JP2004170861A - Fresnel lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Fresnel lens which prevents the occurrence of a double image owing to stray light and is free from problems of manufacture. <P>SOLUTION: The Fresnel lens has a plurality of 1st prisms 4 which are arranged on a light incidence surface and has refracting surfaces 5 and total reflecting surfaces 6 and linearly extend and a plurality of 2nd prisms 7 which are arranged on a light exit surface and has lens surfaces 8 and non-lens surfaces 9 and arcuately extend. The 1st prisms 4 are so formed that luminous flux projected from the 1st prisms 4 are divergent in a plane perpendicular to the ridges of the 1st prisms, and 1st prisms disposed in some area among the plurality of 1st prisms have larger vertical angles δ than 1st prisms 4 disposed adjacent to the above 1st prisms and closer to a light source than the above 1st prisms 4. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a Fresnel lens, and in particular, can be suitably applied to a Fresnel lens constituting a transmission screen used for a rear projection television.
[0002]
[Prior art]
One of the large screen televisions is a rear projection television. This rear projection television projects video light (projection light) onto the transmission screen from a projection device arranged on the side opposite to the viewer of the transmission screen to provide an image to the viewer. .
[0003]
In such a rear projection television, since the projection light is enlarged and projected from the projection device, it is necessary to secure a certain distance between the projection device and the transmissive screen, and therefore, the space in the depth direction increases. Problems arise. For this reason, as shown in FIG. 8, the projection device E is disposed obliquely to the surface of the transmissive screen A, and the projection device E projects the projection light to reduce the space in the depth direction. I am planning.
[0004]
However, a conventional Fresnel lens provided with a plurality of arc-shaped prisms having a lens surface and a non-lens surface on a light-emitting surface (light-emitting surface) as a Fresnel lens constituting a transmission screen in the projection system shown in FIG. If used, the following problems occur. That is, when projecting the projection light from an oblique direction with respect to the surface of the transmission screen, the incident angle θ of the projection light to the transmission screen increases, so that the reflection amount of the projection light on the lens surface increases, As a result, there is a problem that the image projected on the transmission screen becomes dark and it becomes difficult to observe.
[0005]
In order to solve this problem, a plurality of first prisms extending linearly on a light incident surface (light incident surface) and a plurality of first prisms extending in an arc shape on a light exit surface are used as Fresnel lenses constituting a transmission screen. It is effective to use a Fresnel lens having two prisms (see Patent Document 1).
[0006]
That is, first, the first prism converts the projection light projected from the oblique direction to the transmission screen into light having the same diffused light beam as when projected from the front of the transmission screen, and then converts the light into the second light. Since the light is supplied to the prism, reflection on the lens surface of the second prism can be reduced, and a high-contrast image can be provided.
[Patent Document 1]
JP-A-63-30837 (page 3, FIG. 1)
[0007]
[Problems to be solved by the invention]
However, even when a transmission type screen is configured using the conventional Fresnel lens disclosed in Patent Document 1, the problem described below occurs.
[0008]
First, for the sake of convenience, the specifications of the projection system used to describe the following problems are defined (see FIG. 9). First, a projection device is disposed obliquely below the transmission screen, and the projection light projected from the projection device enters the first prism (the upper end first prism) located at the uppermost end of the Fresnel lens at an incident angle of 70 °. It is assumed that the light enters the first prism located at the lowermost end (first prism at the lower end) at an incident angle of 30 °. Also, the light that has been totally reflected by the first prism forms a divergent ray bundle that becomes 25 ° upward at the upper first prism and 25 ° downward at the lower first prism in a plane perpendicular to the ridge line of the prism element. Shall be shown. Further, the material constituting the Fresnel lens has a refractive index of 1.55. Based on the above assumptions, the problems will be described below.
[0009]
First, considering the lens design of the first prism at the lower end, the angle (vertical angle) δ between the refraction surface and the total reflection surface must be 45 in order to make the light entering at an incident angle of 30 ° emit at a downward angle of 25 °. °, the angle between the total reflection surface and the Fresnel lens surface (total reflection surface angle) β is 83.3 °, and the angle between the refraction surface and the Fresnel lens surface (refraction surface angle) α is 51.7 °. It becomes.
[0010]
However, in this case, a large amount of light (stray light) Z that does not reach the total reflection surface is generated, and the ratio (light emission efficiency) of the normal light X (which means light that is emitted normally other than stray light) to the incoming light is 57%. (See FIG. 10). Since this stray light causes a double image or the like, it is necessary to minimize generation of the stray light.
[0011]
Therefore, in order to make the light output efficiency 90%, the apex angle δ needs to be 30 °. At this time, the total reflection surface angle β is 79.3 °, and the refraction surface angle α is 70.7 °.
[0012]
Next, the lens design of the upper first prism will be considered. As described above, when the apex angle δ is 30 °, in order to cause light incident at an incident angle of 70 ° to exit upward at 25 °, the total reflection surface angle β is 33.5 ° and the refraction surface angle α is 116. 0.5 °.
[0013]
However, in this case, since the refracting surface angle α exceeds 90 °, when the Fresnel lens is molded using the mold C (the molding by the mold is indispensable for mass production), the molded body W is formed. In the mold C, and it is difficult to release the molded body W from the mold C (see FIG. 11).
[0014]
The present invention has been made in view of such problems, and an object of the present invention is to provide a Fresnel lens that can prevent a double image from being generated due to stray light and that does not cause a manufacturing defect. To provide.
[0015]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, a Fresnel lens that receives light projected from a light source from a light incident surface side and emits light to a light emitting surface side, wherein the light is arranged on the light incident surface and projected from the light source A plurality of first prisms formed so as to extend linearly with a refracting surface for refracting light and a total reflecting surface for totally reflecting light refracted by the refracting surface; and a total reflecting surface disposed on the light emitting surface. A plurality of second prisms formed so as to extend in an arc shape with a lens surface that refracts the light reflected by the lens and a non-lens surface that connects the lens surfaces to each other. The first light beam emitted from the first prism is formed so as to become a divergent light beam in a plane perpendicular to the ridge line of the first prism, and the first light beam located in at least a partial region of the plurality of first prisms. The angle between the refracting surface and the total reflecting surface Top angle is to form a Fresnel lens to be greater than the top angle of the first prism is located next to the side closer to the light source than the first prism 其 s (claim 1).
[0016]
According to this, since the apex angle becomes smaller as the first prism is closer to the light source, it is possible to prevent or reduce the generation of stray light and to prevent the generation of a double image.
[0017]
When changing the vertex angle to increase as the distance from the light source increases, it is desirable to change the vertex angle to increase gradually. According to this, the apex angle does not suddenly greatly change at a certain position, and it is possible to prevent the observer from feeling uncomfortable.
[0018]
Further, in all the first prisms, it is desirable to form the apex angle of each first prism to be larger than the apex angle of the first prism located on the side closer to the light source than the first prism. . According to this, generation of stray light can be more reliably prevented or reduced, and generation of a double image can be reliably prevented.
[0019]
In order to solve the above-mentioned problem, in the Freren lens according to the first aspect, it is preferable that a refracting surface angle, which is an angle formed between the refracting surface and the Fresnel lens surface, be 90 ° or less ( Claim 2).
[0020]
According to this, since the refraction surface angle is set to 90 ° or less, when a Fresnel lens is molded using a mold, the molded body does not easily enter the mold and becomes difficult to release, which causes a problem. It is possible to manufacture a Fresnel lens without using a lens. The surface of the Fresnel lens refers to a surface when the Fresnel lens is viewed macroscopically, that is, a surface perpendicular to the thickness direction of the Fresnel lens. In this specification, the surface of a Fresnel lens may be referred to as a Fresnel lens surface.
[0021]
In order to solve the above-mentioned problem, the Fresnel lens according to claim 1 or 2, further comprising a coating layer made of a material having a smaller refractive index than a material forming the first prism on at least a surface of the first prism. It is desirable to form them (claim 3).
[0022]
According to this, when light enters the Fresnel lens, the coating layer made of a material having a small refractive index is disposed on the surface thereof, so that the reflectance of the incident light can be reduced, and the Fresnel lens It is possible to prevent the occurrence of a double image caused by the light reflected on the surface of the image.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
A preferred embodiment of the present invention will be described with reference to the drawings.
[0024]
1. First Fresnel Lens Configuration of First Fresnel Lens FIG. 1 is a perspective view of the first Fresnel lens viewed from an oblique direction on the light exit surface side, and FIG. 2 is a cross-sectional view taken along line xx of FIG. .
[0025]
As shown in FIGS. 1 and 2, the Fresnel lens 1 includes a first prism group 2 on a light incident surface and a second prism group 3 on a light exit surface, and the diagonally lower part of the Fresnel lens 1 on the light incident surface side. First, the first prism group 2 forms a divergent light beam having a virtual focal point I at a position vertically separated from substantially the center of the surface of the Fresnel lens 1 by the first prism group 2. The light is converted into light (divergent light flux light) D, and then converted by the second prism group 3 into light (parallel light flux light) P indicating a parallel light flux substantially perpendicular to the surface of the Fresnel lens 1. I do.
[0026]
3A and 3B are partially enlarged views of FIG. 2, wherein FIG. 3A shows the Fresnel lens 1 at a portion far from the projection device E, and FIG. 3B shows the Fresnel lens 1 at a portion near the projection device E. As shown in FIG. 3, the first prism group 2 is formed by regularly arranging vertically extending first prisms 4 each having a triangular cross section and extending linearly in the horizontal direction. The first prism 4 includes a refraction surface 5 that refracts the projection light S and a total reflection surface 6 that totally reflects the light refracted by the refraction surface 5.
[0027]
Then, the first prism 4 converts the projection light S into the divergent light flux D and forms the refraction surface 5 and the total reflection surface 6 as follows in order to exert the effect of the present invention. The refracting surface 5 and the total reflecting surface 6 are specified by a refracting surface angle α, a total reflecting surface angle β, and a vertex angle δ, and each of them will be described below.
[0028]
First, the total reflection surface angle β is determined so as to be maximum at the lower first prism 4a closest to the projection device E and minimum at the upper first prism 4b farthest from the projection device E. That is, in the lower first prism 4a, the incident angle θ of the incident light S is minimum, and the light emission angle γ from the first prism 4 is maximum downward, so that the total reflection surface angle β is set to the maximum. Then, the incident angle θ increases and the light emission angle γ increases upward as the first prism 4 is located at the upper part, so that the total reflection surface angle β is set to gradually decrease. Since the incident angle θ is maximum and the light emission angle γ is maximum upward, the total reflection surface angle β is set to the minimum.
[0029]
Next, the vertex angle δ is set so as to be the smallest at the lower end first prism 4a, and gradually increase as approaching the upper end first prism 4b. That is, the apex angle δ is set small in the lower first prism 4a where the incident angle θ is small and stray light (light that does not reach the total reflection surface 6) is most likely to occur, and the height H of the first prism 4 is reduced as shown in FIG. When it is constant, the pitch P of the first prism 4 is made small, so that stray light hardly occurs. Note that, unlike FIG. 3, when the pitch P of the first prisms 4 is constant, the height H of the first prisms 4 increases, and stray light hardly occurs. Furthermore, it is possible to gradually increase the apex angle δ from the lower first prism 4a to the upper first prism 4b, so that the refraction surface angle α becomes 90 ° or less as described later. To prevent the observer from feeling uncomfortable by eliminating sudden changes in.
[0030]
Finally, the refraction surface angle α is set to be 90 ° or less so as not to cause a problem at the time of manufacturing using a mold described later.
[0031]
As shown in FIG. 3, the second prism group 3 is formed by regularly arranging concentrically arranged second prisms 7 each having a triangular cross section and extending in an arc shape. The second prism 7 includes a lens surface 8 that refracts light totally reflected by the total reflection surface 6 of the first prism 4 and a non-lens surface 9 that connects adjacent lens surfaces.
[0032]
The lens surface 8 is formed at an angle ε at which the divergent light beam D is refracted and emitted as a parallel light beam P. The non-lens surface 9 is formed to be substantially perpendicular to the Fresnel lens surface so as to connect the adjacent lens surfaces 8 to each other.
[0033]
B. The first method for fabricating a Fresnel lens <br/> Next, a method of manufacturing the Fresnel lens 1 described above.
[0034]
The Fresnel lens 1 is molded by a resin molding method using a light-transmissive resin material such as an acrylic resin, a styrene resin, a polycarbonate resin, and an epoxy resin. As the resin molding method, (1) a method in which a mold formed in a shape corresponding to the Fresnel lens 1 is filled with a light-transmitting resin material in a molten state, cured, and then released (casting method); ▼ A method in which the mold is filled with a heated light-transmissive resin material, molded by pressurizing, and then released (hot press method). There is a method of releasing after curing by irradiation (ultraviolet curable resin method), and the like, but in all cases, after molding with a mold, the molded body is released from the mold.
[0035]
Hereinafter, the ultraviolet curing resin method will be described in detail. In the following, an example is shown in which the light incident surface side of the Fresnel lens 1 is formed first, and then the light output surface side is formed. However, it is also possible to mold from the light output surface side.
[0036]
First, the light incident surface side of the Fresnel lens 1 is formed. First, a first mold C is prepared by engraving a mold material such as aluminum, brass, copper, steel or the like into a shape corresponding to the light incident surface of the Fresnel lens 1 with a cutting tool or the like. Then, an ultraviolet curable resin is applied to the first mold C. The application of the ultraviolet curable resin may be performed by a roll coating method, a gravure method, a dispenser method, a die coating method, or the like.
[0037]
Next, a substantially transparent substrate that transmits ultraviolet light is laminated on the applied ultraviolet curable resin, and the substrate is brought into close contact with the ultraviolet curable resin by pressing with a pressure roller or the like. Then, ultraviolet rays are irradiated from above the laminated substrates to cure the ultraviolet curable resin.
[0038]
Next, the cured ultraviolet-curing resin is released from the first mold C. At this time, since the refraction surface angle α is set to 90 ° or less as described above, the molded body W comes into the first mold C and comes off. It is possible to release the mold without causing such a problem that it does not exist (see FIG. 4).
[0039]
Next, the light exit surface side of the Fresnel lens 1 is molded.
[0040]
It should be noted that, even when the Fresnel lens 1 after molding is molded by a casting method or a hot press method, it is the same in that troubles such as difficulty in removing the Fresnel lens 1 from the mold are not caused.
[0041]
C. Transmission screen using first Fresnel lens FIG. 5 is a perspective view showing a transmission screen using the Fresnel lens 1 described above.
[0042]
As shown in FIG. 5, the transmission screen 10 is formed by arranging a lenticular lens 11 on the light exit surface side of the Fresnel lens 1.
[0043]
The lenticular lens 11 is formed by providing a semi-cylindrical lens 12 extending linearly in a direction perpendicular to the light incident surface, forming a light absorbing layer 13 on the surface of the semi-cylindrical lens 12, and further dispersing a diffusing agent 14 therein. Is done.
[0044]
The transmissive screen 10 projects image light S projected from a projection device E provided below the light incident surface side and provides the image light S to an observer. At this time, as described above, the Fresnel lens 1 does not generate stray light, or even if the stray light is generated, the amount thereof is small. Therefore, the image projected on the transmissive screen 10 has uniform brightness and double There is no image degradation such as an image.
[0045]
In addition, a known lenticular lens can be used for the transmissive screen 10 in addition to the lenticular lens 11 described above. In this case, the same effect can be obtained.
[0046]
2. Second Fresnel lens The second Fresnel lens 21 is obtained by forming a coating layer 22 made of a material having a low refractive index on the surface of the first Fresnel lens. In the following, only differences from the first Fresnel lens 1 will be described. The same parts as those of the first Fresnel lens 1 will be described using the same reference numerals.
[0047]
A. Structure of second Fresnel lens FIG. 6 is a sectional view of the second Fresnel lens 21 in the thickness direction. As shown in FIG. 6, the second Fresnel lens 21 has a coating layer 22 on the surface of the first prism 4.
[0048]
The coating layer 22 is formed of a material having a lower refractive index than the material forming the first prism 4. Specifically, examples of a material for forming the coating layer 22 include a fluorine resin and a silicone resin.
[0049]
The Fresnel lens 21 having the coating layer 22 reduces the amount of reflection of incoming light, and in particular, prevents generation of ghost light in a projection system using a mirror M as described below.
[0050]
B. 2. Method for manufacturing second Fresnel lens The Fresnel lens 21 is manufactured by forming a coating layer 22 on the surface of the first prism 4.
[0051]
That is, first, a molded body including the first prism 4 and the second prism 7 is formed by the same method as that of the first Fresnel lens 1 described above. Next, the above-mentioned fluororesin, silicone resin, or the like is applied to the surface of the first prism 4 of the molded body by flow coating or dipping coating to form a coating layer 22, thereby completing the Fresnel lens 21.
[0052]
C. Transmission screen using second Fresnel lens The Fresnel lens 21 described above is used as the transmission screen 31 together with the lenticular lens 11 arranged on the light exit surface side.
[0053]
FIG. 7 is a diagram illustrating a projection system using the transmission screen 31.
[0054]
As shown in FIG. 7, the transmissive screen 31 projects image light S projected from a projection device E provided below the light incident surface side and then reflected by a mirror M, and provides the image light S to an observer. . At this time, since the Fresnel lens 21 does not generate stray light or does not generate any stray light, the image projected on the transmissive screen has uniform brightness as a whole and image deterioration such as a double image. This is the same as the transmission type screen using the first Fresnel lens 1 in that the first type does not occur.
[0055]
Further, in the transmission type screen 31 using the second Fresnel lens 21, since the reflection of the projection light is reduced by the coating layer 22, the light Y reflected on the light incident surface of the transmission type screen 31 is reflected by the mirror M. It is possible to prevent a double image caused by re-entering the transmission screen.
[0056]
3. Modification In the above embodiment, the case where the first prism 4 is formed so as to extend in the horizontal direction has been described. However, the present invention is not limited to this, and the first prism 4 may be formed so as to extend in the vertical direction. May be. When the first prism 4 is formed to extend in the vertical direction, light is projected by disposing the projection device E in the oblique side direction.
[0057]
Further, in the above embodiment, in all the first prisms 4 of the Fresnel lenses 1 and 21, the apex angle δ is changed so as to gradually increase from the side closer to the projector E to the side farther from the projector E. Without being limited, the apex angle δ may be changed as described above only for the first prism 4 located in a part of the area. In particular, when the apex angle δ is changed as described above only for the first prism 4 located in a region near the projection device E where stray light is likely to occur, stray light can be efficiently prevented.
[0058]
In the above embodiment, the case where the coating layer 22 is formed on the surface of the first prism 4 of the second Fresnel lens 21 has been described. However, the present invention is not limited to this, and the surface of the second prism 7 may be coated. The layer 22 may be formed. In this case, in order to make the coating layer 22 function effectively, it is preferable to form the coating layer with a material having a lower refractive index than the material forming the second prism 7.
[0059]
Further, in the above embodiment, the case where the second prism 7 is formed so that the light emitted from the second prism 7 becomes the parallel light flux P perpendicular to the transmission type screen surface has been described. Instead, the second prism 7 may be formed so as to be a divergent light beam or a convergent light beam.
[0060]
【Example】
Using a Fresnel lens and a lenticular lens shown below, a screen having a screen size of 50 inches was formed, and the screen was moved 400 mm in a direction perpendicular to the screen from a point 250 mm below the center of the screen in the horizontal direction and 250 mm below the lower end of the screen. A projection device (projector) was arranged at the point, and image light was projected toward the transmission screen.
[0061]
{Circle around (1)} A first prism having a refraction surface and a total reflection surface on the light incident surface of the Fresnel lens and extending linearly in the horizontal direction was formed at a pitch of 0.08 mm. The first prism was formed such that the light emitted from the first prism became a diffused light beam having a virtual focus at a point moved 400 mm toward the projection device in a direction perpendicular to the screen passing through the center of the screen.
[0062]
Here, the apex angle of the first prism is set to 30 ° for the lower first prism and 55 ° for the upper first prism, and the first prism therebetween is set so as to gradually increase in proportion to the distance from the lower first prism. did.
[0063]
Further, a second prism having a lens surface and a non-lens surface on the light emitting surface and extending in an arc shape is formed concentrically at a pitch of 0.112 mm. The focal length of the second prism was 400 mm, and the emitted light was a parallel light beam.
[0064]
The first prism 4 and the second prism 7 were formed using a material having a refractive index of 1.55.
[0065]
{Circle around (2)} Lenticular lens A semi-cylindrical lens extending linearly in the direction perpendicular to the light entrance surface was formed at a pitch of 0.143 mm. The thickness of the lenticular lens was 1 mm, and a diffusing agent was dispersed inside the lenticular lens such that the half angle of horizontal diffusion was 25 ° and the half angle of vertical diffusion was 10 °.
[0066]
Further, a light absorbing layer having a thickness of 20 μm was provided on the surface of the kamaboko lens. The absorptance of the light absorbing layer was 40%.
[0067]
As described above, by projecting image light from the projection device onto the formed transmission screen and observing the image projected on the transmission screen, the brightness of the entire screen is uniform, and a double image due to stray light or the like is observed. A good image could be observed without deterioration of the image.
[0068]
【The invention's effect】
As described above, according to the present invention, the first prism on the light incident surface is formed so as to reduce the apex angle on the side close to the light source, so that the occurrence of stray light can be prevented or reduced, and therefore, In addition, generation of a double image due to stray light can be prevented.
[0069]
In addition, since the first prism is formed so that the refraction surface angle is 90 ° or less, the first prism portion does not easily enter the mold when removing the Fresnel lens molded product from the mold. Fresnel lenses can be manufactured without problems.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an overall configuration of a first Fresnel lens of the present invention.
FIG. 2 is a sectional view taken along line xx of FIG. 1;
3A is a partial enlarged view of a portion of the first Fresnel lens of the present invention far from the projection device, and FIG. 3B is a partial enlarged view of a portion of the first Fresnel lens of the present invention near the projection device. FIG.
FIG. 4 is a diagram illustrating release from a mold for molding the first Fresnel lens of the present invention.
FIG. 5 is a perspective view showing a transmission screen using the Fresnel lens of the present invention.
FIG. 6 is a sectional view in a thickness direction of a second Fresnel lens of the present invention.
FIG. 7 is a diagram showing an example of a projection system of a transmission screen using a second Fresnel lens of the present invention.
FIG. 8 is a diagram showing a projection system that projects light from an oblique direction.
FIG. 9 is a diagram illustrating a projection system used for describing a problem of a conventional Fresnel lens.
FIG. 10 is a diagram illustrating generation of stray light in a conventional Fresnel lens.
FIG. 11 is a diagram illustrating a problem in manufacturing a conventional Fresnel lens.
[Explanation of symbols]
Reference Signs List 1 Fresnel lens 2 First prism group 3 Second prism group 4 First prism 5 Refraction surface 6 Total reflection surface 7 Second prism 8 Lens surface 9 Non-lens surface α Refraction surface angle β Total reflection surface angle δ Apex angle E Projection device S Image light (projection light)
D Divergent light beam P Parallel light beam

Claims (3)

A Fresnel lens that receives light projected from a light source from a light incident surface side and emits light to a light exit surface side,
A plurality of light-reflecting surfaces arranged on the light-entering surface, the light-reflecting surface being formed by a refraction surface that refracts light projected by the light source and a total reflection surface that totally reflects light refracted by the refraction surface are formed. A first prism of
A plurality of second prisms arranged on the light exit surface and including a lens surface that refracts light reflected by the total reflection surface and a non-lens surface that connects the lens surfaces to each other and formed to extend in an arc shape; And
The first prism is formed such that a light beam emitted from the plurality of first prisms becomes a divergent light beam in a plane perpendicular to a ridge line of the first prism,
The first prism located in at least a partial region of the plurality of first prisms has a vertex angle, which is an angle formed between the refraction surface and the total reflection surface, more than the first prism. Formed so as to be larger than the apex angle of the first prism located next to the side close to the light source,
A Fresnel lens characterized in that: The Freren lens according to claim 1,
A Fresnel lens, wherein a refraction surface angle, which is an angle formed between the refraction surface and the surface of the Fresnel lens, is 90 ° or less. The Fresnel lens according to claim 1 or 2,
A Fresnel lens, comprising: a coating layer made of a material having a smaller refractive index than a material forming the first prism on at least a surface of the first prism.

Images