Diffraction lens, optical system and vehicle

Abstract

To provide a diffraction lens that can be efficiently made both thin and achromatic at the same time, an optical system and a vehicle.SOLUTION: A diffraction lens 8 includes, on its incidence surface 8a, a first diffraction lens structure 20 such that diffraction grating of diffracted light of fifth order or larger in absolute value of diffraction order is maximum relative to light having a wavelength within a wavelength spectrum range of a light source, and a second diffraction lens structure 22 such that diffraction efficiency of diffracted light of first order or less in absolute value of diffraction order is maximum. The incidence surface 8a is formed as a diffraction lens surface where the second diffraction lens structure 22 is located adjacently to surround an outer periphery of the first diffraction lens structure 20 located in the center.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a diffractive lens, and more particularly to a diffractive lens suitable for automobile headlights (headlamps), an optical system including the diffractive lens, and a vehicle equipped with the optical system. [Background technology]

[0002] FIG. 23 shows a conceptual diagram of the structure of an optical system (optical system) for a typical projector-type headlamp (for low beam) used in automobile headlamps. Here, FIG. 23(a) is a top view of the optical system, and FIG. 23(b) is a vertical cross-sectional view. As shown, the optical system 100 includes an LED 102 as a light source, a reflector 104, a shade 106, and a lens 108. The optical system 100 may also include a heat sink and an LED control circuit, but these are not shown here. In the figure, with respect to the coordinate system XYZ, the Z direction is the horizontal direction of vehicle travel, the X direction is the horizontal width direction, and the Y direction is the vertical direction, with the -Z end of the reflector 104 being the origin. In this case, FIG. 23(b) is a YZ cross-sectional view of the reflector 104 at X=0.

[0003] As shown in the figure, the reflector 104 is an aspherical surface or a free-form surface based on an ellipse. The LED 102 is installed so that the center of its light-emitting surface emits light in the Y direction toward a first focal point 110 of the ellipse of the reflector 104. In this case, the light emitted from the LED 102 is condensed at a second focal point 120 of the reflector 104. The lens 108 is installed so that the second focal point 120 of the reflector 104 becomes the focal point of the lens 108, and projects the luminance distribution of the light spot formed at the second focal point 120 of the reflector 104.

[0004] For low beams, it is necessary to form an area in the horizontal direction where the luminous intensity changes suddenly, called a cut-off line, but in low beam projector-type headlamps, a shade 106 is provided near the second focal point 120, and the luminance distribution of the formed light spot is adjusted (a region in the horizontal direction where the luminance changes suddenly) and the cut-off line is created by projecting this luminance distribution with a lens 108. The shade 106 is provided in a horizontal plane, and a metal film such as aluminum is formed on its surface by vapor deposition or the like, so that the light that reaches the shade 106 can also be used.

[0005] However, in such a conventional optical system 100, the thickness of the lens 108 is large. This is because a high light utilization efficiency is required, which requires a high NA (at least 0.5 or more, approximately 0.70), which in turn reduces the radius of curvature (hence increases the lens thickness). As a result, the lens 108 is heavy, requires a long molding time, and increases manufacturing costs.

[0006] For this reason, a technology has been proposed and implemented to make the lens 108 a diffractive lens by adding a diffractive lens structure having the same deflection power as the convex surface to one surface of the lens 108. This allows the convex surface to be made flat or concave, making it possible to reduce the thickness of the lens. Specifically, as an example, the diffractive lens structure is a stepped annular zone structure that maximizes the diffraction efficiency at high diffraction orders, the absolute value of which is 5 or higher, for light of a certain wavelength within the wavelength spectrum range of the light source (see, for example, Patent Document 1). In this case, the optical path difference at the step is longer than the coherence length of the light source, and the lens essentially functions as a Fresnel lens, thereby effectively achieving a reduction in thickness.

[0007] Furthermore, in the conventional optical system 100 described above, coloring occurs on the low-beam cutoff line due to chromatic aberration of the lens 108, and a solution to this problem is also desired. The refractive index of lens materials varies depending on the wavelength of light (the refractive index increases as the wavelength of light becomes shorter). For example, in a lens that converts green light into parallel light, when light from a light source enters the lens, the blue light of the emitted light is narrowed and the red light is diverged. To correct this chromatic aberration, so-called achromatic lenses have been known, which combine a concave lens and a convex lens made of lens materials whose refractive indexes differ by different amounts depending on the wavelength of light.

[0008] Another solution to remove color (for achromatization) is to use a diffractive lens with a stepped ring-shaped structure that maximizes the diffraction efficiency at diffraction order 1 for light of a wavelength within the wavelength spectrum of the light source. In this case, the diffraction angle θ2 of the mth diffracted light satisfies the following equation:

number

[0009] For the same m, the diffraction angle increases as the wavelength λ increases, which is the opposite of the tendency of the refraction angle. Therefore, if a diffraction grating is provided on the surface of a refractive lens and an optical design is performed in which part of the refractive power of the refractive lens is replaced by diffraction, it becomes possible to correct the chromatic aberration of the lens (achromatism). [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 6297932 Summary of the Invention [Problem to be solved by the invention]

[0011] As described above, although it has been possible to thin a lens and achromatize a lens in the past, it has been difficult to say that both thin a lens and achromatization have been achieved simultaneously and efficiently in a single lens.

[0012] The present invention has been made in view of the above circumstances, and has an object to provide a diffractive lens, an optical system, and a vehicle that can simultaneously and efficiently achieve both lens thinning and achromatism. [Means for solving the problem]

[0013] In order to solve the above-mentioned problems, the present invention provides a diffractive lens for use in a vehicle headlamp, comprising: The lens includes, on either the light entrance surface or the light exit surface, a first diffractive lens structure that maximizes the diffraction efficiency of diffracted light whose absolute value of the diffraction order is 5 or more for light of a wavelength within the wavelength spectrum range of the light source, and a second diffractive lens structure that maximizes the diffraction efficiency of diffracted light whose absolute value of the diffraction order is 1 or less, The one surface of the lens is formed as a diffractive lens surface in which the second diffractive lens structure is positioned adjacent to the first diffractive lens structure so as to surround the outer periphery of the first diffractive lens structure positioned in the center. It is characterized by:

[0014] According to the above-described configuration of the present invention, the first diffractive lens structure maximizes the diffraction efficiency of diffracted light with an absolute value of the fifth or higher diffraction order for light of a wavelength within the wavelength spectrum range of the light source, thereby effectively thinning the lens at its inner surface. Furthermore, the second diffractive lens structure maximizes the diffraction efficiency of diffracted light with an absolute value of the first or lower diffraction order at the outer periphery of the lens, where chromatic aberration is significant (the coloring of the cut line is significantly affected by the lens periphery), thereby effectively achieving achromatization. Providing the second diffractive lens structure at the outer periphery of the lens is particularly beneficial for projector-type headlamps, where intense light is incident on the outer periphery of the lens. Furthermore, providing both the first and second diffractive lens structures simultaneously on one surface of the lens is particularly beneficial for automobile headlight lenses, where one surface is often modified with free-form surfaces and dimples, etc., to meet specified standards. This allows for both thinning the lens and achromatization to be achieved simultaneously and efficiently.

[0015] In the above configuration, as long as there is a first diffractive lens structure on the inside of the lens and a second diffractive lens structure on the outer periphery of the lens, the extension form of these diffractive lens structures on the lens may be any form, and these diffractive lens structures may or may not be adjacent to each other so as to define a boundary.

[0016] Furthermore, in the above-described configuration of the present invention, when the first diffractive lens structure forms a circle at the center of the lens and the second diffractive lens structure forms an annular shape around it, the radius of the circular boundary line forming the boundary between the first diffractive lens structure and the second diffractive lens structure is preferably 0.4 times or more the effective radius of the lens. Here, the "effective radius of the lens" refers to half the effective diameter of the lens, and the "effective diameter" refers to the portion of the lens that has the specified and acceptable optical characteristics and can actually be used as a lens, and is expressed as the diameter of a circle centered on the optical axis of the lens. Specifically, the "effective diameter" refers to the diameter of a circle whose radius is the distance from the optical axis of the light ray passing through the lens surface at the position farthest from the optical axis. In this configuration, the boundary itself may form a circular line, or the boundary may extend so as to form a circular line at any radial position within that region. In the above configuration, it is preferable that the shift amount of the boundary portion in the first diffractive lens structure is approximately the same as the shift amount of the boundary portion in the second diffractive lens structure, where "approximately the same" means that the difference is within 10 nm or less.

[0017] In the above configuration, the first diffractive lens structure and the second diffractive lens structure may form a stepped annular structure, in which case it is preferable that the transition from the first diffractive lens structure to the second diffractive lens structure occurs at a step position of the first diffractive lens structure that is lower than the maximum height of the step of the second diffractive lens structure. This makes it possible to prevent a step from occurring at the transition portion or to minimize the step, thereby preventing light loss.

[0018] The present invention also provides an optical system having a diffractive lens of the above configuration, and a vehicle equipped with such an optical system. [Effects of the Invention]

[0019] According to the present invention, both lens thinning and achromatism can be achieved simultaneously and efficiently. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an optical system 1 according to one embodiment of the present invention that constitutes a vehicle headlamp. [Figure 2] The stepped annular zone structure of the diffractive lens structure in the diffractive lens of the optical system of FIG. 1 is shown in cross section as a development view. [Figure 3] 2 shows a cross-sectional view of the detailed shape of a diffractive lens in the optical system of FIG. 1; [Figure 4] 1 shows an example of the dependency of light on the light output angle from the optical axis in a typical refractive lens. [Figure 5] This shows the wavelength dependency of the first-order diffraction efficiency when visible light is incident on a diffraction grating that generates one diffracted light for light with a wavelength of 0.52 μm. [Figure 6] This shows the wavelength dependency of the fifth-order diffraction efficiency when visible light is incident on a diffraction grating that generates fifth-order diffracted light for light with a wavelength of 0.52 μm. [Figure 7] This shows the wavelength dependency of the 10th-order diffraction efficiency when visible light is incident on a diffraction grating that generates 10th-order diffracted light for light with a wavelength of 0.52 μm. [Figure 8] This shows the wavelength dependence of the diffraction efficiency of each order, including the fifth order, when visible light is incident on a diffraction grating that generates fifth-order diffracted light for light with a wavelength of 0.52 μm. [Figure 9] This shows the wavelength dependence of the diffraction efficiency of each order, including the 10th order, when visible light is incident on a diffraction grating that generates 10th-order diffracted light for light with a wavelength of 0.52 μm. [Figure 10] The wavelength dependency of the diffraction angle when visible light is incident on a diffraction grating with a pitch of 59.588 μm that generates m-th order diffracted light for light with a wavelength of 0.52 μm is shown for m=1. [Figure 11] The wavelength dependency of the diffraction angle when visible light is incident on a diffraction grating with a pitch of 59.588 μm that generates m-th order diffracted light for light with a wavelength of 0.52 μm is shown for m=5. [Figure 12]The wavelength dependence of the diffraction angle when visible light is incident on a diffraction grating with a pitch of 59.588 μm that generates m-th order diffracted light for light with a wavelength of 0.52 μm is shown for m=10. [Figure 13] 2 is a detailed explanatory diagram of the boundary positions of the diffractive lens structure of the optical system 1 of FIG. 1. FIG. [Figure 14] 1 shows lens cross-sectional shapes related to Example 1, where (a) shows the cross-sectional shape of a diffractive lens 8 according to an embodiment of the present invention, and (b) shows the cross-sectional shape of a refractive lens as a comparative example. [Figure 15] 1 shows numerical data of the cross-sectional shapes of the diffractive lens and the refractive lens related to Example 1. [Figure 16] 1A shows a graph illustrating the light output angle dependence of the diffractive lens and the refractive lens confirmed by the phase function in Example 1, and FIG. 1B shows a green spot diagram of the diffractive lens and the refractive lens confirmed by the phase function. [Figure 17] 1 shows lens cross-sectional shapes related to Example 2, where (a) shows the cross-sectional shape of a diffractive lens 8 according to an embodiment of the present invention, and (b) shows the cross-sectional shape of a refractive lens as a comparative example. [Figure 18] 10 shows numerical data of the cross-sectional shapes of the diffractive lens and the refractive lens related to Example 2. [Figure 19] 1A shows a graph illustrating the light output angle dependence of the diffractive lens and the refractive lens confirmed by the phase function in Example 2, and FIG. 1B shows a green spot diagram of the diffractive lens and the refractive lens confirmed by the phase function. [Figure 20] 1 shows lens cross-sectional shapes related to Example 3, where (a) shows the cross-sectional shape of a diffractive lens 8 according to an embodiment of the present invention, and (b) shows the cross-sectional shape of a refractive lens as a comparative example. [Figure 21] 10 shows numerical data of the cross-sectional shapes of the diffractive lens and the refractive lens related to Example 3. [Figure 22] 10(a) shows a graph illustrating the light output angle dependence of the diffractive lens and the refractive lens confirmed by the phase function in Example 3, and FIG. 10(b) shows a green spot diagram of the diffractive lens and the refractive lens confirmed by the phase function. [Figure 23] These are conceptual diagrams of the structure of the optical system of a typical projector-type headlamp (for low beam) used in automobile headlamps, where (a) is a top view of the optical system and (b) is a vertical cross-sectional view. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. This embodiment can realize a highly reliable system, particularly in a sensing system, and contributes to the development of resilient infrastructure. The target is "9. Industry, innovation and infrastructure" of the Sustainable Development Goals (SDGs) advocated by the United Nations, which states, "9.1 Develop quality, reliable, sustainable and resilient infrastructure, including regional and transborder infrastructure, to support economic development and human well-being, with a focus on affordable and equitable access for all."

[0022] 1 shows a schematic diagram of an optical system 1 according to one embodiment of the present invention, which constitutes a vehicle headlamp mounted on a vehicle (not shown). As shown in the figure, the optical system 1 includes an LED 2 as a light source that emits light, a reflector 4 that reflects and collects the light emitted from the LED 2, a shade 6 that blocks a portion of the light reflected and collected by the reflector 4, and a diffractive lens 8 that receives the light that has passed through the shade 6 and directs it forward of the vehicle.

[0023] 3 shows a more detailed shape and structure of a diffractive lens 8 according to one embodiment of the present invention. As shown in the figure, the diffractive lens (hereinafter simply referred to as lens) 8 has an incident surface 8a on one side, onto which light from the LED 2 is incident (via the reflector 4 and the shade 6), and an exit surface 8b on the other side, from which the incident light is emitted toward the front of the vehicle. Either the incident surface 8a or the exit surface 8b of the lens 8 (in this figure, the incident surface 8a of the lens 8) includes a first diffractive lens structure 20 that maximizes the diffraction efficiency of diffracted light whose absolute value of the diffraction order is 5 or greater for light of a certain wavelength within the wavelength spectrum range of the LED 2, and a second diffractive lens structure 22 that maximizes the diffraction efficiency of diffracted light whose absolute value of the diffraction order is 1 or less.

[0024] Specifically, the entrance surface 8a is formed as a diffractive lens surface in which the second diffractive lens structure 22 is positioned adjacent to the first diffractive lens structure 20 located at the center so as to surround the outer periphery. In particular, in this embodiment, the first diffractive lens structure 20 is added to a circular region 8aa in the center of the entrance surface 8a of the lens 8 to form a circle, which contributes to thinning the lens. In addition, the second diffractive lens structure 22 is added to an annular region 8ab around the circular region 8aa of the entrance surface 8a of the lens 8 to form an annular shape, which contributes to achromatism (chromatic aberration correction).

[0025] 2, the first diffractive lens structure 20 and the second diffractive lens structure 22 form a stepped annular zone structure. In particular, in this embodiment, the first diffractive lens structure 20 transitions to the second diffractive lens structure 22 at the position of the step (maximum height H1) of the first diffractive lens structure 20, which is lower than the maximum height H2 of the step of the second diffractive lens structure 22. In this case, the radius r of the circular boundary line B (shown by a dashed line in FIG. 2) that forms the boundary 30 (transition portion from the first diffractive lens structure 20 to the second diffractive lens structure 22) between the circular first diffractive lens structure 20 and the annular second diffractive lens structure 22 is set to be 0.4 times or more the effective radius of the lens 8. Furthermore, as will be described later, the first lens structure 20 and the second lens structure 22 can have a shape in which a shift is applied to a macroscopic structure (base surface) represented by an aspherical formula or the like. 2 shows a case where the envelope surface connecting the lower ends of each annular zone is the base surface 42, but it is preferable that the shift amount of the boundary portion 30 in the first diffractive lens structure 20 is approximately the same as the shift amount of the boundary portion 30 in the second diffractive lens structure 22. In FIG. 2, the shift amount of the boundary portion 30 of the first diffractive lens structure 20 and the second diffractive lens structure 22 is H3, which is the same.

[0026] Furthermore, as shown in Figure 1, the circular boundary line B (in the cross sections of Figures 1 and 3, the boundary line B is shown as a dot on the incident surface 8a) that forms the boundary 30 between the circular first diffractive lens structure 20 and the annular second diffractive lens structure 22 is preferably positioned radially inward of the position P at which the light ray L2 traveling from the end 6a of the shade 6 toward the diffractive lens 8 reaches the diffractive lens structure of the diffractive lens 8 at an angle θ2 (1.73θ1 or -1.73θ1) that is 1.73 times the angle θ1 with respect to the horizontal plane 40 of the light ray L1 that is emitted from the center 0 of the LED 2 in a direction perpendicular to the light emission surface 2a of the LED 2, reflected by the reflector 4, and passes through the end 6a of the shade 6 toward the diffractive lens 8.

[0027] Thus, with the above-described configuration of this embodiment, the first diffractive lens structure 20, which maximizes the diffraction efficiency of diffracted light with absolute values ​​of diffraction orders of 5 or higher for light of a certain wavelength within the wavelength spectrum of the LED 2, effectively thins the lens 8 at its inner side. Furthermore, the second diffractive lens structure 22, which maximizes the diffraction efficiency of diffracted light with absolute values ​​of diffraction orders of 1 or lower, is provided at the outer periphery of the lens 8, where chromatic aberration is significant (the coloring of the cut line is significantly affected by the lens periphery). Figure 4 clearly shows that a typical refractive lens without the second diffractive lens structure 22 or a diffractive lens with only the first diffractive structure exhibits significant chromatic aberration at the outer periphery of the lens. Figure 4 shows the degree of divergence at the output side of green light (shown by curve L3), blue light (shown by curve L4), and red light (shown by curve L5) for a refractive lens that converts green light into parallel light, with the horizontal axis representing the lens radius (mm) and the vertical axis representing the light output angle (deg). In Figure 4, when the light output angle is 0°, the light travels in the direction of the central axis (optical axis) of the lens; when the value is positive, the light travels in the positive radial direction relative to the optical axis; and when the value is negative, the light travels in the negative radial direction relative to the optical axis. Therefore, Figure 4 shows that blue light is focused toward the inner radius of the lens, while red light diverges toward the outer radius of the lens. Furthermore, the divergence angle on the output side of blue and red light increases as the absolute value of the lens radius increases, that is, toward the outer periphery. The divergence angle on the output side indicates the magnitude of chromatic aberration, and Figure 4 shows that chromatic aberration is greater at the outer periphery of the refractive lens.

[0028] Next, the above characteristic configuration will be described in more detail. First, the selection of the diffraction order in the diffraction for achromatization by the second diffractive lens structure 22 (hereinafter referred to as achromatic diffraction) and the diffraction for thinning by the first diffractive lens structure 20 (hereinafter referred to as thinning diffraction) and the boundary between the first diffractive lens structure 20 and the second diffractive lens structure 22 will be described in detail.

[0029] As described above, a diffractive lens structure having a ring structure divided by steps is formed on the entrance surface (or exit surface) of the diffractive lens. The diffractive lens structure can be a shape obtained by adding a surface shift (hereinafter simply referred to as a shift) by an optical path difference function or a phase function to a macroscopic shape described by an aspherical formula. Here, the optical path difference function is a function that indicates, with respect to the radial coordinate, the value of the optical path difference applied to light passing through a (macroscopic) surface described by an aspherical formula to which the diffractive lens structure is added. The phase function is a function that indicates, with respect to the radial coordinate, the optical path difference applied to light passing through a (macroscopic) surface described by an aspherical formula to which the diffractive lens structure is added, in terms of a phase value.

[0030] A diffractive lens structure that generates m-th order diffracted light (m is an integer) has an annular structure in which, in the optical path difference function, a step is formed every time the optical path difference of light of a certain wavelength λ0 becomes an integer multiple of λ0 × the diffraction order (design diffraction order) m of the diffracted light to be generated. Alternatively, in the phase function, when the phase value is expressed in radians, a step is formed every time the phase value becomes an integer multiple of 2π × the design diffraction order m. The step at the annular zone boundary is a step where the optical path difference at the step is λ0 × m, or a step where the phase difference of light of wavelength λ0 is 2π × m. Examples of methods for converting aspherical formulas and optical path difference functions to diffractive lens structures are described, for example, in "Introduction to Diffractive Optical Elements" (Optronics Co., Ltd.).

[0031] When light of wavelength λ0 passes through a diffractive lens structure where the optical path difference at the step of the ring zone boundary is λ0×m, m-th order diffracted light is generated. If we consider a case where the light source using the diffractive lens is a light source containing multiple wavelengths, such as white light, the optical path difference generated at the step of the ring zone boundary where the optical path difference is λ0×m for a certain wavelength λ0 will be a different value, where the optical path difference is λ1×m for another wavelength λ1. In this case, diffracted light other than the m-th order diffracted light will be generated at wavelength λ1.

[0032] If the ratio of the energy of m-th order diffracted light to all orders of diffracted light generated by a diffractive lens structure for light of a certain wavelength λ is defined as the m-th order diffraction efficiency, then the m-th order diffraction efficiency of a diffractive lens structure that generates m-th order diffracted light for a wavelength λ0 is 1 for the wavelength λ0, but less than 1 for the wavelength λ1. Here, we will consider the generation of diffracted light when using a diffractive lens structure that generates 1st order diffracted light (m=1) for a light source that emits light in the visible light range, and when using a diffractive lens structure that generates higher order diffracted light (m>1).

[0033] Figure 5 shows the wavelength dependence of the first-order diffraction efficiency when visible light is incident on a diffraction grating made of PMMA (polymethyl methacrylate) such that the diffraction efficiency of first-order diffracted light for light with a wavelength of 0.52 μm is 1. Figure 6 shows the wavelength dependence of the fifth-order diffraction efficiency when visible light is incident on a diffraction grating such that the diffraction efficiency of fifth-order diffracted light for light with a wavelength of 0.52 μm is 1. Figure 7 shows the wavelength dependence of the tenth-order diffraction efficiency when visible light is incident on a diffraction grating such that the diffraction efficiency of tenth-order diffracted light for light with a wavelength of 0.52 μm is 1.

[0034] As shown in Figures 5 to 7, the m-th order diffraction efficiency is 1 for light with a wavelength λ0 of 0.52 μm, but for wavelengths other than λ0 of 0.52 μm, the m-th order diffraction efficiency is less than 1. Furthermore, when light is incident on a diffractive lens that generates m-th order diffracted light for light with a wavelength λ0, the wavelength range over which high m-th order diffraction efficiency is obtained becomes wider as m becomes smaller. In particular, a diffractive lens that generates 1st order diffracted light can obtain high 1st order diffraction efficiency across the entire visible light range.

[0035] Figure 8 shows the wavelength dependence of the diffraction efficiency of each order, including the 5th order, when visible light is incident on a diffraction grating whose diffraction efficiency for 5th order diffracted light is 1 for light with a wavelength of 0.52 μm, using PMMA as the diffractive lens material. Figure 9 shows the wavelength dependence of the diffraction efficiency of each order, including the 10th order, when visible light is incident on a diffraction grating whose diffraction efficiency for 10th order diffracted light is 1 for light with a wavelength of 0.52 μm.

[0036] 8 and 9, at wavelengths where the m-th order diffraction efficiency is low, the a-th order diffraction efficiency (≠m) may be higher than the m-th order diffraction efficiency, and in particular, at wavelengths where the m-th order diffraction efficiency is nearly 0, a peak appears where the a-th order diffraction efficiency is nearly 1 (for example, according to FIG. 8, at a wavelength of 0.44 μm, the 5th order diffraction efficiency is nearly 0, but the 6th order diffraction efficiency is nearly 1). In other words, when a diffractive lens structure that generates high-order diffracted light where m>1 is used, within the visible light range, there will be multiple wavelengths where the diffraction efficiency is nearly 1, even though the diffraction orders of the generated diffracted light will differ. Furthermore, the larger the value of m, the greater the number of wavelengths where the diffraction efficiency is nearly 1.

[0037] The diffraction angle dependence of visible light incident on a diffraction grating with a pitch of 59.588 μm and a PMMA lens, which produces a diffraction efficiency of 1 for the m-th order diffracted light for 0.52 μm wavelength, is shown in Figure 10 for m = 1, Figure 11 for m = 5, and Figure 12 for m = 10. The diffraction angle for each wavelength is the diffraction angle for the order producing the maximum diffraction efficiency. A blazed diffraction grating is known as a diffraction grating that produces a high diffraction efficiency for the m-th order diffracted light for a wavelength λ0. Its structure consists of repeated triangular grooves with a sawtooth cross section. A blazed diffraction grating is defined as a grating that produces a diffraction efficiency of 1 for the m-th order diffracted light for 0.52 μm wavelength. The refraction angle is also shown as the angle of refraction when light is incident on a surface with a continuous sawtooth wave slope. 11 and 12, the wavelengths at which the m-th or a-th order diffraction efficiency becomes 1 in FIGS. 8 and 9 are indicated by dashed lines.

[0038] According to Figure 10, when m = 1, the diffraction angle changes continuously within the visible light range, but becomes larger as the wavelength becomes longer, which is the opposite trend to when refraction occurs. According to Figures 11 and 12, in the case of a diffraction grating that generates higher-order diffracted light, the diffraction angle changes continuously in the wavelength range where the diffraction order with maximum diffraction efficiency is the same, but the diffraction angle changes discontinuously at the wavelength where the diffraction order with maximum efficiency changes. The diffraction angle at the wavelength where the diffraction efficiency is nearly 1 is equal to the refraction angle, and if only the diffraction angle at the wavelength where the diffraction efficiency is nearly 1 is extracted, a dependency similar to the wavelength dependency of the refraction angle occurs (the refraction angle (diffraction angle) becomes larger as the wavelength becomes shorter).

[0039] Furthermore, we consider the generation of diffracted light due to the step between the light source and the diffractive lens structure. As mentioned above, the optical path difference at the boundary of the ring zone of a diffractive lens structure that generates m-th order diffracted light (m is an integer) for a given wavelength λ0 is λ0 × m. In particular, in a diffractive lens structure that generates higher-order diffracted light, the step becomes larger in proportion to m.

[0040] In the discussion up to now, we have considered the case where the coherence length of the light source is sufficiently long so that light interference occurs between the ring zones, meaning that diffraction is inevitable. However, if the step at the ring zone boundary exceeds the coherence length of the light source and the coherence between the ring zones becomes negligible, the light interference required for a diffractive lens does not occur, and instead a geometrical optical refraction effect occurs in each ring zone, causing the lens to essentially behave like a Fresnel lens.

[0041] The coherence length of the light source varies depending on the light source, but when using a white light source in particular, the coherence length can be as short as 2 μm. Therefore, if the step at the annular boundary exceeds 2 μm, the lens may behave as a Fresnel lens.

[0042] When a lens behaves as a Fresnel lens, the diffraction angle of a light ray is the same as the refraction angle in geometric optics. For example, when visible light from a light source whose coherence length is shorter than the step difference at the ring boundary is incident on a diffraction grating with a pitch of 59.588 μm, which gives a diffraction efficiency of 1 for 5th-order diffracted light with a wavelength of 0.52 μm, the deflection angle is the refraction angle when the light is incident on a surface with a continuous series of sloped surfaces of the sawtooth slope of the blazed diffraction grating in Figure 11.

[0043] From the above, it can be seen that a diffractive lens structure that generates first-order diffracted light has a maximum first-order diffraction efficiency within the visible light range, and is a diffractive lens with a diffraction angle wavelength dependency in which the diffraction angle increases as the wavelength becomes longer. When a diffractive lens structure that generates higher-order diffracted light (m>1) is used, there are multiple wavelengths within the visible light range where the diffraction efficiency is nearly 1, and the wavelength dependency of the diffraction angle at those wavelengths is the same as that of a refractive lens, or the diffractive lens behaves essentially as a Fresnel lens, which is a refractive lens.

[0044] Next, we will consider the diffraction lens structure for achromatization (achromatic diffraction) and the diffraction lens structure for thinning (thinning diffraction). To achieve achromatism, it is necessary for the wavelength dependence of the diffraction angle to continuously exhibit a tendency opposite to that of a refractive lens within the visible light range. As shown in Figures 5 and 10, a diffractive lens structure that generates first-order diffracted light (m = 1) can increase the first-order diffraction efficiency throughout the entire visible light range, and the diffraction angle of the first-order diffracted light increases continuously as the wavelength increases, i.e., exhibits a characteristic that exhibits a tendency opposite to that of refraction. Therefore, when achieving achromatism, it is preferable to use a diffractive lens structure that generates first-order diffracted light (m = 1 or less). Furthermore, since the performance focused on in this study (wavelength range and diffraction angle with high diffraction efficiency) can be considered similar even when the diffraction order is m = -1, a diffractive structure that generates -1st-order diffracted light (m = -1) can also be used.

[0045] Furthermore, when thinning the lens, it is not necessary to provide optical characteristics such as achromatism, but it is necessary to achieve performance equivalent to that of a refractive lens, that is, to minimize the loss in light utilization efficiency due to the addition of diffraction, and it is desirable to have wavelengths over a wide range within the visible light range where the diffraction efficiency is nearly 1. If the diffractive lens structure generates high-order diffracted light, there will be multiple wavelengths within the visible light range where the diffraction efficiency is nearly 1, as shown in Figures 8 and 9, or the diffractive lens will be one that behaves essentially as a Fresnel lens, which is a refractive lens.

[0046] For example, the light used in automobile headlamps is white. If a light source is used that generates white light by combining light of wavelengths where the diffraction efficiency in Figures 8 and 9 is approximately 1, or if a white light source whose spectrum spreads widely within the visible light range is used, a diffractive lens structure that generates diffracted light of a diffraction order where the step at the annular boundary is longer than the coherence length can be used, and thinning by adding diffraction with high light utilization efficiency is possible.

[0047] Therefore, when thinning the lens, it is preferable to use a diffractive lens structure that generates high-order diffracted light of m>1. In particular, as shown in Figures 8 and 9, when m≧5, it is possible to provide wavelengths in the red, green, and red regions where the diffraction efficiency is 1, so it is desirable to use a diffractive lens structure that generates high-order diffracted light of m≧5. Furthermore, since this consideration can be considered similar when m≦−5, a diffractive structure that generates high-order diffracted light of m≦−5 is also acceptable.

[0048] Furthermore, when considering the boundary between a diffractive lens structure that maximizes m1 (|m1|≧5)-order diffracted light and is provided on the inner periphery of the lens surface for the purpose of thinning, and a diffractive lens structure that maximizes m2=1-order diffracted light and is provided on the outer periphery of the lens for the purpose of achromatization, it is desirable that the shift amount of the boundary in the diffractive lens structure that maximizes m1-order diffracted light be approximately the same as the shift amount of the boundary in the diffractive lens structure that maximizes m2=1-order diffracted light. If they are not the same, a step, or a so-called vertical wall, will appear in the optical axis direction at the boundary, generating noise due to light passing through the vertical wall and degrading performance, which is undesirable. Furthermore, when manufacturing a diffractive lens by injection molding, fabricating the vertical wall is difficult, and manufacturing errors due to the vertical wall are likely to occur.

[0049] For example, if the aspherical formula, optical path difference function, or phase function is such that it satisfies the following condition, it is possible to make the shift amount of the boundary in the diffractive lens structure in which the m1-order diffracted light is maximized approximately the same as the shift amount of the boundary in the diffractive lens structure in which the m2=1-order diffracted light is maximized. 1) The value of the macroscopic aspheric formula at the boundary radius (radius of the circular boundary line) is the same. 2) The optical path difference value of the optical path difference function or the phase value of the phase function at the boundary radius (the radius of the circular boundary line) is the same. 3) The absolute value of the shift amount from the macroscopic shape at the boundary radius (radius of the circular boundary line) of the diffractive lens structure that produces m1-order diffracted light is smaller than the step at which the optical path difference in the diffractive lens structure that produces m2-order diffracted light becomes λ0×1.

[0050] If the above conditions are met, the optical path difference at the boundary in a diffractive lens structure in which the m1-order diffracted light is maximized and which is provided on the inside (center) of the lens for the purpose of thinning can be made the same value as the optical path difference at the boundary in a diffractive lens structure in which the m2=1-order diffracted light is maximized and which is provided on the outer periphery of the lens for the purpose of achromatization.This makes the shift amount of the optical path difference function from the macroscopic surface at the boundary approximately the same, preventing a step due to a mismatch in the shift amount at the boundary, and further preventing the generation of unnecessary noise light caused by that step.

[0051] Next, the boundary between the thinned diffraction and the achromatic diffraction and the structure of the automobile headlamp will be described in more detail. Here, we consider the boundary radius of a diffractive lens structure that maximizes m1 (|m1|≧5)-order diffracted light and is provided on the inside (center) of the lens surface for the purpose of thinning, and a diffractive lens structure that maximizes m2=1-order diffracted light and is provided on the periphery of the lens for the purpose of achromatization.

[0052] If the boundary radius is set closer to the inner periphery, the thinning effect due to diffraction decreases. The extent to which thinning is possible by adding diffraction is determined by the performance of the lens after thinning (such as the magnitude of aberration). For example, in the case of a lens for an automobile headlamp, the lens thickness that can be thinned is the one that provides performance at wavelength λ0, which is used to calculate the optical path difference between the ring zones of the diffractive lens structure, that is equivalent to or better than that of a refractive lens with the same focal length. The thickness of a diffractive lens that provides performance equivalent to that of a refractive lens with the same focal length depends on the boundary radius; the closer the boundary radius is to the inner periphery, the thicker the diffractive lens will be, approaching the thickness of a refractive lens.

[0053] As a result of extensive research, the inventors have found that a sufficient thinning effect can be obtained if the boundary radius is set to 0.4 times or more the effective radius of the lens. Therefore, it is desirable that the boundary position between thinning diffraction and achromatic diffraction be 0.4 times or more the effective radius of the lens.

[0054] Furthermore, moving the boundary radius setting toward the outer periphery reduces the achromatic effect. The magnitude of the effect of chromatic aberration depends not only on the chromatic aberration of the lens itself, but also on the in-plane distribution of the light beam entering the lens. Therefore, the allowable effect of chromatic aberration varies not only with the lens itself, but also with the entire optical system and the application of the lens.

[0055] Here, let us consider the case where a diffractive lens is used as a lens for a projector-type headlamp for an automobile headlight. The structure of a typical projector type headlamp (for low beam) has already been explained, but here, the optical system 1 will be explained in more detail with reference to FIG.

[0056] As shown in the figure, the reflector 4 is an aspherical or free-form surface based on an ellipse. The LED 2 is installed so that the center of its light-emitting surface is at a first focal point 50 of the ellipse of the reflector 4, and light is emitted in the Y direction. In this case, the light emitted from the LED 2 is condensed at a second focal point 52 of the reflector 4. The diffractive lens 8 is installed so that the second focal point 52 of the reflector 4 becomes the focal point of the lens 8, and projects the brightness distribution of the light spot formed at the second focal point 52 of the reflector 4.

[0057] For low beams, it is necessary to form an area in the horizontal direction where the luminous intensity changes suddenly, called a cut-off line, but in low beam projector-type headlamps, a shade 6 is provided near the second focal point 52, and the brightness distribution of the formed light spot is adjusted (a region in the horizontal direction where the brightness changes suddenly) and the cut-off line is created by projecting this brightness distribution with a lens 8. The shade 6 is provided in a horizontal plane, and a metal film such as aluminum is formed on its surface by vapor deposition, so that the light that reaches the shade 6 can also be used.

[0058] In addition, low beams need to irradiate light up to approximately ±40° horizontally, and if it is necessary to project light while providing a good cutoff line over such a wide angle of view, an optical system must be constructed that takes aberrations into consideration. When considering aberrations, due to coma, astigmatism, and field curvature, the projection surface that creates the low beam luminous intensity distribution should be a curved surface that is closer to the lens than the focal point of lens 8 the further away from the Z axis it is, in order to provide a good cutoff line over a wide angle of view. Therefore, the shade becomes U-shaped in the ZX plane. The detailed shape is determined taking into consideration the aberrations of lens 8.

[0059] Furthermore, in order to obtain the required luminance distribution in the shade 6, the shape of which is determined taking into account the aberration of the lens 8, the shape of the reflector 4 is made aspherical or free-form based on an ellipse, and the luminance distribution in the shade 6 is adjusted so that the required luminous intensity distribution of the low beam is obtained when projected by the lens 8.

[0060] Here, we will show the behavior of light and a design example of the required reflector and lens shape in the YZ cross section shown in the figure. When the reflector 4 is elliptical, the YZ cross section shape can be expressed by the following equation.

number

[0061] Here, c is the curvature, which is the reciprocal of the radius of curvature. K is the conic coefficient, and when this value is between 0 and -1, it becomes an ellipse with its major axis in the Z direction. In a typical projector-type headlamp, K takes a value between 0 and -1.

[0062] When the reflector shape is expressed by the above formula, the Z coordinate of the first focal point 50 of the reflector 4 is expressed as follows:

number

[0063] The Z coordinate of the second focal point 52 is

number

[0064] Incidentally, the LED 2 emits the most intense light in the direction perpendicular to its light-emitting surface 2a. Let us consider the light emitted from the LED 2 in the perpendicular direction. The light emitted perpendicularly from the first focal point 50 contained in the LED 2 is expressed in the above formula ([Number 2]) as follows:

number

number

number

[0065] The light that reaches the above position is directed toward the second focal point 52, and therefore, with respect to the Z axis,

number

[0066] Next, consider the setting of the focal length of lens 8. Considering the vertical luminous intensity distribution of the low beam, light is emitted from approximately 0° to -8° vertically. Of this, strong luminous intensity is required from 0° to -3° vertically near 0° horizontally.

[0067] Here, considering the light-emitting area of ​​LED2, the light-emitting area is finite, and if its half width in the Z direction is d, then light is emitted from the area whose Z direction position is the first focal point ±d. X=0, Y=0,

number

number

number

[0068] For example, the focal length of lens 8 is from Y=0 to Y= 2d The light in the region of (Y 2d The focal length f of the lens 8 is set so that β = f tan β. Note that β differs depending on the conditions, for example, when the low beam is formed by one optical system or when it is formed by two optical systems, and so β ​​must be set appropriately.

[0069] Therefore, the focal length of the lens 8 and the base elliptical reflector shape that serves as the basis before adjusting the luminous intensity distribution (making the lens and reflector aspherical or free-form) are determined from the light-emitting width of the LED 2 and the required luminous intensity distribution in the vertical direction of the low beam. The lens 8 and shade 6 are designed from the determined information, and the reflector shape is then adjusted (from elliptical to aspherical or free-form) to obtain the required luminous intensity distribution, resulting in the optimal shapes of the reflector 4, shade 6 and lens 8.

[0070] Now, with further reference to FIG. 13, let us consider the use of a diffractive lens 8 according to an embodiment of the invention in an optical system designed as above. The luminous intensity distribution of the white LED 2 is generally a Lambertian distribution, and the luminous intensity of the light L1 emitted vertically from the LED 2 is the highest. Therefore, on the lens surface, the angle θ of the light L1 emitted vertically from the LED 2 with respect to the Z axis just before it reaches the lens surface

number

[0071] In particular, when a general elliptical reflector is used, the light L1 emitted vertically from the LED 2 reaches the lens surface from the position on the lens surface toward the outer periphery of the lens at an angle α

number

[0072] As mentioned above, the magnitude of the effect of chromatic aberration depends not only on the lens's own chromatic aberration tendency (which increases toward the outer periphery of the lens) but also on the in-plane distribution of the light beam incident on the lens 8. Effective achromatization is achieved by achromatizing the light at the outer periphery of the lens, where high-energy light reaches up to an angle α. Therefore, by positioning the boundary between achromatic diffraction and thin-wall diffraction more inward than the position where light from the shade 6 at an angle α with respect to the Z axis reaches, achromatization can be achieved in the region where the high-energy light reaches, resulting in effective achromatization (the boundary radius between achromatic diffraction and thin-wall diffraction does not exceed radius r in Figure 13). Note that in optical systems, the optical axis of the lens may not coincide with the edge of the shade. 13, in an optical system in which a lens is installed such that its optical axis is shifted in the Y direction relative to the shade end, the lens radius at the position where light from the shade end, making an angle α with respect to the Z axis in the YZ cross section, reaches is different from the lens radius at the position where light from the shade end, making an angle -α with respect to the Z axis, reaches. In this case, the lens radius with the largest absolute value is the position where light from shade 6, making an angle α with respect to the Z axis, reaches. For example, in an optical system in which a lens is installed such that its optical axis is shifted in the +Y direction relative to the shade end, the lens radius at the position where light from shade 6, making an angle α with respect to the Z axis in the YZ cross section, reaches (the position where X = 0 and Y < 0) is the position where light from shade 6, making an angle α with respect to the Z axis, reaches. Conversely, in an optical system in which the lens is installed so that its optical axis moves in the -Y direction relative to the end of the shade, the lens radius at the position where light from the shade 6 reaches with an angle -α relative to the Z axis in the YZ cross section (the position where X = 0 and Y > 0) is the position where light from the shade 6 reaches with an angle α relative to the Z axis.

[0073] Considering the YZ cross section again, when a reflector 4 with c=0.05 (1 / mm) and K=-0.55 is used, the Z coordinate of the first focal point 50 is 11.484 (mm), the Z coordinate of the second focal point 52 is 77.405 (mm), and the angle θ of the light emitted vertically from the first focal point 50 with respect to the Z axis is -16.877°. When the half width of the LED 2 in the Z direction is 0.5, the Y2d = 1.826, and when β is set to 2°, the focal length required for lens 8 can be set to 52.30°. Also, θ = -16.877°, and α = -29.179°.

[0074] Example 3, described below, shows a lens design example with a lens effective radius of 30 mm and a focal length of 52.69 mm, in which a thinned diffraction is located within a radius of 15.326 mm on the exit side and an achromatic diffraction is located outside of that radius. In this design example, the radius of the boundary between the thinned diffraction and the achromatic diffraction, 15.263 mm, is larger than 0.4 times the effective lens radius (12 mm). Furthermore, when the optical axis of the lens coincides with the edge of the shade, the position where light passing through the shade 6 at θ = 16.877° reaches the exit side of the lens is a radius of 15.367 mm, while the position where light at α = 29.1977° reaches the exit side of the lens is a radius of 26.039 mm. The radius of the boundary between the thinned diffraction and the achromatic diffraction, 15.263 mm, is smaller than the radius where light at angle α reaches. Therefore, while maintaining the performance of a refractive lens with an equivalent focal length, the thickness is reduced by 0.9 times, achieving effective thinning.

[0075] Next, three examples will be described, each involving a first and a second diffractive lens structure according to an embodiment of the present invention, the boundary positions of which are defined as described above.

[0076] Example 1 In Example 1, the design was carried out under the conditions of Φ60 mm, NA = 0.57, and the material being PMMA, and its feasibility and effects were confirmed. In Fig. 14(a), the cross-sectional shape of the diffractive lens 8 according to the embodiment of the present invention is shown, and in Fig. 14(b), the cross-sectional shape of the refractive lens designed under the same conditions as the comparative example is shown. In the diffractive lens 8 according to the present embodiment, the first and second diffractive lens structures are added to its incident surface. Specifically, at the central portion of the incident surface of the diffractive lens 8, a first diffractive lens structure 20 with a diffraction order (designed diffraction order) of 50 that is generated with respect to the design wavelength λ0 for forming a thin diffractive lens surface with a radius of 20 mm or less is added, and at its outer peripheral portion, a second diffractive lens structure 22 with a designed diffraction order of 1 for forming an achromatic diffractive lens surface with a radius exceeding 20 mm is added. In this Example 1, the thickness (central thickness) T of the refractive lens is 21.325 mm, and the thickness (central thickness) T of the diffractive lens is 17.060 mm. Therefore, a thinning of about 20% was achieved.

[0077] The surface shape of the refractive surface of the refractive lens and the diffractive lens is described by the following formula.

Equation

[0078] The inner peripheral side (r ≤ r1) surface shape of the diffractive surface (serving as the base surface) of the diffractive lens is described by the following formula.

Equation

[0079] The outer peripheral side (r1 ≤ r < r2) surface shape of the diffractive surface (serving as the base surface) of the diffractive lens is described by the following formula.

Equation

[0080] Here, z is the amount of sag of the surface with the positive optical axis direction, r is the radial coordinate, c is the surface curvature (the reciprocal of the surface curvature radius R), k is the conic coefficient, A4, A6, A8, A10, A14, A16, A18, A110, A24, A26, A28, A210 are aspheric coefficients. z0 is a coefficient for correcting so that the sag is continuous at the radial coordinate r1.

[0081] The phase function (the value of the phase added to the light transmitted through the surface) on the inner peripheral side (r ≤ r1) of the diffractive surface of the diffractive lens is described by the following equation.

Equation

Equation

[0082] The phase function (the value of the phase added to the light transmitted through the surface) on the outer peripheral side (r1 ≤ r < r2) of the diffractive surface of the diffractive lens is described by the following equation.

Equation

Equation

[0083] FIG. 15 shows numerical data on the cross-sectional shapes of the diffractive lens and the refractive lens (in the table, the inner circumferential diffraction corresponds to the first diffractive lens structure, and the outer circumferential diffraction corresponds to the second diffractive lens structure). The phase values ​​calculated using the phase function coefficients shown in FIG. 15 are expressed in radians. FIG. 16(a) also shows a graph showing the dependence of the diffractive lens and the refractive lens on the light output angle as determined by the phase function. In the figure, curve L6 indicates the output angle of green light depending on the lens radius of the refractive lens, curve L7 indicates the output angle of blue light depending on the lens radius of the refractive lens, curve L8 indicates the output angle of red light depending on the lens radius of the refractive lens, curve L9 indicates the output angle of green light depending on the lens radius of the diffractive lens, curve L10 indicates the output angle of blue light depending on the lens radius of the diffractive lens for a light beam passing through the second diffractive lens structure, and curve L11 indicates the output angle of red light depending on the lens radius of the diffractive lens for a light beam passing through the second diffractive lens structure. As can be seen from this graph, in a refractive lens, red light and blue light are largely diverged outward and inward in the radial direction of the lens, but in the diffractive lens of Example 1, the divergence of all green light, red light, and blue light is suppressed, and a sufficient achromatic effect (reduction of the colored width) is obtained in the outer periphery of the lens (and therefore in the achromatic diffractive section (second diffractive lens structure)).

[0084] The upper part of Figure 16(b) shows a spot diagram of green light (wavelength 550 nm) from a diffractive lens, confirmed by ray tracing using a phase function, and the lower part of Figure 16(b) shows a spot diagram of green light (wavelength 550 nm) from a refractive lens. This graph is a diagram in which a number of rays from a point light source are traced and the angle of each ray after exiting the lens is plotted. More specifically, for rays emitted from a point light source and entering a lens, the angle at which the ray (principal ray) that reaches the vertex of the lens's exit surface (the position where it intersects with the optical axis) exits the lens is taken as the reference. The exit angles from the lens of each ray that enters the lens at a position other than the vertex of the lens's exit surface (each ray enters the lens at a different position) are calculated as the difference in horizontal and vertical angles from the reference, and points are plotted at corresponding positions on a graph with the difference in horizontal angle on the horizontal axis and the difference in vertical angle on the vertical axis. A spot diagram can be used to confirm the degree of divergence of the emitted light, which is one of the lens performance indicators. For example, in the case of a lens designed to convert light from a certain point into parallel light, ideally, all light rays from that point would be emitted at the same angle as the reference chief ray. In this case, all light rays are plotted at the same position in the spot diagram, so there is no divergence at the plotted positions for each ray. If aberration occurs due to a point light source being different from the position used in the design or an insufficient design, light rays other than the chief ray will be emitted from the lens at angles different from the chief ray. Therefore, in this spot diagram, the plotted positions for light rays other than the chief ray will differ from the plotted position for the chief ray, and each plotted position will have a divergence. This distribution represents the degree of divergence of the light emitted from the lens, and the distribution's state (how far it diverges, is the distribution uniform, are there many rays near the chief ray, etc.) can be confirmed, thereby confirming the lens's performance. In a low-beam projector headlamp, as described above, the shade end is installed so as to be the focal point of the lens, and the lens converts the light from the shade end (focal point) into parallel light.Therefore, in the spot diagram of Figure 16(b), the narrower the spread of the plotted positions for each light ray and the more plotted positions there are in the center, the better the lens's performance can be determined. The diffractive lens of the present invention is expected to be used as a lens for a projector-type headlamp, and considering that shades and reflectors will be adjusted to the lens, it is necessary for the lens to have performance equivalent to or better than that of the refractive lens of the comparative example. The diagrams on the left in the upper and lower rows of Figure 16(b) show the spread of light emitted from the lens for light rays emitted from a point on the optical axis on the focal plane (a plane perpendicular to the optical axis including the focal point). The diagrams in the center of the upper and lower rows show the spread of light emitted from the lens for light rays emitted from a point located 5.00 mm horizontally from the optical axis on the focal plane. The diagrams on the right in the upper and lower rows show the spread of light emitted from a point located 10.00 mm horizontally from the optical axis on the focal plane. While spot diagrams vary depending on the wavelength of light, Figure 16(b) compares green light (wavelength 550 nm), which has a high luminosity factor and a large effect on the luminous intensity distribution after emission from the lens, and which is set to have a high diffraction efficiency in diffractive lenses. Each spot diagram is the result of ray tracing of 1,519 rays, and shows a range of ±2.86° horizontally and ±2.86° vertically from a reference angle, which is the angle of emission of the chief ray. As can be seen from these, the green light performance of a diffractive lens is no different from that of a refractive lens. In other words, the diffractive lens in Example 1 achieved sufficient achromatic effect and thinning without changing the green light performance.

[0085] Example 2 In Example 2, a design was performed under the conditions of Φ60 mm, NA=0.57, and PMMA material, and its feasibility and effectiveness were confirmed. Figure 17(a) shows the cross-sectional shape of a diffractive lens 8 according to an embodiment of the present invention, and Figure 17(b) shows the cross-sectional shape of a refractive lens designed under the same conditions as a comparative example. The diffractive lens 8 according to this embodiment has first and second diffractive lens structures added to its exit surface. Specifically, a first diffractive lens structure 20 with a designed diffraction order of 50, which forms a thinned diffractive lens surface with a radius of 20.054 mm or less, is added to the center of the entrance surface of the diffractive lens 8. A second diffractive lens structure 22 with a designed diffraction order of 1, which forms an achromatic diffractive lens surface with a radius of more than 20.054 mm, is added to the outer periphery of the first diffractive lens structure. In this Example 2, the thickness (center thickness) T of the refractive lens was 21.325 mm, and the thickness (center thickness) T of the diffractive lens was 17.060 mm, thus realizing a reduction in thickness of about 20%.

[0086] FIG. 18 shows numerical data for the cross-sectional shapes of the diffractive lens and the refractive lens (in the table, the inner circumferential diffraction corresponds to the first diffractive lens structure, and the outer circumferential diffraction corresponds to the second diffractive lens structure). The phase values ​​calculated using the phase function coefficients in FIG. 18 are in radian units. In FIG. 18, z0 is a coefficient used for correction so that the sag becomes continuous at the radial coordinate r1. δ0 is also a coefficient used for correction so that the phase becomes continuous at the radial coordinate r1. Conditions 1), 2), and 3) were listed above as conditions for preventing the generation of a step at the boundary between the first and second diffractive structures. However, condition 1) is satisfied by adding z0 to the design, and condition 2) is satisfied by adding δ0 to the design. Here, the phase value of the boundary between the first diffractive lens structure and the second diffractive lens structure can be found from the phase function: Φ1(r=20.054)=-20734.69215 radians for α12, α14, α16, and α18 of the inner diffractive lens (first diffractive lens structure) in the table. If the diffractive lens structure that generates m-th order diffracted light is structured so that annular zones are formed every time the phase value in the phase function becomes an integer multiple of 2π × the designed diffraction order m, and the phase difference of the step at the annular zone boundary is 2π × m, the shift amount of the base surface at r is the phase value of the phase function at r minus the integer multiple of 2π × m. Therefore, since the diffraction order of the first lens structure is 50, the shift amount from the base surface corresponds to a phase difference of -0.18064 radians, which is the phase value Φ1' (r = 20.054) obtained by subtracting an integer multiple of 2π × 50 from Φ1 (r = 20.054). Since the diffraction order of the first lens structure is 1, the shift amount at the boundary between the first and second diffractive lens structures in the outer diffractive lens (second diffractive lens) corresponds to a phase difference of Φ'2, which is the phase value Φ2 (r = 20.054) obtained by subtracting an integer multiple of 2π × 1 from the phase value Φ2 obtained from the phase function. The possible values ​​of Φ'2 are between -2π × 1 and 2π × 1, taking into account whether the sign of the phase function near r = 0 is positive or negative. Therefore, by setting the lens radius position at the boundary so that the value of Φ1' is between -2π × 1 and 2π × 1, the phase value can be within the possible range of Φ2'.In this case, the optical axis direction position of the first diffractive lens structure at the boundary between the first and second diffractive lens structures is set to a position that is included in the range of the step of the second diffractive lens structure. Therefore, by appropriately setting the lens radius that forms the boundary, the above-mentioned condition 3) can be satisfied. The phase value Φ1' (r = 20.054) = -0.18064 radians is in the range of -2π × 1 to 2π × 1, which is one of the possible values ​​for Φ2', and the optical axis direction position of the first diffractive lens structure at the boundary between the first and second diffractive lens structures is set to a position that is included in the range of the step of the second diffractive lens structure. In other words, the above-mentioned condition 3) is satisfied. Furthermore, when Φ2 is calculated from the phase function, for δ0, α22, α24, α26, and α28 of the outer diffraction side of the diffractive lens (second diffractive lens structure) in the table, Φ2(r=20.054)=-20734.69215 radians, and the phase value Φ2'(r=20.054)=-0.18064 radians, the design results in Φ1' and Φ2' being the same value. The shift amount corresponding to the phase difference Φ' varies depending on the angle of incidence when light is incident obliquely on the base surface having the diffractive lens structure, and on the wavelength λ0 at which 50th-order diffracted light is generated in the first diffractive structure and 1st-order diffracted light in the second diffractive structure, but will be approximately the same if the values ​​of Φ1' and Φ2' are the same. In the case of FIG. 18 (λ0=550 nm), the shift amount of the first lens structure at the boundary between the first and second diffractive lens structures is 35 nm, and the shift amount of the second lens structure is 35 nm, which are the same value, preventing the occurrence of a step at the boundary between the first and second diffractive lens structures and further preventing light loss due to the occurrence of the step. Therefore, in the diffractive lens according to Example 2, the boundary between the first and second diffractive lens structures is set at a position where the optical axis position of the first diffractive lens structure is included in the range of the step of the second diffractive lens structure. In other words, the transition from the first diffractive lens structure to the second diffractive lens structure occurs at the position of the step of the first diffractive lens structure that is lower than the step of the second diffractive lens structure. Furthermore, the shift amount of the boundary in the first diffractive lens structure and the shift amount of the boundary in the second diffractive lens structure are set to be approximately the same.Therefore, the step at the boundary between the first diffractive lens structure and the second diffractive lens structure is minimized, and light loss can be prevented.

[0087] 19(a) shows a graph illustrating the light output angle dependence of the diffractive lens and the refractive lens confirmed by the phase function. In the figure, curve L6 shows the output angle of green light depending on the lens radius of the refractive lens, curve L7 shows the output angle of blue light depending on the lens radius of the refractive lens, curve L8 shows the output angle of red light depending on the lens radius of the refractive lens, curve L9 shows the output angle of green light depending on the lens radius of the diffractive lens, curve L10 shows the output angle of blue light depending on the lens radius of the light passing through the second diffractive lens structure of the diffractive lens, and curve L11 shows the output angle of red light depending on the lens radius of the diffractive lens. As can be seen from this graph, in a refractive lens, red light and blue light are largely diverged outward and inward in the radial direction of the lens, but in the diffractive lens of Example 2, the divergence of all green light, red light, and blue light is suppressed, and a sufficient achromatic effect (reduction of the colored width) is obtained in the outer periphery of the lens (and therefore in the achromatic diffractive section (second diffractive lens structure)).

[0088] The top row of Figure 19(b) shows a green spot diagram of a diffractive lens confirmed by the phase function, and the bottom row of Figure 19(b) shows a green spot diagram of a refractive lens. In Figure 19(b), the left diagrams in the top and bottom rows show the divergence of light emitted from a point on the optical axis on the focal plane (a plane perpendicular to the optical axis and including the focal point). The center diagrams in the top and bottom rows show the divergence of light emitted from a point 5.00 mm horizontally from the optical axis on the focal plane. The right diagrams in the top and bottom rows show the divergence of light emitted from a point 10.00 mm horizontally from the optical axis on the focal plane. Spot diagrams vary depending on the wavelength of light, but Figure 19(b) compares green light (wavelength 550 nm), which has a high luminosity factor, has a significant impact on the luminous intensity distribution after exiting the lens, and is designed to provide high diffraction efficiency for diffractive lenses. Each spot diagram is the result of ray tracing of 1,519 rays, and shows a range of ±2.86° horizontally and ±2.86° vertically from a reference angle, which is the angle of emergence of the chief ray. As can be seen from these, the performance of the diffractive lens for green light is the same as that of a refractive lens. In other words, the diffractive lens in Example 2 suppresses the generation of unnecessary noise light and achieves sufficient achromatic effect and thinning without changing the performance for green light.

[0089] Example 3 In Example 3, we confirmed the feasibility and effectiveness of an optical system for a vehicle lamp equipped with a diffractive lens of the present invention. As described above, the optical system of a typical projector-type headlamp includes a light source, a reflector that reflects and condenses the light emitted from the light source, a shade that blocks a portion of the light reflected and condensed by the reflector, and a lens that receives the light that has passed through the shade and directs it toward the front of the vehicle. Furthermore, if the reflector has a free-form surface based on an ellipse, and the vertical cross section of the lens optical axis is an ellipse with c = 0.05 (1 / mm) and K = -0.55 in [Equation 2], and the light source has a half width in the optical axis direction of 0.5 mm and is located at the first focus of the reflector ellipse, the lens will have a focal length of approximately 52.30 mm. In typical projector-type headlamps, the lens is generally a refractive lens. The shape of the reflector other than the vertical cross section of the lens optical axis and the shape of the shade are adjusted (free-formed) to meet low-beam standards, taking into account lens performance. Here, we designed a diffractive lens of the present invention with a diameter of 60 mm, numerical aperture of 0.56, and a focal length of 52.67 mm for light with a wavelength of 550 nm, and a shade whose shape has been adjusted when using this lens, and further, the vertical cross section of the lens at the optical axis is an ellipse with c = 0.05 (1 / mm) and K = -0.55 in [Equation 2], and the other shapes include a reflector with a free-form surface that takes into account the lens and shade and complies with low-beam standards, and the optical axis of the diffractive lens coincides with the edge of the shade, and confirmed the feasibility and effectiveness of this optical system for a projector-type headlamp.The diffractive lens of the present invention was designed with a diameter of 60 mm, numerical aperture of 0.56, a focal length of 52.67 mm for light with a wavelength of 550 nm, and made of PMMA. Figure 20(a) shows the cross-sectional shape of a diffractive lens 8 according to an embodiment of the present invention, and Figure 20(b) shows, as a comparative example, the cross-sectional shape of a refractive lens designed under the same conditions and used to adjust the shape of a shade or reflector in an optical system using a refractive lens. In the diffractive lens 8 according to this embodiment, first and second diffractive lens structures are added to its exit surface.Specifically, a first diffractive lens structure 20 with a design diffraction order of 50 that forms a thinned diffractive lens surface with a radius of 15.326 mm or less is added to the center of the entrance surface of the diffractive lens 8, and a second diffractive lens structure 22 with a design diffraction order of 1 that forms an achromatic diffractive lens surface with a radius of more than 15.326 mm is added to the outer periphery. In this Example 3, the thickness (center thickness) T of the refractive lens is 21.593 mm, and the thickness (center thickness) T of the diffractive lens is 19.192 mm, thus achieving a thinning of about 11%.

[0090] FIG. 21 shows numerical data for the cross-sectional shapes of the diffractive lens and the refractive lens (in the table, the inner circumferential diffraction corresponds to the first diffractive lens structure, and the outer circumferential diffraction corresponds to the second diffractive lens structure). The phase values ​​calculated using the phase function coefficients in FIG. 21 are in radian units. Here, the phase value at the boundary between the first diffractive lens structure and the second diffractive lens structure is calculated from the phase function: Φ1(r=15.326)=-7854.49414 radians for α12, α14, α16, and α18 of the inner circumferential diffraction (first diffractive lens structure) of the diffractive lens in the table. Since the diffraction order of the first lens structure is 50, the shift amount from the base surface of the first lens structure corresponds to a phase difference of Φ1'(r=15.326)=-0.51251 radians, which is calculated by subtracting an integer multiple of 2π×50 from Φ1(r=15.326). Similarly, the shift amount from the base surface in the second lens structure corresponds to a phase difference of Φ2' (r = 15.326) = -0.51251 radians. When light is incident obliquely onto a surface having a diffractive lens structure, the shift amount varies depending on the angle of incidence. However, if the values ​​of Φ1' and Φ2' are the same, the values ​​are approximately the same. This design prevents the step at the boundary between the first and second diffractive structures and the loss of light due to that step. As mentioned above, for a reflector with c = 0.05 mm and K = -0.55, the absolute value θ of the angle of light emitted vertically from the first focal point relative to the optical axis of the lens is 16.877°. Therefore, as mentioned above, high-energy light is incident from the shade up to an angle of α = 1.73θ. In Example 3, α = -29.198°. In Example 3, taking into consideration that the optical axis of the diffractive lens coincides with the edge of the shade, and also by ray tracing simulation, the absolute value of the angle from the optical axis of the light ray that reaches the 15.326 mm radius of the diffractive lens exit surface from the focal point was found to be 16.800°. Therefore, 15.326 mm, which is the boundary between the first and second diffractive lens structures, is smaller than the radius (15.368 mm) of light that is emitted perpendicularly from the light source and enters the lens from the shade at an angle θ = -16.877° and reaches the exit surface, and is further smaller than the radius (26.041 mm) of light that is incident from the shade at an angle α = -29.198° and reaches the exit surface.Therefore, the outer periphery of the region where high light energy exists on the lens exit surface, which is more affected by chromatic aberration, reaches the second lens structure that performs achromatic diffraction, so that achromatism can be performed more effectively.

[0091] 22(a) shows a graph illustrating the light output angle dependence of the diffractive lens and the refractive lens confirmed by the phase function. In the figure, curve L6 shows the output angle of green light depending on the lens radius of the refractive lens, curve L7 shows the output angle of blue light depending on the lens radius of the refractive lens, curve L8 shows the output angle of red light depending on the lens radius of the refractive lens, curve L9 shows the output angle of green light depending on the lens radius of the diffractive lens, curve L10 shows the output angle of blue light depending on the lens radius of the light passing through the second diffractive lens structure of the diffractive lens, and curve L11 shows the output angle of red light depending on the lens radius of the light passing through the second diffractive lens structure of the diffractive lens. As can be seen from this graph, in a refractive lens, red light and blue light are largely diverged outward and inward in the radial direction of the lens, but in the diffractive lens of Example 3, the divergence of all green light, red light, and blue light is suppressed, and a sufficient achromatic effect (reduction of the colored width) is obtained in the outer periphery of the lens (and therefore in the achromatic diffractive section (second diffractive lens structure)).

[0092] The top row of Figure 22(b) shows a green spot diagram of a diffractive lens confirmed by the phase function, and the bottom row of Figure 22(b) shows a green spot diagram of a refractive lens. In Figure 22(b), the left diagrams in the top and bottom rows show the divergence of light emitted from a point on the optical axis on the focal plane (a plane perpendicular to the optical axis and including the focal point), the center diagrams in the top and bottom rows show the divergence of light emitted from a point 5.00 mm horizontally from the optical axis on the focal plane, and the right diagrams in the top and bottom rows show the divergence of light emitted from a point 10.00 mm horizontally from the optical axis on the focal plane. Spot diagrams vary depending on the wavelength of light, but Figure 22(b) compares green light (wavelength 550 nm), which has a high luminosity factor, has a significant impact on the luminous intensity distribution after exiting the lens, and is designed to provide high diffraction efficiency for diffractive lenses. Each spot diagram is the result of ray tracing of 1,519 rays and shows a range of ±2.86° horizontally and ±2.86° vertically from a reference angle, which is the angle of emergence of the chief ray. As can be seen from these figures, the green light performance of the diffractive lens is the same as that of a refractive lens. In other words, the diffractive lens of Example 3 achieves sufficient achromatic effect and thinning without changing the green light performance. Furthermore, the fact that the green light performance is the same as that of a refractive lens indicates that the diffractive lens of Example 3 can be used in optical systems. Furthermore, by using the diffractive lens of the present invention in an optical system, color performance due to chromatic aberration is improved, and the lens can be made thinner, thereby realizing a smaller, lighter, and more cost-effective optical system.

[0093] Although the present invention has been described above in relation to various embodiments, the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the present invention. For example, the shapes of lenses and the like in the present invention are not limited to those of the above-described embodiments. Furthermore, some or all of the above-described embodiments may be combined, or part of the configuration of one of the above-described embodiments may be omitted, without departing from the spirit of the present invention. [Explanation of symbols]

[0094] 1 Optical System 2 LED (light source) 4 Reflector 6 Shades 8. Diffractive Lenses 8a Incidence plane 8b Output surface 20 First diffractive lens structure 22 Second diffractive lens structure

Claims

1. A diffractive lens for use in a vehicle headlight, The lens includes, on either the light entrance surface or the light exit surface, a first diffractive lens structure that maximizes the diffraction efficiency of diffracted light having an absolute value of a diffraction order of 5 or more for light of a wavelength within the wavelength spectrum range of a light source, and a second diffractive lens structure that maximizes the diffraction efficiency of diffracted light having an absolute value of a diffraction order of 1 or less, the one surface of the lens is formed as a diffractive lens surface in which the second diffractive lens structure is positioned adjacent to the first diffractive lens structure so as to surround the outer periphery of the first diffractive lens structure positioned in the center, A diffractive lens characterized by:

2. 2. The diffractive lens according to claim 1, wherein the radius of a circular boundary line forming the boundary between the circular first diffractive lens structure and the annular second diffractive lens structure is 0.4 times or more the effective radius of the lens.

3. 2. The diffractive lens according to claim 1, wherein the shift amount of the boundary portion in the first diffractive lens structure is substantially the same as the shift amount of the boundary portion in the second diffractive lens structure.

4. The diffractive lens according to claim 1, characterized in that the first diffractive lens structure and the second diffractive lens structure form a stepped annular structure, and a transition from the first diffractive lens structure to the second diffractive lens structure occurs at a step position of the first diffractive lens structure that is lower than the maximum height of the step of the second diffractive lens structure.

5. 2. A diffractive lens according to claim 1, wherein said one surface is said light entrance surface of said lens.

6. a light source that emits light; a reflector that reflects and collects the light emitted from the light source; a shade that blocks a portion of the light reflected and collected by the reflector; and a diffractive lens that receives the light that has passed through the shade and irradiates it toward a front of the vehicle; the diffractive lens includes, on either a light entrance surface or a light exit surface of the lens, a first diffractive lens structure that maximizes the diffraction efficiency of diffracted light whose absolute value of the diffraction order is 5 or more for light of a wavelength within the wavelength spectrum range of a light source, and a second diffractive lens structure that maximizes the diffraction efficiency of diffracted light whose absolute value of the diffraction order is 1 or less; the one surface of the lens is formed as a diffractive lens surface in which the second diffractive lens structure is positioned adjacent to the first diffractive lens structure so as to surround the outer periphery of the first diffractive lens structure positioned in the center, An optical system characterized by:

7. 7. The optical system of claim 6, wherein the circular boundary line forming the boundary between the circular first diffractive lens structure and the annular second diffractive lens structure is positioned radially inward of the position at which a ray of light traveling from the end of the shade toward the diffractive lens reaches the diffractive lens structure of the diffractive lens at an angle 1.73 times the angle with respect to the horizontal plane of a ray of light that is emitted from the center of the light source in a direction perpendicular to the light emission surface of the light source, reflected by the reflector, and passes through the end of the shade toward the diffractive lens.

8. 7. The optical system according to claim 6, wherein the shift amount of the boundary portion in the first diffractive lens structure is substantially the same as the shift amount of the boundary portion in the second diffractive lens structure.

9. 7. The optical system of claim 6, wherein the first diffractive lens structure and the second diffractive lens structure form a stepped annular structure, and a transition from the first diffractive lens structure to the second diffractive lens structure occurs at a step position of the first diffractive lens structure that is lower than the maximum height of the step of the second diffractive lens structure.

10. 7. The optical system according to claim 6, wherein said one surface is the light entrance surface of said diffractive lens.

11. A vehicle comprising an optical system according to any one of claims 6 to 10.

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