JP2016218181A - Projection device using laser beam and head-up display using projection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a projection device using a laser beam, which can reduce a speckle noise and can display a high-quality image.SOLUTION: A projection device 10 using a laser beam includes: a light source part 100 for generating a plurality of laser beams at different wavelengths in a predetermined polarization direction; a combination optical part 200 for combining the plurality of laser beams; an image forming part 300 for forming a projection image from the plurality of laser beams; and a liquid crystal element 400 for changing the polarization state of the plurality of laser beams exiting from the combination optical part 200. In the projection device 10, the liquid crystal element 400 includes a liquid crystal layer between a pair of substrates; the liquid crystal element 400 generates a single polarization state in a transmission region of the plurality of laser beams; the polarization state varies while one projection image is formed; a liquid crystal molecule in the liquid crystal layer has a helical structure between the pair of substrates; and a product of a helical pitch of the liquid crystal molecule with a refractive index anisotropy of the liquid crystal layer is 2500 nm or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a projection device using laser light and a head-up display using the projection device.

  In recent years, a projection apparatus using a small and highly efficient laser beam as a light source has been developed. Laser light has high brightness and excellent color reproducibility, and can be applied to head-mounted displays and head-up displays as a projection device because of its wide angle of view, low power consumption, and high contrast. Yes.

  Since the laser light has high coherence, when the laser light is scattered by the unevenness of the surface (projection surface) projected by the projection device, the scattered light interferes with the image (display image) displayed on the projection surface. Speckle noise that is a speckled pattern may appear. The quality of the displayed image is significantly impaired by speckle noise. Speckle noise is a problem peculiar to a projection apparatus using a laser light source, and various attempts have been made so far.

  Patent Document 1 discloses an image display device including a polarization distribution conversion unit that converts a spatial polarization distribution of laser light emitted from a laser light source. This polarization distribution means converts the polarization distribution so that the polarization directions of the laser light incident on adjacent pixels of the spatial light modulator are orthogonal to each other. The laser light after the polarization distribution is converted is incident on the spatial light modulator, and the screen is irradiated with the laser light modulated by the spatial light modulator to display an image. In the image displayed on the screen, the laser light irradiated to the area corresponding to the adjacent pixel of the spatial light modulator does not cause interference because the polarization directions are orthogonal to each other. As a result, speckle noise Does not occur. Therefore, it is possible to display a high-quality image in which generation of speckle noise is significantly suppressed.

  Patent Document 2 discloses a display optical system that includes a polarization control unit that temporally changes the polarization state of laser light emitted from a laser light source and projects an image. The polarization control unit is configured to change the polarization state with time by rotating the half-wave plate with a motor. In the display optical system, since the polarization state of the laser light changes with time, a plurality of images with different speckle noises are sequentially displayed on the screen. For example, when each image is displayed at a frame rate of 60 Hz, the image in which speckle noise occurs is also switched every 1/60 seconds. Since this is faster than the afterimage time of the human eye, speckle noise generated in each frame due to the afterimage effect of the eye is smoothed and recognized. This reduces speckle noise in the display image.

  Patent Document 3 discloses a projection-type image display device including a polarization state modulation unit that switches the polarization state of laser light emitted from a laser light source. The polarization state modulation unit sequentially switches a plurality of specific polarization states in units of frames of the video signal according to a predetermined order. A liquid crystal cell (liquid crystal element) is used for the polarization state modulation unit, and the specific polarization state is switched by changing the magnitude of the AC voltage applied to the liquid crystal cell in units of frames. Since the location where speckle noise occurs varies depending on the polarization state of the laser light, in the image display device, images with speckle noise occurring at different locations are sequentially displayed on the screen in units of frames. Thereby, speckle noise in the display image is reduced.

  In Patent Document 4, a laser beam emitted from a laser light source is two-dimensionally scanned to form an image of one frame, and a projected image is displayed on a screen with a blanking period between each frame drawing. A laser projection apparatus is disclosed. The laser projection device includes a polarization converter that changes the polarization state of light emitted from the laser light source during the blanking period. The polarization conversion unit includes a drive control device and a liquid crystal element, and switches the polarization state in units of frames by switching on and off of the alternating voltage applied to the liquid crystal element during the blanking period. By providing such a drive control device, although the polarization state changes in units of frames, the polarization state does not change during drawing of each frame, so that speckle noise can be reduced and the luminance changes within one frame. No good image can be projected.

JP 2002-066252 A JP 2006-047422 A International Publication No. 2010/116838 International Publication No. 2011/037039

  In Patent Document 1, although the polarization distribution is converted so that the polarization directions of laser light incident on adjacent pixels of the spatial light modulator are orthogonal to each other, the polarization direction of each pixel changes with the passage of time. In addition, since the polarization direction is the same as that of every other pixel or pixels in an oblique direction, speckle noise may occur between such pixels. In Patent Document 2, the polarization state is configured to change with time. However, since the half-wave plate is rotated by a motor to change the polarization state, a drive source is required, and the display optical system is There is a risk of enlargement.

  In Patent Documents 3 and 4, the polarization state is changed in units of frames using a liquid crystal element, and the polarization state is changed at the frame switching timing without changing the polarization state during drawing of each frame. When the liquid crystal element is used, the apparatus can be reduced in size, but the response speed of the liquid crystal element to the applied voltage of the liquid crystal molecules is slow, and it is difficult to actually change the polarization state at the frame switching timing. If the change state cannot be switched in units of frames, a plurality of frames are drawn in the same polarization state, so there is a possibility that the effect of reducing speckle noise cannot be obtained sufficiently.

  In addition, when there are a plurality of light sources that generate light of different colors such as red, green, and blue, for example, the wavelength of the light varies depending on the color, so that the light transmitted through the liquid crystal element has different optical rotation dispersion depending on the color, The polarization direction characteristics with respect to the applied voltage are also different. If the characteristics of the polarization direction with respect to the applied voltage are different, color changes occur in the plurality of pixels (panel pixels) that should display the same color in the projected image due to differences in the characteristics of the polarization direction depending on the color of the light. There is a risk of giving a strange feeling to the person who is watching.

  Thus, there is a need for a projection device using laser light that can reduce speckle noise and display a high-quality image.

  One embodiment of a projection device using laser light according to the present invention includes a light source unit that generates a plurality of laser beams having different wavelengths in a predetermined polarization direction, a combining optical unit that combines the plurality of laser beams, An image forming unit that forms a projection image from the plurality of laser beams; and a liquid crystal element that changes a polarization state of the plurality of laser beams emitted from the combining optical unit, and the liquid crystal device is disposed between a pair of substrates. A liquid crystal layer, and the liquid crystal element generates a single polarization state in the transmission region of the plurality of laser beams, and the polarization state changes during formation of one projection image, and the liquid crystal of the liquid crystal layer The molecules have a spiral structure between the pair of substrates, and the product of the spiral pitch of the liquid crystal molecules and the refractive index anisotropy of the liquid crystal layer is 2500 nm or more.

  As described above, in the projection apparatus including the liquid crystal element that satisfies the relationship that the product of the spiral pitch of the liquid crystal molecules and the refractive index anisotropy of the liquid crystal layer is 2500 nm or more, the polarization direction of the synthesized light changes in units of frames. Speckle noise in the display image projected on the screen is reduced, and a high-quality image with little noise can be displayed. In addition, since the liquid crystal element is configured so that the optical rotation dispersion of a plurality of laser beams having different wavelengths is reduced and the polarization characteristics are the same, even if the polarization direction changes during the formation of one projection image, the applied voltage In contrast, all the polarization directions of a plurality of laser beams having different wavelengths are changed in the same manner, so that the color hardly changes. Therefore, the projection apparatus can display a high-quality image with little speckle noise.

  In one embodiment of the projection device, the refractive index anisotropy is greater than 0.25.

  When the refractive index anisotropy is larger than 0.25, the helical pitch in which the product of the helical pitch and the refractive index anisotropy of the liquid crystal layer is 2500 nm or more, that is, the thickness of the liquid crystal layer can be reduced. The response speed of the liquid crystal molecules in the layer can be increased, and the change in the polarization direction of the liquid crystal element can be completed in a short time.

  In one embodiment of the projection device, the twist angle of the liquid crystal molecules in the liquid crystal layer is 90 degrees.

  With such a configuration, the polarization direction of light transmitted through the liquid crystal layer changes by 90 degrees depending on whether voltage application to the liquid crystal layer is turned on or off. Thereby, the effect of reducing speckle noise due to the change in the polarization direction can be maximized.

  In one embodiment of the projection device, the liquid crystal layer is formed by laminating a plurality of layers. Preferably, the liquid crystal layer is formed by laminating a first layer and a second layer, and the twist angle in the first layer and the twist angle in the second layer are both 45 degrees.

  In general, when the liquid crystal element is composed of a plurality of N liquid crystal layers, in order to change the polarization direction of light passing through the liquid crystal element by φ degrees, the voltage applied to each liquid crystal layer By performing the application with the same voltage and at the same timing, the change of the polarization direction in each liquid crystal layer due to turning on and off of the applied voltage is only φ / N degrees. At this time, the twist angle of the liquid crystal molecules in each liquid crystal layer may be θ (= φ / N) degrees, and the helical pitch p of the liquid crystal molecules in each liquid crystal layer is dx when the thickness of each liquid crystal layer is dx. p = dx × 360 / θ.

  In each of the plurality of liquid crystal layers, the product of the helical pitch of the liquid crystal molecules of each liquid crystal layer and the refractive index anisotropy of each liquid crystal layer is 2500 nm or more.

  For example, when the polarization direction of transmitted light is changed by 90 degrees, a spiral pitch of a liquid crystal element (single layer element) having a single liquid crystal layer and a liquid crystal layer having the same thickness as the single liquid crystal layer are stacked. The spiral pitch of a plurality of liquid crystal elements (multi-layer elements) is larger because the multi-layer element has a smaller twist angle of the liquid crystal molecules of each liquid crystal layer. When the product of the spiral pitch of the liquid crystal molecules of the liquid crystal element of the single layer element and the refractive index anisotropy of the liquid crystal layer is 2500 nm, the refractive index anisotropy of the single layer element and the multilayer element is equal, The product of the helical pitch of the liquid crystal molecules of each liquid crystal element of the multi-layer element and the refractive index anisotropy of each liquid crystal layer is greater than 2500 nm as the helical pitch is larger than that of the single layer element. Thereby, the optical rotation dispersion of a plurality of laser beams having different wavelengths can be further reduced, and the polarization characteristics can be made the same.

  Further, if the multilayer device is configured so that the product of the spiral pitch of the liquid crystal molecules in each liquid crystal layer and the refractive index anisotropy of each liquid crystal layer is 2500 nm, the spiral pitch of the liquid crystal molecules in each liquid crystal layer is The value of the refractive index anisotropy of each liquid crystal layer can be reduced by an amount larger than the spiral pitch of the single layer element. As a result, it is possible to use a liquid crystal having a small value of refractive index anisotropy for each liquid crystal layer, and the range of liquid crystal material selection can be expanded.

  Further, when the multilayer device is configured such that the product of the helical pitch of the liquid crystal molecules of each liquid crystal layer and the refractive index anisotropy of each liquid crystal layer is 2500 nm, the refractive index is different between the single layer device and the multilayer device. If the directionality is the same, the thickness of each liquid crystal layer can be reduced until the helical pitch is equivalent to the helical pitch of the single layer element while maintaining the twist angle of the liquid crystal molecules of each liquid crystal layer in the multilayer device. it can. If the thickness of each liquid crystal layer is reduced, the response speed of the liquid crystal molecules in each liquid crystal layer can be increased (continuum theory), so the change of the polarization direction between 0 degree and 90 degrees can be completed in a short time as a liquid crystal element. The time during which the polarization direction is 0 degree and 90 degrees can be lengthened during the formation of the projection image. As a result, the projection apparatus can display a higher quality image with less speckle noise.

  As in this one embodiment, when the liquid crystal element includes a first layer and a second layer having a twist angle of 45 degrees as a liquid crystal layer, a voltage is applied to the first layer and the second layer by an AC power source. By performing the same voltage at the same timing, the change in the polarization direction of the light transmitted through the first layer and the second layer due to turning on and off of the applied voltage is 45 degrees. Therefore, the change in the polarization direction of 0 degrees and 90 degrees can be completed in a short time as a liquid crystal element. As a result, the time during which the polarization direction is 0 degree and 90 degrees can be lengthened during the formation of the projection image, so that the projection apparatus can display a higher quality image with less speckle noise.

  In one embodiment of the projection apparatus, an antireflection film is formed on the surface of the pair of substrates at least near the light source unit. The antireflection film is preferably a laminated film made of an inorganic material.

  With such a configuration, since the light emitted from the light source unit can be effectively incident on the liquid crystal element, the projection apparatus can display a high-luminance image.

  In one embodiment of the projection device, a diffusion film is formed on the surface of at least the substrate far from the light source unit of the pair of substrates. Irregularities for diffusing the plurality of laser beams may be formed instead of the diffusion film.

  With such a configuration, the coherence of the light transmitted through the liquid crystal element is lowered, so that speckle noise appearing in the image projected on the screen can be further reduced. In addition, it is possible to display a uniform luminance image with little luminance unevenness on the screen.

  In one embodiment of the projection apparatus, the light source unit includes a red laser light source that generates red light, a green laser light source that generates green light, and a blue laser light source that generates blue light.

  With such a configuration, the projection apparatus can project a color image.

  In one embodiment of the projection apparatus, the image forming unit includes a scanning mechanism that scans the plurality of laser beams. At this time, the liquid crystal element is disposed between the combining optical unit and the image forming unit.

  With such a configuration, an image can be projected using the scanning mechanism.

  In one embodiment of the projection apparatus, the image forming unit includes a spatial light modulation element that modulates the plurality of laser beams for each pixel. At this time, the image forming unit is disposed between the light source unit and the liquid crystal element.

  With such a configuration, a projection apparatus using a spatial light modulation element can be configured.

  One embodiment of an in-vehicle head-up display according to the present invention includes a projection device using the laser beam described above.

  With such a characteristic configuration, it is possible to obtain a head mounted display that can display a high-quality image with little speckle noise.

It is a block diagram showing the structure of the projection apparatus which concerns on 1st Embodiment. It is sectional drawing showing the structure of the liquid crystal element which concerns on 1st Embodiment. It is the schematic showing the change of a polarization state when light permeate | transmits the liquid crystal element which concerns on 1st Embodiment. It is a graph showing the change of the polarization state at the time of image projection. It is a graph showing a polarization characteristic when red light, green light, and blue light permeate | transmit the liquid crystal element whose product of the helical pitch of a liquid crystal molecule and the refractive index anisotropy of a liquid crystal layer is 2500 nm or more. It is a graph showing a polarization characteristic when red light, green light, and blue light permeate | transmit the liquid crystal element whose product of the helical pitch of a liquid crystal molecule and the refractive index anisotropy of a liquid crystal layer is less than 2500 nm. It is a graph showing the saturation difference in a L * a * b * color system. It is a graph showing the relationship between the product of the helical pitch of a liquid crystal molecule and the refractive index anisotropy of a liquid crystal layer, and the difference in saturation between two colors. It is sectional drawing showing the structure of the liquid crystal element which concerns on the modification of 1st Embodiment. It is the schematic showing the change of a polarization state when light permeate | transmits the liquid crystal element which concerns on the modification of 1st Embodiment. It is sectional drawing showing the structure of the liquid crystal element which concerns on the modification of 1st Embodiment. It is sectional drawing showing the structure of the liquid crystal element which concerns on the modification of 1st Embodiment. It is sectional drawing showing the structure of the liquid crystal element which concerns on the modification of 1st Embodiment. It is a block diagram showing the structure of the projection apparatus which concerns on 2nd Embodiment. It is a block diagram showing the structure of the head-up display using the projection apparatus which concerns on 1st Embodiment.

[First Embodiment]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the projection apparatus 10 according to the first embodiment. The projection device 10 includes a light source unit 100, a combining optical unit 200, an image forming unit 300, and a liquid crystal element 400. The projection device 10 combines the light generated by the light source unit 100 with the combining optical unit 200 to generate combined light, and changes the polarization state of the combined light (hereinafter also referred to as “polarization direction”) with the liquid crystal element 400. Then, the image forming unit 300 forms a projection image from the combined light and projects it on the screen 500 to display the image (display image).

  Speckle noise is generated by the scattered light of the synthesized light interfering with each other due to the unevenness of the surface of the screen 500. Speckle noise appears as a bright and dark speckled pattern on the display image on the screen 500, thereby reducing the quality of the display image. In the projection apparatus 10 according to the present embodiment, speckle noise is reduced by changing the polarization state of the combined light transmitted through the liquid crystal element 400 in units of frames. Details will be described later.

  The light source unit 100 includes a laser diode (LD) 110 that generates green light, an LD 120 that generates blue light, and an LD 130 that generates red light. The light of each color is modulated and emitted in a predetermined linear polarization state and corresponding to the pixel (panel pixel) of the image displayed on the screen 500. By providing the LDs 110 to 130, the projection device 10 can project a color image. The LD 110 is an example of a green laser light source, the LD 120 is an example of a blue laser light source, and the LD 130 is an example of a red laser light source.

  In the present embodiment, a dedicated LD is used for each of the three color light sources, but a two-color or three-color light source may be configured by separating the LD generating white light by a dichroic mirror or the like. In the present embodiment, the LDs 110 to 130 are used, but the light source unit 100 may be configured using LDs that generate other colors. Further, the number of light emitted from the light source unit 100 is not limited to three, and may be two or four or more.

  The combining optical unit 200 includes a mirror 210 that reflects green light generated from the LD 110, a dichroic mirror 220 that transmits green light and reflects blue light generated from the LD 120, and transmits green light and blue light and is generated from the LD 130. And a dichroic mirror 230 that reflects red light. The combining optical unit 200 combines the green light, the blue light, and the red light to generate combined light. In the combined light state, green light, blue light, and red light all have the same polarization direction.

  The image forming unit 300 includes a horizontal scanning mirror 310 and a vertical scanning mirror 320, which are examples of a scanning mechanism, and scans incident combined light in a two-dimensional direction. In the present embodiment, the horizontal scanning mirror 310 is a MEMS (Micro Electro Mechanical System) mirror manufactured using semiconductor manufacturing technology or the like, and can be reciprocally oscillated (resonant vibration) using electromagnetic force or the like. Use what you can. Further, a galvanometer mirror is used as the vertical scanning mirror 320. The horizontal scanning mirror 310 and the vertical scanning mirror 320 are driven by a driving source (not shown). As a result, the combined light is scanned from the upper left end on the screen 500 to the lower right end by sequentially repeating the scanning in the horizontal direction and the scanning in the vertical direction, and one frame image formed by the image forming unit 300 is projected. Displayed. Thereafter, the scanning position returns to the upper left end again, and scanning for projecting and displaying the image of the next frame starts. Note that one frame of the projected image may be displayed by scanning the combined light from the upper right end to the lower left end. Further, as another example of the scanning mechanism, the image forming unit 300 may be configured to swing so as to scan in the horizontal direction and the vertical direction with a single mirror.

  As shown in FIG. 2, the liquid crystal element 400 includes a light-transmitting liquid crystal layer 441 between the light-transmitting substrates 411 and 412, and a liquid crystal layer 441 between the liquid crystal layer 441 and the substrates 411 and 412. Light-transmitting alignment layers 431 and 432 for aligning molecules, and electrode layers 421 and 422 made of a light-transmitting material such as ITO (Indium Tin Oxide) and generating an electric field applied to the liquid crystal layer 441 by applying a voltage. I have. The liquid crystal element 400 is disposed between the combining optical unit 200 and the image forming unit 300, and transmits the combined light emitted from the combining optical unit 200 to enter the image forming unit 300. When a voltage is applied to the electrode layers 421 and 422 by the AC power source 450, the alignment state of the liquid crystal molecules in the liquid crystal layer 441 of the liquid crystal element 400 is changed by the generated electric field, and thereby the polarization state of the transmitted synthesized light is changed. To do. Note that Δn in FIG. 2 represents the refractive index anisotropy of the liquid crystal layer 441, and d represents the thickness of the liquid crystal layer 441. In the figure, the arrow from the left to the right represents the traveling direction of the combined light.

  FIG. 3 shows a change in the polarization state of light transmitted through the liquid crystal element 400 when no voltage is applied. The alignment layers 431 and 432 of the liquid crystal element 400 have a function of aligning liquid crystal molecules in a certain direction. In the present embodiment, the alignment layers 431 and 432 are arranged to face each other so as to align the liquid crystal molecules in an orthogonal direction. Therefore, the liquid crystal molecules in the liquid crystal layer 441 have a helical structure in which the liquid crystal molecules are twisted by 90 degrees and arranged from the alignment layer 431 toward the alignment layer 432. The polarization direction of light transmitted through the liquid crystal layer 441 rotates along the alignment of liquid crystal molecules.

  As shown in FIG. 3, when the polarization direction of the light X indicated by the arrow A is the same as the alignment direction of the liquid crystal molecules in the vicinity of the alignment layer 431 indicated by the arrow B, the light X passes through the alignment layer 431 and enters the liquid crystal layer 441. To do. While the light X is transmitted through the liquid crystal layer 441, the polarization direction is rotated by 90 degrees as indicated by the arrow D along the alignment of the liquid crystal molecules, and the light X is transmitted through the alignment layer 432 indicated by the arrow C while being in the polarization state indicated by the arrow D. To do.

  On the other hand, when a voltage is applied to the electrode layers 421 and 422 by the AC power source 450, the liquid crystal molecules are aligned along the traveling direction of the light X under the action of the generated electric field. The polarization direction of X remains the direction of arrow A and does not change. As described above, the polarization direction of the light X transmitted through the liquid crystal element 400 changes by 90 degrees by turning on and off the voltage applied by the AC power supply 450. This change in the polarization direction also occurs in the combined light.

  In the present embodiment, the liquid crystal layer 441 of the liquid crystal element 400 has a structure that forms a single pixel in order to simultaneously and similarly change the polarization state of the combined light. According to the liquid crystal element 400 having such a configuration, the polarization state of the transmitted composite light is the same regardless of the transmission position, and the change in the polarization state occurs simultaneously and similarly. In other words, the liquid crystal element 400 generates only one polarization state in the combined light.

  In the present embodiment, as shown in FIG. 4, the drawing time of a projected image of one frame (hereinafter referred to as “frame period T”, when representing each frame period T, the frame period T1, the frame period T2, etc. In other words, the polarization direction of the combined light changes during the formation time of the projection image in the image forming unit 300. As shown in FIG. 4, the polarization direction changes from 0 degrees to 90 degrees at the first change time τr in the frame period T1, and the polarization direction changes from 90 degrees to 0 at the second change time τd in the next frame period T2. Change in degrees. The frame period T1 and the frame period T2 have the same time, and the sum of the first change time τr and the second change time τd is shorter than the frame period T. A voltage is applied by the AC power source 450 during the frame period T1, and the voltage application is stopped during the frame period T2. As described above, since the response speed of the liquid crystal molecules with respect to the applied voltage is slow, the liquid crystal molecules are not aligned in the traveling direction of the synthesized light unless the first change time τr elapses even when the voltage is applied by the AC power supply 450. Even if the voltage application is stopped, the liquid crystal molecules do not return to the helical structure unless the second change time τd elapses. That is, the first change time τr and the second change time τd are required for the polarization direction to change by 90 degrees.

  By adopting such a configuration, a projection image with a polarization direction of 0 degrees and a projection image with a 90 degree polarization are alternately displayed on the screen 500 in units of frames. Change. Thereby, speckle noise generated in each frame is smoothed, and speckle noise appearing in the image displayed on the screen 500 is reduced. In this embodiment, since the polarization direction changes by 90 degrees, the effect of reducing speckle noise is maximized compared to changes in other angles. Even if the polarization direction changes in the middle of the frame period T, the scanning direction of the horizontal scanning mirror 310 and the vertical scanning mirror 320 is one direction (from the upper left end to the lower right end). The frames are not projected continuously.

  As described above, in the projection apparatus 10 of the present embodiment, the polarization direction changes by 90 degrees during the frame period T. In general, when the color (wavelength) of light is different, different optical rotation dispersion occurs in the liquid crystal layer 441, and the characteristic of the polarization direction with respect to the applied voltage (hereinafter referred to as “polarization characteristic”) is different. As shown in FIG. 6, green light, blue light, and red light, which are components of the combined light, have different wavelengths, and therefore have different optical rotation dispersion and different polarization characteristics. When the polarization characteristics are different, when the polarization direction is changed during the frame period T as in this embodiment, depending on the value of the applied voltage, a plurality of panel pixels that should display the same color in the projected image, A change in color is caused by the difference in the polarization characteristics of green light, blue light, and red light, which may give a strange feeling to a person viewing the image. In order to prevent color change, it is necessary to form and project a projection image with the same polarization characteristics of green light, blue light, and red light.

  Therefore, in this embodiment, p × Δn ≧ 2500 nm (Δn is the refractive index anisotropy of the liquid crystal layer 441 described above) where p is the helical pitch of the helical structure of the liquid crystal molecules of the liquid crystal layer 441 of the liquid crystal element 400. It is configured to satisfy. The helical pitch p has a relationship of p = d × 360 / θ where d is the twist angle of the liquid crystal molecules (d is the thickness of the liquid crystal layer 441 described above). In the present embodiment, the twist angle θ is 90 degrees from the relationship between the arrow B and the arrow C shown in FIG.

  When the liquid crystal element 400 satisfies the relationship of p × Δn ≧ 2500 nm, as shown in FIG. 5, the optical rotatory dispersion of green light, blue light, and red light is reduced and the polarization characteristics are the same. Thus, even if the polarization direction changes during the frame period T as in the present embodiment, all the polarization directions of green light, blue light, and red light change in the same manner with respect to the applied voltage. Therefore, even if the same color is displayed on a plurality of panel pixels in the projection image during the frame period T, no color change occurs.

  Strictly speaking, even when the liquid crystal element 400 satisfies the relationship of p × Δn ≧ 2500 nm, the polarization characteristics of green light, blue light, and red light are slightly different as shown in FIG. Therefore, when the same color is displayed on a plurality of panel pixels in the projection image during the frame period T, the color may slightly change between the plurality of panel pixels. However, if the person who sees the projected display image cannot recognize the difference in color, it will be recognized that the person looks the same color and no color change has occurred.

Hereinafter, in the projection apparatus 10 based on the L * a * b * color system standardized in 1976 by the International Commission on Illumination (CIE) and adopted in Japanese Industrial Standards JIS Z8781-4, It will be described that the color change between the panel pixels is not recognized by a person viewing the display image. In the L * a * b * color system, L * represents lightness, and a * and b * represent chromaticity indicating hue and saturation. a * and b * are values uniquely determined by the color, and the saturation (c *) is obtained by c * = ((a *) ² + (b *) ² ) ^1/2 . The difference in saturation between two colors (Δc *) is Δc * = ((Δa *) ² + (Δb *) ² ) ^1/2 (Δa * is the difference in a * between the two colors, and Δb * is (B * difference between two colors).

  FIG. 7 is a graph showing the saturation difference in the L * a * b * color system. Δa * and Δb * indicate the direction of the difference between the two colors, + Δa * indicates the red direction, −Δa * indicates the green direction, + Δb * indicates the yellow direction, and −Δb * indicates the blue direction. When there is no difference between the two colors, since a * and b * of the two colors are the same value, Δa * = 0, Δb * = 0, and Δc * = 0. This is the intersection of the Δa * axis and the Δb * axis. In the reliability evaluation and the commercial transaction in the display device such as the projection device 10 and the projector according to the present embodiment, if Δc * ≦ 1.2, that is, if Δc * is inside the circle indicated by the broken line in FIG. It is said that humans cannot discriminate between the two colors and look the same color.

  As shown in FIGS. 7 and 8, when p × Δn = 2500 nm in the liquid crystal element 400, the color difference between two panel pixels displaying the same color in the projected image is Δa * = − 0.8, Δb *. = 0.9 and Δc * = 1.2. When p × Δn = 2550 nm, the difference in color between two panel pixels displaying the same color in the projected image is Δa * = − 0.6, Δb * = 0.6, and Δc * = 0.8. It becomes. All of these are Δc * ≦ 1.2, and when the same color is displayed on a plurality of panel pixels in the projection image during the frame period T using the projection device 10, the display image projected on the screen 500 is viewed. Humans cannot recognize the difference in color, and it is recognized that no color change has occurred.

  On the other hand, when p × Δn = 2000 nm, the color difference between two panel pixels displaying the same color in the projected image is Δa * = 1.3, Δb * = 1.5, and Δc * = 2.0. Therefore, when the same color is displayed on the plurality of panel pixels in the projection image during the frame period T, the person who sees the projected display image recognizes the difference in color, and the display image has color unevenness. You will feel uncomfortable.

  As described above, in the projection device 10 including the liquid crystal element 400 satisfying the relationship of p × Δn ≧ 2500 nm, the polarization direction of the synthesized light changes in units of frames, and thus the speckle in the display image projected on the screen 500 is displayed. Noise is reduced, and a high-quality image with less noise can be displayed. In addition, since the liquid crystal element 400 is configured so that the optical rotation dispersion of green light, blue light, and red light is reduced and the polarization characteristics are the same, even if the polarization direction changes during the frame period T, the applied voltage is changed. On the other hand, all the polarization directions of green light, blue light, and red light change in the same manner, and almost no color change occurs. Since Δc * ≦ 1.2 at this time, even if a color change occurs, a person viewing the display image projected on the screen 500 cannot recognize the difference, and the color change does not occur. It is recognized as a thing. Therefore, the projection device 10 can display a high-quality image with little speckle noise.

  Usually, the refractive index anisotropy Δn of the liquid crystal layer used in the projection apparatus 10 is about 0.11 to 0.22, but Δn is preferably larger than 0.25. When Δn is larger than 0.25, p satisfying p × Δn ≧ 2500 nm, that is, d can be reduced, so that the response speed of the liquid crystal molecules in the liquid crystal layer 441 can be increased (continuum theory). The liquid crystal element 400 can complete the change in the polarization direction of 0 degrees and 90 degrees in a short time. Therefore, since the first change time τr and the second change time τd in the frame period T shown in FIG. 4 can be shortened and the time in which the polarization direction is 0 degrees and 90 degrees can be lengthened, Higher quality images can be displayed with less speckle noise.

[Modification of First Embodiment]
Hereinafter, a projection apparatus 10 according to a modification of the first embodiment of the present invention will be described in detail based on the drawings. In the description of this modified example, the same reference numerals are given to the same components as those in the first embodiment, and the descriptions regarding the same components are omitted.

  In the projection apparatus 10 according to this modification, the configuration of the liquid crystal element 400 is different from that of the first embodiment. Specifically, as illustrated in FIG. 9, the liquid crystal element 400 includes a light-transmitting liquid crystal layer 441 and a light-transmitting liquid crystal layer 442. The liquid crystal layer 441 is provided between the light-transmitting substrates 411 and 413, and the liquid crystal layer 442 is provided between the substrates 413 and 412. The refractive index anisotropy of the liquid crystal layer 441 and the liquid crystal layer 442 may be the same value or different values. The thickness of the liquid crystal layer 441 is d1, and the thickness of the liquid crystal layer 442 is d2. In the figure, the arrow from the left to the right represents the traveling direction of the combined light. In this modification, the liquid crystal layer 441 is an example of a first layer, and the liquid crystal layer 442 is an example of a second layer. The thickness d1 of the liquid crystal layer 441 and the thickness d2 of the liquid crystal layer 442 do not have to be the same, but when the characteristics of the liquid crystal layer 441 and the liquid crystal layer 442 are made equal, the thickness of the liquid crystal layer 441 is It is desirable that d1 and the thickness d2 of the liquid crystal layer 442 be the same.

  A translucent alignment layer 431 and an alignment layer 433 that align liquid crystal molecules of the liquid crystal layer 441 are provided on both sides of the liquid crystal layer 441, and a translucent property that aligns the liquid crystal molecules of the liquid crystal layer 442 on both sides of the liquid crystal layer 442. The alignment layer 433 and the alignment layer 432 are provided. Further, outside the alignment layer 431 and the alignment layer 433 and outside the alignment layer 433 and the alignment layer 432, electrode layers 421 and 422 made of a light-transmitting material such as ITO and applying a voltage to the liquid crystal layers 441 and 442 are provided. Are provided.

  FIG. 10 shows a change in the polarization state of light transmitted through the liquid crystal element 400 when no voltage is applied. The alignment layer 433 is arranged so that the liquid crystal molecules are aligned at 45 degrees with respect to the alignment layers 431 and 432. As shown in FIG. 10, when the polarization direction of the light X indicated by the arrow A is the same as the alignment direction of the liquid crystal molecules in the vicinity of the alignment layer 431 indicated by the arrow B, the light X passes through the alignment layer 431 and enters the liquid crystal layer 441. To do. While the light X is transmitted through the liquid crystal layer 441, the polarization direction is rotated by 45 degrees as indicated by the arrow E along the alignment of the liquid crystal molecules, and the light is transmitted through the two alignment layers 433 indicated by the arrow E in the polarization state. Incident on layer 442. Then, while passing through the liquid crystal layer 442, the polarization direction is further rotated by 45 degrees along the arrangement of the liquid crystal molecules, and is transmitted through the alignment layer 432 indicated by the arrow C in the polarization state indicated by the arrow D and emitted. That is, when the light X is transmitted through the liquid crystal element 400, the polarization direction of the light X is rotated by 90 degrees as in the first embodiment.

  When the liquid crystal element 400 includes the liquid crystal layers 441 and 442 as in this modification, voltage application to the liquid crystal layers 441 and 442 by the AC power source 450 is performed at the same timing and at the same timing, thereby turning on and off the applied voltage. The change in the polarization direction of the light transmitted through the liquid crystal layers 441 and 442 is 45 degrees. Therefore, the change in the polarization direction of 0 degrees and 90 degrees can be completed in a short time as the liquid crystal element 400. Therefore, since the first change time τr and the second change time τd in the frame period T shown in FIG. 4 can be shortened and the time in which the polarization direction is 0 degrees and 90 degrees can be lengthened, Higher quality images can be displayed with less speckle noise.

  In general, when the liquid crystal element is composed of a plurality of N liquid crystal layers, in order to change the polarization direction of light passing through the liquid crystal element by φ degrees, the voltage applied to each liquid crystal layer By performing the application with the same voltage and at the same timing, the change of the polarization direction in each liquid crystal layer due to turning on and off of the applied voltage is only φ / N degrees. At this time, the twist angle of the liquid crystal molecules in each liquid crystal layer may be θ (= φ / N) degrees, and the helical pitch p of the liquid crystal molecules in each liquid crystal layer is dx when the thickness of each liquid crystal layer is dx. p = dx × 360 / θ.

  In each of the plurality of liquid crystal layers, each liquid crystal layer satisfies the relationship of p × Δn ≧ 2500 nm.

  For example, when the polarization direction of transmitted light is changed by 90 degrees, the helical pitch p1 of a liquid crystal element (single layer element) having a single liquid crystal layer having a thickness dx and the same thickness dx as the single liquid crystal layer The spiral pitch p2 of the liquid crystal element (multi-layer element) having N layers by laminating the liquid crystal layers of the multi-layer element is that the twist angle θ of the liquid crystal molecules of each liquid crystal layer can be smaller. Becomes larger than the helical pitch p1. When the liquid crystal layer of the single layer element satisfies the relationship of p1 × Δn = 2500 nm and the refractive index anisotropy Δn is the same between the single layer element and the multilayer element, p2 × of each liquid crystal element of the multilayer element. Δn becomes larger than 2500 nm by the amount that the helical pitch p2 becomes larger than that of the single layer element. Thereby, the optical rotation dispersion of a plurality of laser beams having different wavelengths can be further reduced, and the polarization characteristics can be made the same.

  Further, if each liquid crystal layer of the multilayer element is configured to have a relationship of p2 × Δn = 2500 nm, each liquid crystal layer is increased by the amount that the helical pitch p2 of the liquid crystal molecules of each liquid crystal layer is larger than the helical pitch p1. The value of the refractive index anisotropy Δn can be reduced. As a result, it is possible to use liquid crystals having a small refractive index anisotropy Δn for each liquid crystal layer, and the range of liquid crystal material selection can be expanded.

  Further, when each liquid crystal layer of the multilayer element satisfies the relationship of p × Δn = 2500 nm, if the refractive index anisotropy Δn is the same between the single layer element and the multilayer element, each liquid crystal layer in the multilayer element While maintaining the twist angle θ of the liquid crystal molecules, the thickness dx of each liquid crystal layer can be reduced until the helical pitch p2 becomes the helical pitch p1. If the thickness dx of each liquid crystal layer is reduced, the response speed of the liquid crystal molecules in each liquid crystal layer can be increased (continuum theory), so that the change in polarization direction between 0 degree and 90 degrees as a liquid crystal element can be achieved in a short time. It can be completed, and the time when the polarization direction is 0 degree and 90 degrees can be lengthened during the formation of the projection image. As a result, the projection apparatus can display a higher quality image with less speckle noise.

[Other Modifications of First Embodiment]
As shown in FIG. 11, in this modification, an antireflection film 460 is formed by alternately laminating two types of inorganic materials having different refractive indexes on the outer surface of the substrate 411 of the liquid crystal element 400. In the figure, the arrow from the left to the right represents the traveling direction of the combined light. By providing the antireflection film 460 on the surface of the substrate 411 in this manner, the light emitted from the light source unit 100 can be effectively incident on the liquid crystal element 400, so that the projection apparatus 10 displays a high-luminance image. Can be made. Note that the antireflection film 460 may be provided on the inner surface of the substrate 411 or on both surfaces of the substrate 411.

[Other Modifications of First Embodiment]
As shown in FIG. 12, in this modification, a diffusion film 470 is formed on the outer surface of the substrate 412 of the liquid crystal element 400. Alternatively, as shown in FIG. 13, an uneven portion 480 is formed on the outer surface of the substrate 412. In the figure, the arrow from the left to the right represents the traveling direction of the combined light. By providing the diffusion film 470 or the uneven portion 480 in this way, the light transmitted through the liquid crystal element 400 is diffused and its coherence is lowered, so that speckle noise appearing in the image projected on the screen 500 is further reduced. Can be made. In addition, since the light transmitted through the liquid crystal element 400 can be uniformly incident on the image forming unit 300 and an image can be formed, an image with uniform luminance with little luminance unevenness can be displayed on the screen 500. Note that the diffusion film 470 or the uneven portion 480 may be provided on the inner surface of the substrate 412.

[Second Embodiment]
Hereinafter, the projection apparatus 20 according to the second embodiment of the present invention will be described in detail with reference to the drawings. In the description of the present embodiment, the same reference numerals are given to the same components as those in the first embodiment and the modifications thereof, and the description regarding the same components is omitted. FIG. 14 is a block diagram showing the configuration of the projection apparatus 20 of this embodiment.

  The projection device 20 includes an image forming unit 300 between the light source unit 100 and the combining optical unit 200, and includes a liquid crystal element 400 and a projection lens 600 subsequent to the combining optical unit 200.

  In the present embodiment, the combining optical unit 200 includes a dichroic prism 240. Further, the image forming unit 300 includes a spatial light modulation element 330. The LD 110, LD 120, and LD 130 of the light source unit 100 are arranged to face each other in three directions of the dichroic prism 240, and the spatial light modulator 330 is arranged between the LD 110, LD 120, and LD 130 and the dichroic prism 240. Yes.

  In the projection device 20 of the present embodiment, the light generated by the LD 110, LD 120, and LD 130 is spread by a lens (not shown) or the like and is uniformly incident on all the pixels of each spatial light modulator 330. Each spatial light modulation element 330 modulates each pixel to form and emit an image of each color. Light (images of each color) emitted from the spatial light modulation element 330 is incident on the combining optical unit 200, where it is combined to generate combined light (projected image). The combined light emitted from the combining optical unit 200 enters the liquid crystal element 400. The combined light changes its polarization state in units of frames while being transmitted through the liquid crystal element 400. The combined light emitted from the liquid crystal element 400 is enlarged and projected by the projection lens 600, and an image is displayed on the screen 500.

  Also in the present embodiment, as in the first embodiment, the projection apparatus 20 includes the liquid crystal element 400 that satisfies the relationship of p × Δn ≧ 2500 nm, and the polarization direction of the combined light changes in units of frames. Speckle noise in the display image projected on the screen 500 is reduced, and a high-quality image with little noise can be displayed. In addition, since the liquid crystal element 400 is configured so that the optical rotation dispersion of green light, blue light, and red light is reduced and the polarization characteristics are the same, even if the polarization direction changes during the frame period T, the applied voltage is changed. On the other hand, all the polarization directions of green light, blue light, and red light change in the same manner, and almost no color change occurs. Since Δc * ≦ 1.2 at this time, even if a color change occurs, a person viewing the display image projected on the screen 500 cannot recognize the difference, and the color change does not occur. It is recognized as a thing. As described above, the projection device 20 can also display a high-quality image with less speckle noise.

[Application to head-up display]
FIG. 15 is a schematic diagram illustrating a state in which the projection device 10 according to the first embodiment is applied to the head-up display 50. A head-up display is a device that displays information superimposed on the human visual field. Thereby, for example, information is displayed on the windshield in the field of view of the driver of the vehicle, and the driver can recognize the information while driving.

  The head-up display 50 is configured by including a reflection mirror 340 in the projection apparatus 10. The head-up display 50 according to this embodiment is embedded in the dashboard of the vehicle, and the combined light projected from the image forming unit 300 of the projection device 10 is reflected by the reflection mirror 340 and projected onto the windshield 700 of the vehicle. The

  The projected image (information) is imaged at a point at infinity, and it appears to the driver that the information is displayed on a virtual screen 510 located far away. Therefore, the driver can recognize the information by focusing on the information while paying attention to the distance in the traveling direction while driving the vehicle.

  In addition, in this embodiment, although the application to the head-up display 50 of the projection apparatus 10 was demonstrated, the projection apparatus 10 which concerns on the modification of 1st Embodiment, or the projection apparatus 20 which concerns on 2nd Embodiment is used as a head-up display. Needless to say, it can be applied to 50.

  The projection devices 10 and 20 can be applied not only to the head-up display 50 but also to any projection device or display device that uses laser light as a light source. For example, the present invention can be applied to a projector, a projector television, a projection mapping device, a digital signage, a head mounted display, a micro projector, a portable information terminal incorporating the projector, and the like.

  The present invention can be used for a projection device using laser light and a head-up display using the projection device.

10, 20 Projection device 50 Head-up display 100 Light source 110 Laser diode (green laser light source)
120 Laser diode (blue laser light source)
130 Laser diode (red laser light source)
200 Synthetic Optical Unit 300 Image Forming Unit 310 Horizontal Scanning Mirror (Scanning Mechanism)
320 Vertical scanning mirror (scanning mechanism)
330 Spatial Light Modulator 400 Liquid Crystal Element 411 Substrate 412 Substrate 441 Liquid Crystal Layer (First Layer)
442 Liquid crystal layer (second layer)
460 Antireflection film 470 Diffusion film 480 Concavity and convexity Δn Refractive index anisotropy p Spiral pitch

Claims (15)

A light source unit that generates a plurality of laser beams having different wavelengths in a predetermined polarization direction;
A combining optical unit for combining the plurality of laser beams;
An image forming unit that forms a projection image from the plurality of laser beams;
A liquid crystal element that changes a polarization state of the plurality of laser beams emitted from the combining optical unit, and
The liquid crystal element has a liquid crystal layer between a pair of substrates,
The liquid crystal element generates a single polarization state in the transmission region of the plurality of laser beams,
The polarization state changes during the formation of one of the projected images,
The liquid crystal molecules of the liquid crystal layer have a spiral structure between the pair of substrates,
A projection apparatus using laser light, wherein a product of the helical pitch of the liquid crystal molecules and the refractive index anisotropy of the liquid crystal layer is 2500 nm or more.   The projection apparatus using laser light according to claim 1, wherein the refractive index anisotropy is larger than 0.25.   The projection apparatus using a laser beam according to claim 1 or 2, wherein a twist angle of the liquid crystal molecules in the liquid crystal layer is 90 degrees.   The projection apparatus using laser light according to claim 1, wherein the liquid crystal layer is configured by laminating a plurality of layers.   5. The liquid crystal layer according to claim 4, wherein the liquid crystal layer is configured by laminating a first layer and a second layer, and the twist angle in the first layer and the twist angle in the second layer are both 45 degrees. Projection device using laser light.   6. The projection device using laser light according to claim 1, wherein an antireflection film is formed on a surface of at least a side of the pair of substrates close to the light source unit.   The projection apparatus using laser light according to claim 6, wherein the antireflection film is a laminated film made of an inorganic material.   8. The projection device using laser light according to claim 1, wherein a diffusion film is formed on a surface of a substrate far from the light source unit of the pair of substrates. 9.   The laser beam according to any one of claims 1 to 7, wherein an uneven portion for diffusing the plurality of laser beams is formed on a surface of the substrate far from the light source unit of the pair of substrates. Projection device using.   10. The light source unit according to claim 1, comprising: a red laser light source that generates red light; a green laser light source that generates green light; and a blue laser light source that generates blue light. Projection device using laser light.   The projection device using laser light according to claim 1, wherein the image forming unit includes a scanning mechanism that scans the plurality of laser beams.   The projection apparatus using laser light according to claim 11, wherein the liquid crystal element is disposed between the combining optical unit and the image forming unit.   11. The projection apparatus using laser light according to claim 1, wherein the image forming unit includes a spatial light modulation element that modulates the plurality of laser lights for each pixel. 11.   The projection device using laser light according to claim 13, wherein the image forming unit is disposed between the light source unit and the liquid crystal element.   An in-vehicle head-up display comprising the projection device using the laser beam according to claim 1.

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