JP2006146093A - Front plate and display apparatus using same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a front plate for improving bright-room contrast and reducing deterioration in picture quality and decrease in contrast due to crosstalk between pixels or stray light, and to provide a display apparatus such as a plasma display using the front plate. <P>SOLUTION: Acicular light guides in a truncated cone form having a predetermined curvature in an inclined surface are arrayed, so that exiting light from a display element is propagated inside the acicular light guides by total reflection while external light is attenuated by repetition of multiple reflection on the surfaces forming wedge faces. By applying a light absorbing film onto the surface of the acicular light guides, the contrast ratio is further increased. The above structure results in the bright-room contrast. By lengthening the arrayed acicular light guides to bring the light entrance faces of the acicular light guides into close contact with the display screen of a display panel, crosstalk in adjacent pixels is decreased and decrease in contrast by display light is reduced. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a display device that reduces a decrease in contrast (hereinafter referred to as bright room contrast) due to external light when a display screen is viewed in a bright room.

  2. Description of the Related Art Conventionally, in a display device using a display panel in which self-luminous pixels such as a plasma display (hereinafter referred to as PDP) are arranged in a matrix, light incident on a front plate (hereinafter, “ Output light from the display panel by attenuating the external light while the light is reflected on the opposite surface of the front plate or reflected by the display panel (that is, while reciprocating the front plate twice). There is a technique for improving the contrast by increasing the ratio of the light to outside light. Such a technique is disclosed, for example, in page 2, right column, paragraph number [0009] of Patent Document 1.

  Other bright room contrast enhancement means include a technology to provide an anti-reflection film that does not reflect external light on the external light incident surface of the front plate, all of which improve the bright room contrast by preventing reflection by external light. It is something to be made. These techniques are disclosed in FIG. 4 of Patent Document 2, for example.

  Patent Document 3 discloses a front plate having a role of reducing unnecessary radiation, a color temperature adjusting function, an infrared ray reducing effect, a panel glass protecting function, and the like in addition to adjusting the transmittance.

  Furthermore, another means for improving the contrast in a bright room is similar to a wedge-shaped projection that absorbs sound and radio waves without reflecting them, for example, a frustum-shaped projection such as a truncated cone is emitted from the transmission screen. There is technology to form on the surface. These techniques are disclosed in, for example, Patent Document 4 and Patent Document 5.

JP 2000-206932 A (page 2, right column, paragraph number [0009]) JP 2002-83548 A (FIG. 4) Japanese Patent Laid-Open No. 10-283939 JP-A-6-194741 Japanese Patent Laid-Open No. 2003-50307

  However, in the techniques disclosed in Patent Documents 1 to 3, the output light is reduced by the transmittance τ together with the external light, and it is necessary to increase the emitted light in advance. In any case, the function of improving the bright room contrast by adjusting the transmittance is not changed, but there is a problem that the light emitted from the light emitting element is attenuated. Therefore, even if the technique disclosed in the above-mentioned patent document is combined with the original display panel with low output luminance or display panel with low light output efficiency, it is not a drastic solution for improving bright room contrast.

  Further, in Patent Documents 4 to 5, a technique for forming a truncated cone-like projection such as a truncated cone on the exit surface of the transmission screen is disclosed, but directional light is emitted on the exit surface of the transmission screen. It is assumed that it is input. On the other hand, for example, in a self-luminous display panel in which a light emission pattern such as a PDP follows a Lambertian distribution, the emitted light is scattered. Cannot be improved. Hereinafter, this will be described in detail with reference to FIG.

  FIG. 8 is a view showing an example in which the optical plate on which the above-described convex protrusions are formed is used as a front plate. As shown in FIG. 8, for example, an optical plate on which a protrusion 110 ′ is formed is pasted via an adhesive layer 3 on two display surfaces such as a PDP and a field emission display (hereinafter abbreviated as “FED”). In this case, since the light emitted from the display pixel 400 of the PDP or FED has no directivity (Lambert distribution), the light emitted from the display pixel 400 is formed on the adjacent display pixel 401, for example, adjacent protrusions And becomes a light leakage 500. Alternatively, the light emitted from the display pixel 400 hits the flat portion 150 and repeats multiple reflection to become stray light 501. For this reason, as described below, there is a problem that the image is blurred as a result or the contrast is lowered.

  For example, when the protrusion 110 ′ is formed as shown in FIG. 8, light incident on the protrusion 110 ′ serving as a light guide path is reduced. For example, of the light emitted from the display pixel 400, light that cannot be incident on the protrusion corresponding to the display pixel 400 becomes stray light 501 or leakage light 500, and the display light itself deteriorates the contrast of the display image and lowers the image quality (blur). Let

  Further, for example, as shown in FIG. 5 of Patent Document 4, the incident portion (the bottom portion of the protrusion) is shaped like the bottom of a pyramid, and the incident light has directivity even when the flat portion is eliminated. If the light is not close to parallel light, the light intake into the protrusion 110 ′ is disturbed and optical crosstalk occurs.

  Moreover, although the said shape makes the incident part a pyramid shape and makes the output part a conical shape, the reflection of light becomes complicated. Therefore, when light having no directivity or light having directivity is incident, conditions such as total reflection are disturbed and light loss occurs. This adversely affects image quality.

  Problems such as degradation of image quality due to stray light generation, optical crosstalk, and loss of light are caused by light having no directivity (broad directivity) in the structure in which the light incident portion of each projection 110 ′ is separated from the light source. (Including the case of sex). In particular, in the case of Patent Documents 4 and 5, for example, due to restrictions on the manufacturing method, the thickness of the base sheet is large, so the influence is large. Originally, the technique disclosed in this document is effective for directional light such as parallel light, and is considered as an improvement technique for a transmissive screen of a projection display that handles directional light. Therefore, for omnidirectional light such as display light of PDP and FED, generation of stray light due to disturbance in the protrusions, loss of light, increase in crosstalk, and the like become major problems.

  Further, as shown in FIG. 8, when the distance (indicated by t1) between the light capturing side of the protrusion 110 ′ and the display pixel 400 serving as the light source is increased, the radius r of the light capturing surface of the protrusion 110 ′ is increased. If it is not large, the uptake will be small. Conversely, when the radius r is increased, the amount of capture increases, but the amount of leakage from the adjacent pixels also increases, crosstalk increases, image quality degradation and resolution degradation occur. In order to prevent a reduction in resolution and reduce crosstalk, r on the bottom surface of the protrusion must be sufficiently small with respect to the size of the display pixel, and at the same time, a light intake port (incident surface of the protrusion) from the light source. ) To be small.

  Further, in Patent Documents 4 and 5 mentioned above, a specific method of making a sheet is also mentioned, but transfer by a roll is proposed because of the simplicity of the manufacturing method. While transfer by roll is suitable for mass production and cost reduction, it is not suitable for handling light with no directivity, and the proposed method is applied to reduce image quality due to stray light generation, optical crosstalk, And it has been difficult to improve performance, which is a problem such as light loss.

  The present invention has been made in view of the circumstances described above, and an object of the present invention is to improve a bright room contrast and to use a front plate that can reduce image quality deterioration and contrast reduction due to crosstalk between pixels and stray light. It is to provide a display device.

  In order to solve the above problems, the present invention is a front plate disposed on the front surface of a display panel having a plurality of pixels, which prevents external light reflection, and has a cross-sectional area in the direction of light emitted from the display panel. A region corresponding to a unit pixel area of the display panel, wherein a plurality of frustum-like projections for preventing reflection of external light that are gradually reduced are provided, and a slope of the frustum-like projection has a curvature of a higher-order function. It is set as the structure included in multiple. Then, the frustum-like projections whose projection slopes have a convex function curvature are optically mounted on the surface of the display panel.

  ADVANTAGE OF THE INVENTION According to this invention, the front plate which can improve bright room contrast, can reduce image quality degradation by contrast between pixels, stray light, and a contrast fall, and a display apparatus using the same can be provided.

  Hereinafter, the present embodiment will be described in detail with reference to the drawings. Note that, in each drawing, parts having common functions are denoted by the same reference numerals, and once described, the repeated description of those parts is omitted.

  This embodiment improves the bright room contrast in the display device, and the image quality degradation and contrast reduction due to inter-pixel crosstalk and stray light that occurs when the light emitted from the display device has no directivity or broad directivity characteristics. In order to reduce the amount of light, the property that the amount of light attenuation is large for multiple reflection of light and small for total reflection is applied. A sheet-like front plate with a tapered frustum-like needle-like protrusion (hereinafter referred to as a “frustum-like protrusion”) having a curvature that can be approximated by a convex function on the side is closely attached. It is a feature.

  FIG. 1 is a schematic vertical cross-sectional configuration diagram in the thickness direction of a front plate according to the present embodiment.

  In FIG. 1, a front plate according to the present invention is a needle-shaped optical sheet 1 in which a fine tapered frustum-like projection, which will be described later, stands toward the outgoing light 35, and is a front surface of a display panel 2 of a display device. Are bonded with an optically transparent adhesive layer 3.

  The display device uses a display panel in which self-luminous pixels whose light emission patterns follow a Lambertian distribution are arranged in a matrix. Examples of the display panel include CRT, PDP, FED, LED (Light Emitting Diode). ) In a matrix, or an EL panel in which EL (Electro Luminescence) is arranged in a matrix.

  In this embodiment, as will be described later, since the external light 20 is attenuated by the fine frustum-shaped projections, reflection on the display device or the display panel can be reduced, and at the same time the bright room contrast can be improved. In order to reduce crosstalk between display pixels serving as light sources, the protrusion shape is devised. In FIG. 1, the emitted light 35 is image light emitted from the display panel. Needless to say, the refractive index of the display panel 2 and the needle-shaped optical sheet 1 is larger than that of air.

  FIG. 2 is a diagram illustrating the first embodiment, and is a schematic diagram of a needle-shaped optical sheet that is a front plate obtained by enlarging part A of FIG. FIG. 3 is an enlarged schematic view of a tapered frustum-shaped protrusion having a shape that can be approximated by a convex function of an inclined portion that is an individual element of the needle-shaped optical sheet. In this embodiment, a tapered truncated cone-shaped protrusion having a different diameter on both end faces is used as the truncated cone-shaped protrusion.

  2 and 3, the needle-shaped optical sheet 1 is a sheet shape in which truncated cone-shaped protrusions as the needle-shaped light guide body 110 that guides light from the display panel 2 are formed with a predetermined density per unit pixel. It is a front plate. The front plate of the needle-shaped optical sheet 1 is bonded to the front surface of the display panel 2 with a transparent adhesive layer 3 on the end surface side with the larger diameter (thickness) of the frustoconical protrusion. Further, a diffusing surface 4 for diffusing light is provided on the exit surface of the frustoconical protrusion so that the exit distribution of the exit light 35 can be controlled to a predetermined characteristic.

  With this configuration, crosstalk between pixels can be reduced and contrast can be improved, as will be described later, even for display elements having no directivity such as PDP and FED or broad directivity. Both effects can be achieved simultaneously.

  If the diameter (thickness) of the needle-shaped light guide body 110 is made sufficiently smaller than the unit pixel of the display panel 2 (approximately 1/5 or less), the problem related to pixel sampling can be reduced and function effectively. Needless to say.

  Here, the shape of the inclined portion on the side surface of the needle-shaped light guide body 110 (conical truncated projection) will be described with reference to FIG. As shown in FIG. 3, the shape of the inclined portion 115 on the side surface side of the needle-shaped light guide body 110 can be realized by a linear shape 116. However, in this embodiment, a convex function (hereinafter, “convex” It is assumed that it has a curvature that can be approximated by "function").

  Now, assuming the bottom part between the needle-shaped light guide bodies closest to the display panel 2 as the origin, the inclined portion 115 is obtained by taking orthogonal coordinates with the light emission direction as the Y axis and the direction perpendicular thereto as the X axis. The shape of the inclined portion 115 can be approximated by a convex function defined by Equation 1 with respect to the x coordinates x1 and x2 in the region.

  It goes without saying that a smooth curve is desirable as the convex function in consideration of irregular reflection.

There are various convex functions, and specifically, there are those expressed by Formula 2.
y = | a × x ⁿ | (Formula 2) (where n is a positive number greater than 1 and a is an arbitrary coefficient)
If way of example, x ² is one the typical. Of course, the region of the inclined portion may be divided into a plurality of regions so as to satisfy Equations 1 and 2, and different functions may be combined for each region. However, it is desirable to connect smoothly in order to reduce irregular reflection at the boundary of the region. Needless to say, smooth also includes mathematical meaning.

  The shape of the inclined portion 115 of the needle-shaped light guide body 110 is set to a curved shape that can be approximated by Equations 1 and 2 rather than the shape of the straight line 116, and light from the display panel surface is transmitted to the needle-shaped light guide 110. The amount of reflection inside increases, and the loss of light from the display panel decreases. In addition, the height of the needle-shaped light guide 110 in the Y-axis direction (for example, t1 + t2 in FIG. 4) is lower than that in which the inclined portion is linear because of the angle of the inclined portion with respect to light from the display panel. I can do it.

  As a result, the depth of the needle-shaped light guide 110 can be reduced, and the manufacturing is greatly facilitated. In addition, since the sheet shape is adopted, the needle-shaped optical sheet can be easily handled. Furthermore, reducing the height of the needle-shaped light guide 110 in the Y-axis direction contributes to improving the strength of the front plate.

Next, the principle of reducing crosstalk between pixels in this embodiment will be described with reference to FIG. 4 or FIG. FIG. 5 shows a distribution of light emitted from a light source (light emitting point) which is a pixel of the display panel. The distribution 410 of light emitted from the pixels 400 and 401 of the PDP taken up for explanation in this example is called a Lambertian distribution as shown in FIG. 5, when the intensity of light in the center and I _0, the intensity of the angle θ direction of the light is represented by I ₀ cos [theta]. With this distribution, PDP, FED, CRT, and the like can see images in a considerably wide range (wide viewing angle).

  In this embodiment, the distance t1 from the light source that is the display pixel (for example, 400) to the light intake port (incident surface of the protrusion) is smaller than that in FIG. 8 showing the prior art as shown in FIG. . This is because, in the case of a transmissive screen, a certain amount of thickness is indispensable for maintaining a predetermined strength, and in the case of a plasma display panel, a base such as a display panel is formed adjacent to the front plate. This is because a certain level of strength can be maintained even if the distance t1 is reduced. Furthermore, since t1 becomes smaller, the amount of light leaking from adjacent pixels (for example, 401) on the bottom surface of the needle-shaped light guide body is reduced, and there is an advantage that the bright room contrast is improved.

  In addition, since a plurality of needle-shaped light guide bodies are included in one display pixel 400 region, the flat portion of the bottom surface of the needle-shaped light guide body can be reduced as compared with the conventional case, and the bottom surface of the needle-like light guide path has a curvature. Since it has a curved surface, the amount of light emitted from the pixel 400 or 401 is reflected again is very small, so that stray light is small. Further, as the diameter of the frustoconical protrusion serving as the needle-shaped light guide path is made sufficiently small with respect to the display pixel (for example, 1/5 or less), the image quality deterioration and the contrast reduction can be sufficiently ignored. .

  FIG. 6 is a partially enlarged detail view of the needle-shaped light guide body 110 shown in FIG. 4 and is a diagram for explaining the operation thereof. In FIG. 6, the needle-shaped light guide body 110 includes an inclined portion 115 in which the cross-sectional area perpendicular to the main optical axis 8 of the incident light 30 incident on the needle-shaped light guide body 110 from the display panel gradually decreases in the emission direction. , A frustoconical space having a shape defined by Equation 2 is formed, and incident light 30 incident on the needle-shaped light guide body 110 passes through the truncated cone-shaped space inside the needle-shaped light guide body 110. It propagates without being greatly attenuated while repeating total reflection as shown in the figure at the interface with air, diffuses from the diffusion surface 4 and exits as outgoing light 35.

  At this time, since the shape of the inclined portion 115 on the side surface of the needle-shaped light guide body 110 is a convex function expressed by Equations 1 and 2, the inclination of the inclined portion 115 is steeper than the linear shape 116 except for the vicinity of the bottom surface portion. The incident light 30 having no directivity or broad directivity from the display pixels of the display panel (that is, including a light component obliquely intersecting the main optical axis 8) is efficiently shown in the figure. It can be taken into the needle-shaped light guide 110.

  Note that the cross-sectional area perpendicular to the main optical axis 8 of the needle-shaped light guide body 110 is sufficiently smaller than the unit pixel area of the display panel, and if there is no diffusion surface 4 on the exit side surface of the needle-shaped light guide body 110. Since the output light 35 becomes light substantially parallel to the main optical axis 8, it is necessary to diffuse it on the diffusion surface 4.

  On the other hand, the external light 20 has a concave shape formed between the surfaces of the needle-shaped light guide body 110 having a refractive index larger than that of the two airs (that is, between the needle-shaped light guide body 110 and the adjacent needle-shaped light guide body 110. As shown in the groove 7), multiple reflections occur, and a reflection coefficient is applied and attenuated each time reflection is performed. The degree of attenuation is significant and is specifically shown below.

Assuming that the brightness of the external light 20 is b, the surface reflectance of the needle light guide 110 is k (k <1), and the number of reflections is n, the brightness of the external light reflected n times is expressed by the following formula 3 Indicated by
b × (k) ⁿ (Equation 3)
For example, even if the external light 20 is incident on the surface of the needle-shaped light guide body 110 with a large incident angle, the incident angle gradually decreases and the number of reflections increases as shown in the figure. Then, the direction reverses in the middle and returns to the viewer side. During this time, the external light 20 is sufficiently attenuated, so that the bright room contrast can be improved as compared with the conventional case.

  In the description so far, the processing of the slope on the side surface of the needle-shaped light guide body 110 has not been mentioned, but in order to further improve the contrast performance, as shown in FIG. A light reflecting film 5 may be provided on the light reflecting film 5, and a light absorbing film 6 may be further provided on the light reflecting film 5. By processing in this way, light leaking from the needle-shaped optical path body 110 without satisfying the total reflection condition is reduced, and waste of light emitted from the panel can be suppressed.

  In addition, by attaching the light absorption film 6 on the light reflection film 5, external light is also absorbed, and the effect is further increased.

  Further, the diffusing surface 4 has an optical function of diffusing the emitted light in accordance with the purpose. In the simplest case, the surface of the emitting surface may be made fine to scatter the light. Further, in order to control the diffusion amount, a micro prism shape or a lens shape may be provided on the exit surface. In short, it goes without saying that various optical means may be applied depending on the viewing angle required for the display device. Needless to say, the diffusion surface 4 is not necessary when the viewing angle is narrowed and used in an application where high luminance is required in a specific region. In addition, the shape can be optically designed to align the light emission direction.

  In this way, the ratio of the light emitted from the front plate arranged on the front surface of the display panel and the external light that enters the front plate and returns to the viewer side is increased. A high image will be obtained.

  As described above, according to the present embodiment, it is not necessary to attenuate the light from the display panel of the display device by decreasing the transmittance, while maintaining the light emitted from the display panel at substantially the same brightness, Since reflected light of outside light can be reduced, the bright room contrast can be improved satisfactorily. Furthermore, crosstalk between pixels, image degradation (blur) due to stray light, and contrast reduction can be solved simultaneously, and an image with high sharpness and contrast can be realized.

  If the black light absorption film 6 is made of a conductive material (for example, carbon) and is grounded to a ground potential, it can be used as a shield for reducing noise such as unnecessary radiation.

  FIG. 9 is a view of the above-described front plate viewed from the input direction of external light. FIG. 9A is a diagram in which a plurality of circular diffusion surfaces 4 are in contact with each other in a lattice shape, and a hatched portion a indicates a region that can be seen from a gap between the diffusion surfaces. FIG. 9B is a diagram in which the center positions of the circles of the respective diffusion surfaces 4 are shifted and contacted, and the hatched portion b indicates a region that can be seen from the gaps of the diffusion surfaces. These regions are preferably smaller in consideration of stray light and light leakage. Therefore, it is needless to say that it is preferable to configure the front plate as shown in FIG. 9 (2) rather than FIG. 9 (1).

  FIG. 7 shows the needle-shaped light guide path portion of the front plate according to the second embodiment. The present embodiment is different from the first embodiment in that the light absorber 9 is filled in the portion of the concave groove 7 between the adjacent needle-shaped light guide bodies to absorb external light.

  In FIG. 7, the concave groove 7 between the forested needle light guides 110 is filled with, for example, a black light absorber 9 that absorbs light, and the light absorber 9 absorbs the external light 20. Therefore, the contrast can be further improved. Further, the filling of the light absorber 9 can also increase the mechanical strength of the needle-shaped light guide path portion.

  Further, if the light absorber 8 is made of a conductive substance (for example, carbon) and is grounded to a ground potential, it can also be used as a shield for reducing noise such as unnecessary radiation.

  In the embodiment described above, the needle-shaped light guide body has been described as having good transmittance and does not attenuate the light emitted from the display panel. However, the present invention is not limited to this. Needless to say, various optical characteristics can be obtained.

  The present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the invention. Furthermore, the above-described embodiments include various inventions, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. For example, even if some constituent features are deleted from the previous constituent features shown in the embodiment, at least one of the issues described in the column of the problem to be solved by the invention can be solved, and described in the column of the effect of the invention In a case where at least one of the obtained effects can be obtained, a configuration in which this configuration requirement is deleted is an invention.

1 is a schematic configuration diagram of an entire embodiment. It is a schematic diagram of the acicular optical sheet which is a front plate which shows a 1st Example. It is an expansion schematic diagram near the needle-shaped light guide way of the 1st example. It is a figure explaining the principle which reduces the crosstalk between pixels. It is a distribution pattern figure of the light output from a pixel. It is an enlarged detail drawing of the acicular light guide path body part of a 1st Example. It is a figure which shows the acicular light guide path body part of the front plate which is a 2nd Example. It is a figure explaining the function of the conventional front plate. It is the figure which looked at the front board in a present Example from the emitted light side.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Needle-shaped optical sheet, 3 Transparent adhesive layer, 4 Diffusion surface, 5 Light reflection film, 6 Light absorption film, 7 Concave groove, 8 Main optical axis, 9 Light absorber, 20 External light, 30 Incident light, 35 Outgoing light,
110 Needle-shaped light guide body, 115 inclined portion, 116 linear shape, 150 flat portion, 200 front plate, 210 antireflection film, 400 display pixel, 401 adjacent display pixel, 500 light leakage, 501 stray light

Claims (10)

A front plate disposed in front of the display panel,
With multiple frustum-shaped protrusions,
The frustoconical protrusion is disposed so that a cross-sectional area in a direction perpendicular to the optical axis is gradually reduced with respect to the light emitting direction,
A plurality of the frustum-shaped projections are included in a region corresponding to the unit pixel area of the display panel,
A front plate characterized in that the shape of the frustum-shaped projection is configured based on a curvature expressed by a convex function. The front plate according to claim 1,
The shape of the inclined part of the frustum-shaped protrusion is y = | a × x ⁿ | (n is a positive number larger than 1). The front plate according to claim 1 or 2,
The front plate, wherein the shape of the inclined portion of the frustum-shaped protrusion is configured by a curvature obtained by combining a plurality of functions. The front plate according to any one of claims 1 to 3,
A front plate comprising a light reflecting film that reflects light so as to cover a surface of the frustum-shaped protrusion excluding the protrusion surface. The front plate according to any one of claims 1 to 4,
A front plate comprising a light absorbing film that absorbs light so as to cover a surface of the frustum-shaped protrusion except for a protruding surface. The front plate according to any one of claims 1 to 5,
A front plate comprising a light absorption layer that absorbs external light so as to fill a space between adjacent frustum-shaped protrusions. The front plate according to any one of claims 1 to 6,
The front plate, wherein the frustum-shaped projection has a scattering surface for scattering light on the projection surface.   A display device comprising the front plate according to any one of claims 1 to 7, which is disposed on the front surface of the display panel as an integral part of the display panel or as a separate body. A front plate for preventing external light reflection disposed on the front surface of a display panel having a plurality of pixels,
A plurality of frustum-shaped protrusions for preventing reflection of outside light,
The frustoconical protrusions are arranged such that the length in the direction perpendicular to the optical axis is gradually reduced with respect to the light emitting direction,
A front plate, wherein the shape of the inclined portion of the frustum-shaped projection is configured with a curvature represented by a convex function. A front plate for preventing external light reflection disposed on the front surface of a display panel having a plurality of pixels,
A plurality of frustum-shaped protrusions for preventing reflection of outside light,
A front plate, characterized in that the frustum-shaped projections are arranged so that the length in the direction perpendicular to the optical axis becomes gradually smaller with respect to the light emitting direction.

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