JP2011170028A - Diffractive optical element and imaging optical system having the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffractive optical element capable of obtaining an excellent image by reducing flare generated from the grating wall surface of a diffraction grating, when the diffractive optical element is used to reduce the chromatic aberration of an imaging optical system and to provide the imaging optical system having the diffractive optical element. <P>SOLUTION: The diffractive optical element is provided in the optical path of the imaging optical system. The diffractive optical element has a diffraction surface including a grating surface and the grating wall surface arranged at a predetermined grating pitch in at least a part of the boundary between a first material and a second material different from each other in dispersion and refractive index. When the side of the first material is set as a light incident side, the refractive index of the first material is smaller than the refractive index of the second material; and when a position where the grating wall surface is in parallel with the center axis of the diffractive optical element is set as the reference position of the grating wall surface, the grating wall surface is inclined in a direction where a vertex angle of grating formed by the grating wall surface and the grating surface is smaller than that in the reference position in at least a part of the range of the diffraction surface. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a diffractive optical element and an imaging optical system using the diffractive optical element, and more particularly to a digital camera, a video camera, a TV camera, and a monitor capable of reducing flare light generated from a diffraction grating forming the diffractive optical element and obtaining good optical performance. It is suitable for a camera.

  Conventionally, as a method of reducing chromatic aberration of an imaging optical system, a method of providing a diffractive optical element having a diffractive action in a part of the imaging optical system is known (Non-Patent Document 1). When the diffractive optical element is used in the imaging optical system, it is necessary that the diffraction efficiency of the light beam of the designed order is sufficiently high over the entire use wavelength range. When the diffraction efficiency is low, that is, when there are many light beams having diffraction orders other than the design order, these light beams are imaged at a place different from the light of the design order, and flare light that adversely affects the image formation. Become.

  In order to prevent a reduction in diffraction efficiency, it is important to appropriately set the grating pitch of the diffraction surface composed of the grating wall surface as the grating surface constituting the diffraction grating. In addition, as a factor that lowers the diffraction efficiency, there is a large amount of light reflected or transmitted by the grating wall surface that does not contribute to the diffraction action of the diffraction grating. When a diffractive optical element is used in an imaging optical system, flare light is generated when light reflected or transmitted by a grating wall surface that does not contribute to the diffractive action of the diffraction grating reaches the image plane. For this reason, it is important that the light reflected or transmitted by the grating wall surface does not reach the image plane.

  A diffractive optical element optimized for the luminous flux within the effective angle of view is known by reducing the vignetting on the grating wall surface by appropriately setting the grating wall angle to increase the diffraction efficiency of the diffraction grating. (Patent Documents 1 and 2). Further, there is known a diffractive optical element that reduces the reflection flare from the grating wall surface to reach the image plane by tilting the grating wall angle so that the vertex angle of the grating becomes an obtuse angle direction (patent) Reference 3). Further, there is known a diffractive optical element in which a light shielding means is provided on a grating wall surface in a diffractive optical element in which a plurality of diffraction gratings are closely attached (Patent Document 4).

JP 2003-294924 A JP 10-186118 A JP 2005-292571 A JP 2004-126394 A SPIE Vol.1354 International Lens Design Conference (1990)

  When a diffractive optical element is used in an imaging optical system, a method for increasing the diffraction efficiency of light within the effective screen to reduce flare light generated from the diffraction grating is to suppress flare light generated from the grating wall surface with respect to the light within the effective screen. Is effective. However, flare light increases in a real scene of a backlight scene in which an object with strong brightness such as sunlight is outside the effective screen.

  In general, when the incident angle of a light beam incident on the diffraction grating deviates from the angle of the grating wall surface, the light reflected by the grating wall surface and reaching the image plane has a bad influence on the image as flare. As a countermeasure, if the angle of the grating wall surface is tilted so that the grating apex angle becomes a more obtuse angle, it is possible to reduce the reflection flare from reaching the image plane. However, the inventors have recently found that it is difficult to greatly reduce the influence of unnecessary light even if the position of transmitted light is changed by tilting the grating wall surface so that the grating apex angle becomes an obtuse angle direction. This is because the generation of unnecessary light is not only the geometrical optical behavior but also the fact that the light reaches a position that cannot be reached by the geometrical optical ray tracing by causing a diffraction phenomenon. Also, the method of providing the light shielding means on the grating wall surface of the diffraction grating is difficult to process.

  When a diffractive optical element is used in the optical path of the imaging optical system, in many cases, light from outside the effective field angle other than the light beam within the effective field angle used for image formation is incident on the diffractive optical element. In particular, when correcting chromatic aberration by using a diffractive optical element closer to the object side than the stop of the imaging optical system, light from outside the effective angle of view easily enters the diffractive optical element and causes a lot of flare. . In particular, when strong light such as sunlight that is incident from a relatively low angle of 30 degrees or less outside the maximum angle of view within the effective screen is directly incident on the diffractive optical element, the diffractive optical element is used as a secondary light source. It causes a lot of flare.

  The present invention reduces a flare generated from the grating wall surface of a diffraction grating when a diffractive optical element is used to reduce chromatic aberration of the imaging optical system, and a diffractive optical element capable of obtaining a good image and an imaging optical system having the diffractive optical element. For the purpose of provision.

  The diffractive optical element of the present invention is a diffractive optical element provided in the optical path of the imaging optical system, and the diffractive optical element is at least one of the boundaries between the first material and the second material having different dispersion and refractive index. When the first material side is a light incident side, the refractive index of the first material is the second refractive index when the portion has a diffraction surface including a grating surface and a grating wall surface arranged at a predetermined grating pitch. When the position where the grating wall surface is parallel to the central axis of the diffractive optical element is the reference position of the grating wall surface, the grating wall surface is at least part of the diffraction surface. Within the range, the lattice apex angle formed by the lattice wall surface and the lattice surface is inclined in a direction to be smaller than that at the reference position.

  In addition, the diffractive optical element of the present invention is a diffractive optical element provided in the optical path of the imaging optical system, and the diffractive optical element has a boundary between the first material and the second material having different dispersion and refractive index. When a diffraction surface including a grating surface and a grating wall surface arranged at a predetermined grating pitch at least in part and the first material side is a light incident side, the refractive index of the first material is When the position of the grating wall surface that is larger than the refractive index of the second material and is parallel to the central axis of the diffractive optical element is a reference position of the grating wall surface, the grating wall surface is at least one of the diffraction surfaces. It is characterized in that the lattice apex angle formed by the lattice wall surface and the lattice plane is inclined in a direction larger than that at the reference position within the range of the portion.

  According to the present invention, when a diffractive optical element is used to reduce chromatic aberration of an image pickup optical system, flare generated from the grating wall surface of the diffraction grating is reduced, and a diffractive optical element capable of obtaining a good image and an image pickup optical having the same A system is obtained.

Explanatory drawing of the imaging optical system of Example 1 of this invention Sectional drawing of the diffractive optical element of Example 1 of this invention Explanatory drawing of the wall surface angle of the diffraction grating at the time of not implementing Example 1 of this invention Explanatory drawing of the angle of the grating | lattice wall surface of the diffraction grating of Example 1 of this invention Explanatory drawing of light incident angle to diffractive optical element Explanatory drawing of the flare occurrence situation when not implementing Example 1 of the present invention Explanatory drawing of the effect of the inclination of the lattice wall surface of Example 1 of this invention Flare explanatory diagram of light incident on the lower side of the diffractive optical element of the present invention Explanatory diagram of the occurrence of flare of light incident on the lower side of the diffractive optical element of the present invention Explanatory drawing of the diffractive optical element of Example 2 of this invention Enlarged view of the diffractive optical element when the second embodiment of the present invention is not carried out Enlarged view of a diffractive optical element when Example 2 of the present invention is implemented Explanatory drawing of the flare occurrence situation when not implementing Example 2 of the present invention Flare explanatory drawing of a light ray incident on the lower side of the diffractive optical element of Example 2 of the present invention Explanatory drawing of the flare generation | occurrence | production state of the light ray which injected into the lower side of the diffractive optical element of Example 2 of this invention Explanatory drawing of diffraction efficiency change with respect to the angle incident on the diffractive optical element Explanatory drawing of the diffractive optical element of Example 3 of this invention Explanatory drawing of the diffractive optical element of Example 4 of this invention

  The diffractive optical element 1 of the present invention is provided in an optical path of an imaging optical system of an imaging apparatus such as a digital camera or a video camera. The diffractive optical element 1 has a diffractive surface including a grating surface 46 and a grating wall surface 44 arranged at a predetermined grating pitch on at least a part of the boundary between the first material 31 and the second material 32 having different dispersion and refractive index. Have. The first material 31 and the second material 32 are laminated on the lens surface of the lens member. When the first material 41 side is the light incident side, the refractive index of the first material 41 is smaller than the refractive index of the second material 42. A position where the grating wall surface 44 is parallel to the central axis (optical axis) 21 of the diffractive optical element 1 is set as a reference position of the grating wall surface 44. At this time, the grating wall surface 44 is inclined in a direction where the grating apex angle 40 formed by the grating wall surface 44 and the grating surface 46 is smaller than that at the reference position within at least a part of the diffraction surface. Further, when the refractive index of the first material 41 is larger than the refractive index of the second material 42, the lattice apex angle is inclined in a direction larger than that at the reference position. Then, at least a part of the diffraction surface on which the grating wall surface 44 is inclined relative to the reference position is in a range including the central axis 21 of the diffractive optical element 1 or in a range not including the central axis 21. Further, the inclination of the grating wall surface in the range where the grating wall surface 44 is not inclined is the same as the reference position.

  FIG. 1 is a lens cross-sectional view when the diffractive optical element according to Example 1 of the present invention is applied to a telephoto lens (imaging optical system) having a focal length of 400 mm. In FIG. 1, 1 is a diffractive optical element, 2 is an aperture stop, 3a is an image plane on which an image pickup device 3 such as a CCD is disposed. Reference numeral 4 denotes a light beam having the maximum field angle, 5 denotes an optical axis of the imaging optical system 1, and 6 denotes a part of parallel outside effective field angle light such as sunlight from outside the effective field angle (outside the effective field angle). In the diffractive optical element 1, a diffraction grating having a grating pitch narrowed from the center of the grating to the outside so as to have a positive power is added to the lens surface. This improves the chromatic aberration of the imaging optical system.

  In this embodiment, the diffractive optical element 1 includes a first lens G1 and a second lens G2 on the object side of the aperture stop 2, and a diffractive optical part (diffraction grating) provided therebetween. For this reason, incident light such as sunlight enters the diffractive optical element 1 almost without being shielded by the lens barrel. In FIG. 1, for the sake of explanation, light having a wavelength of 550 nm is -30th-order in incident light at 5 degrees outside the effective angle of view that is incident on the entire first surface of the first surface 6 from outside the effective angle of view. The light diffracted in the direction is shown. The light incident on the actual imaging optical system is diffracted by the diffraction grating of the diffractive optical element 1 over a wide order range. Therefore, an image formed on the CCD 3 arranged on the image plane 3a is formed by integrating these lights. For example, since the diffraction angle and the intensity of the diffracted light change when the wavelength of the light 6 outside the effective angle of view in FIG. 1 is different, the width, position, and intensity of the light beam reaching the CCD 3 change. Further, the intensity and angle of light change depending on the diffraction order, and the width, position and intensity of the light beam reaching the CCD 3 change.

  FIG. 2 is an explanatory diagram of the diffractive optical element 1 used in FIG. In FIG. 2, 21 is an optical axis (center axis), 22 is a first lens, 23 is a second lens, 24 is a first material with low refractive index and high dispersion, and 25 is a second material with high refractive index and low dispersion. is there. The first material is made of a material having a lower refractive index than the second material. Reference numerals 24a and 25a denote diffraction gratings. In this way, high diffraction efficiency is obtained in a wide wavelength region by combining the first material 24 with low refractive index and high dispersion and the second material 25 with high refractive index and low dispersion. Here, the first material side is the light incident side. Reference numeral 26 denotes a light beam outside the effective screen (outside the effective field angle) that enters the annular grating wall surface 24b1 of the diffraction gratings 24a and 25a above the optical axis (center axis) 21, and 27 denotes the optical axis (center axis) 21. Light rays outside the effective screen incident on the grating wall surface 24b2 of the annular zone of the lower diffraction grating are shown. A light beam 26 and a light beam 27 indicate a part of the parallel light flux. In the case where the light beam is incident from above and incident on the upper side of the diffractive optical element 1 and in the light beam incident from above and incident on the lower side of the diffractive optical element 1 beyond the optical axis 21, the light beams are grating wall surfaces 24 b 1 and 24 b 2. The situation where the light enters and reflects is different.

  FIG. 3 is an explanatory diagram of the angle of the grating wall surface 34 of the diffraction grating of the conventional diffractive optical element. FIG. 4 is an explanatory diagram of the angle of the grating wall surface 44 of the diffraction grating according to the present embodiment. 3 and 4, reference numerals 31 and 41 denote a first material (first diffraction grating) with a low refractive index and high dispersion, and reference numerals 32 and 42 denote a second material (second diffraction grating) with a high refractive index and low dispersion. is there. Reference numerals 33 and 43 denote auxiliary lines (center angle light beams) indicating the average angles of the maximum light rays 39 and 49 and the minimum angle light beams 38 and 48 within the effective field angle (effective screen) of the light rays forming the image. Reference numerals 35 and 45 denote light rays (outside the effective field angle) that are incident on the grating wall surfaces 34 and 44 of the diffraction grating from outside the effective field angle (outside the effective field angle). Reference numerals 36 and 46 denote lattice planes.

  As a method for determining the average angle, the angle of light incident on the diffractive optical element 1 is determined as shown in FIG. 5 shows the incident angle with respect to the surface on which the diffractive optical element 1 is formed (surface 28 in FIG. 2). The horizontal axis represents the incident light height when the center (optical axis) of the diffractive optical element 1 is 0, and The ray angle is shown on the axis. Reference numeral 51 denotes light rays 39 and 49 having the maximum angle within the effective angle of view. Reference numeral 52 denotes the light rays 38 and 48 having the minimum angle within the effective angle of view. Reference numeral 53 denotes light rays 33 and 43 having an average angle. In order to maximize the diffraction efficiency with respect to the average angle of light rays within the effective angle of view, it is better to incline the grating wall surface 34 of the diffraction grating 31 in the direction that coincides with the light rays 33 having this average angle. FIG. 3 is an explanatory view showing the state at this time.

  The direction of the grating wall surface 34 in FIG. 3 is in the direction of an auxiliary line (light ray of average angle) 33 that is the average value of the light beam 39 having the maximum angle and the light beam 38 having the minimum angle within the effective field angle of the incident light on the diffraction grating 31. They are parallel. At this time, the light beam 35 outside the effective angle of view incident on the diffractive optical element 1 passes through the first material 31 having a low refractive index and then enters the second material 32 having a high refractive index, as shown in FIG. When the light enters the diffraction grating at a low angle, it is totally reflected at the grating wall surface 34. FIG. 6 shows a light amount distribution when the flare generation state at this time is when the incident angle to the diffractive optical element 1 is 10 ° and the horizontal axis indicates the diffraction angle and the vertical axis indicates the total light amount as 100%. The diffraction grating used in the calculation is calculated as an optimized diffraction grating so that the first-order diffraction efficiency is the highest. The angle of the horizontal axis of the graph is an angle with respect to the surface normal of the diffractive optical element 1 and takes the + side as shown in FIG.

  In FIG. 6, there is a saturated peak around 10 degrees, which is the position where the first-order diffracted light is generated. Moreover, although there is a light amount peak in the vicinity of −10 degrees, this is a flare generated by the total reflected light on the grating wall surface. It can be seen that flare due to diffraction occurs around this peak. In FIG. 6, the angle at which the light reaches the image plane is flare around 0 ° ± 1 ° to 0 ° ± 2 °, and the total reflection peak near −10 ° does not reach the image plane. In addition, the diffracted light from the totally reflected light has a shape with a skirt extending to around 0 degrees. As a method for preventing this, the present embodiment is configured as shown in FIG. The grating wall surface 44 of the diffraction grating is tilted in an acute angle direction with respect to the average value direction 43 (the central angle light beam) 43 of the maximum light beam 49 and the minimum light beam 48 to the diffractive optical element 1 among the light beams in the effective screen. ing. That is, the lattice wall surface 44 is inclined in the direction in which the lattice apex angle 30 in FIG. 3 becomes smaller so that the lattice apex angle 40 in FIG. 4 is obtained.

  FIG. 7 is an explanatory diagram showing changes in the amount of flare with respect to the inclination angle of the grating wall surface 44. In FIG. 7, the horizontal axis represents the diffraction angle, and the vertical axis represents the flare light quantity. In FIG. 7, the flare generation | occurrence | production situation when the wall surface angle of the lattice wall surface 44 is changed to an acute angle direction to -10 degree | times with respect to the direction 43 is shown. From the graph, it can be seen that the flare peak near -10 degrees when incident at 0 degrees is away from 0 degrees, and the flare caused by the skirt portion also decreases. Further, the effect is better for flare by increasing the tilt angle. However, if the tilt angle is increased, the problem arises that the diffraction efficiency deteriorates.

  The amount of degradation in diffraction efficiency depends on the pitch and tilt angle of the grating. The mechanism of this deterioration is the same as the mechanism of occurrence when the incident angle of the light beam incident on the diffraction grating changes because the light beam incident on the wall surface of the diffraction grating is kicked by the wall surface. Accordingly, the ratio of kicking decreases as the grating pitch increases, so that the degradation of diffraction efficiency is reduced even at the same inclination angle of the grating wall surface. FIG. 16 shows a graph showing the state of deterioration of diffraction efficiency with respect to the incident angle to the diffraction grating. As shown in FIG. 16, it can be seen that the diffraction efficiency when the incident angle to the diffractive optical element is increased is greatly improved as the pitch is increased to 50 μm, 100 μm, and 350 μm.

  In general, a diffraction grating used for a diffractive optical element used in an imaging optical system has a pitch of 100 μm or more, and in this embodiment, the grating pitch is 200 μm or more. Further, the grating pitch increases to the millimeter unit as it goes to the inner (optical axis side) ring zone. Therefore, even if the grating wall surface is tilted by about 10 degrees, the deterioration of the diffraction efficiency is 2% or less, which is sufficiently acceptable. The above description is about the light ray 26 in FIG. 2 in which strong light such as sunlight from outside the effective angle of view (light outside the effective angle of view) hits the upper side of the diffractive optical element. The case where the light enters the lower annular zone of the diffractive optical element 1 beyond the optical axis 21 in FIG. 2 will be described below.

  FIG. 8 is an explanatory diagram showing a situation when the light beam 86 incident on the lower side of the diffractive optical element 1 in FIG. In FIG. 8, 81 is a first material (first diffraction grating) having a low refractive index and high dispersion, and 82 is a second material (second diffraction grating) having a high refractive index and low dispersion. Reference numeral 83 denotes an auxiliary line indicating the average angle between the maximum angle ray and the minimum angle ray within the effective field angle of the light rays forming the image. Reference numeral 86 denotes light rays that enter the grating wall surface 89 of the diffraction grating from outside the effective angle of view. As shown in FIG. 8, the light ray 86 that has passed through the first material 81 having the low refractive index is reflected and transmitted at the boundary with the second material 82 having the high refractive index, so that two light rays 87 and 88 are formed. To separate. In the close contact type diffractive optical element with improved diffraction efficiency for visible light, the difference in refractive index between the first material 81 and the second material 82 is usually 0.1 or less, Therefore, most of the light is transmitted light.

  For example, when incident on the grating wall surface 89 of the diffraction grating at 80 degrees with respect to the normal direction of the grating wall surface 89, about 94% is transmitted light and 6% is reflected light. The flare light that actually reaches the CCD surface is flare that travels in the direction of the reflected light, and is not the reflected light itself but the diffracted light generated around the reflected light. Accordingly, the occurrence of flare due to the light incident on the lower side of the diffractive optical element across the optical axis is very small and does not cause a problem with respect to image degradation.

  FIG. 9 shows the light quantity distribution when the incident angle to the diffractive optical element is 10 ° at this time and the horizontal axis represents the diffraction angle and the vertical axis represents the total light quantity as 100%. The diffraction grating used in the calculation is calculated as an optimized diffraction grating so that the first-order diffraction efficiency is the highest. The angle of the horizontal axis of the graph is an angle with respect to the surface normal of the diffractive optical element, and takes the + side as shown in FIG.

  In FIG. 9, there is a saturated peak around −10 degrees, which is the position where the first-order diffracted light is generated. Further, there is a light amount peak near +10 degrees, which is a flare generated by Fresnel reflected light on the grating wall surface. Flares due to diffraction occur around this peak, but in any case, the amount of flare is weak light. Further, in FIG. 9, the angle at which the light reaches the image plane is light in the vicinity of 0 ° ± 1 ° to 0 ° ± 2 °, and the amount of flare is so weak that it does not deteriorate the image. Therefore, as described in the present embodiment, flare is sufficiently suppressed by taking measures against the incident side of light from outside the effective angle of view, that is, the upper side of the diffractive optical element in this embodiment. can do.

  FIG. 10 is an explanatory diagram of a diffractive optical element according to Example 2 of the present invention. In FIG. 10, 101 is an optical axis (center axis), 102 is a first lens, 103 is a second lens, 106 is a first material having a high refractive index and low dispersion, and 107 is a second material having a low refractive index and high dispersion. is there. The first material is made of a material having a higher refractive index than that of the second material. In this way, a high diffraction efficiency is obtained in a wide wavelength range by combining the second material 107 having a low refractive index and a high dispersion and the first material 106 having a high refractive index and a low dispersion. In addition, 108 is a light beam outside the effective screen incident on the annular grating wall surface 106b1 of the diffraction grating above the optical axis 101, and 109 is effective incident on the grating wall surface 106b2 of the annular ring below the optical axis 101. Shows off-screen rays. A light beam 108 and a light beam 109 indicate a part of the parallel light flux. In the case where the light beam is incident from above and hits the upper side of the diffractive optical element 1 and in the light beam incident from above and incident on the lower side of the diffractive optical element 1 beyond the optical axis 101, the light beam is incident on the grating wall surfaces 106b1 and 106b2. And the situation of reflection is different.

  FIG. 11 is an explanatory diagram of the angle of the grating wall surface 101b1 of the diffraction grating of the conventional diffractive optical element. FIG. 12 is an explanatory diagram of the angle of the grating wall surface 101b1 of the diffraction grating according to the present embodiment. 11 and 12, reference numerals 111 and 121 denote a first material having a high refractive index and a low dispersion, and reference numerals 112 and 122 denote a second material having a low refractive index and a high dispersion. Reference numerals 113 and 123 denote auxiliary lines (light beams having a central angle) indicating the average angles of the light beams 116 and 126 having the maximum angle and the light beams 117 and 127 having the minimum angle within the effective field angle of the light beam forming the image. Reference numerals 115 and 125 denote light rays (outside the effective angle of view) that enter the grating wall surface 101b1 of the diffraction grating from outside the effective angle of view (outside the effective angle of view).

  In FIG. 11, the light beam 115 outside the effective screen passes through the first material 111 having a high refractive index, and the grating wall surface 101b1 of the diffraction grating of the second material 111 having a high refractive index and the first material 112 having a low refractive index. Totally reflected. Due to total reflection, the amount of flare at this time is strong, but it does not reach the CCD surface in terms of angle. FIG. 13 is a graph showing the state at this time, where the incident angle to the diffractive optical element 1 is 10 °, and the horizontal axis represents the diffraction angle and the vertical axis represents the total light amount as 100%. The diffraction grating used in the calculation is calculated as an optimized diffraction grating so that the first-order diffraction efficiency is the highest. The angle on the horizontal axis of the graph is the angle with respect to the surface normal of the diffractive optical element 1, and is on the + side as shown in FIG.

  In FIG. 13, there is a saturated peak in the vicinity of −10 degrees, which is the generation position of the first-order diffracted light. In addition, there is a light amount peak near +10 degrees, which is a flare generated by the total reflected light on the grating wall surface. It can be seen that flare due to diffraction occurs around this peak. In the graph of FIG. 13, the angle at which light reaches the image plane is a flare near 0 ° ± 1 ° to 0 ° ± 2 °, and the total reflection peak near + 10 ° does not reach the image plane. Further, the diffracted light from the totally reflected light has a shape with a skirt extending to near 0 degrees. As a method for preventing this, in the present embodiment, the grating wall surface of the diffraction grating is formed on the effective screen with the average angle direction 113 of the maximum angle rays 116 and 126 and the minimum angle rays 117 and 127 to the diffractive optical element 1. It is inclined in an obtuse angle direction with respect to 123.

  That is, the lattice wall surface 101b1 is inclined in the direction in which the lattice apex angle 110 increases. FIG. 12 is an explanatory view showing a state of a diffraction grating in which the grating apex angle 120 is inclined in an obtuse angle direction. As shown in FIG. 12, by tilting the grating wall surface 101b1 of the diffraction grating in the obtuse angle direction, the total reflected light reflected by the grating wall surface 101b1 of the light beam 125 moves in a direction away from 0 degrees in FIG. The generated flare light near 0 degrees is also reduced.

  FIG. 14 is an explanatory diagram showing a situation when the light beam incident on the lower side of the diffractive optical element 1 in FIG. 10 enters the grating wall surface 106b2. In FIG. 14, 141 is a first material having a high refractive index and low dispersion, and 142 is a second material having a low refractive index and high dispersion. Reference numeral 143 denotes an auxiliary line indicating the average angle between the maximum angle ray and the minimum angle ray within the effective field angle of the light ray forming the image. Reference numeral 145 denotes light rays that enter the grating wall surface 106b2 of the diffraction grating from outside the effective angle of view. As shown in FIG. 14, the light beam that has passed through the first material 141 having a high refractive index passes through the boundary with the second material 142 having a low refractive index. Thereafter, the light beam is separated into two light beams 146 and 147 by Fresnel reflection and transmission on the grating wall surface 106b2 of the diffraction grating at the boundary with the first material 141 having a high refractive index.

  In the close contact type diffractive optical element with enhanced diffraction efficiency with respect to light in the visible range, the refractive index difference between the first material 141 and the second material 142 is usually 0.1 or less, Therefore, most of the light is transmitted light. For example, when light is incident on the grating wall surface 106b2 of the diffraction grating at 80 degrees with respect to the normal direction of the grating wall surface 106b2, about 94% is transmitted light and 6% is reflected light. The flare light that actually reaches the CCD surface is flare that travels in the direction of the reflected light, and is not the reflected light itself but the diffracted light generated around the reflected light. Accordingly, the occurrence of flare due to the light incident on the lower side of the diffractive optical element across the optical axis is very small and does not cause a problem with respect to image degradation.

  FIG. 15 is a light amount distribution when the incident angle to the diffractive optical element is 10 ° at this time, with the diffraction angle on the horizontal axis and the total light amount on the vertical axis as 100%. The diffraction grating used in the calculation is calculated as an optimized diffraction grating so that the first-order diffraction efficiency is the highest. The angle on the horizontal axis of the graph is the angle with respect to the surface normal of the diffractive optical element, and is negative as shown in FIG. In FIG. 15, there is a saturated peak around +10 degrees, which is the position where the first-order diffracted light is generated. Further, although there is a light amount peak near -10 degrees, this is flare generated by Fresnel reflected light on the grating wall surface. Flares due to diffraction occur around this peak, but in any case, the amount of flare is weak light. Further, in FIG. 15, the angle at which the light reaches the image plane is light in the vicinity of 0 ° ± 1 ° to 0 ° ± 2 °, and the amount of flare is so weak that it does not deteriorate the image. Therefore, as described in the present embodiment, flare is suppressed by taking measures against the light incident side from outside the effective angle of view, that is, the light beam incident on the upper side of the diffractive optical element in this embodiment. be able to.

  FIG. 17 is an explanatory diagram of a diffractive optical element according to Example 3 of the present invention. FIG. 17 is an explanatory view of the diffraction grating as viewed from the front. In FIG. 17, reference numeral 171 denotes a range in which the grating wall surface is tilted with respect to the average angle of the maximum and minimum angle rays within the effective field angle of the light rays forming the image. Reference numeral 172 denotes a range in which the angle of the grating wall surface of the diffraction grating is the average angle of the maximum and minimum angles within the effective field angle of the light rays forming the image.

  In the range 171, since the grating pitch of the diffraction grating is large, the diffraction efficiency is hardly deteriorated with respect to the light within the effective angle of view even if the grating is inclined at a relatively large angle. On the other hand, in the range 172, if the grating pitch of the diffraction grating is relatively small and tilted greatly, there is a concern that the diffraction efficiency will deteriorate. On the other hand, in the range 172, there are many regions where light outside the effective angle of view is kicked by the optical barrel when no light enters the diffractive optical element and no light enters the diffractive optical element. For this reason, when the lens is arranged on the CCD side from the first lens, it is difficult for light outside the screen to hit and the occurrence of flare is reduced. Therefore, by tilting the grating wall surface only in the vicinity of the optical axis of the diffractive optical element, high diffraction efficiency can be maintained while suppressing flare.

  FIG. 18 is an explanatory diagram of a diffractive optical element according to Example 4 of the present invention. In FIG. 18, reference numeral 181 denotes a range in which the angle of the grating wall surface of the diffraction grating is tilted with respect to the average angle of the maximum light ray and the minimum light ray within the effective field angle of the light ray forming the image. Reference numeral 182 denotes a range in which the grating wall surface of the diffraction grating is tilted from the average angle of the light beam having the maximum angle and the light beam having the minimum angle within the effective field angle of the light beam forming the image to suppress flare. In the range 183, the angle of the grating wall surface of the diffraction grating is configured as the average angle of the maximum light ray and the minimum light ray within the effective field angle of the light ray forming the image. The difference from the diffractive optical element in FIG. 17 is the angle of the grating wall surface of the diffraction grating near the center. This is because the flare of the diffraction grating in the range 181 has a large grating pitch and the order of the flare light reaching the image plane is higher than that in the range 182 so that the generation of flare is very small. This is because priority is given to maintaining efficiency.

  As described above, according to each embodiment of the present invention, when a diffractive optical element is used in the imaging optical system, an image of light reaching the wall surface of the diffraction grating by a light beam incident from the outside with respect to the maximum field angle. A good photographed image with little fogging can be obtained by preventing the surface from reaching the surface.

DESCRIPTION OF SYMBOLS 1 Diffractive optical element, 2 Diaphragm, 3 CCD image surface, 4 Light flux of maximum field angle, 5 Optical axis of imaging optical system, 6 Part of parallel light, such as sunlight from outside effective field angle, 21 light Axis 22 First lens 23 Second lens 24 Low refractive index high dispersion first material 25 High refractive index low dispersion second material 31, 41 Low refractive index high dispersion first material, 32, 42 Second material with high refractive index and low dispersion, 33, 43 Auxiliary line showing average angle between maximum angle and minimum angle within effective field angle of ray forming image, 35, 45 Outside effective field angle Rays incident on the diffraction grating wall surface, 31, 41 First material with low refractive index and high dispersion, 32, 42 Second material with high refractive index and low dispersion, 33, 43 Within the effective angle of view of rays forming an image Auxiliary line showing average angle of maximum and minimum angles, 35, 45 Diffracted from outside effective field angle Ray incident on child wall 51 maximum angle of the effective angle of view, 52 minimum angle of the effective angle of view, 53 average angle

Claims (10)

  A diffractive optical element provided in an optical path of an imaging optical system, the diffractive optical element being arranged at a predetermined grating pitch on at least a part of a boundary between a first material and a second material having different dispersion and refractive index When the first material side is the light incident side, the refractive index of the first material is smaller than the refractive index of the second material. When the position where the grating wall surface is parallel to the central axis of the diffractive optical element is a reference position of the grating wall surface, the grating wall surface is separated from the grating wall surface and the grating within at least a part of the diffraction surface. A diffractive optical element, wherein a lattice apex angle formed with a surface is inclined in a direction smaller than that at a reference position.   A diffractive optical element provided in an optical path of an imaging optical system, the diffractive optical element being arranged at a predetermined grating pitch on at least a part of a boundary between a first material and a second material having different dispersion and refractive index When the first material side is a light incident side, the refractive index of the first material is larger than the refractive index of the second material. When the position where the grating wall surface is parallel to the central axis of the diffractive optical element is a reference position of the grating wall surface, the grating wall surface is separated from the grating wall surface and the grating within at least a part of the diffraction surface. A diffractive optical element characterized in that a lattice apex angle formed with a surface is inclined in a direction larger than that at a reference position.   3. The diffractive optical element according to claim 1, wherein at least a part of the diffractive surface on which the grating wall surface is inclined relative to a reference position is within a range including a central axis of the diffractive optical element.   3. The diffractive optical element according to claim 1, wherein at least a part of the diffractive surface on which the grating wall surface is inclined relative to a reference position is within a range not including the central axis of the diffractive optical element.   5. The diffractive optical element according to claim 1, wherein the first material and the second material are laminated on a lens surface of a lens member.   6. The diffractive optical element according to claim 1, wherein an inclination of the grating wall surface in a range where the grating wall surface is not inclined relative to a reference position in the diffraction surface is the same as the reference position. .   An imaging optical system comprising the diffractive optical element according to any one of claims 1 to 6.   8. The imaging optical system according to claim 7, wherein the diffractive optical element is disposed on the object side of the aperture stop.   In an imaging apparatus having a diffractive optical element in the optical path having a diffraction grating at the boundary between a first material and a second material having different dispersion and refractive index, the first material is a lower refractive index material than the second material The diffractive optical element has a grating surface on which a light beam forming an image is incident and a grating wall surface, and has a central angle that is an average of the maximum angle and the minimum angle within the effective field angle of the light beam forming the image. An imaging apparatus, wherein an angle of a grating wall surface is inclined with respect to a light beam in a direction in which a grating apex angle formed by the grating surface and the grating wall surface is reduced.   In an imaging apparatus having a diffractive optical element in the optical path having a diffraction grating at the boundary between a first material and a second material having different dispersion and refractive index, the first material is a material having a higher refractive index than the second material. The diffractive optical element has a grating surface on which a light beam forming an image is incident and a grating wall surface, and has a central angle that is an average of the maximum angle and the minimum angle within the effective field angle of the light beam forming the image. An imaging apparatus, wherein an angle of a grating wall surface is inclined with respect to a light beam in a direction in which a grating apex angle formed by the grating surface and the grating wall surface is increased.