JP2009092765A - Image display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image display device where flare light (unnecessary diffracted light) generated from a diffraction optical element is hardly recognized by an observer. <P>SOLUTION: The image display device includes a display element 1 displaying an original picture, and an optical system OS guiding light from the display element to an exit pupil. The optical system includes the diffraction optical element 3. An axis leading to the center of the exit pupil from the center of a display area of the original picture on the display element is defined as an optical axis AXL of the optical system, a point on the diffraction optical element is defined as P, and a diffracted light beam having specified wavelength and specified diffraction order where scalar diffraction efficiency is the highest out of diffracted light beams from the point P is defined as a specified diffracted light beam. At such a time, the shape of a grating to which the point P belongs on the diffraction optical element is set so that the specified diffracted light beam may turn to a predetermined point more separate from the diffraction optical element than the exit pupil on the optical axis. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image display device such as a head mounted display (HMD).

  The image display apparatus as described above is required to have a wide angle of view and to be small and light with little discomfort and fatigue even when worn on the head. However, in general, the wider the angle of view, the larger the size and weight of the apparatus, and the more difficult it becomes to correct aberrations in the optical system.

As an example of an optical system for HMD, Patent Document 1 discloses an optical system configured by a single prism using both total internal reflection and a free-form surface. In this optical system, the image light from the display element is refracted by the incident surface (first surface) of the prism, totally internally reflected by the second surface, reflected by the third surface, and further the fourth surface (second surface). ) And refract it to the observer's eye. Since this optical system uses a single prism, it is impossible to correct chromatic aberration caused by dispersion of the material of the prism when image light passes through the refractive surface.
As a means for achromatic, a method of combining a plurality of lenses having different dispersions is known. However, when selecting a material having a dispersion that can provide an achromatic effect, glass must be used substantially, making it difficult to reduce the weight of the optical system. On the other hand, an optical system (see Patent Document 2) using a diffractive optical element (DOE) capable of effective achromatization while suppressing volume and weight is disclosed. However, when light in a wide wavelength range is used to display a color image, it is difficult to suppress flare light caused by unnecessary diffracted light (diffracted light having a diffraction order different from the design order) over the entire angle of view.

  Since DOE has a negative dispersion due to its characteristics, chromatic aberration generated in a general optical material can be effectively corrected. However, the diffraction efficiency is not 100% except for a specific wavelength and a specific incident angle, and unnecessary diffracted light is generated. In this case, when an image with high contrast is displayed, unpleasant flare light is visually recognized in the image. In addition, when displaying a high-frequency image, the contrast is lowered.

On the other hand, a so-called laminated DOE that realizes high diffraction efficiency in a wide wavelength region by arranging a plurality of DOE layers made of different materials and having different design diffraction orders in the vicinity has been proposed.
Japanese Patent No. 2911750 Japanese Patent Laid-Open No. 9-65246

  When an DOE is used in an optical system for an image display device such as an HMD (hereinafter referred to as an HMD optical system), the exit pupil (observation) of the entire HMD optical system from the viewpoint of DOE placement space and DOE manufacturing. It is desirable to install the DOE at a position close to the eyes of the person. This is because the HMD optical system often uses a decentered optical system or a rotationally asymmetric curved surface, so that if the DOE is formed on these complicated optical curved surfaces, it becomes difficult to manufacture the DOE itself. . Further, even if a planar DOE is arranged at a position away from the exit pupil in the HMD optical system, the lattice shape (annular zone shape) becomes complicated, making it difficult to manufacture. By disposing the DOE at a position closest to the exit pupil in the HMD optical system, a rotationally symmetric lattice ring zone can be formed on the planar substrate, and the DOE can be easily manufactured.

  However, when the DOE is installed at a position closest to the exit pupil in the entire HMD optical system, the angle of the diffracted light beam (exit angle or diffraction angle) from the DOE to the exit pupil is close to the angle of view formed by each light beam. It becomes. For example, the exit angle of the light beam from the DOE on the optical axis of the HMD optical system (the straight line followed by the light beam passing through the center of the display element and the center of the exit pupil) is zero. On the other hand, in an HMD optical system with an angle of view of 50 degrees, the exit angle (diffraction angle) of the light beam constituting the maximum angle of view from the DOE is a large angle close to 25 degrees.

  Also, if the DOE is provided with optical power sufficient to carry out an achromatic effect alone, the lattice pitch at a high angle of view is reduced. Therefore, flare light is more likely to be generated.

  As described above, by using the laminated DOE, it is possible to ensure high diffraction efficiency in a wide wavelength range. However, even in an HMD optical system using a conventional laminated DOE, it cannot be said that the generation of flare light is sufficiently suppressed, and further improvements for reducing flare light are required.

  The present invention provides an image display device capable of making it difficult for an observer to recognize flare light (unnecessary diffracted light) generated from a diffractive optical element.

  An image display device according to one aspect of the present invention includes a display element that displays an original image, and an optical system that guides light from the display element to an exit pupil. The optical system includes a diffractive optical element. The axis from the center of the display area of the original image on the display element to the center of the exit pupil is the optical axis of the optical system, the point on the diffractive optical element is P, and the scalar diffraction efficiency is highest among the diffracted light rays from the point P. When a diffracted light beam having a specific wavelength and a specific diffraction order is a specific diffracted light beam, the shape of the grating to which the point P in the diffractive optical element belongs is more distant from the diffractive optical element than the exit pupil on the optical axis. It is set to go to a predetermined point.

  In the present invention, the grating shape is set so that the specific diffracted light beam used for image display is not at the center of the exit pupil but rather toward a predetermined point (for example, a point inside the eyeball) farther from the diffractive optical element. Is done. Thereby, even when the observer rotates the eyeball and observes the peripheral region of the image, it is possible to perform a good image display in which flare light due to unnecessary diffracted light generated by the diffractive optical element is hardly visually recognized.

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

  First, prior to the description of a specific embodiment, the basic concept of the embodiment will be described. The image display apparatus according to the embodiment is an apparatus for performing image observation such as a head mounted display (HMD) or an electronic viewfinder of a camera. The apparatus includes a display element such as an LCD panel or a self-luminous element that displays an original image, and an optical system (hereinafter referred to as an HMD optical system) that guides light (visible light) from the display element to an exit pupil. The HMD optical system includes a single-layer or stacked-type diffractive optical element.

  In the embodiment, the axis from the center of the display area of the original image on the display element to the center of the exit pupil of the HMD optical system is referred to as the optical axis of the HMD optical system. The original image is an image corresponding to image information input from the outside to the apparatus. An enlarged image of the original image by the HMD optical system is presented to the observer.

  Further, in this description, when a certain point on the diffractive optical element is P, the scalar diffraction efficiency is the highest among the diffracted light beams from the point P (hereinafter also referred to as the maximum) and the specific wavelength and the specific diffraction order. Is called a specific diffracted light beam. The scalar diffraction efficiency will be described later.

  The following description includes a description using a forward ray trace (hereinafter referred to as a forward ray trace) and a description using a reverse ray trace. In the forward ray tracing, light rays are emitted from the display element and reach the exit pupil through an HMD optical system including a diffractive optical element (hereinafter referred to as DOE). On the other hand, in the reverse ray tracing, it is assumed that the light ray travels in a direction opposite to the traveling direction of the light ray in the forward ray tracing.

  1A and 1B schematically show a basic configuration of an image display device according to the related art and an embodiment. In these image display devices, the distance (eye relief) between the eyeball 5 of the observer and the forefront of the HMD optical system OS viewed from the eyeball side is set as long as possible. Therefore, the position of the exit pupil 4 of the HMD optical system OS The pupil of the eyeball 5 is disposed on the screen. AXL is the optical axis of the HMD optical system OS described above.

  The HMD optical system OS is designed so that the display element 1 has a predetermined angle of view from the position of the exit pupil 4 along the optical axis AXL or with respect to the optical axis AXL (at an angle with respect to the optical axis AXL). For each ray that goes. This is the same when the phase function of the diffraction grating surface of DOE 3 is determined.

The DOE 3 has a rotationally symmetric shape. Specific shapes of a plurality of gratings (a plurality of concentric annular zones: also referred to as grating annular zones) formed on the diffraction grating surface of the DOE 3 are determined based on the phase function. Each lattice has a lattice surface inclined with respect to a predetermined lattice base surface, and a lattice side surface formed between the lattice surface and a lattice surface of a lattice adjacent to the lattice (see FIG. To explain).
The shape of the lattice mainly means the height of the lattice, that is, the height from the lattice base surface to the apex where the lattice surface of the lattice intersects with the lattice side surface. The grating height is set so that the diffraction efficiency of the diffracted light (specific diffracted light) generated from the light incident on the grating and used as image display light (image light) is the highest (maximum). .

  Here, on the DOE 3 (diffraction grating plane), the height d of the grating (grating ring zone) to which the point P having a radial distance r from the center (position on the optical axis AXL) belongs (FIG. 4A and FIG. Consider the optimization of FIG. 4Bx).

The wavelength (for example, visible wavelength) of light incident on a certain grating is λ, and the incident angle (angle formed with respect to the grating base surface) is θ ₁ . Further, the pitch of the lattice of DOE 3 is p. Further, in the reverse ray tracing, the refractive index on the incident side of the DOE 3 is n ₁ ( _{1 in} the case of air), the refractive index on the outgoing side is n ₂ , the diffraction order is m, and the diffraction angle of the m-th order diffracted light beam. Is θ ₂ , the optical path difference function Φ is expressed by the following equation (1).
Φ (r, λ) = {n ₁ (λ) · cos θ ₁ −n ₂ (λ) · cos θ ₂ } · d (r)
... (1)
However, θ ₂ = sin ⁻¹ [{n ₁ (λ) · sin θ ₁ −m · λ / p} / n ₂ ].

The scalar diffraction efficiency (scalar approximation diffraction efficiency: hereinafter simply referred to as diffraction efficiency) η is
η (r, λ) − = sinc ² [π · {m−Φ (r, λ) / λ}] (2)
It becomes.

  The value of the height d of each grating is optimized so that η is maximized for a specific diffracted ray (for example, m = 1). The maximum diffraction efficiency η may be 100% or slightly lower than 100%.

Conventionally, in FIG. 1A, a light ray incident on the point P is a light ray directed from the center O of the exit pupil 4 of the HMD optical system OS toward the point P, and η is maximized using θ ₁ and θ ₂ in this case. D was set as follows. This maximizes the diffraction efficiency of the specific diffracted light that enters the pupil and reaches the retina when the observer's line of sight is directed in the direction of the optical axis, so that unwanted diffracted light (for example, m = 0 or 2) is maximized. The diffraction efficiency is the lowest (minimum).

However, when an observer observes a certain area (for example, a peripheral area of the display image) in the display image, the observer often turns the eyeball in the direction of the area and stares at the area. At this time, in a normal ray tracing, the light from the point P toward the center of the pupil (the center of the exit pupil 4) is not a ray passing through the center of the exit pupil 4 of the HMD optical system OS, as shown in FIG. It is a light ray that passes through the rotation center Q of the eyeball 5 (or its vicinity). Further, in backward ray tracing, the ray incidence angle at the time of entering the point P on DOE 3, a different θ _{1 '(<θ1)} and theta ₁ shown in Figure 1A.

Conventionally, the value of d at the point P is optimized for the incident angle θ ₁ and is not optimized for the light ray incident at θ ₁ ′. For this reason, when the eyeball 5 rotates from the state in which it is directed to the optical axis direction and the other direction is observed, the intensity of unnecessary diffracted light that enters the eyeball 5 through the pupil increases in the normal ray tracing.

In addition, when the eyeball 5 is oriented in the optical axis direction (gazing at the center area of the display image), the light beam passing from the point P through the center of the pupil has the highest sensitivity and spatial resolution in the retina. The region outside the part called the fovea 6 is irradiated. In this case, even if a slight amount of unnecessary diffracted light remains, the degree of visibility is low.

  On the other hand, when the unnecessary diffracted light from the point P is incident on the pupil by rotating the eyeball 5, the unnecessary diffracted light reaches the fovea 6. Therefore, even a small amount of unnecessary diffracted light is visually recognized as flare light. It becomes easy to prevent good image observation.

In order to solve this problem, in the embodiment, when determining the height of each grating on the DOE ₃ , θ ₁ and θ ₂ in the equation (1) are set as follows. That is, θ ₁ and θ ₂ are incident angles of light rays (specific diffracted light rays) incident on a point P on the lattice from a predetermined point Q ′ located farther from the exit pupil 4 than the exit pupil 4 on the optical axis. Is the diffraction angle. Then, the grating height d is determined so that the diffraction efficiency for the specific diffracted light beam is maximized.

  In other words, in the forward ray trace, the specific diffracted light beam having the highest diffraction efficiency is directed to the predetermined point Q ′ farther from the exit pupil 4 on the optical axis AXL than the DOE 3 in the forward ray trace. Set as follows.

  As a result, when the eyeball 5 rotates from a state in which it is oriented in the optical axis direction and the other direction is observed, unnecessary diffracted light incident on the pupil is suppressed, and observation is performed even if unnecessary diffracted light is incident on the fovea 6. It becomes difficult for a person to see. Further, when the image center region is observed with the eyeball 5 facing the optical axis direction, even if a small amount of unnecessary diffracted light incident at an angle on the center of the pupil (center of the exit pupil 4) is generated, the retina Since the light is incident on a region with low visibility outside the fovea 6, it is difficult to visually recognize as flare light. Actually, since unnecessary diffracted light other than the specific diffracted light is defocused on the retina, the visibility is further lowered.

  The distance from the pupil to the rotation center Q of the eyeball 5 is generally said to be around 10 mm. The “predetermined point Q ′” on the optical axis AXL described above corresponds to a point separated from the exit pupil 4 of the HMD optical system OS by a distance between the pupil and the rotation center Q of the eyeball 5, that is, the rotation center Q. It is good to set to the point to do. The point corresponding to the rotation center Q here means not only a point that completely coincides with the rotation center Q but also a point that can be regarded as coincident with the rotation center Q or a point that is close to the rotation center Q.

  In other words, the distance (eye relief) from the forefront of the HMD optical system OS to the exit pupil 4 viewed from the eyeball side is L, and the distance from the forefront of the HMD optical system OS to the predetermined point Q ′ is When L ′ is satisfied, a certain effect can be obtained if L ′> L is satisfied.

  In this embodiment, it is better to use a laminated DOE composed of a plurality of diffraction grating layers made of different materials and having different designed diffraction orders as the DOE 3. In the following description, a laminated DOE of a type in which an air layer as an intermediate layer is interposed between a plurality of diffraction grating layers is used. However, the grating surfaces of the plurality of diffraction grating layers may be in close contact with each other.

  DOE can basically achieve the maximum diffraction efficiency only for a specific diffracted light beam of a single wavelength and diffraction order in a single layer. For this reason, when a display element that displays an image of the three primary colors of visible light is used, high diffraction efficiency cannot be obtained for all colors (wavelengths), and as a result, colored flare light is generated. Become.

  On the other hand, by using the above-described laminated DOE, the maximum diffraction efficiency can be obtained for each of the specific diffracted light beams of the three primary colors (that is, in a wide wavelength region). FIG. 2 shows diffraction efficiency characteristics of a single-layer DOE and a laminated (two-layer) DOE.

At this time, the grating ring zones of the two diffraction grating layers in the laminated DOE are both rotationally symmetric. The heights of the gratings to which the point P having a radial distance r from the center on the diffraction grating surface in these two diffraction grating layers (hereinafter referred to as the first layer and the second layer in order from the exit pupil side) are d _1. , D ₂ (see FIGS. 6A and 6B). In the reverse ray tracing, the wavelength of light incident on the first layer is λ, the incident angle is θ _1, and the grating pitch of the first layer and the second layer is p. The first layer the refractive index of the (incident side grating layer) and n _1, the refractive index of the intermediate layer (air layer) and n _2, a second layer of refractive index of the (exit side grating layer) n ₃ And Further, the diffraction order at the first layer is m ₁ , the m _1st order diffraction angle is θ ₂ , the diffraction order at the 2nd layer is m ₂ , and the m _2nd order diffraction angle is θ ₃ . . In this case, the optical path difference functions Φ ₁ and Φ ₂ of the first layer and the second layer are respectively expressed as follows.

Φ ₁ (r, λ) = {n ₁ (λ) · cos θ ₁ −n ₂ (λ) · cos θ ₂ } · d ₁ (r)
However, θ ₂ = sin ⁻¹ [{n ₁ (λ) · sin θ ₁ −m ₁ · λ / p} / n ₂ ]. ... (3a)
Φ ₂ (r, λ) = {n ₂ (λ) · cos θ ₂ −n ₃ (λ) · cos θ ₃ } · d ₂ (r)
However, θ ₃ = sin ⁻¹ [{n ₂ (λ) · sin θ ₂ −m ₂ · λ / p} / n ₃ ]. ... (3b)
The diffraction efficiency (scalar approximation diffraction efficiency) η for the two layers is
η (r, λ) = sinc ² [π · {m− {Φ ₁ (r, λ) + Φ ₂ (r, λ)} / λ}] (4)
However, m = m ₁ + m ₂ .

Even when such a stacked (two-layer) DOE is used, the height of each lattice of the first and second layers is determined in the same manner as when a single-layer DOE is used. That is, the theta ₁ through? ₃ in the formula (3a) (3b), from a predetermined point Q 'on the optical axis AXL away against DOE3 than the exit pupil 4 of the HMD optical system OS to the point P on the grid The incident angle and diffraction angle of the incident light beam (specific diffracted light beam) are used. Then, the height of the grating is determined so that the diffraction efficiency for the specific diffracted light beam is maximized.

  In other words, in the forward ray trace, the specific diffracted light beam having the highest diffraction efficiency is directed to the predetermined point Q ′ farther from the exit pupil 4 on the optical axis AXL than the DOE 3 in the forward ray trace. Set as follows.

  In this embodiment, the DOE 3 is preferably arranged so as to constitute an optical surface closest to the exit pupil 4 among a plurality of optical surfaces each having optical power in the HMD optical system OS.

  The HMD optical system OS of the present embodiment also uses an eccentric optical system and a rotationally asymmetric curved surface. For this reason, if it is going to form DOE on these complicated optical curved surfaces, manufacture of DOE itself will become difficult. Furthermore, even if a planar DOE is disposed at a position away from the exit pupil 4 in the HMD optical system OS, the lattice shape (annular zone shape) becomes complicated, making it difficult to manufacture.

  On the other hand, by arranging the DOE 3 so as to constitute an optical surface closest to the exit pupil 4 among the plurality of optical surfaces having optical power in the HMD optical system OS, a rotationally symmetric lattice is formed on the planar substrate. A DOE can be easily manufactured by forming an annular zone.

Here, let us consider a light beam having a high angle of view when the DOE 3 is arranged in this manner. For example, the distance on the optical axis AXL from the exit pupil 4 to the diffraction grating surface of the DOE 3 is 20 mm, and the maximum angle of view (one side) in the diagonal direction from the center of the exit pupil 4 to the diffraction grating surface of the DOE 3 is 40 degrees. And In the reverse ray tracing, the incident angle of the light beam with the maximum field angle to the diffraction grating surface is 40 degrees as it is, and the distance from the optical axis AXL at the incident position on the DOE 3 is
20 × tan (40 °) = 16.78 mm
It becomes.

On the other hand, when the light beam having the maximum angle of view is not a light beam from the center of the exit pupil 4 but a light beam passing through a predetermined point Q ′ that is 10 mm away from the exit pupil 4 with respect to the DOE 3, diffraction of the light beam is performed. The angle of incidence on the grating surface is
tan ⁻¹ {16.78 / (20 + 10)} = 29.22 °
It becomes. This incident angle differs by 10 degrees or more (small) with respect to the former.

  Therefore, assuming that the grating height is optimized so that the diffraction efficiency is maximized for the light ray passing through the center of the exit pupil 4 as in the former case, the eyeball 5 is actually rotated in the direction of the field angle of the light ray. Sometimes, a non-negligible level of unwanted diffracted light reaches the fovea 6 through the center of the pupil. In order to prevent this, as in the latter case, the diffraction efficiency is maximized for light rays passing through a predetermined point Q ′ (point corresponding to the rotation center Q of the eyeball 5) 10 mm away from the DOE 3 than the exit pupil 4. It is desirable to optimize the grid height so that

  Next, the shape of each lattice will be described. 3A and 3B show a lattice shape on a conventional DOE 3. Each grating 3a has a grating surface 3b that is inclined with respect to a grating base surface 3d as a reference surface on which the grating 3a is formed and diffracts light passing through the grating 3a (that is, that causes a diffraction effect). Each lattice 3a has a lattice side surface 3c formed between the lattice surface 3b of the lattice 3a and the lattice surface 3b 'of the lattice 3a' adjacent to the lattice 3a.

  FIG. 3A shows a grating 3a formed in a low angle of view area (area on and near the optical axis AXL) and light rays passing therethrough, and FIG. 3B shows a grating 3a formed in a high angle of view area ( 3a ') and rays passing through it. 3A and 3B are shown by normal ray tracing.

  3A and 3B, the grating side surface 3c is formed in parallel to the normal 3n (optical axis AXL) of the grating base surface 3d. When the grating side surface 8 is formed in parallel to the normal 3n, the light beam in the low field angle region shown in FIG. 3A travels substantially parallel to the grating side surface 3c. Further, the lattice pitch in the low field angle region is coarser than the lattice pitch in the high field angle region. For this reason, almost no light is incident on the grating side surface 3c, and there is little flare light generated when the light beam is reflected (or refracted) by the grating side surface 3c.

  On the other hand, the light in the high angle of view region shown in FIG. 3B is incident on the DOE 3 obliquely, and the lattice pitch in this region is fine. For this reason, light rays incident on the grating side surface 3c increase, and flare light (unnecessary light) generated when the light rays are reflected by the grating side surface 8 also increases. In particular, as described above, when the DOE 3 is arranged so as to form an optical surface closest to the exit pupil 4 among a plurality of optical surfaces having optical power in the HMD optical system OS, light rays in a high angle of view range. Since the incident angle to the DOE 3 increases, flare light generated on the grating side surface 8 tends to increase.

  Here, in Japanese Patent Laid-Open No. 2003-294924, as shown in FIG. 4A, the grating side surface 3c is tilted to an angle equal to the incident angle or diffraction angle of the light beam with respect to the grating 3a, and the light beam is incident on the grating side surface 3c. A method for avoiding and preventing a decrease in diffraction efficiency is disclosed.

  As shown in FIG. 4A, it is assumed that the pupil of the eyeball 5 is at the position of the exit pupil 4 of the HMD optical system, and the eyeball 5 faces the optical axis direction (image center direction). A method of determining the inclination angle of the lattice side surface 3c of the lattice 3a on the DOE 3 at this time (the angle formed with respect to the normal 3n of the lattice base surface 3d: hereinafter referred to as the lattice side surface angle) will be described.

Angle at which rays from the point P on the grid 3a towards the center of the exit pupil 4 (principal ray) with respect to the normal line 3n of the lattice base surface 3d (hereinafter, light that angle) is defined as theta _1. When the grating side surface angle is equal to the incident angle (θ ₁ ) of the light beam from the grating 3a, the light beam from the point P passes through the center of the exit pupil 4 (pupil) without being reflected by the grating side surface 3c. To reach the retina.

However, since the observer actually rotates the eyeball 5 in the direction of the area when observing each area of the image, as shown in FIG. 4B, the light beam passing through the center of the pupil is centered on the exit pupil 4. It is not a light beam that passes through, but a light beam that passes through the rotation center Q of the eyeball 5 (or its vicinity). Ray angle at which the light beam is directed toward the rotation center Q of the eye 5 (or the vicinity thereof) from the point P, the theta ₁ shown in FIG. 4A is a different _{θ 1 '(<θ 1)} .

If the light beam from the point P on the grating 3a shown in FIG. 4A has the light beam angle θ ₁ ′, the light beam is reflected or refracted by the grating side surface 3c and becomes unnecessary light. For this reason, when an observer rotates the eyeball 5 and observes the direction of the point P, the intensity | strength of the unnecessary light observed increases.

  Further, when the eyeball 5 is directed in the optical axis direction, the light beam from the point P to the center of the pupil reaches the region outside the fovea 6 in the retina, so even if there is a little unnecessary light. The visibility is low. On the other hand, when the eyeball 5 is rotated in the direction of the point P, the light beam reaches the central fovea 6, so that even a few unnecessary light beams are easily visible.

For this reason, in the present embodiment, as shown in FIG. 4B, the light beam directed to the predetermined point Q ′ that is farther from the DOE 3 than the exit pupil 4 through the lattice surface 3 b passes through the lattice side surface 3 c of the lattice 3 a. It is inclined with respect to the normal 3n of the grating base surface 3d so as not to enter the side surface 3c.

More specifically, an angle (ray angle) formed by a light ray traveling toward the predetermined point Q ′ after passing through the lattice surface 3b with respect to the normal line n of the lattice base surface 3d is defined as θ ₁ ′. The angle (grating side angle) formed by the grating side surface 3c with respect to the normal n of the grating base surface 3d is set to an angle (θ ₁ ′) equal to the ray angle. As a result, light rays in a high angle of view also travel in parallel with the grating side surface 3c, and flare light generated when the light rays are reflected or refracted by the grating side surface 3c is reduced.

  However, the light ray angle and the grating side surface angle do not necessarily have to be equal, and the grating side surface angle may be set so that the light beam toward the predetermined point Q ′ does not enter the grating side surface 3c.

  By setting the grating side surface angle in this way, even when the eyeball 5 rotates in a direction other than the optical axis direction, unnecessary light incident on the fovea 6 is reduced, and thus it is difficult to be visually recognized as flare light. Conversely, when the eyeball 5 is oriented in the optical axis direction, even if a small amount of unnecessary light is incident on the center of the pupil (the center of the exit pupil 4) at an angle, the unnecessary light is transmitted through the retina. Of these, it reaches a region with low visibility outside the fovea 6, so that it is difficult to see as flare light. Actually, since unnecessary light is defocused on the retina, the visibility is further reduced.

  Note that the above-described setting of the grating side surface angle is also effective for suppressing generation of unnecessary light in the stacked DOE.

For example, as shown in FIG. 5, consider a stacked DOE having a second layer, an air layer, and a first layer in order from the display element side (right side in the figure) to the exit pupil side (left side in the figure). At this time, the incident angle of the light beam on the diffraction grating surface of the second layer is θ _3, and the m _2nd- order diffraction angle (incident angle on the first layer) from the diffraction grating surface of the second layer is θ ₂ . To do. In addition, the m _1st- order diffraction angle from the diffraction grating surface of the first layer is defined as θ ₁ ′. The light beam emitted from the first layer at the diffraction angle θ ₁ ′ is directed to the predetermined point Q ′.

In such a stacked DOE, for example, by setting the lattice side angle of the second layer to θ ₂ and the lattice side angle of the first layer to θ ₁ ′, as shown in FIG. Reflection and refraction of light rays can be substantially avoided, and generation of unnecessary light can be suppressed.

In the laminated DOE, the grating side surface angle may be set to be equal to the incident angle of the light beam to each layer, not the diffraction angle from each layer. Specifically, as shown in FIG. 6B, the lattice side angle of the second layer may be θ ₃ and the lattice side angle of the first layer may be θ ₂ . Further, the grating side surface angle of the second layer may be θ ₃ and the grating side surface angle of the first layer may be θ ₁ ′. Even in these cases, it is possible to suppress generation of unnecessary light due to reflection and refraction at the grating side surfaces.

  Hereinafter, more specific examples of the present invention will be described.

  FIG. 7 shows the configuration of an image display apparatus (HMD) that is Embodiment 1 of the present invention. Reference numeral 1 denotes a display element constituted by a self-luminous element such as a transmissive LCD or an organic EL. The display element 1 is driven by a panel driver 11 that receives image information (input image) from an image supply device 10 such as a personal computer, a DVD player, or a TV tuner. As a result, an original image corresponding to the input image is displayed on the display element 1.

  Reference numeral 2 denotes a magnifying optical system composed of a prism as an optical element. The magnifying optical system 2 has three decentered free-form surfaces as the entrance surface 2a, the reflection / transmission surface 2b, and the reflection surface 2c.

  Reference numeral 3 denotes a single-layer DOE made of a resin material and formed on the glass substrate 3e. The magnifying optical system 2 and the DOE 3 constitute an HMD optical system OS. Reference numeral 4 denotes an exit pupil of the HMD optical system OS.

  The DOE 3 is disposed at a position closest to the exit pupil 4 among the four surfaces (incident surface 2a, reflection / transmission dual-purpose surface 2b, reflection surface 2c, and DOE 3) having optical power constituting the HMD optical system OS.

  In this embodiment, the optical design is performed by setting the angle of view of the HMD optical system OS to 50 degrees, setting the exit pupil 4 at a position 20 mm from the diffraction grating surface of the DOE 3, and setting the pupil diameter to 14 mm.

The grating ring zone of DOE 3 has a rotationally symmetric shape (concentric shape) around the optical axis AXL, and the phase function φ at the diffraction grating surface is
φ (r) = C ₁ · r ² + C ₂ · r ⁴ + C ₃ · r ⁶ + C ₄ · r ⁸
However, C ₁ = −9.88792 × 10 ⁻⁴
C ₂ = 1.5989 × 10 ⁻⁷
C ₃ = 2.9680 × 10 ⁻⁹
C ₄ = −4.182 × 10 ⁻¹²
r is the distance in the radial direction from the optical axis AXL.

Here, the radius R (k) of each lattice ring zone is
φ (R (k)) = − k · λ
R (k) satisfying the above, and the pitch between the lattice rings (lattice pitch) p (k) is
p (k) = R (k) -R (k-1)
It is represented by

  Now, in reverse ray tracing, the height (grid height) d of k = 290th lattice ring zone is obtained.

Design wavelength (specific wavelength) of DOE: λ = 587.56 nm
Refractive index of DOE material: n ₁ (λ) = 1.5709
Then, the radius R (290) of the lattice ring zone (k = 290) is 13.958 [mm] from the above phase function, and the lattice pitch p (290) is 29.667 [μm]. Then, the incident angle of the light beam incident on the grating ring zone (k = 290) from the predetermined point Q ′ on the optical axis AXL farther from the DOE 3 than the exit pupil 4 is θ ₁ ′, and the specific diffraction order is the first order. It is assumed that the first-order diffraction angle is θ ₂ and the predetermined point Q ′ is 10 mm from the exit pupil 4 in the optical axis direction.

In this case, the incident angle θ ₁ ′ is
θ ₁ ′ = tan ⁻¹ {13.958 / (20 + 10)}
= 24.951 [°]
It becomes.

The diffraction angle θ ₂ is
θ ₂ = sin ⁻¹ [{1 · sin θ ₁ −1 · λ / p} / n ₁ ]
= 14.829 [°]
It becomes.

The optical path difference function Φ at this time follows the equation (1):
Φ (R (290), λ) = {1 · cos θ ₁ −n ₁ (λ) · cos θ ₂ } · d (290)
It is represented by

The diffraction efficiency η is
η (R (290), λ) − = sinc ² [π · {1-Φ (R (100), λ) / λ}]
It becomes.

At this time, in order to set η to 100%, the height d (290) of the lattice ring zone (k = 290) is
d (290) = 1 · λ / {1 · cos θ ₁ −n ₁ (λ) · cos θ ₂ } (5)
= 0.960 [μm]
And it is sufficient.

Assuming that the incident angle of a light ray incident on the lattice ring zone (k = 290) from the center of the exit pupil 4 is θ ₁ ,
θ ₁ = tan ⁻¹ (13.958 / 20)
= 34.912 [°]
It becomes. In this case, θ ₂ = 20.592 [°], and the grating height at which the diffraction efficiency is 100% is d = 0.903 [μm]. However, when this grating height is set, the diffraction efficiency η (λ) of the first-order diffracted light becomes 98.85% for the light incident at θ ₁ = 24.951 °, and the zero-order light and the second-order light. Unwanted diffracted light as light is generated with diffraction efficiencies of 0.35% and 0.45%, respectively. This is because when the eyeball is rotated in the direction of the lattice ring zone, the unnecessary diffracted light having the above amount reaches the central fovea 6 of the retina, and the visibility of the unnecessary diffracted light (flare light) is increased. End up.

The inclination angle (grid side surface angle) of the lattice side surface of the lattice ring zone (k = 290) is 24.951 [°] equal to θ ₁ ′.

  With the above setting method, the lattice height and the lattice side surface angle are set for all lattice ring zones.

  FIG. 8 shows the configuration of an image display apparatus (HMD) that is Embodiment 2 of the present invention. Reference numeral 1 denotes the same display element as in the first embodiment, and reference numeral 2 also denotes the same magnification optical system as in the first embodiment. Therefore, the magnifying optical system 2 has three decentered free-form surfaces as the entrance surface 2a, the reflection / transmission surface 2b, and the reflection surface 2c.

  3 is a laminated DOE having a first diffraction grating layer (hereinafter referred to as a first layer) I and a second diffraction grating layer (hereinafter referred to as a second layer) II made of a resin material formed on the glass substrates 3f and 3g. It is. The magnifying optical system 2 and the DOE 3 constitute an HMD optical system OS. Reference numeral 4 denotes an exit pupil of the HMD optical system OS.

  Also in the present embodiment, the DOE 3 is located at the position closest to the exit pupil 4 among the four surfaces (incident surface 2a, reflection / transmission surface 2b, reflection surface 2c, and DOE 3) having the optical power constituting the HMD optical system OS. Has been placed.

  In this embodiment, the optical design is performed with the angle of view of the HMD optical system OS, the position of the exit pupil 4 from the diffraction grating surface of the DOE 3 and the pupil diameter as those of the first embodiment. The grating ring zone of DOE 3 has a rotationally symmetric shape (concentric circle shape) around the optical axis AXL, and the phase function φ on the diffraction grating surface is the same as that described in the first embodiment.

  In the present embodiment, the diffraction grating layer on the exit pupil side of the stacked DOE 3 is referred to as a first layer I, and the diffraction grating layer on the display element side is referred to as a second layer II. In the present embodiment, the first layer I has a positive power and the second layer II has a negative power.

The material of the first layer I has a refractive index n ₁ (λ) of 1.524 and a dispersion of 51.57. The material of the second layer II has a refractive index n ₂ (λ) of 1.635 and a dispersion of 22.75. The first and second layers I and II are arranged so as to be close to each other with an air layer between them and the diffraction grating surfaces face each other. The glass substrates 3f and 3g that hold the first and second layers of the laminated DOE 3 respectively holding I and II have a thickness of 3 mm and a refractive index of 1.516.

Assuming that the position of the exit pupil 4 of the HMD optical system OS is 20 mm from the exit pupil side surface of the glass substrate 3f on the first layer side, the lattice height of the k = 290th lattice ring zone in the reverse ray trace d ₁ and d ₂ are obtained.

  The radius R (290) of the grating ring zone is 13.958 [mm] from the above phase function, and the grating pitch p (290) is 40.145 [μm].

The incident angle of a light beam incident on the grating ring zone (k = 290) from a predetermined point Q ′ on the optical axis AXL farther from the DOE 3 than the exit pupil 4 is θ ₁ ′, the specific diffraction order is first order, 1 It is assumed that the next diffraction angle is θ ₂ and the predetermined point Q ′ is at a position 10 mm from the exit pupil 4 in the optical axis direction.

In this case, the incident angle θ ₁ ′ from the predetermined point Q ′ to the R (290) position on the diffraction grating surface of the first layer I has the glass substrate 3 f interposed therebetween,
θ ₁ ′ = 15.258 [°]
It becomes.

At this time, the diffraction orders of the first and second layers I and II are set so that the diffraction efficiency of the laminated DOE obtained from the equation (4) is maximized in a wide wavelength range.
m ₁ = 8.5
m ₂ = −7.5
That is, m = m ₁ + m ₂ = 1.

And like (5) Formula, the lattice height of each layer is
d ₁ = 9.875 [μm]
d ₂ = 7.087 [μm]
It is obtained.

On the other hand, the incident angle of the light ray that enters the lattice ring zone (k = 290) from the center of the exit pupil 4 is θ ₁ = 20.733 [°]. In this case, the diffraction orders of the first and second layers are set to m ₁ = 8 and m ₂ = −7, and m ₁ + m ₂ = 1 so that the diffraction efficiency is maximized in a wide wavelength range. And The lattice height of each layer at this time is
d ₁ = 9.368 [μm]
d ₂ = 6.630 [μm]
It becomes.

FIG. 9A shows first-order, zero-order, and second-order diffracted light in a stacked DOE in which the grating height is optimized with respect to the _first -order diffracted light having a diffraction angle θ ₁ = 20.733 [°] in the forward ray trace. The diffraction efficiency is shown by a dotted line. In this laminated DOE, a diffraction efficiency close to 100% is obtained for the first-order diffracted light in the blue, red, and green wavelength bands.

However, for the _first -order diffracted light having the diffraction angle θ ₁ ′ = 15.258 [°] shown by the solid line in the figure, the diffraction efficiency is reduced by 2-3% in the same wavelength band. On the contrary, the diffraction efficiency of the 0th-order and 2nd-order unnecessary diffracted light exceeds 1%. This is because unnecessary diffracted light that has entered the pupil from the direction of the grating ring zone of k = 290 cannot be seen when the eyeball 5 is oriented in the optical axis direction, but is not necessary if the eyeball 5 is rotated in the direction of the grating ring zone. It means that the diffracted light becomes noticeable.
On the other hand, FIG. 9B shows the first-order, zero-order, and zero-order in the laminated DOE in which the grating height is optimized with respect to the first-order diffracted light having the diffraction angle θ ₁ ′ = 15.258 [°] in the forward ray trace. The diffraction efficiency of the second-order diffracted light is indicated by a dotted line.

In this laminated DOE, for the _first -order diffracted light having a diffraction angle θ ₁ ′ = 15.258 [°], a diffraction efficiency close to 100% is obtained in the blue, red, and green wavelength bands (specific wavelength region). In addition, the diffraction efficiencies of the zero-order and second-order unnecessary diffracted light are lower than 1% and close to 0%. Therefore, when the eyeball 5 is directed in the direction of the lattice ring zone of k = 290, the unnecessary diffracted light is hardly visually recognized.

In this laminated DOE, as shown by a solid line in FIG. 9B, the diffraction efficiency of the 0th order and second order diffracted light at θ ₁ = 20.733 [°] is 0.1 in the blue and green wavelength bands. Although it is very small, less than 1%, it exceeds 1% in the red wavelength band. However, since the eyeball when the red unnecessary diffracted light is incident on the pupil is oriented in the optical axis direction, the unnecessary diffracted light reaches a region outside the central fovea 6 of the retina. Therefore, the visibility of the unnecessary red diffracted light is very small and does not cause a problem.

  As described above, in the above embodiment, the lattice shape of the DOE 3 is set so that the specific diffracted light beam used for image display is not directed to the center of the exit pupil 4 but to a predetermined point farther from the DOE 3 than that. The Therefore, the visibility of unnecessary diffracted light (flare light) generated in the DOE 3 can be reduced.

  Each embodiment described above is only a representative example, and various modifications and changes can be made to each embodiment in carrying out the present invention.

The block diagram of the image display apparatus using DOE by which lattice height is determined by the conventional method. 1 is a configuration diagram of an image display device using a DOE whose lattice height is determined by the method of an embodiment of the present invention. The figure which shows the diffraction efficiency characteristic of single layer DOE and lamination | stacking DOE. Sectional drawing which shows the grating | lattice shape in the optical axis vicinity part of the conventional single layer DOE. Sectional drawing which shows the lattice shape in the peripheral part of the conventional single layer DOE. Sectional drawing which shows the grating | lattice side surface angle in the conventional single layer DOE (Unexamined-Japanese-Patent No. 2003-294924). Sectional drawing which shows the grating | lattice side surface angle in the single layer DOE of an Example. The figure which shows the mode of the diffraction of the light beam in lamination | stacking DOE. Sectional drawing which shows the example of the grating | lattice side surface angle in the lamination | stacking DOE of an Example. Sectional drawing which shows the other example of the grating | lattice side surface angle in the lamination | stacking DOE of an Example. The block diagram of the image display apparatus of the Example using single layer DOE. The block diagram of the image display apparatus of the Example using laminated | multilayer DOE. The figure which shows the example of the diffraction efficiency of the conventional lamination | stacking DOE. The figure which shows the example of the diffraction efficiency of the lamination | stacking DOE of an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Display element 2 Magnification optical system 3 DOE
3a Lattice 3b Lattice surface 3c Lattice side surface 3d Lattice base surface 4 Exit pupil 5 Eyeball 6 Fovea Q Q Center of rotation of the eye Q 'Predetermined point

Claims (5)

A display element for displaying the original image;
An optical system for guiding the light from the display element to the exit pupil,
The optical system includes a diffractive optical element,
An axis from the center of the display area of the original image to the center of the exit pupil in the display element is an optical axis of the optical system, a point on the diffractive optical element is P, and a scalar out of diffracted rays from the point P When a specific wavelength and a specific diffraction order diffracted light beam having the highest diffraction efficiency are used as the specific diffracted light beam,
The shape of the grating to which the point P in the diffractive optical element belongs is set so that the specific diffracted light beam is directed to a predetermined point farther from the diffractive optical element than the exit pupil on the optical axis. An image display device characterized by the above.   The image display apparatus according to claim 1, wherein the predetermined point corresponds to a rotation center when an eyeball of an observer arranged at the position of the exit pupil rotates.   The image display apparatus according to claim 1, wherein the diffractive optical element is configured by stacking a plurality of diffraction grating layers.   The diffractive optical element constitutes an optical surface closest to the exit pupil among a plurality of optical surfaces each having an optical power in the optical system. Image display device. Each grating is inclined with respect to a grating base surface in the diffractive optical element to generate a diffraction action, and a grating side surface formed between the grating surface and a grating surface of a grating adjacent to the grating Have
5. The grating side surface is inclined with respect to a normal line of the grating base surface so that a light beam directed to the predetermined point through the grating surface does not enter the grating side surface. The image display apparatus as described in any one of these.