JP2018163359A - Optical element and head-up display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical element for allowing a user to surely view the whole virtual image even in any viewpoint position.SOLUTION: An optical element is used as an optical screen, for example, and diffuses and outputs laser light incident from a laser light source. The optical element includes a micro-lens array or a diffraction grating, and its pitches are set based on an incident numerical aperture of the laser light and wavelength of the laser light such that distribution of the diffracted lights output from the optical element becomes uniform.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an optical screen of a laser projector.

  A head-up display (hereinafter also referred to as “HUD”) is an image (real image) projected on a screen of a liquid crystal display or a laser projector by a half mirror called a combiner placed in front of the driver's field of view. It is a device that allows the driver to visually recognize it as a virtual image. As a result, the driver can view the instrument, navigation information, and the like superimposed on the scenery without looking down while looking forward.

  In HUD using a laser projector, laser light from a light source is projected onto an optical screen to generate an image that is visually recognized by the driver as a virtual image. Patent Documents 1 and 2 are cited as prior arts related to an HUD optical screen using a laser projector.

  Patent Document 1 includes a plurality of optical element portions in which an optical element constituting an optical screen has a curved surface, and the pitch of the optical element portion is set so that the diffraction width of the light beam diffused by the optical element is equal to or smaller than the pupil diameter of the viewer. The method of setting is described.

  Patent Document 2 describes a technique in which speckle noise is considered to be generated by the edge of the lens, and the lens pitch is made larger than the diameter of the laser light so that the laser light does not strike the edge of the lens.

JP2013-64985A Japanese Patent No. 5075595

  In Patent Document 1, in setting the pitch of the optical element portion constituting the optical element, the incident NA (numerical aperture) of the laser light incident on the optical element is not taken into consideration. Further, although the viewer's pupil diameter is set as a reference in setting the pitch, there is a problem that the appearance changes due to a change in the position of the viewer, particularly a change in the position in the front-rear direction.

  In Patent Document 2, since the laser beam is prevented from hitting the edge of the lens, there is a problem that the entire lens cannot be effectively used.

  Examples of the problems to be solved by the present invention include those described above. An object of the present invention is to provide an optical element that can reliably see the entire virtual image at any viewpoint position.

  The invention described in claim is an optical element that diffuses laser light incident from a laser light source, and includes a microlens array or a diffraction grating, and a pitch of the microlens array or diffraction grating is an incident aperture of the laser light. The distribution of diffracted light emitted from the optical element is determined to be uniform based on the number and the wavelength of the laser light.

The schematic structure of HUD which concerns on an Example is shown. A mode that a laser beam is scanned on an optical screen is shown typically. It is a figure explaining lens pitch and incident NA of a laser beam. It is a figure explaining the mode of the emitted light from an optical screen. The relationship between a lens array and diffracted light distribution is shown. The relationship between the incident NA of laser light and diffracted light is shown. The distribution of the diffracted light by an Example is shown. It is a figure explaining other conditions for obtaining desired emitted light. The structure of a reflective optical screen is shown. The structure of the optical screen using a diffraction grating is shown.

  A preferred embodiment of the present invention is an optical element that diffuses laser light incident from a laser light source, and includes a microlens array or a diffraction grating, and the pitch of the microlens array or diffraction grating is the incidence of the laser light. Based on the numerical aperture and the wavelength of the laser light, the distribution of the diffracted light emitted from the optical element is set to be uniform.

  The optical element is used as an optical screen, for example, and diffuses and outputs laser light incident from a laser light source. The optical element includes a microlens array or a diffraction grating, and the pitch is set so that the distribution of the diffracted light emitted from the optical element is uniform based on the incident numerical aperture of the laser light and the wavelength of the laser light. Has been. Thereby, the light quantity distribution of the emitted light from the optical element can be made uniform regardless of the viewpoint position and without depending on the optical system arranged in the subsequent stage.

  In one aspect of the above optical element, the optical element is a microlens array or a diffraction grating, and the microlens array or the diffraction grating is formed on the basis of the spot diameter of the laser light applied to the microlens array or the diffraction grating. The pitch is smaller than the spot diameter. Thereby, the performance of the microlens itself designed to a desired specification can be exhibited.

  In a preferred example, when the pitch is p, the incident numerical aperture is NAinc, and the wavelength is λ, the pitch p is

Satisfied.

In another preferable example, the laser beam includes laser beams of three colors of red, green, and blue, the pitch is p, the incident numerical aperture is NAinc, and the wavelength of the red laser beam is λ _RED , When the wavelength of the blue laser light is λ _BLUE , the pitch p is

Satisfied.

  In another preferred embodiment of the present invention, a head-up display includes a laser light source, the optical element described above, an optical screen that emits laser light emitted from the laser light source, and the optical screen emitted from the optical screen. And a combiner that reflects the laser light.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

[Example]
FIG. 1 shows a configuration of a HUD according to an embodiment of the present invention. The HUD includes a laser projector 10 and a combiner 5. Laser light constituting an image to be visually recognized by the driver is emitted from the laser projector 10, reflected by the combiner 5 which is a concave mirror, and reaches the viewpoint of the driver. As a result, the driver visually recognizes the image as a virtual image.

  The laser projector 10 includes a laser light source 11, an optical screen 12, and a MEMS mirror 13. The laser light source 11 includes laser light sources of three colors R (red), G (green), and B (blue), and emits RGB laser light L corresponding to an image to be visually recognized by the driver. As shown in FIG. 3A, the optical screen 12 includes a lens array in which a plurality of microlenses (hereinafter simply referred to as “lenses”) 12x are arranged.

  FIG. 2 shows how the laser beam L is scanned on the optical screen 12. The laser light L emitted from the laser light source 11 is scanned on the optical screen 12 by the MEMS mirror 13. Thereby, an image to be visually recognized by the driver is drawn on the optical screen 12.

  In this embodiment, the lens pitch p of the lens array constituting the optical screen 12 is condensed on the lens array so that the driver can surely see the entire virtual image at any viewpoint position. It is characterized in that it is determined on the basis of the incident NA (numerical aperture) of the laser beam (incident light) and the wavelength of the laser beam. Specifically, the lens pitch p of the lens array constituting the optical screen 12 is determined so as to satisfy the following expression (1).

  Here, as shown in FIG. 3A, the lens pitch p is defined as a distance p between the centers of adjacent lenses 12x among the plurality of lenses 12x constituting the lens array. Further, as shown in FIG. 3B, the incident NA is given as “incident NA = sin θ” using an angle θ indicating the spread of the incident light L incident on the optical screen 12.

  Hereinafter, Formula (1) is demonstrated. FIG. 4A shows the emitted light when an ideal optical screen is used. Since an ideal optical screen diffuses incident light uniformly, the laser spot for one pixel spreads uniformly in the emitted light. Therefore, an image by incident light can be viewed in the same way at any viewpoint position.

  FIG. 4B shows emitted light when a lens array is used as the optical screen. When the lens array is used, diffraction occurs due to the periodic structure of the microlens constituting the lens array, and the emitted light does not spread evenly. Therefore, depending on the viewpoint position, there are cases where an image can be seen and where it cannot be seen.

  FIG. 5A shows the lens pitch p of the lens array, and FIG. 5B shows the distribution of diffracted light emitted from the lens array. According to the grating law, the interval of the diffracted light emitted from the lens array depends on the lens period. For example, when a lens array having a lens pitch p shown in FIG. 5A is used, the interval of diffracted light is (2 / √3) · (λ / p).

  On the other hand, according to the scalar diffraction theory, the size of each diffracted light is determined by the incident NA of the incident light. Specifically, as shown in FIG. 6A, when the incident NA is large, the magnitude (angle θ) of the diffracted light becomes large. Further, as shown in FIG. 6B, when the incident NA is small, the magnitude (angle θ ′) of the diffracted light is small.

  As described above, as shown in FIG. 7, when the incident NA is increased to fill the gap of the diffracted light, the emitted light in which the diffracted light is distributed almost uniformly can be obtained. That is, under the condition that the size of the diffracted light is greater than or equal to the interval of the diffracted light, that is, when the relationship of the following expression (2) is established, the light quantity (brightness) distribution of the emitted light is uniform and almost ideal You can get a screen.

  In the emitted light, the emission NA as shown in FIG. 8B is also an important design element. If the emission NA is too small, the viewpoint range becomes narrow, and if it is too large, the brightness of the virtual image becomes insufficient. In order to obtain an injection NA that meets the system specifications, the following condition (A) is further required.

  (A) The spot of incident light needs to be larger than the diameter of a single lens. This is because the performance of a single lens cannot be fully exhibited unless incident light is incident on the entire surface of each lens. Therefore, as shown in FIG. 8A, the diameter of the incident light L is made larger than the diameter of the lens 12x. Here, when the diameter of the Airy disk (r = 1.22λ / NA) is used as the spot diameter of the laser light, the following formula (3) needs to be established.

  From the above formulas (2) and (3), when the formula (1) is established, the light quantity distribution of the emitted light becomes uniform and a desired emission NA can be obtained.

Equation (1) includes a laser wavelength parameter. When the laser light from the laser light source 11 is RGB color laser light, the laser wavelength has a relationship of red>green> blue, so the following equation (4) is obtained. Here, λ _RED is the wavelength of the red laser light, and λ _BLUE is the wavelength of the blue laser light.

Next, a numerical example satisfying the above equation (4) will be examined. As an example, the incident NA of laser light is set to 0.003. Assuming that the wavelength λ _RED of the red laser light of formula (4) is 0.638 nm, the lens pitch p ≧ about 123 μm. Further, when the blue laser wavelength λ _BLUE in Expression (4) is 0.450 nm, the lens pitch p ≦ about 183 μm. Accordingly, the lens pitch p of the optical screen 12 for making the emitted light uniform is 123 to 183 μm.

  As described above, by setting the lens pitch p so as to satisfy the above formula (1) or (4), an optical system (for example, disposed at the rear stage of the optical screen 12 regardless of the viewpoint position). The light quantity distribution of the light emitted from the optical screen 12 can be made uniform without depending on a combiner or the like.

  Ideally, the diffracted light should be arranged without a gap, but in reality, there is a gap between adjacent diffracted lights, or some of the adjacent diffracted lights overlap each other, and this Such gaps or overlapping portions are not strictly uniform. However, since the human eye is not a point, and it looks at the target object with a certain extent, such small gaps and overlaps are visually averaged and recognized. It will look uniform. In the present invention, the diffracted light is assumed to be uniform in this sense.

(Modification)
In the above embodiment, the optical screen 12 is configured as a transmission type, but the optical screen 12 may be configured as a reflection type. FIG. 9 shows an example of a reflective optical screen 12R. The reflective optical screen 12R is produced by forming a reflective film 12a on the curved surface of the lens array of the transmissive optical screen 12. The laser light incident on the lens array is reflected by the reflection film 12a on the lens array. By making the optical screen a reflective type, it is possible to increase the degree of freedom of component placement in the HUD.

  In the optical screen 12, each lens 12x constituting the lens array has a hexagonal shape, but the application of the present invention is not limited to this. In other words, the individual lenses 12x may be squares or other polygons.

  In the optical screen 12, the pitches in all directions of the lens array may not be the same. For example, even if the lens 12x is a hexagon that is crushed in the vertical or horizontal direction, if both the vertical lens pitch and the horizontal lens pitch satisfy Expressions (1) and (4), the emitted light Can be made uniform. Further, there may be an interval between adjacent lenses 12x.

[Other embodiments]
In the above embodiment, the optical screen 12 is constituted by a lens array, but the application of the present invention is not limited to this. The optical screen only needs to have a periodic array, and for example, a diffraction grating may be used. FIG. 10 shows a configuration of an optical screen 22 using a diffraction grating. FIG. 10A is a perspective view of the optical screen 22, and FIG. 10B is a plan view of the optical screen 22. The optical screen 22 using a diffraction grating also diffracts incident light and emits it. At this time, by setting the grating pitch p so as to satisfy the expressions (1) and (4) described above, the light quantity distribution of the emitted light can be made uniform as in the embodiment of the lens array.

5 Combiner 10 Laser Projector 11 Laser Light Source 12, 22 Optical Screen 12x Micro Lens 13 MEMS Mirror

Claims (1)

An optical element that diffuses laser light incident from a laser light source,
Comprising a microlens array or diffraction grating,
The pitch p of the microlens array or diffraction grating is NAinc for the incident numerical aperture of the laser beam and λ for the wavelength of the laser beam.

An optical element characterized by satisfying

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