (hereinafter referred to as a light source arrangement surface37b) of a light source circuit board37a. The light source circuit board37ais a rigid circuit board that is shaped in a flat plate form and is formed by using a synthetic resin, such as glass epoxy resin, as a base material. The light source circuit board37ais arranged in such a posture that the light source arrangement surface37bis tilted with respect to the display screen32band the illumination subject surface32aof the liquid crystal panel32at a tilt angle α of, for example, about 10 to 15 degrees. A wiring pattern is formed on the light source arrangement surface37b, so that each of the light source elements37cis connected to a power supply through the wiring pattern.

The plurality of light source elements37care arranged in a one-dimensional array or a two-dimensional array on the planar light source arrangement surface37b. For example, a light emitting diode element, which serves as a point light source, is used in each of the light source elements37c. The light source element37cis formed by sealing a chip-shaped blue light emitting diode with a yellow phosphor in which a yellow fluorescent agent is mixed with a translucent synthetic resin. The yellow phosphor is excited by a blue light emitted from the blue light emitting diode according to the amount of electric current applied thereto, and thereby a yellow light is emitted. As a result of mixing of the blue light and the yellow light, a white (more specifically, pseudo-white) illumination light is emitted from the light source element37ctoward the image display panel. Here, an intensity peak direction, in which a degree of an emission intensity of the illuminating light is maximized, at each light source element37cis a direction that is substantially perpendicular to the light source arrangement surface37b, i.e., a direction along a normal direction which is normal to the light source arrangement surface37b.

The light collecting unit38collects the illumination light, which is emitted from each of the light source elements37c, without substantially changing the intensity peak direction to enhance the directivity of the illumination light, and then the light collecting unit38irradiates the collected illuminating light on the illumination subject surface32aof the liquid crystal panel32. The light collecting unit38includes a convex lens array38aand a convex lens38b. The convex lens array38ais formed by a plurality of convex lens elements which are arranged according to the arrangement of the light source elements37c. The convex lens38bhas a single optical surface.

In this way, the backlight36does not apply the illumination light from the direction perpendicular to the illumination subject surface32aof the liquid crystal panel32but applies the illumination light from an oblique direction that is set based on the above-mentioned tilt angle α. When the liquid crystal panel32linearly transmits the illumination light, the display light, which is outputted from the display screen32b, is also outputted in an oblique direction with respect to the display screen32bbased on the above-mentioned tilt angle α.

According to the present embodiment, in a cross section (i.e., a cross section shown inFIG.2) of the rectangular liquid crystal panel32, which is taken along the transverse direction SD, the liquid crystal panel32is tilted relative to the light source arrangement surface37bat the tilt angle α described above. On the other hand, in the cross section of the liquid crystal panel32, which is taken in the longitudinal direction LD, the tilt of the liquid crystal panel32is more limited than on the cross section which is taken in the transverse direction SD. For example, the liquid crystal panel32is arranged such that the liquid crystal panel32is substantially parallel to the light source arrangement surface37b. That is, the liquid crystal panel32is tilted from a parallel posture of the liquid crystal panel32, which is parallel to the light source arrangement surface37b, to the tilted posture by rotating the liquid crystal panel32about a tilt reference axis TRA, which is parallel to the longitudinal direction LD, by the above-mentioned tilt angle α.

The display light and an external light may travel forward or backward along an optical path, which extends from the projector31to the windshield3. As described above, the display light, which is projected from the projector31, travels to the windshield3through the light guide unit21and the window portion12. On the other hand, the external light, which is, for example, the sunlight applied downward from the upper side Ue to the windshield3located on the lower side Si, passes through the windshield3and travels backward along the optical path. When the external light, which travels backward along the optical path, reaches the display screen32bof the liquid crystal panel32, a part of its energy may be converted into heat. When the liquid crystal panel32receives the considerable amount of heat damage in this way, it becomes difficult to maintain the display quality of the image displayed on the display screen32b.

The external light, which is reflected on the display screen32b, is separated from the optical path due to the tilted posture of the liquid crystal panel32described above. The external light, which is reflected on the display screen32b, can be relatively easily separated from the optical path because the external light is shifted in the transverse direction SD, in which a light bundle width is smaller with respect to the display light.

The polarization adjustment member41is arranged particularly between the projector31and the light guide unit21along the optical path, which extends from the projector31to the windshield3, such that the polarization adjustment member41forms an imaging optical system of the HUD10. The polarization adjustment member41is shaped in a flat plate form and is arranged substantially parallel to the liquid crystal panel32. That is, the polarization adjustment member41is rotated from a parallel posture, which is parallel to the light source arrangement surface37b, to a tilted posture about a tilt reference axis of the polarization adjustment member41, which is parallel to the tilt reference axis TRA of the liquid crystal panel32, by the same tilt angle α as the liquid crystal panel32.

The polarization adjustment member41is formed to be one size larger than the liquid crystal panel32. However, an outer peripheral contour of the polarization adjustment member41forms four recesses41aand four bulges41b,41c. The four recesses41aare formed by inwardly recessing four corner portions of the outer peripheral contour of the polarization adjustment member41relative to an outer peripheral contour of the liquid crystal panel32. The bulges41b,41care formed by outwardly bulging two long sides and two short sides of the outer peripheral contour of the polarization adjustment member41relative to the four recesses41a. Among the four bulges41b,41c, a pair of bulges41b, between which a center of the polarization adjustment member41is interposed in the longitudinal direction LD, are outwardly bulged relative to the outer peripheral contour of the liquid crystal panel32. On the other hand, among the four bulges41b,41c, each of a pair of bulges41c, between which the center of the polarization adjustment member41is interposed in the transverse direction SD, is bulged to a position that is the same as the position of the outer peripheral contour of the liquid crystal panel32.

A shape of the outer peripheral contour of the polarization adjustment member41is asymmetric about an imaginary reference line VRL1(seeFIG.7) that bisects a surface area of the polarization adjustment member41in the longitudinal direction LD of the liquid crystal panel32. Similarly, the shape of the outer peripheral contour of the polarization adjustment member41is asymmetric about an imaginary reference line VRL2(seeFIG.7) that bisects the surface area of the polarization adjustment member41in the transverse direction SD of the liquid crystal panel32.

As shown inFIG.4, the polarization adjustment member41is formed in a state where a polarizing plate42and a support plate43are bonded together. The polarizing plate42has a property of transmitting the light, which is oscillated in a transmission direction along the transmission axis TAp (i.e., an axial direction of the transmission axis TAp), and blocking the light oscillated in a block direction along the block axis SAp (i.e., an axial direction of the block axis SAp). That is, the polarizing plate42has a so-called polarization property.

The polarization adjustment member41is configured to adjust the polarization of at least one of the display light and the external light which travel along the optical path. In this embodiment, the polarization of the external light is mainly adjusted. Specifically, the polarization adjustment member41has a function of inputting only the polarized light, which is oscillated in the transmission direction, onto the liquid crystal panel32among the external light by blocking the light oscillated in the block direction among the external light. That is, the polarization adjustment member41reduces the amount of the external light, which is reached to the liquid crystal panel32and is absorbed by the liquid crystal panel32, to limit a heat damage to the liquid crystal panel32. In the present embodiment, the polarization of the display light outputted from the liquid crystal panel32is not substantially changed by the polarization adjustment member41, but the polarization adjustment member41exhibits the polarization adjusting function in the sense of maintaining the polarization of the liquid crystal panel32.

The polarizing plate42of the present embodiment is an absorption type polarizing plate that is shaped in a thin plate form (film form) and has a multi-layered structure, in which a polarizing layer42ais clamped between a pair of protective layers42b. The respective layers are bonded by a bonding agent that is mainly composed of PVA (polyvinyl alcohol) resin.

The polarizing layer42ais formed by adding iodine, which is a dichroic dye, to, for example, a PVA film made of a PVA resin and then stretching the PVA film. By this stretching treatment, the iodine molecules are oriented in a direction that corresponds to the stretching direction of the PVA film. Therefore, the polarization property can be added to the PVA film such that the stretching direction corresponds to the block axis SAp, and a direction, which is perpendicular to the stretching direction, corresponds to the transmission axis TAp. That is, the polarizing plate42of the present embodiment is an iodine-based polarizing plate, and the block axis SAp of the polarizing plate42is an absorption axis for absorbing the light oscillated in the block direction.

The protective layer42bis formed, for example, by a TAC film made of a TAC (triacetyl cellulose) resin, a PET film made of a PET (polyethylene terephthalate) resin, or a stack of these films. Such a protective layer42bmay be accompanied by a hard coat, an antiglare coat, an antireflection coat, an antistatic coat, or a coat having a combination of these functions.

The polarizing plate42, particularly the polarizing layer42aof the polarizing plate42is in a state where a residual stress, a degree of which is maximized in the direction along the stretching direction, is left in the polarizing plate42(particularly the polarizing layer42aof the polarizing plate42) due to the stretching treatment. This direction, in which the maximum residual stress is exerted in the polarizing plate42, will be defined as a first residual stress direction RSD1in the present embodiment. The residual stress has uniformity in each portion of the polarizing plate42.

The support plate43is shaped in a flat plate form and has translucency. The support plate43is mainly made of a synthetic resin having excellent processability, such as PMMA (polymethyl methacrylate) resin or PC (polycarbonate) resin. For example, the support plate43is formed by extruding a resin raw material from a mold in the extruding direction to form an extruded product having a plate-shaped cross section (extrusion molding), and then punching out individual pieces from the extruded product. Alternatively, for example, the support plate43is formed by rolling a resin raw material in a rolling direction with a roller to form a large flat plate (rolling process), and then punching out individual pieces from this flat plate.

Such a support plate43is in a state where a residual stress, a degree of which is maximized in the direction along the extruding direction or the rolling direction, is left in the support plate43due to the extrusion molding treatment or the rolling treatment. The direction, which is along a plane of the support plate43and in which the maximum residual stress is left in the support plate43, will be defined as a second residual stress direction RSD2in the present embodiment. The residual stress has uniformity in each portion of the support plate43.

Due to the extrusion molding treatment or the rolling treatment, the support plate43substantially functions as a retardation plate. A fast axis of the support plate43is substantially one of the second residual stress direction RSD2and the perpendicular direction, which is perpendicular to the second residual stress direction RSD2, among the directions along the plane of the support plate43, and a slow axis of the support plate43is substantially the other one of the second residual stress direction RSD2and the perpendicular direction, which is perpendicular to the second residual stress direction RSD2.

Then, the polarizing plate42and the support plate43are bonded together such that the first residual stress direction RSD1is along the second residual stress direction RSD2, i.e., the first residual stress direction RSD1and the second residual stress direction RSD2coincide with each other. By doing so, even if the temperature of the polarization adjustment member41changes from a low temperature to a high temperature due to the incident of the external light, the effect of relieving the residual stress on the polarizing plate42and the effect of relieving the residual stress on the support plate43can be generated in the same direction as each other. That is, a pattern of deforming the polarization adjustment member41in response to the relieving of the residual stress is controlled to be a pattern that corresponds to the first residual stress direction RSD1and the second residual stress direction RSD2which coincide with each other.

Furthermore, since the fast axis and the slow axis of the support plate43coincide with the transmission axis TAp and the block axis SAp of the polarizing plate42, respectively, polarization rotation on the support plate43is unlikely to occur. That is, an influence of a retardation value of the support plate43on the polarization adjusting function is limited. It is possible to simplify the manufacturing operation for reducing the manufacturing error of the retardation value of the support plate43.

Furthermore, in the present embodiment, the first residual stress direction RSD1and the second residual stress direction RSD2of the polarization adjustment member41are aligned in the direction parallel to the tilt reference axis TRA of the liquid crystal panel32. In this way, the tilted posture of the liquid crystal panel32and the direction of the pattern, in which the polarization adjustment member41is about to be deformed, are harmonized.

Further, the support plate43preferably satisfies the following performances. Specifically, the support plate43preferably has a transmittance of 85% or more. It is possible to limit attenuation of the display light when the display light passes through the polarization adjustment member41. A thickness of the support plate43is preferably 10 times or more a thickness of the polarizing plate42. When the residual stress of the thin plate-shaped polarizing plate42is relieved, it is possible to limit the support plate43from being integrally warped together with the polarizing plate42in response to an exertion of a contraction force of the polarizing plate42. A glass transition temperature of the support plate43is preferably 105 degrees Celsius or higher. Since a temperature increase inside the HUD10installed at the automobile1is theoretically about 105 degrees Celsius at the maximum, the strength of the support plate43can be maintained to the extent that the support plate43does not warp. A coefficient of linear expansion of the support plate43is preferably 5.0 to 7.0×10−5/K. By bringing the linear expansion coefficient of the polarizing plate42and the linear expansion coefficient of the support plate43close to each other, it is possible to limit an occurrence of warpage of the polarization adjustment member41.

The case51holds both the projector31and the polarization adjustment member41such that a relative positional relationship between the projector31and the polarization adjustment member41can be maintained. The case51includes a backlight receiver52, a cover59and a plurality of cushion materials67,68.

The backlight receiver52is shaped in a substantially rectangular tubular form and is made of a synthetic resin having a light-shielding property. The backlight receiver52receives the backlight36at an inside of a tubular hole56of the backlight receiver52. An end portion of a tubular main body53of the backlight receiver52, which is opposite to the cover59, is an opening53ahaving an opening plane formed substantially parallel to the light source circuit board37a(and the light source arrangement surface37b). The opening53ais closed by a heat sink69that is made of metal having a thermal conductivity which is higher than that of the backlight receiver52. By placing a back surface of the light source circuit board37ain close contact with the heat sink69, the heat, which is generated by the light source circuit board37a, can be released to the outside of the case51that is spaced away from the liquid crystal panel32and the polarization adjustment member41.

The backlight receiver52has a plurality of flanges54, each of which is shaped in a plate form and laterally projects from the tubular main body53. Each of the flanges54has a plurality of screw holes for fastening the backlight receiver52to the housing11. Each of the flanges54is provided with a plurality of reinforcing ribs55for increasing a strength of the flange54such that the reinforcing ribs55connect between the tubular main body53and the flange54.

An end portion of the tubular main body53, which is located on the cover59side, is an opening53bwhile an opening plane of the opening53bis tilted relative to the light source arrangement surface37bin conformity with the tilt angle α of the liquid crystal panel32and is substantially parallel to the liquid crystal panel32. The tubular hole56of the tubular main body53may be closed by a diffusion plate39that is placed adjacent to the opening53b.

As shown inFIG.5, the opening53bis stepped to have an increasing size that is increased in a stepwise manner toward the polarization adjustment member41from a size, which corresponds to a size of the light collecting unit38at the tubular hole56, to sizes, which respectively correspond to a size of the liquid crystal panel32and a size of the polarization adjustment member41. As a result, the opening53bhas: a pair of outer peripheral side pedestal surfaces53c, which are located on an outer peripheral side and the polarization adjustment member41side; and a pair of inner peripheral side pedestal surfaces53d, which are located on an inner peripheral side and the light source arrangement surface37bside. The pair of outer peripheral side pedestal surfaces53care respectively located on two sides of the liquid crystal panel32, which are opposite to each other in the longitudinal direction LD, and the pair of inner peripheral side pedestal surfaces53dare also respectively located on the two sides of the liquid crystal panel32, which are opposite to each other in the longitudinal direction LD. Each of the outer peripheral side pedestal surfaces53cand the inner peripheral side pedestal surfaces53dis shaped in an elongated form that is elongated along the opening plane (i.e., elongated in the transverse direction SD). Each of the outer peripheral side pedestal surfaces53cand the inner peripheral side pedestal surfaces53dis in a form of a flat surface.

The liquid crystal panel32is placed in a stepped space SS, which is formed between the outer peripheral side pedestal surfaces53cand also between the inner peripheral side pedestal surfaces53d, and the liquid crystal panel32is clamped between the inner peripheral side pedestal surfaces53dand the cover59. The polarization adjustment member41is placed in a cover space CS, which is formed between the outer peripheral side pedestal surfaces53cand the cover59, and the polarization adjustment member41is clamped between the outer peripheral side pedestal surfaces53cand the cover59.

Furthermore, as shown inFIG.3, the backlight receiver52has a pair of engaging hole portions57. The engaging hole portions57are transversely bulged from the opening53band hold the liquid crystal panel32from two opposite sides of the liquid crystal panel32which are opposite to each other in the transverse direction SD. Each of the engaging hole portions57has two engaging holes57awhich are spaced from each other by a predetermined interval in the longitudinal direction LD of the liquid crystal panel32. Each of the engaging holes57ais configured to be engaged with the cover59.

The cover59is made of, for example, a synthetic resin having a light-shielding property. The cover59covers the liquid crystal panel32, which is placed on the inner peripheral side pedestal surfaces53d, and the polarization adjustment member41, which is placed on the outer peripheral side pedestal surface53c, while the cover59is configured to output the display light to the outside of the projector unit30.

A cover main body60of the cover59is shaped in a rectangular frame plate form that surrounds the rectangular opening window60athat matches the size of the display screen32b. The cover main body60is tilted relative to the light source arrangement surface37bin conformity with the tilt angle α of the liquid crystal panel32and the polarization adjustment member41. In other words, the cover main body60is arranged substantially parallel to the inner peripheral side pedestal surfaces53d, the liquid crystal panel32, the outer peripheral side pedestal surfaces53cand the polarization adjustment member41. The cover main body60has: an exposed portion61, which is exposed to the outside of the projector unit30; and an opposing portion62, which is located on a side opposite to the exposed portion61and is opposed to the polarization adjustment member41and the liquid crystal panel32.

The cover59has a pair of engaging claw portions65which are laterally bulged from the cover main body60and are opposed to each other in the transverse direction SD to clamp the liquid crystal panel32therebetween in the transverse direction SD. Each of the engaging claw portions65has two engaging claws65awhich are spaced from each other by a predetermined interval in the longitudinal direction LD of the liquid crystal panel32. Each of the engaging claws65aprojects toward the backlight receiver52in a normal direction, which is normal to the light source arrangement surface37b, and each of the engaging claws65ais engaged with a corresponding one of the engaging holes57aby snap-fit. The backlight receiver52and the cover59are coupled with each other such that a cover space CS of a predetermined size is formed between the backlight receiver52and the cover59.

Hereinafter, the clamping of the polarization adjustment member41and the liquid crystal panel32will be described in detail with reference toFIGS.6to8. Among the four bulges41b,41cof the polarization adjustment member41, the pair of bulges41b, between which the center of the polarization adjustment member41is interposed in the longitudinal direction LD, contact the outer peripheral side pedestal surfaces53cthrough the cushion materials68, respectively, on the backlight receiver52side. Each of the cushion materials68is realized by, for example, an elastically deformable elastomer, an urethane sponge or the like and is configured to absorb vibrations and an assembly error of the automobile1.

The bulges41bof the polarization adjustment member41contact a pair of contact surfaces62awhich are respectively shaped in a flat surface form while the pair of contact surfaces62aare formed at the opposing portion62such that the opening window60ais interposed between the contact surfaces62ain the longitudinal direction LD. Among the four bulges41b,41cof the polarization adjustment member41, the pair of bulges41c, between which the center of the polarization adjustment member41is interposed in the transverse direction SD, contact a pair of elongated hold structures63of the opposing portion62on the cover59side.

Each of the elongated hold structures63has a contact portion63a. The contact portion63ais in contact with the polarization adjustment member41and is elongated in the longitudinal direction LD of the liquid crystal panel32, i.e., a direction which is substantially perpendicular to the first residual stress direction RSD1and the second residual stress direction RSD2. Each of the elongated hold structures63of the first embodiment is realized by an elongated double-sided adhesive tape fixed to the cover59. Furthermore, the contact portion63aof each of the elongated hold structures63is adhered to a corresponding one of the bulges41cof the polarization adjustment member41.

In the liquid crystal panel32, the rectangular frame32ccontacts each of the inner peripheral side pedestal surfaces53dthrough a corresponding one of the cushion materials67on the backlight receiver52side. In the liquid crystal panel32, four corner portions of the rectangular frame32ccontact four projecting hold structures64, respectively, of the opposing portion62on the cover59side.

Each of the projecting hold structures64is made of the synthetic resin that is integrated with the cover59in one-piece. Each of the projecting hold structures64is formed at a location which corresponds to the corresponding one of the four recesses41aof the polarization adjustment member41and is deviated from the corresponding recess41atoward the outer peripheral side of the cover59. Furthermore, each of the projecting hold structures64projects to a position, which is closer to the corresponding one of the inner peripheral side pedestal surfaces53dthan the corresponding one of the contact surfaces62aof the opposing portion62, to hold the liquid crystal panel32. Also, each of the projecting hold structures64includes a guide portion64aand a hemispherical protrusion64b. The guide portion64aprojects to a position closer to the backlight receiver52than the contact surfaces62aand the elongated hold structures63, such that the guide portion64aextends along an outer peripheral contour of the corresponding recess41ato guide (position) the outer peripheral contour of the polarization adjustment member41. The hemispherical protrusion64bis shaped in a hemispherical form that further protrudes from the guide portion64atoward the corresponding inner peripheral side pedestal surface53d, and the hemispherical protrusion64bis in point contact with the corresponding corner portion of the rectangular frame32cof the liquid crystal panel32.

Effects and Advantages

Hereinafter, effects and advantages of the first embodiment will be described.

According to the first embodiment, at the polarizing plate42and the support plate43, which are bonded together, the first residual stress direction RSD1of the polarizing plate42is along the second residual stress direction RSD2of the support plate43. That is, the direction, in which the degree of the residual stress left in the polarizing plate42is maximized, and the direction, in which the degree of the residual stress left in the support plate43is maximized, coincide with each other. Thus, even if the residual stress of the polarizing plate42and the residual stress of the support plate43are relieved in response to a temperature change of the polarization adjustment member41due to, for example, the incident of the external light on the polarization adjustment member41, it is possible to limit an occurrence of deformation of the polarizing plate42and deformation of the support plate43in different directions, respectively. In other words, an occurrence of the unintentional deformation, such as unintentional warpage, of the polarization adjustment member41is limited.

For example, since the complicated deformation of the polarization adjustment member41is limited, an occurrence of local unevenness of the optical action of the polarization adjustment member41and also an occurrence of local unevenness (for example, luminance unevenness) in the display quality of the virtual image VRI can be limited. Furthermore, for example, the undeformed state of the polarization adjustment member41can be easily maintained if the minimum deformation countermeasure corresponding to the directions RSD1, RSD2, which coincide with each other, is taken. As described above, it is possible to stably exert the optical action in the display of the virtual image VRI.

Further, according to the first embodiment, the polarization adjustment member41is held in the common case51, which commonly holds the polarization adjustment member41and the projector31, so that the projector unit30, in which the polarization adjustment member41and the projector31are integrated together, is formed. By doing so, the relative positional relationship between the projector31and the polarization adjustment member41can be easily maintained. Therefore, it is possible to limit an occurrence of an abnormality in the optical action, such as the polarization adjustment.

Furthermore, according to the first embodiment, the polarization adjustment member41is held through the elongated hold structures63that respectively has the contact portion63awhile the contact portion63ais in contact with the polarization adjustment member41and is elongated in the direction which is parallel to or perpendicular to the first residual stress direction RSD1and the second residual stress direction RSD2. With respect to the polarization adjustment member41, in which the unintended deformation, such as the unintended warpage, in a direction oblique to the first residual stress direction RSD1and the second residual stress direction RSD2is limited, the deformation, such as the warpage, of the polarization adjustment member41in a non-oblique direction, which is not oblique to the first residual stress direction RSD1and the second residual stress direction RSD2, is also limited by the above-described elongating mode of the respective contact portions63a. Since the maintenance performance for maintaining the undeformed state of the polarization adjustment member41is efficiently maintained, the stability of the optical action at the time of displaying the virtual image VRI is remarkably improved.

Furthermore, according to the first embodiment, the polarization adjustment member41is arranged such that the first residual stress direction RSD1and the second residual stress direction RSD2are set to be along the parallel direction, which is parallel to the tilt reference axis TRA, or the perpendicular direction, which is perpendicular to the tilt reference axis TRA. Therefore, it is possible to limit complex and anisotropic action of the influence of the tilt of the liquid crystal panel32and the influence of the deformation of the polarization adjustment member41, which is controlled in the mode corresponding to the first residual stress direction RSD1and the second residual stress direction RSD2. That is, it is possible to limit the occurrence of local unevenness in the display quality of the virtual image VRI.

If an optical material, such as glass, which is not excellent in processability, is used for the support plate43, the shape of the outer peripheral contour of the support plate43must be simplified in order to reduce the cost. On the other hand, according to the first embodiment, since the synthetic resin, which has the excellent processability, is used for the support plate43, the shape of the outer peripheral contour of the polarization adjustment member41can be made more complex. The shape of the outer peripheral contour of the polarization adjustment member41is asymmetric about at least one of: the imaginary reference line VRL1which bisects the surface area of the polarization adjustment member41in the perpendicular direction that is perpendicular to the first residual stress direction RSD1and the second residual stress direction RSD2; and the other imaginary reference line VRL2which bisects the surface area of the polarization adjustment member in the parallel direction that is parallel to the first residual stress direction RSD1and the second residual stress direction RSD2. Due to such a shape that is line asymmetric, i.e., is not line symmetric, for example, it is possible to limit an occurrence of misorientation of the polarization adjustment member41at the time of placing the polarization adjustment member41on the optical path during the manufacturing. Therefore, since the first residual stress direction RSD1can be correctly arranged relative to the hold structures, unintended deformation, such as unintended warpage, is limited. At the same time, since the optical axis, such as the block axis SAp, which is associated with the first residual stress direction RSD1, is correctly arranged, it is possible to provide the polarization adjustment member41that correctly exhibits the expected polarization adjusting function.

Other Embodiments

Although the one embodiment has been described above, the present disclosure should not be limited to this embodiment and may be applied to various other embodiments without departing from the gist of the present disclosure.

Specifically, as a first modification, the elongated hold structures63may be realized as urging ribs that are formed integrally with the cover59in one-piece. The urging ribs, which protrude from the opposing portion62toward the outer peripheral side pedestal surfaces53c, may extend along the longitudinal direction LD or the transverse direction SD of the liquid crystal panel32, so that distal end portions of the urging ribs, may serve as the contact portions63awhich extend in the direction that is substantially parallel to the first residual stress direction RSD1and the second residual stress direction RSD2and contact the polarization adjustment member41. Each of the urging ribs urges the corresponding one of the bulges41cof the polarization adjustment member41through the contact portion63a. In this way, the polarization adjustment member41is clamped between the backlight receiver52and the cover59in the state where the cushion materials68are interposed between the polarization adjustment member41and the backlight receiver52.

As a second modification, the contact portions63aof the elongated hold structures63may be elongated along a direction substantially parallel to the first residual stress direction RSD1and the second residual stress direction RSD2.

As a third modification, the polarization adjustment member41may be held by using a hold structure(s) other than the elongated hold structures63.

As a fourth modification, the polarization adjustment member41does not have to be held by the common case51, which is common to the polarization adjustment member41and the projector31, as long as the polarization adjustment member41is placed along the optical path which extends from the projector31to the windshield3. The polarization adjustment member41may be arranged at a position, which is spaced away from the projector31, for example, such that the polarization adjustment member41is installed to the window portion12of the housing11and also functions as the dustproof sheet. The polarization adjustment member41does not have to be arranged in parallel with the liquid crystal panel32.

As a fifth modification, the polarization adjustment member41may be any type of polarization adjustment member as long as it adjusts the polarization of at least one of the display light and the external light. For example, the polarization adjustment member41may change the polarization direction of the display light outputted from the liquid crystal panel32such that the occupant wearing polarized sunglasses can easily see the virtual image VRI.

As a sixth modification, the polarizing plate42does not have to be the iodine-based polarizing plate. For example, since there is a type of reflective polarizing plate in which a residual stress is left along a plane of the reflective polarizing plate, such a reflective polarizing plate may be used. In a case where the first residual stress direction RSD1of the reflective polarizing plate is along the transmission axis TAp, the first residual stress direction RSD1and the second residual stress direction RSD2may be along the direction that is parallel to the tilt reference axis TRA.

As a seventh modification, the transmission axis TAd of the liquid crystal panel32may be set in an oblique direction that defines an angle of, for example, 45 degrees relative to the longitudinal direction LD. Further, the projector31may project an unpolarized display light.

It should be noted that in the present disclosure, the perpendicular direction, which is perpendicular to the predetermined direction, or the perpendicular direction, which is perpendicular to the predetermined axis may not actually intersect the predetermined direction or the predetermined axis, as in the concept of “perpendicular” of the translation invariant vector.