Display device that projects a virtual image display

A display device includes a light source that supplies light, a phase modulator that modulates the phase of the light from the light source to form an image, and an imaging optical system that focuses the image formed through the modulation performed by the phase modulator. The phase modulator produces diffracted light according to a phase modulation pattern while changing the phase of the light so as to form an intermediate image between the phase modulator and the imaging optical system, and the phase modulator also changes the position of the intermediate image.

BACKGROUND

1. Technical Field

The present invention relates to a display device, and particularly to a technology for a display device that projects a virtual image display.

2. Related Art

There have been proposed head-up displays that directly project a display in the field of view of a user. A known example of such a head-up display is an in-vehicle display device that projects a display of an indicator or the like in the field of view of the driver. By projecting a display in the field of view of the driver, the amount of sightline movement of during driving can be reduced. Further, by setting the position where the display is focused to be distant from the driver, it is possible to reduce the difference between the position of an object viewed through the windshield and the position where the virtual image is focused, allowing the display to be more easily viewed. Moreover, by changing the position where the virtual image is focused, the image can be displayed as if it moves as the vehicle travels. For example, JP-A-6-43392 proposes a technology for changing the position where the virtual image is focused. In the technology proposed in JP-A-6-43392, movement of the modulation unit of a CRT or a lens that is part of the projection optical system is used to change the position where the virtual image is focused.

SUMMARY

In the technology proposed in JP-A-6-43392, a moving mechanism is essential to move at least one of optical elements. Not only to simplify the configuration, but also to provide reliable, high optical performance, it is desirable to employ a configuration in which the position where the virtual image is focused can be changed with no mechanically moving mechanism. An advantage of some aspects of the invention is to provide a display device that displays a virtual image at various reproduction positions with no mechanically moving mechanism.

A display device according to an aspect of the invention includes a light source that supplies light, a phase modulator that modulates the phase of the light from the light source to form an image, and an imaging optical system that focuses the image formed through the modulation performed by the phase modulator. The phase modulator produces diffracted light according to a phase modulation pattern while changing the phase of the light so as to form an intermediate image between the phase modulator and the imaging optical system. The phase modulator also changes the position of the intermediate image.

The phase modulator not only modulates the phase of the light to form an image but also changes the position of the intermediate image. The intermediate image formed between the phase modulator and the imaging optical system is observed as a virtual image through the imaging optical system. By changing the position and the size of the intermediate image, the virtual image can be displayed at various reproduction positions with various sizes. The use of the phase modulator to modulate the phase of the light allows the position where the virtual image is reproduced to be changed with no mechanically moving mechanism. The use of the phase modulator allows the position and the size of the intermediate image to be easily changed, so that the reproduction position and the size of the virtual image can be freely set. There is thus provided a display device that displays a virtual image at various reproduction positions with no mechanically moving mechanism.

It is preferable that the phase modulator desirably includes liquid crystal elements. The use of the liquid crystal elements allows the light phase modulation pattern to be easily changed by controlling voltage application.

It is preferable that each of the liquid crystal elements is desirably a liquid crystal element driven in the field control birefringence mode. Such a liquid crystal element allows high-speed response of the phase modulator and sufficient phase modulation with a low drive voltage.

It is preferable that the light source desirably supplies coherent light. The use of coherent light allows the phase modulator to provide an excellent diffraction characteristic.

It is preferable that the display device desirably further includes a partial transmission mirror that transmits part of the light from the imaging optical system and reflects the other part of the light. This configuration allows the viewer to observe the space on the opposite side of the partial transmission mirror and the virtual image at the same time.

It is preferable that the first-order diffracted light is desirably used to form an image. The use of the first-order diffracted light to form an image can prevent only part of the image from being bright when the amount of the zero-order diffracted light is greater than that of the first-order diffracted light. Thus, an image having an excellent light intensity distribution can be obtained.

It is preferable that the imaging optical system is desirably disposed at a position where the zero-order diffracted light from the phase modulator is not incident. This configuration allows an image to be formed by using the first-order diffracted light.

It is preferable that the display device desirably further includes an optical separator that separates the zero-order diffracted light and the first-order diffracted light coming from the phase modulator from each other. In this configuration, the zero-order diffracted light is deviated from the imaging optical system, and only the first-order diffracted light can easily travel toward the imaging optical system.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

An embodiment of the invention will be described below in detail with reference to the drawings.

FIG. 1shows a schematic configuration of a display device10according to the embodiment of the invention. A light source11is a semiconductor laser that supplies coherent laser light. The light source11may have a configuration using a wavelength converter, such as a second harmonic generation (SHG) element, which converts the wavelength of the laser light from a semiconductor laser. The light source11may be a diode pumped solid state (DPSS) laser, solid state laser, liquid laser, gas laser or the like instead of the semiconductor laser.

The laser light from the light source11, which is collimated light, is incident on a phase modulator12. The phase modulator12is a transmissive liquid crystal panel with a plurality of liquid crystal elements (not shown) corresponding to pixels. The phase modulator12modulates the phase of the laser light from the light source11to form an image. The phase modulator12produces diffracted light according to a phase modulation pattern. Since coherent laser light is incident on the phase modulator12, the phase modulator12can provide an excellent diffracted characteristic.

FIG. 2shows a cross-sectional configuration of the liquid crystal element20provided in the phase modulator12shown inFIG. 1. The liquid crystal element20has a configuration in which a first transparent substrate21and a second transparent substrate24encapsulate a liquid crystal layer32. A first transparent electrode22is formed between the first transparent substrate21and the liquid crystal layer32. The first transparent electrode22can be made of, for example, ITO or IZO, which is a metal oxide. A light blocking layer23is provided in part of the region between the first transparent electrode22and the liquid crystal layer32. Additionally, a rubbed orientation film (not shown) is formed.

A semiconductor layer25is formed in part of the region above the second transparent substrate24. A gate insulating film26is formed on the semiconductor layer25, and a gate electrode27is formed on the gate insulating film26. These portions form a thin film transistor (TFT), and portions of the semiconductor layer25are used as the source and the drain of the TFT. An insulating film28made of silicon oxide or the like is formed on the second transparent substrate24and the TFT. A signal line29is provided above the TFT via the insulating film28. The signal line29is connected to the source of the TFT. A scan line (not shown) is connected to the gate electrode27, which is the gate of the TFT.

A PSG (phosphosilicate glass) film30is formed on the insulating film28, the signal line29, and the scan line. A second transparent electrode31is provided on the portion of the PSG film30other than the portion corresponding to the TFT. The second transparent electrode31can be made of ITO or IZO as in the first transparent electrode22. The TFT and the second transparent electrode31are formed in each of the pixels. Additionally, an orientation film is also formed on the second transparent substrate24side. The rubbing direction of the orientation film on the second transparent substrate24side is parallel but opposite to the rubbing direction of the orientation film on the first transparent substrate21side, allowing the liquid crystal elements20to be uniformly oriented as a whole.

Each of the liquid crystal elements20transmits the light sequentially through the first transparent substrate21, the first transparent electrode22, the liquid crystal layer32, and the second transparent electrode31, and the light exits from the second transparent substrate24. The liquid crystal elements20are disposed in such a way that the direction in which the laser light incident on the first transparent substrate21is polarized coincides with the direction in which the liquid crystal molecules on the first transparent substrate21side are orientated. The laser light from the light source11(seeFIG. 1) can thus be efficiently incident on the liquid crystal elements20.

The phase modulator12is driven, for example, in the active matrix method. The liquid crystal element20can use a liquid crystal material driven in the field control birefringence mode, such as a nematic liquid crystal material. Use of a liquid crystal material having a low molecular weight and relatively high birefringence allows high-speed response of the phase modulator12and sufficient phase modulation with a low drive voltage. The liquid crystal element20may use any of liquid crystal materials other than a nematic liquid crystal material as long as they are driven in the field control birefringence mode.

Referring back toFIG. 1, the phase modulator12produces diffracted light according to the phase modulation pattern while changing the phase of the light so as to form an intermediate image13. The intermediate image13is formed between the phase modulator12and an imaging optical system14. The light from the phase modulator12is incident on the imaging optical system14. The imaging optical system14focuses the image formed through the modulation performed by the phase modulator12. By forming the intermediate image13at a position closer to the imaging optical system14than the focal point thereof, a virtual image16of the intermediate image13is reproduced through the imaging optical system14. The imaging optical system14may have a configuration using a focusing lens.

The light from the imaging optical system14is incident on a partial transmission mirror15. The partial transmission mirror15transmits part of the light from the imaging optical system14and reflects the other part of the light. The partial transmission mirror15can be formed by coating a dielectric multilayer film on a transparent glass member or the like. The portion of the light from the imaging optical system14that is reflected off the partial transmission mirror15travels toward the viewer. Such a configuration allows the viewer to observe the virtual image16of the intermediate image13formed by the phase modulator12. The display of the virtual image16is presented at a position on the opposite side of the partial transmission mirror15along the extension of the reflected light ray from the partial transmission mirror15.

The portion of the light incident on the partial transmission mirror15from the opposite side to the viewer that passes through the partial transmission mirror15travels toward the viewer. Such a configuration allows the viewer to observe the space on the opposite side of the partial transmission mirror15and the virtual image16at the same time. By thus projecting the display in the field of view of the viewer, it is possible, for example, to reduce the amount of movement of the sightline of the viewer. Further, by setting the position where the virtual image is reproduced to be distant from the viewer, it is possible to reduce the distance between objects on the opposite side of the partial transmission mirror15and the virtual image16, allowing the display to be more easily viewed.

The phase modulator12changes the phase of the light to change the position of the intermediate image13. For example, by setting in advance phase modulation patterns, through which the intermediate image13is formed at different positions, in the driver of the phase modulator12, the phase modulator12can change the position of the intermediate image13. By shifting the position of the intermediate image13forward and backward along the light traveling direction as indicated by the double-headed arrow in the figure, the virtual image16can be focused and displayed at a different position. By changing the position where the virtual image16is focused, the position where the virtual image16is reproduced can be changed in the depth direction when viewed from the viewer. Further, by changing the size of the intermediate image13, the size of the virtual image16can be changed.

The display device10of the invention uses the phase modulator12to modulate the phase of the light, allowing the position where the virtual image16is reproduced to be changed with no mechanically moving mechanism. The use of the phase modulator12allows the position and the size of the intermediate image13to be easily changed. When the position and the size of the intermediate image13can be easily changed, the reproduction position and the size of the virtual image16can be freely set. There is thus provided an advantage of displaying the virtual image16at various reproduction positions with no mechanically moving mechanism.

The display device10of the invention is useful, for example, when applied to a navigation system that displays route guidance information and the like on the windshield of a vehicle as well as a night vision system that projects human figures and the like captured by an infrared camera onto the windshield of a vehicle. The use of the phase modulator12is not limited to the case where the position of the intermediate image13is changed along the light traveling direction, but the phase modulator12may change the position of the intermediate image13along two-dimensional directions including another direction substantially perpendicular to the light traveling direction. In this case, the position where the virtual image16is reproduced can be changed in the two-dimensional directions. The display device10may have a configuration in which the partial transmission mirror15is omitted and the viewer directly looks at the light from the imaging optical system14.

FIG. 3shows a schematic configuration of a display device40according to a first variation of this embodiment. The display device40of this variation is characterized in that the first-order diffracted light from the phase modulator12is used to form the intermediate image13. In the phase modulator12, the zero-order diffracted light travels straight forward as indicated by the broken lines, while the first-order diffracted light travels in an oblique direction as indicated by the solid lines, so that the zero-order diffracted light and the first-order diffracted light are separated from each other. The phase modulator12uses the first-order diffracted light traveling in the oblique direction to form the intermediate image13.

The imaging optical system14is disposed at a position where the first-order diffracted light from the phase modulator12is incident but the zero-order diffracted light from the phase modulator12is not incident. The phase modulator12may have a configuration in which a large diffraction angle is obtained for the first-order diffracted light so that only diffracted light is incident on the imaging optical system14. The phase modulator12can provide a large diffraction angle for the first-order diffracted light by arranging the liquid crystal elements20(seeFIG. 2) at small intervals. The use of the first-order diffracted light to form the intermediate image13can prevent the zero-order diffracted light and the first-order diffracted light from being superimposed and only part of the virtual image16from being bright when the amount of the zero-order diffracted light is greater than that of the first-order diffracted light. A virtual image16with an excellent light intensity distribution can be thus obtained.

A display device50shown inFIG. 4has a prism51provided between the phase modulator12and the imaging optical system14. The prism51is a triangular prism formed of a transparent member. Due to the refraction occurring in the prism51, the zero-order diffracted light from the phase modulator12travels in an oblique direction, while the first-order diffracted light from the phase modulator12travels straight forward. The prism51is an optical separator that separates the zero-order diffracted light and the first-order diffracted light coming from the phase modulator12from each other.

The display device50using the prism51may have a configuration in which the first-order diffracted light travels straight forward. Such a configuration easily allows only the first-order diffracted light to travel toward the imaging optical system14. The prism51may be a prism other than a triangular prism. The optical separator may use optical elements other than the prism51as long as they can separate the zero-order diffracted light and the first-order diffracted light from each other. The optical separator is not limited to that having a configuration in which the first-order diffracted light travels straight forward, but may have a configuration in which the first-order diffracted light travels in an oblique direction. The optical separator is useful particularly when it is difficult to obtain a large diffraction angle in the phase modulator12.

FIG. 5shows a schematic configuration of a display device60according to a second variation of this embodiment. The display device60of this variation includes a phase modulator61, which is a reflective liquid crystal panel. The phase modulator61reflects the laser light from the light source11to produce diffracted light according to a phase modulation pattern. The display device60, which uses the phase modulator61to reflect the laser light and produce diffracted light, can also display a virtual image16at various reproduction positions.

As described above, the display devices according to the invention are useful as a display device for projecting a virtual image display.

The entire disclosure of Japanese Patent Application No. 2007-012271, filed Jan. 23, 2007 is expressly incorporated by reference herein.