JP6413651B2 - Fresnel lens and method of manufacturing Fresnel lens - Google Patents

Description

  The present invention relates to a Fresnel lens and a manufacturing method thereof, and more particularly to a resin-made Fresnel lens having a prism surface inside a curved surface and a manufacturing method thereof.

  In a concentrating solar power generation apparatus, sunlight is often collected using a Fresnel lens having a curved surface shape. For example, as shown in Patent Documents 1 and 2, in this type of Fresnel lens, the outer surface of the curved surface is a smooth incident surface. A plurality of concentric protrusions are formed on the inner surface of the curved surface, and the surface on the side away from the optical axis constituting the protrusion is a prism surface. The light emitted from the prism surface is collected at a focal point on the optical axis.

  When such a Fresnel lens is manufactured from resin using mold molding, the prism surface formed inside the curved surface becomes undercut when the mold is opened in the optical axis direction. In order to remove the mold from the inner surface having such an undercut structure, as shown in Patent Document 1, the mold is divided into a plurality of parts. In other words, the mold is divided into a center core disposed at a portion including the optical axis and a slide core disposed around the center core, and after removing the center core, the slide core is slid in the radial direction and removed. .

JP 2006-35817 A JP 2012-9846 A

  As shown in Patent Document 1, when a mold is divided and removed, the prism surface of the Fresnel lens located at the division boundary of the mold and the edge of the divided mold slide, and this prism The surface may be damaged. Then, the optical characteristics of the prism surface change, and there is a possibility that the light transmittance (lens efficiency) and the light collection rate as designed cannot be obtained. Further, even if the prism surface is not damaged by sliding, the mechanical strength of the prism surface may be lowered.

  The problem to be solved by the present invention is to provide a Fresnel lens in which optical properties are prevented from deteriorating due to damage to the prism surface or reduction in mechanical strength caused by sliding with the mold during mold molding. And providing a method of manufacturing such a Fresnel lens.

  In order to solve the above problems, a Fresnel lens according to the present invention is made of resin, and has a plurality of prism surfaces inside a curved surface centered on the optical axis, and the prism surfaces are in the optical axis direction. A non-undercut portion that is not undercut, and a portion that is adjacent to the non-undercut portion and further away from the optical axis than the non-undercut portion, and the prism surface is formed with respect to the optical axis. An undercut portion that is an undercut, and along the adjacent direction of the non-undercut portion and the undercut portion of the prism surface located at the boundary between the non-undercut portion and the undercut portion The gist is that the width is at least twice the smaller one of the two adjacent prism surfaces.

  Here, it is preferable that the Fresnel lens has an equal thickness at the non-undercut portion, the undercut portion, and the boundary.

  In this case, the Fresnel lens is a pentagon having a pentagon formed in a cross section including a parallelogram having a first side and a second side parallel to each other and a triangle having the first side of the parallelogram as a base. Each pentagonal prism has an equal distance between the first side and the second side of the parallelogram, and the second side of the parallelogram is the entrance surface. It is preferable that the sides of the parallelogram are adjacent to each other at the side intersecting the first side and the second side so that one of the sides other than the base of the triangle becomes the prism surface.

  On the other hand, the manufacturing method of the Fresnel lens according to the present invention is a method of manufacturing a Fresnel lens as described above, and when the mold is removed from the inside of the curved surface shape, at the boundary between the undercut portion and the non-undercut portion. The gist is to divide the mold, and after removing the mold located in the non-undercut portion in the optical axis direction, the mold located in the undercut portion is shifted to the optical axis side and removed. .

  After the molding, the Fresnel lens according to the above invention is formed by dividing the mold at the boundary between the non-undercut portion and the undercut portion, and after removing the mold corresponding to the non-undercut portion, the mold corresponding to the undercut portion. It can be manufactured by shifting the mold to the optical axis side and removing it. At this time, the prism surface located at the boundary between the non-undercut portion and the undercut portion slides with the edge of the divided mold, and this prism surface has a width twice or more that of the adjacent prism surface. Therefore, even if the prism surface is damaged or the strength is reduced over the same width region as the other prism surface during sliding, the remaining width having the same or wider width than the other prism surface There is no damage or loss of mechanical strength in the area. As a result, compared with the case where a wide prism portion is not provided between the non-undercut portion and the undercut portion, the Fresnel lens is caused by sliding at the time of releasing, such as a decrease in transmittance (lens efficiency) and light collection rate. Deterioration of the optical characteristics is less likely to occur.

  Here, when the Fresnel lens has the same thickness at the non-undercut portion, the undercut portion, and the boundary, a constant mechanical strength can be obtained in the entire Fresnel lens.

  In this case, if the Fresnel lens is formed as an assembly of pentagonal prisms as described above, the distance between the first side and the second side of the parallelogram that is the thickness of the Fresnel lens is constant throughout the Fresnel lens. However, it is possible to design a Fresnel lens that can collect the light emitted from each prism surface on the focal point.

  On the other hand, in the method for manufacturing a Fresnel lens according to the invention, the mold is divided at the boundary between the non-undercut portion and the undercut portion where the prism surface having a width twice that of the adjacent prism surface is formed. Even if the edge of the divided mold and the prism surface slide, it is possible to avoid damaging the entire prism surface at the boundary and reducing the mechanical strength due to the sliding. Thereby, in the manufactured Fresnel lens, it can suppress that an optical characteristic falls.

It is a top view which shows the outline of the Fresnel lens concerning one Embodiment of this invention. It is a perspective view of the Fresnel lens. It is sectional drawing of the said Fresnel lens, (a) is a general view, (b) is an enlarged view which shows a non-undercut part. In FIG. (B), a further enlarged portion surrounded by a circle is also shown. It is sectional drawing of the said Fresnel lens, (a) is a general view, (b) is an enlarged view which shows an undercut part. It is a figure which shows the boundary vicinity of a non-undercut part and an undercut part, (a) is the state which looked at the lens inner surface along the optical axis, (b) has shown the cross section. It is a perspective view which shows the metal mold | die for manufacturing the said Fresnel lens, (a) is an inner side type | mold, (b) is an outer side type | mold. It is a schematic diagram which shows the manufacturing method of the said Fresnel lens, (a), (b) has shown the release process in order. A schematic perspective view of only the mold (inner mold) is shown, and the relationship between the mold and the lens is shown in cross-sectional views (XX and YY cross sections). FIG. 8 is a schematic diagram sequentially illustrating steps subsequent to FIG. 7 in the manufacturing method of the Fresnel lens. It is a schematic diagram which shows the measuring method of lens efficiency. It is a figure which shows the measurement result of the lens efficiency in the said Fresnel lens, (a) shows the lens concerning embodiment of this invention, (b) has shown the conventional lens. FIG. 2B also shows auxiliary lines and symbols for identifying each region. (A) Schematic showing a pentagonal prism, (b) Schematic showing a Fresnel lens designed by assembling pentagonal prisms. It is a figure which shows an example of the solar power generation device using the said Fresnel lens.

  Hereinafter, a Fresnel lens according to an embodiment of the present invention will be described with reference to the drawings.

[Solar power generator]
First, the solar power generation device 50 in which the Fresnel lens 10 according to the embodiment of the present invention is preferably used will be briefly described. In FIG. 12, the outline of the solar power generation device 50 is shown. The solar power generation device 50 converts the light energy of incident sunlight into electrical energy and outputs it. The solar power generation device 50 includes a plurality of Fresnel lenses 10 and a plurality of power generation receivers 51 arranged on a support plate 53 in a matrix. Each Fresnel lens 10 has a curved surface shape, and a large number are arranged in a matrix corresponding to each power generation receiver 51. In the solar power generation device 50, a wall member is provided on the outer peripheral portion to prevent insects and raindrops from entering, and the power generation receiver 51 is accommodated in a housing formed by the Fresnel lens 10, the support plate 53, and the wall member. However, in FIG. 12, the wall surface member is excluded.

  The Fresnel lens 10 serves as a primary optical system for concentrating sunlight on the power generation receiver 51. The power generation receiver 51 includes a solar power generation element 52 made of a semiconductor, and converts the light energy of the collected sunlight into electric energy and outputs the electric energy. In addition to the solar power generation element 52, the power generation receiver 51 is an optical member (referred to as a secondary optical system or a homogenizer; not shown) for spatially uniforming the collected sunlight and guiding it to the surface of the solar power generation element 52; And a circuit system (not shown) used for taking out the current obtained by the photovoltaic power generation element 52 and the like.

  The solar power generation element 52 is disposed so that the center of the semiconductor surface (light receiving surface) is the focal position of the Fresnel lens 10 and the semiconductor surface is perpendicular to the optical axis of the Fresnel lens 10. When sunlight enters parallel to the optical axis (rotation symmetry axis) of the Fresnel lens 10, it is condensed in a spot shape at the focal position on the semiconductor surface of the photovoltaic power generation element 52, and power generation is performed at a high current density. . Therefore, it is possible to perform power generation with high efficiency using the photovoltaic power generation element 52 having a small area, as compared with a flat panel photovoltaic power generation apparatus that does not include the Fresnel lens 10.

  Thus, since the solar power generation device 50 collects sunlight with the Fresnel lens 10 and generates power, when the optical axis of the Fresnel lens 10 is directed to the incident direction of sunlight, The power generation efficiency in the solar power generation device 50 is the highest. For this reason, the solar power generation device 50 is supported by a solar tracking device (not shown) that can change the azimuth angle direction and the altitude direction, and the optical axis of the Fresnel lens 10 is always directed to the incident direction of sunlight. So that it is controlled. Thereby, even if the position of the sun on the celestial sphere changes, power generation with high efficiency is maintained.

[Configuration of Fresnel lens]
Hereinafter, the configuration of the Fresnel lens 10 will be described with reference to FIGS.

  As shown in FIGS. 1, 2, 3 (a), and 4 (a), the Fresnel lens 10 is made of an acrylic or polycarbonate resin material typified by polymethyl methacrylate (PMMA). And has a curved surface shape (for example, a substantially partial spherical shape) that swells in a dome shape around the optical axis A. A curved outer surface (convex surface) 14 serving as an incident surface has a smooth surface. On the other hand, the inner side surface (concave surface) 15 is formed with a number of concentric ridges centered on the optical axis A and protruding inward from the smooth base surface 15a. Each protrusion has a sawtooth shape in cross section perpendicular to the radial direction of the Fresnel lens 10, and a radially outer plane constituting each protrusion refracts incident sunlight and lies on the optical axis A. The prism surfaces P1 to P3 are focused on the focal point. On the other hand, the radially inner surface of each protrusion is divided surfaces S1 to S3 that divide each prism surface. Over the entire Fresnel lens 10, prism surfaces and split surfaces are alternately formed in the radial direction of the base surface 15 a of the inner surface 15. The Fresnel lens 10 according to the present embodiment is characterized by setting widths W1 to W3 of the prism surfaces P1 to P3 (lengths along the radial direction in the surface of the Fresnel lens 10). Further, it is preferable that the thickness d of the Fresnel lens 10, that is, the distance between the outer side surface 14 and the base surface 15 a of the inner side surface 15 is equal throughout the entire area.

  As shown in FIGS. 1 and 2, the Fresnel lens 10 includes a non-undercut portion 11, a first undercut portion 12, and a second undercut portion 13. The non-undercut portion 11 is provided in the central portion of the Fresnel lens 10 as a substantially circular portion including the optical axis A. And the four 1st undercut parts 12 are arrange | positioned adjacent to the radial direction outer side of the non-undercut part 11. FIG. Further, a total of four second undercut portions 13 are arranged between the first undercut portions 12. The non-undercut portion 11, the first undercut portion 12, and the second undercut portion 13 are all integrally formed from a resin material and have a discontinuous breakage or joint at the boundary. However, as will be described later, since the mold used at the time of molding is divided at the boundary between these regions, the inner surface 15 of the Fresnel lens 10 hits the boundary of each part as shown in FIGS. A streak-like trace 16 formed by the dividing boundary of the mold appears at the site.

  FIG. 3 shows the configuration of the non-undercut portion 11. The non-undercut portion 11 is formed on the top of the Fresnel lens 10 including the optical axis A. The top of the Fresnel lens 10 has a shape that is recessed toward the inside. This is because the split surface S1 described later can be directed inward by making the light rays refracted outward at the outer surface 14, which makes it easier to remove the mold when the Fresnel lens 10 is manufactured. The prism surface P1 of the non-undercut portion 11 has an angle θ1 with respect to the optical axis A. Here, the angle θ1 is defined as an angle passing from the optical axis A inside the Fresnel lens 10 to the optical axis A outside the Fresnel lens 10 through the radially outer side. Hereinafter, the same applies to all angles (φ1, θ2, φ2) described with reference to the optical axis A. Since the light passing through the prism surface P1 needs to be collected at a point on the optical axis A, the angle θ1 of the prism surface P1 with respect to the optical axis A is 90 ° ≦ θ1 <180 °. In FIG. 3, the light incident on the position away from the optical axis A is also collected at the focal point on the optical axis A with high efficiency, and therefore the angle θ1 of the prism surface P1 increases toward the outer side in the radial direction. On the other hand, the angle φ1 of the dividing surface S1 with respect to the optical axis A is φ1 ≧ 0 °. In FIG. 3, φ1≈0 °, and in the enlarged view surrounded by a circle in FIG. 3B, φ1 is shown larger than the actual one for easy understanding. Since the angle θ1 of the prism surface P1 and the angle φ1 of the division surface S1 take such values, the prism surface P1 of the non-undercut portion 11 is not undercut with respect to the optical axis A direction. That is, the protrusion formed by the prism surface P1 and the dividing surface S1 at the time of molding the mold does not form an undercut that forms a catch when the mold is pulled along the optical axis A. The prism surface P1 has a width W1. Note that the width W1 and the angles θ1 and φ1 of the prism surface P1 may not be the same in all the prism surfaces P1 and the divided surfaces S1, and may change continuously, for example.

  FIG. 4 shows the configuration of the undercut portions 12 and 13. As shown in the drawing, the undercut portions 12 and 13 are composed of a first undercut portion 12 and a second undercut portion 13, and are adjacent to the non-undercut portion 11 and the Fresnel lens 10 in the radial direction, and the optical axis A. It is formed in the part away from. The first undercut portion 12 and the second undercut portion 13 are integrally continuous as described above, and have continuous protrusions on the inner side surface 15 having the same cross-sectional shape. The undercut portions 12 and 13 are formed on a curved surface inclined radially outward of the Fresnel lens 10. The prism surface P2 of the undercut portions 12 and 13 has an angle θ2 with respect to the optical axis A. Further, the dividing surface S2 has an angle φ2 with respect to the optical axis A. The angle θ2 of the prism surface P2 is 90 ° ≦ θ2 <180 ° because light passing through the prism surface P2 needs to be collected at a point on the optical axis A. Further, in order to condense the light incident on the undercut portions 12 and 13 existing at a position away from the optical axis A at the focal point on the optical axis A, the angle θ2 of the prism surface P2 of the undercut portions 12 and 13 is: The angle θ1 of the prism surface P1 of the non-undercut portion 11 is larger. On the other hand, the angle φ2 of the split surface S2 with respect to the optical axis A is φ2 <0 ° in order to avoid total reflection of light on the split surface S2. Since the angle θ2 of the prism surface P2 and the angle φ2 of the dividing surface S2 take the above values, the prism surface P2 of the undercut portions 12 and 13 is undercut with respect to the optical axis A direction. . That is, when the mold is formed, the ridge formed by the prism surface P2 and the dividing surface S2 becomes an undercut that forms a catch when the mold is pulled along the optical axis A.

  The prism surface P2 of the undercut portions 12 and 13 has a width W2. The width W2 of the prism surface P2 of the undercut portions 12 and 13 may be the same as or different from the width W1 of the prism surface P1 of the non-undercut portion 11, but in this embodiment, the width W2 is different. Is narrower than the width W1. The width W2 and the angles θ2 and φ2 of the prism surface P2 may not be the same in all the prism surfaces P2 and the divided surfaces S2, and may change continuously, for example.

  In addition, from the optical request, the entire Fresnel lens 10 may be formed by the undercut portions 12 and 13. However, as will be described later, in the mold molding, the mold can be released by dividing the mold. Thus, the non-undercut part 11 is formed in the area | region of the top part.

  FIG. 5 shows a configuration near the boundary B between the non-undercut portion 11 and the undercut portions 12 and 13. The boundary prism surface P3 has a width W3. The widths W1, W2, and W3 of the prism surfaces P1, P2, and P3 are the lengths measured in the radial direction of the Fresnel lens 10 along the actual prism surfaces P1 to P3. In the figure viewed parallel to the optical axis A, the width of each part is displayed as widths W1 ′, W2 ′, and W3 ′ when projected onto a plane orthogonal to the optical axis A.

The width W3 of the boundary prism surface P3 is more than twice the smaller one of the width W1 of the prism surface P1 of the non-undercut portion 11 and the width W2 of the prism surface P2 of the undercut portions 12 and 13. Here, when the widths W1 and W2 of the prism surfaces P1 and P2 are not the same in all the prism surfaces P1 and P2, respectively, the width W3 of the boundary prism surface P3 is the width of the prism surface P1 adjacent to the boundary prism surface P3. It is twice or more of the smaller one of W1 and the width W2 of the prism surface P2. More preferably, the width W3 of the boundary prism surface P3 is not less than twice the width W1 of the prism surface P1 of the non-undercut portion 11 and the width W2 of the prism surface P2 of the undercut portions 12 and 13. .

[Fresnel lens manufacturing method]
Next, the manufacturing method of the Fresnel lens concerning one Embodiment of this invention for manufacturing the above Fresnel lenses 10 is demonstrated. The Fresnel lens 10 is formed by injection molding using a mold as shown in FIG. That is, the inner mold 20 (FIG. 6 (a)) and the outer mold 30 (FIG. 6 (b)) formed so as to have a space between them when they are overlapped, and molten resin is put into the space. The Fresnel lens 10 can be manufactured by pouring and removing the mold (releasing) after solidification. The inner surface 15 of the Fresnel lens 10 is molded by the inner mold 20 having the mountain shape, and the outer surface 14 is molded by the outer mold 30 having the depression shape. In the present specification, description of details of the manufacturing apparatus and the manufacturing process is omitted, but the same manufacturing apparatus and manufacturing process as described in Patent Document 1 can be applied.

  This manufacturing method is characterized by how to remove the inner mold 20. 7 and 8 schematically show the process of removing the inner mold 20. In the figure, for each step, a schematic perspective view of only the inner mold 20, and an XX sectional view and a YY sectional view showing the relationship between the inner mold 20 and the Fresnel lens 10 are shown. In the figure, each process is shown in an easy-to-understand manner, and the positional relationship of each member does not necessarily match the actual one.

  As shown in FIG. 6A and FIG. 7A, the inner mold 20 used in this manufacturing method includes a center core 21, a first slide core 22, and a second slide core 23. The center core 21 corresponds to the non-undercut portion 11 of the Fresnel lens 10, the first slide core 22 corresponds to the first undercut portion 12, and the second slide core 23 corresponds to the second undercut portion 13, respectively. 15 is molded. The boundary between the center core 21 and the first slide core 22 and the second slide core 23 coincides with the boundary B between the non-undercut portion 11 and the undercut portions 12 and 13 of the Fresnel lens 10. . The first slide core 22 has an elongated shape in the radial direction of the inner mold 20 (the direction from the center to the periphery), but the width is wide at the center and narrows toward the periphery. ing. Each of the cores 21, 22, and 23 is formed by a separated member so that it can be divided at the time of mold release, or is formed as an integral member to form a crack at the boundary portion.

  After forming the Fresnel lens 10 and opening the outer mold as shown in FIG. 7A, the center core 21 and the first slide core 22 and the second slide core 22 of the inner mold 20 are opened as shown in FIG. 7B. The space between the slide core 23 is divided, and the center core 21 is pulled inward along the optical axis A direction (direction D1). Since the undercut is not formed in the non-undercut portion 11, the center core 21 can be pulled out along the optical axis A in this way. On the other hand, since the undercut portions 12 and 13 are formed with undercuts in the optical axis A direction, the first slide core 22 and the second slide core 23 are arranged along the optical axis A direction like the center core 21. I can't pull it out.

  Therefore, next, as shown in FIG. 8A, the first slide core 22 is divided with the second slide core 23 and shifted toward the optical axis A side of the Fresnel lens 10 (direction D2). Since the width of the first slide core 22 is wider toward the center side and narrower toward the peripheral side, the second slide core 22 is shifted toward the center space formed by removing the center core 21 in this way. Easy to operate. Thereby, the first slide core 22 can be removed from the first undercut portion 12 of the Fresnel lens 10.

  Finally, as shown in FIG. 8B, the space generated by removing the center core 22 and the gap generated between the first slide core 22 and the second slide core 23 by shifting the first slide core 22 are formed. Using the second slide core 23, the second slide core 23 is tilted inward in the radial direction (direction D3). Thereby, the second slide core 23 can be removed from the second undercut portion 13. In addition, in the perspective view of FIG.8 (b), the state after extracting two of the four 2nd slide cores 23 is shown for easy viewing. Thus, the Fresnel lens 10 having the undercut portions 12 and 13 can be manufactured by mold molding by dividing the inner mold 20 and removing it.

[Influence of sliding during mold release]
When the Fresnel lens 10 is manufactured as described above, the Fresnel lens 10 is in contact with the edge of the center core 21 when the center core 21 is removed along the optical axis A direction in the step of FIG. The boundary prism surface P3 and the edge of the center core 21 slide at the boundary portion B. 8A, when the first slide core 22 is shifted toward the optical axis A, the boundary prism surface P3 and the edge of the first slide core 22 slide. This sliding occurs over a distance equal to the width W2 of the prism surface P2 of the undercut portions 12 and 13 at the maximum. As described above, if sliding occurs between the boundary prism surface P3 and the edge of the divided inner mold 20 in the steps of FIGS. 7B and 8A, the boundary prism surface P3 is damaged. Damage such as cracks may occur. These damages reduce the light transmittance at the boundary prism surface P3, or cause a decrease in light condensing performance due to a change in the refractive index or the emission direction. It may lead to degradation of optical properties. Even if the boundary prism surface P3 is not obviously damaged by sliding, the mechanical strength of the boundary prism surface P3 may be lowered.

  However, in the present Fresnel lens 10, the width W3 of the boundary prism surface P3 is more than twice the smaller one of the widths W1 and W2 of the prism surfaces P1 and P2, and such sliding temporarily assumes the width W1. Alternatively, the boundary prism surface that occupies a width region that is the same as or wider than the widths W1 and W2 even if damage or a decrease in mechanical strength occurs over the entire region where sliding occurs on the boundary prism surface P3. The remaining portion of P3 is not damaged or deteriorated in mechanical strength, and maintains optical characteristics and mechanical strength comparable to those of the other prism surfaces P1 and P2. Thereby, the influence on the optical characteristic of the Fresnel lens 10 given by sliding on the boundary prism surface P3 can be suppressed to a small level. If the width W3 of the boundary prism surface P3 is at least twice as large as the widths W1 and W2 of the prism surfaces P1 and P2, the influence on the optical characteristics due to sliding can be suppressed with higher accuracy.

  Here, as an optical characteristic of the Fresnel lens 10 as described above, a result of actually measuring a spatial distribution of lens efficiency is shown. The measurement method is schematically shown in FIG. That is, the circularly polarized light L output from the He—Ne laser 91 (with the 3B class, 5 × expander and ¼λ circularly polarizing plate) is irradiated in parallel to the optical axis of the Fresnel lens 10 held in the lens holder 93. did. Then, the light L is converted into the lens of the Fresnel lens 10 by using the XY stage 92 supporting the He-Ne laser 91 and the XYZ controller (or gonio stage) 94 supporting the lens holder 93. The total intensity of the light L transmitted through the Fresnel lens 10 was measured using the integrating sphere 95 and the detector 96 while sweeping the surface. Further, the entire region corresponding to the lens surface of the Fresnel lens 10 subjected to the surface sweep was divided into 625 regions (25 mm × 25 mm per region), and the light intensity was integrated for each divided region. The intensity of the light L that is incident on the integrating sphere without passing through the Fresnel lens 10 (the central axis of the He-Ne laser 91 and the aperture center of the integrating sphere 95 are made to coincide with each other) The spatial distribution of the lens efficiency in each divided region was mapped by normalizing the measurement so that it was perpendicular to the laser optical axis.

  FIG. 10 shows the spatial distribution of the lens efficiency of the Fresnel lens 10 obtained by actual measurement. (A) corresponds to the case where the width W3 of the boundary prism surface P3 is at least twice the widths W1 and W2 of the prism surfaces P1 and P2, as described above. Here, W1 = W2. On the other hand, (b) corresponds to the case where the width of the boundary prism surface P3 is the same as the width W2 of the prism surfaces of the undercut portions 12 and 13.

  In either case, a region having a low lens efficiency is formed in an annular shape in a region where the boundary prism surface P3 corresponding to the boundary B between the non-undercut portion 11 and the undercut portions 12 and 13 is disposed. However, when the width W3 of the boundary prism surface P3 in FIG. 10A is increased, the width of the ring is relatively small and the uniformity of the ring over the entire circumference is also high. On the other hand, when the width of the boundary prism surface P3 in FIG. 10B is not widened, the width of the ring in the region where the lens efficiency is lowered is wider than that in the case of FIG. It has become. Further, the lens efficiency is reduced in a spatially non-uniform manner, such as a large reduction in lens efficiency over a wide area at the lower end of the ring. Comparison between FIGS. 10A and 10B shows that the reduction in lens efficiency due to sliding during mold release can be reduced by increasing the width W3 of the boundary prism surface P3.

[Detailed shape of Fresnel lens]
As described above, in the Fresnel lens 10, it is preferable that the thickness d, that is, the distance between the outer surface 14 and the base surface 15 a of the inner surface 15 is equal throughout the entire area. As described above, the Fresnel lens having a constant thickness and capable of focusing on one focal point in the entire region can be formed as an assembly of a plurality of minute pentagonal prisms.

  FIG. 11A shows an outline of the pentagonal prism 80. The pentagonal prism 80 has a pentagonal ABDEC as a cross section. The pentagon ABDEC has a shape in which a parallelogram (including a rectangle) ABDC and a triangle DEC are joined at a side CD. The side CD is one side (first side) of the parallelogram ABDC and at the same time the bottom side of the triangle DEC. In the pentagonal prism 80 having such a cross-sectional shape, if the internal angles of the parallelogram ABDC and the triangle DEC are appropriately selected, the side AB (second side) of the parallelogram ABDC as shown in FIG. When the light L is incident non-perpendicularly, the light L can be refracted so that the component incident from the end point B passes through the end point E of the side CE of the triangle DEC and can be emitted from the side CE. At this time, the light L incident on at least a part of the side AB can satisfy the minimum deflection angle condition (a condition in which the incident angle to the side AB is equal to the emission angle from the side CE). Therefore, even if the internal angle of the parallelogram ABDC is the same and the thickness d of the prism 80 (the distance between the side AB and the side CD) is the same, the arrangement angle of the parallelogram ABDC with respect to the incident direction of the light L By making the inner angles of the triangle DEC different, the light L can be refracted and emitted toward an arbitrary point. That is, it is possible to arrange a plurality of pentagonal prisms 80 having the same inner angle of the parallelogram ABDC and collect light emitted from all the prisms 80 at a common focal point. By utilizing this fact, the Fresnel lens 10 as described above can be configured as an assembly of a plurality of pentagonal prisms 80.

  Specifically, as schematically shown in FIG. 11 (b), pentagonal prisms 80 (81 to 84) having the same internal angle of the parallelogram portion and the same thickness d but different internal angles of the triangular portion, The Fresnel lens 10 may be designed as a shape in which a plurality of parallelogram portions are adjacent to each other. At this time, as indicated with reference numerals for the pentagonal prisms 82 and 83, the first sides CD and C′D ′ of the parallelograms ABDC and A′B′D′C ′ are formed on the inner surface 15 of the Fresnel lens 10. Two pentagonal prisms 82 and 83 are adjacent to each other on the sides AC and B′D ′ intersecting with the base surface 15 a and the second sides AB and A′B ′ as the outer surface 14. Then, the sides CE and C'E 'which are one side intersecting the bottom side of the triangles DEC and D'E'C' become the prism surfaces P and P 'that refract the incident light L and emit it. These prism surfaces P and P 'are in a state of being divided by dividing surfaces S and S' composed of sides DE and D'E 'which are the other sides intersecting the bottom side of the triangle DEC. In this way, it is possible to design the Fresnel lens 10 having the inner surface 15 having a ridge shape composed of the prism surfaces P and P ′ and the split surfaces S and S ′. In each pentagonal prism 80 (81-84), if the inner angle of the triangle DEC is changed according to the inclination angle of the side AB with respect to the optical axis A of the Fresnel lens 10, each prism surface enters parallel to the optical axis A. Light emitted from (P, P ′) can be collected at a common focal point on the optical axis A.

  In actually designing the Fresnel lens 10, as long as the shape of the outer surface 14 and the position of the focal point and the width of each pentagonal prism (the length of the side AB) are determined, the light L incident on the side AB under the minimum deflection angle condition. Is emitted toward the focal point and the component incident on the end point B passes through the end point E, the inner angle of the parallelogram ABDC common to each pentagonal prism 80 (and the common wall thickness d determined as a result thereof) Each shape parameter of each pentagonal prism 80 including) is automatically determined. That is, by defining the emission direction of the light L from the prism 80, the angle and arrangement of the side CE that is the emission surface with respect to the side AB that is the incidence surface are determined, and the optical path of the light L incident from the end point B is The end point E of the exit surface is determined as the intersection of the exit surfaces. In this way, the cross-sectional shape of the pentagonal prism 80 is uniquely determined.

  As described above, if the Fresnel lens 10 is configured as an assembly of pentagonal prisms 80, the non-undercut portion 11, the undercut portions 12, 13 and the boundary portion B thereof have the same thickness d, In addition, it is possible to obtain the Fresnel lens 10 that can collect light at a common focal point. By having the same thickness d over the entire area, the same mechanical strength can be obtained in the entire Fresnel lens 10. Note that. As a variation in the thickness d for each pentagonal prism 80 due to tolerances in optical design and manufacturing, about ± 10% is allowed. For example, when the Fresnel lens 10 is made of PMMA resin, the thickness d is preferably in the range of 2.5 to 6 mm. If it is thinner than this range, it becomes difficult to obtain sufficient mechanical strength. On the other hand, if it is thicker than this range, the mass becomes too large and the optical loss due to the Fresnel lens 10 cannot be ignored.

  Further, by designing the Fresnel lens 10 as an assembly of pentagonal prisms 80, the parameters of each part can be automatically determined by only specifying a small number of parameters as described above. Therefore, the design and manufacturing process of the Fresnel lens 10 are simplified. In the above-described Fresnel lens, the relationship between the widths W1 to W3 of the prism surfaces P1 to P3 of each part is defined, but the widths W1 to W3 of the prism surfaces P1 to P3 are the widths (sides) of each pentagonal prism. (Length of AB) can be adjusted.

  Also in Patent Document 2 shown above, the Fresnel lens is designed as an aggregate of a plurality of prism portions, but unlike the present embodiment, each prism portion has a trapezoidal shape. In this case, the lens thickness cannot be determined independently of the optical optimization design for condensing incident light at a desired focal point. Therefore, as a result of the optical optimization design, there is a possibility that a portion having a small lens thickness may be generated (for example, FIG. 1 of Patent Document 2).

  As mentioned above, although embodiment of this invention was described in detail, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the summary of this invention.

DESCRIPTION OF SYMBOLS 10 Fresnel lens 11 Non-undercut part 12 First undercut part 13 Second undercut part 14 Outer side face 15 Inner side face 20 Inner side mold 21 Center core 22 First slide core 23 Second slide core 80 Pentagon prism A Optical axis B Boundary P1 Prism surface P2 of non-undercut portion Prismatic surface P3 of undercut portion Boundary prism surface S1 Split surface S2 of non-undercut portion Split surface of undercut portion

Claims (4)

Made of resin, with a plurality of prism surfaces inside the curved shape centered on the optical axis,
The prism surface is formed in a portion that is not undercut with respect to the optical axis direction, and a portion that is adjacent to the non-undercut portion and further away from the optical axis than the non-undercut portion, An undercut portion in which the prism surface is undercut with respect to the optical axis;
The width of the prism surface located at the boundary between the non-undercut portion and the undercut portion along the adjacent direction of the non-undercut portion and the undercut portion is the smaller of the two adjacent prism surfaces. A Fresnel lens characterized by being at least twice the width.   The Fresnel lens according to claim 1, wherein the non-undercut portion, the undercut portion, and the boundary have equal thicknesses. The Fresnel lens includes a plurality of pentagonal prisms having a cross section of a pentagon made up of a parallelogram having first and second sides parallel to each other and a triangle having the first side of the parallelogram as a base. Having a shaped shape,
Each pentagonal prism has the same distance between the first side and the second side of the parallelogram, the second side of the parallelogram is the entrance surface, and one of the sides other than the base of the triangle is The Fresnel lens according to claim 2, wherein the Fresnel lens is adjacent to each other at a side intersecting the first side and the second side of the parallelogram so as to be the prism surface. In the method of manufacturing the Fresnel lens according to any one of claims 1 to 3,
When removing the mold from the inside of the curved surface,
After dividing the mold at the boundary between the undercut part and the non-undercut part, and after removing the mold located in the non-undercut part in the optical axis direction,
A method of manufacturing a Fresnel lens, wherein a mold located at the undercut portion is shifted and removed toward the optical axis side.