JPH10160916A - Optical system with diffractive dunctional surface and optical equipment using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain high diffractive efficiency in average in a whole range of a use wavelength bond by designing an optical system so that at least two diffractive functional surfaces among plural functional surfaces are different in wavelengths of light whose diffractive efficiency becomes highest. SOLUTION: An optical system 1 is a rear focus type zoom lens consisting of four groups of positive, negative, positive and positive. Light 2 from an object is made incident from a left side to be image formed in a solid-state image pickup element 3 being an image surface. The diffractive functional surfaces 5a, 5b are formed respectively on surfaces having the curvatures of the second group and third group of the optical system 1. The diffractive functional surfaces 5a, 5b are a diffraction grating so-called binary obdisk (BO) so that a periodical structure of a structure consisting of eight steps is formed in a ring belt shape around an optical axis 2. In this a case, by changing the design wavelengths of the diffractive functional surfaces 5a, 5b, a spectral transmission characteristics in the whole optical system 1 becomes a proper state, Refractive indexes and heights of structure of material constituting the diffractive functional surfaces 5a, 5b are changed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system having a diffraction surface which is suitably used for a photographing optical system and an observation optical system of an optical device such as a camera.

[0002]

2. Description of the Related Art Conventionally, in an optical system composed only of a refraction system, chromatic aberration is reduced by combining glass materials having different dispersion characteristics. For example, in an objective lens such as a telescope, a glass material having a small dispersion is a positive lens, and a glass material having a large dispersion is a negative lens. By using these in combination, chromatic aberration appearing on the axis is corrected. For this reason, when the number of constituent lenses is limited, or when the usable glass material is limited, the chromatic aberration cannot be sufficiently corrected in some cases.

A method for correcting chromatic aberration by combining glass materials is described in SPIE Vol.1354 International Lens Des.
Documents such as Ign Conference (1990) disclose a method of reducing chromatic aberration by giving a part of an optical system a diffraction effect. This method utilizes the physical phenomenon that chromatic aberration occurs in the opposite direction between the refractive optical system and the diffractive optical system as shown in FIG.

Meanwhile, the diffraction efficiency of a predetermined order of diffracted light on the diffraction surface has a characteristic as shown in FIG. 16, for example. In FIG. 16, the horizontal axis represents wavelength and the vertical axis represents diffraction efficiency. As shown in FIG. 16, the diffraction surface is designed so that the diffraction efficiency of the diffracted light of the predetermined order becomes the highest at the predetermined wavelength λD (λL ≦ λD ≦ λU).
The diffraction efficiency at other wavelengths is relatively lower than the diffraction efficiency at wavelength λD. The wavelength at which the diffraction efficiency is highest, such as λD, is called the design wavelength, and the order of the target diffracted light is called the design order.

For example, when the design wavelength is 550 nm, the design order is 1st, and the diffraction surface is formed by an annular stepped structure of eight steps, the diffraction efficiency at the design wavelength becomes about 95%, and the wavelength at 440 nm is obtained. The diffraction efficiency of the first-order diffracted light is about 80%, and the diffraction efficiency of the first-order diffracted light at a wavelength of 650 nm is about 88%. Therefore, in an optical system having a diffraction surface, it is desirable to set the design wavelength near the center of the wavelength band used by the optical system.

A general optical system has, for example, a spectral transmission characteristic as shown in FIG. In FIG. 17, the horizontal axis represents wavelength, and the vertical axis represents spectral transmittance. This spectral transmission characteristic is determined by the absorption of light by the glass material and the reflection of light on the refraction surface. However, in the case of an optical system having a diffraction action surface, the spectral transmission characteristic considering diffraction efficiency is considered. There must be.

Assuming that the spectral transmission characteristic of the optical system having the diffraction operation surface excluding the diffraction operation surface is η _lens (λ) and the diffraction efficiency of the diffraction operation surface is η _def (λ), the spectral transmission characteristic η ( λ) is expressed as η (λ) = η _lens (λ) × η _def (λ). When a diffractive surface having the diffraction efficiency shown in FIG. 16 is added to the optical system having the spectral transmission characteristics shown in FIG. 17, the spectral transmission characteristics at the design order are as shown in FIG.
become that way.

[0008]

In the case where only one diffraction surface exists in the optical system, by setting the design wavelength near the center of the working wavelength band as described above, the spectral characteristics can be relatively improved. Can be kept. However, when there are a plurality of diffraction action surfaces in the optical system, if the design wavelength on each diffraction action surface is set to a single wavelength near the center of the used wavelength band, the diffraction efficiency of the entire optical system becomes different from each diffraction action. Since the diffraction efficiency is represented by the product of the diffraction efficiencies on the working surface, as shown in FIG. 19, the diffraction efficiency at the design wavelength is kept high, but the diffraction efficiency at wavelengths other than the design wavelength is extremely reduced. Problems arise. In particular, on the wavelength side shorter than the design wavelength λD, the effect on the image cannot be ignored because the diffraction efficiency sharply decreases.

In view of the above problems, it is an object of the present invention to provide an optical system having a plurality of diffractive surfaces to obtain a high average diffraction efficiency over the entire wavelength band used.

[0010]

In order to achieve the above object, the optical system of the present invention has a plurality of diffraction surfaces, and at least two of the plurality of surfaces have the highest diffraction efficiency. It is characterized in that it is designed to have different wavelengths of light.

In the optical system of the present invention, there is a form in which the wavelength of the light having the highest diffraction efficiency is different for each of the plurality of diffraction surfaces.

In addition, any i of a plurality of diffraction action surfaces
The wavelength at which the diffraction efficiency is highest on the
When i is

[0013]

[Outside 2] λL: the shortest wavelength in the wavelength band of light used by the optical system λU: the longest wavelength in the wavelength band of light used by the optical system It is preferable to satisfy the following condition.

The wavelength band used in the optical system of the present invention is considered to be in a visible light region, a visible light region, an infrared light region, and the like.

The optical system of the present invention can be suitably used for an imaging optical system, an observation optical system and the like.

An attachment lens according to another embodiment of the present invention is an attachment lens having a second diffraction operation surface attached to an imaging optical system having a first diffraction operation surface, wherein the second lens has a second diffraction operation surface. The wavelength of the light having the highest diffraction efficiency on the diffraction surface is different from the wavelength of the light having the highest diffraction efficiency on the first diffraction surface.

It is preferable that the diffraction surface of the optical system and the attachment lens of the present invention is formed on the lens surface.

Further, the diffraction surface is preferably a ring-shaped diffraction grating. In this case, the periodic structure of the diffraction surface is a step-like form, a kinoform-like form,
There is a form having a triangular wave shape.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) FIG. 1 is a schematic diagram of a configuration of a zoom lens used in a consumer camcorder or the like. In the figure, reference numeral 1 denotes an optical system, which is a rear focus type zoom lens composed of four groups of positive, negative, positive and positive. 1a is a first lens group as a front lens, 1b is a second lens group as a variator, 1c is a third lens group as an afocal system, and 1d is a fourth lens group serving also as a compensator and a focus. Reference numeral 2 denotes an optical axis, and a light ray from an object enters from the left side in the figure and forms an image on the solid-state imaging device 3 which is an image plane. Reference numeral 4 denotes a stop, reference numerals 5a and 5b denote surfaces on which a structure for exhibiting a diffraction effect is formed (diffraction operation surface), and reference numeral 6 denotes a filter for adjusting visibility and limiting a spatial frequency of an incident image. .

As shown in FIG. 1, the diffraction surfaces 5a, 5a
b is formed on the surface of the optical system 1 having the curvature of the second group and the third group, respectively. In the present embodiment, the diffraction surface 5
By making the design wavelengths of a and 5b different, the optical system 1
The spectral transmission characteristics of the whole are set to be in an appropriate state.

FIG. 2 is an enlarged sectional view of the diffraction action surfaces 5a and 5b, and FIG. 3 is a schematic front view of the diffraction action faces 5a and 5b. The diffraction action surfaces 5a and 5b of the present embodiment have a so-called eight-step stair-like periodic structure formed in an annular shape around the optical axis 2 as shown in FIG. This is a diffraction grating called binary optics (BO). In FIG. 3, a concentric solid line indicates one pitch of the periodic structure, and the pitch is 1 from the optical axis toward the periphery.
The pitch period becomes shorter. By appropriately designing this pitch, it is possible to give power to the diffraction surface.
The sectional shape and the front shape of the diffraction surfaces 5a and 5b are as follows.
Actually, the shape differs depending on the design conditions. In particular, in the present embodiment, since the design wavelength is changed by making the refractive index of the material constituting the diffraction action surfaces 5a and 5b different from each other and the height of the structure, the sectional shape of each diffraction action face is different.

FIGS. 4 (a) and 4 (b) show the diffraction efficiencies of the diffractive surfaces 5a and 5b over the entire useable wavelength band. The design order of the diffracted light is 1st order for both the diffraction action surfaces 5a and 5b,
The diffraction efficiency at the respective design wavelengths of 530 nm and 510 nm is about 95%. The wavelength range used by the optical system of this embodiment is mainly the shortest wavelength (λL) of 380 nm.
From 700nm to the longest wavelength (λU)
4 (a) and 4 (b), 400 nm to
The wavelength range of 700 nm is shown. FIG. 4 (a), (b)
From this, it can be seen that, since the design wavelength on each diffraction surface is different, the characteristic of the diffraction efficiency with respect to the wavelength is different.

FIG. 5 shows the diffraction efficiency of both the diffraction action surfaces 5a and 5b. It can be seen from FIG. 5 that the diffraction efficiency is kept relatively high over the entire use wavelength band.
Therefore, at wavelengths other than the design wavelength, the spectral transmittance of the entire optical system is not extremely reduced, and good spectral transmittance characteristics can be maintained as a whole.

In this embodiment, the case where the diffraction action surface has a staircase-like periodic structure composed of eight steps has been described.
Even in the case of a step-like periodic structure including steps, the same effect can be obtained by changing the design wavelength between the diffraction action surfaces only by changing the diffraction efficiency at the design wavelength.
Further, even if it is not a step-like shape, even if it is a kinoform shape in which one pitch of a periodic structure is continuous as shown in FIG. 7 and a triangular wave shape as shown in FIG. Similar effects can be obtained by setting the design wavelength of each diffraction surface differently.

The lens group forming the diffraction surface is
Groups other than the second and third groups may be used, and the groups and positions to be set are not particularly limited. Furthermore, in the present embodiment, the case of a four-group type zoom lens in which each group is positive, negative, positive, and positive from the object side has been described. However, the present invention is not limited to this zoom type. Even if it is a two-group type zoom lens used for
Similar effects can be obtained with other zoom lenses.

The number of diffractive surfaces in the optical system is not limited to two, but the same effect can be obtained by changing the design wavelength of at least two of the plurality of diffractive surfaces. can get. At this time, it goes without saying that the design wavelengths of all the diffraction surfaces may be different.

In this embodiment, the optical system used in the visible light region has been described. However, the present invention is not limited to this wavelength band. Similar effects can be obtained even when the band is in the region of infrared light or in the region from visible light to infrared light.

The same effect can be obtained by changing the design wavelength between at least two diffractive surfaces even if the diffracted light to be used is not limited to the first-order diffracted light but other diffracted lights such as the second-order diffracted light. Can be

By the way, the design wavelength of each diffraction action surface needs to be determined according to the sensitivity characteristics of the image forming surface. That is, in the present embodiment, an example is shown in which the sensitivity characteristic of the solid-state imaging device is matched, but the present invention is not limited to this wavelength. In the case of an imaging surface having a peak sensitivity with respect to a certain wavelength as in the solid-state imaging device described in the present embodiment, the design wavelength is set at a value close to the wavelength while the diffraction surface is set at a value close to the wavelength. By making them different, high diffraction efficiency can be obtained over the entire use wavelength range, and high diffraction efficiency can be obtained even at a wavelength at which peak sensitivity is obtained. At this time, it is desirable that each design wavelength λi satisfies the following condition.

[0030]

[Outside 3]

(Embodiment 2) FIG. 9 is a schematic structural view of an observation optical system such as binoculars. In the figure, reference numeral 7 denotes an entire observation optical system, 7a denotes an objective lens, 7b denotes a prism for establishing an image, and 7c denotes an eyepiece. 2 is the optical axis, 8
Reference numerals a and 8b denote diffraction surfaces, and reference numeral 9 denotes an image forming surface of the objective lens 7a. The diffraction surface 8a corrects various aberrations including the chromatic aberration on the imaging surface 9, and the diffraction surface 8b corrects various aberrations including the chromatic aberration of the eyepiece 7c.

In the optical system of the present embodiment, the wavelength band to be used is about 400 nm to 700 nm because observation is performed with the naked eye. FIGS. 10A and 10B show the diffraction efficiencies of the diffraction surfaces 8a and 8b at this time. The design wavelengths of the diffraction action surfaces 8a and 8b of this embodiment are 550 nm and 5
00 nm, the design order is the first order.

FIG. 11 shows the diffraction efficiency of both the diffraction surfaces 8a and 8b. It can be seen from FIG. 11 that the diffraction efficiency is kept relatively high over the entire use wavelength band. Therefore, the spectral transmission characteristics of the optical system as a whole are also kept very good.

In this embodiment, the case where the diffractive surface is formed on both the objective lens and the eyepiece is shown. However, the present invention is not limited to this, and the diffractive surface may be formed at another position in the optical system such as the surface of the prism 7b. Even if there is, the same effect can be obtained. However, since there is an effect of reducing chromatic aberration on the image forming surface 9, it is desirable to provide at least one diffractive surface at any place of the objective lens 7a. Further, when the diffraction surface is formed by a ring-shaped diffraction grating as described in the first embodiment, the diffraction surface can have the effect of an aspheric lens by appropriately setting the pitch of the periodic structure. Therefore, it is preferable to provide at least one diffractive surface on the eyepiece lens 7c. That is, in the case of an observation optical system, it is more desirable to provide at least one diffraction action surface on both the objective lens and the eyepiece as in the present embodiment.

The configuration in which the diffraction action surfaces are provided on both the objective lens and the eyepiece is not limited to only the binoculars as in this embodiment, but may be a terrestrial telescope or a telescope for astronomical observation. Further, the same effect can be obtained even with an optical viewfinder such as a lens shutter camera or a video camera.

Similar to the first embodiment, the same effect can be obtained by appropriately designing the structure of the diffractive surface, whether it is a staircase shape, a kinoform shape, a triangular wave shape, or the like.

(Embodiment 3) FIG. 12 schematically shows a photographic lens in which an attachment lens called an afocal converter for changing a focal length by being attached to an object side of a master lens is attached. In the figure, reference numeral 10 denotes an attachment lens, and reference numeral 11 denotes a master lens. Reference numeral 12 denotes an image plane, and an object image is formed on the image plane 12 by the master lens 11 even when the attachment lens 10 is not attached. The attachment lens 10 and the master lens 11
4a and 14b are provided in each optical system.

FIG. 13A shows the diffraction efficiency of the diffraction surface 14 b in the master lens 11. The design wavelength of the diffractive optical surface 14b is 530 nm, and the design order is the first order. Since the master lens 11 is used alone, a wavelength slightly deviated from the center in the main wavelength band of 380 nm to 700 nm is set as the design wavelength.

FIG. 13B shows the attachment lens 1.
It shows the diffraction efficiency of the diffraction surface 14a within 0. Although the attachment lens is not used alone, the aberration is corrected singly even when used alone to maintain good optical performance when combined with the master lens 11. In the present embodiment, the design wavelength of the diffraction action surface 14a is set to 500 nm different from that of the diffraction action surface 14b.

FIG. 14 shows the diffraction efficiency of both the diffraction action surfaces 14a and 14b. It can be seen that even after the attachment lens 10 is attached, relatively high diffraction efficiency is maintained in the used wavelength band.

In such an optical system having a diffractive surface,
It can be seen that when another optical system having a diffraction action surface is added and used, it is better to change the design wavelength of each diffraction action face in order to maintain high diffraction efficiency over the entire used wavelength band.

In this embodiment, the case where the attachment lens is mounted on the object side (front) of the master lens has been described. However, the same effect can be obtained by using a rear converter lens mounted on the image side (rear) of the master lens. .
Further, in the present embodiment, the case where the design wavelength on the attachment lens side is shorter than the design wavelength on the lens side of the master is shown. Can be expected.

Similar to the first and second embodiments, the same effect can be obtained by appropriately designing the structure of the diffractive surface in any of a staircase shape, a kinoform shape, and a triangular wave shape.

In this embodiment, an attachment lens for a photographic lens is shown. However, similar effects can be expected with an attachment lens for a camcorder or a telescope.

[0045]

As described above, according to the present invention,
In an optical system having a plurality of diffraction surfaces, a high diffraction efficiency can be obtained on average over the entire wavelength band used.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a zoom lens according to a first embodiment.

FIG. 2 is an enlarged sectional view of a diffraction action surface.

FIG. 3 is a schematic front view of a diffraction action surface.

FIG. 4 is a diagram showing the diffraction efficiency of each of the diffraction surfaces 5a and 5b.

FIG. 5 is a diagram showing diffraction efficiency obtained by combining the diffraction action surfaces 5a and 5b.

FIG. 6 is another example of an enlarged cross-sectional view of a diffraction action surface (a stepped shape including four steps).

FIG. 7 is another example of an enlarged sectional view of a diffraction action surface (a kinoform shape).

FIG. 8 is another example of an enlarged sectional view of the diffraction surface (triangular wave shape).

FIG. 9 is a schematic configuration diagram of an observation optical system according to a second embodiment.

FIG. 10 is a diagram showing the diffraction efficiency of each of the diffraction action surfaces 8a and 8b.

FIG. 11 is a diagram illustrating diffraction efficiency obtained by combining the diffraction action surfaces 8a and 8b.

FIG. 12 is a schematic configuration diagram of a photographic lens according to a third embodiment.

FIG. 13 is a diagram illustrating the diffraction efficiency of each of the diffraction action surfaces 14b and 14a.

FIG. 14 is a diagram illustrating diffraction efficiency obtained by combining the diffraction surfaces 14a and 14b.

FIG. 15 is a diagram showing a difference in chromatic aberration generated between a refractive optical system and a diffractive optical system.

FIG. 16 is a diagram illustrating an example of a diffraction efficiency of a diffraction action surface.

FIG. 17 is a diagram illustrating a spectral transmission characteristic of a general optical system.

18 is a diagram illustrating the spectral transmission characteristics at the design order when the diffraction surface having the diffraction efficiency illustrated in FIG. 16 is added to the optical system having the spectral transmission characteristics illustrated in FIG. 17;

FIG. 19 is a diagram illustrating the diffraction efficiency of the entire optical system having two diffractive optical surfaces having the same design wavelength.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Zoom lens optical system of Embodiment 1 2 Optical axis 3 Solid-state image sensor 4 Aperture 5a, 5b Diffraction surface 6 Optical filter 7 Observation optical system 8a, 8b Diffraction surface 9 of Embodiment 9 Imaging surface 10 Attachment lens 11 Master Lens 12 Image surface 14a, 14b Diffraction surface

Claims (19)

[Claims] 1. A diffraction action surface having a plurality of diffraction action surfaces, wherein at least two of the plurality of diffraction action surfaces have different wavelengths of light having the highest diffraction efficiency. Having optical system. 2. An optical system having a diffraction surface according to claim 1, wherein the wavelength of light having the highest diffraction efficiency is different for each of the plurality of diffraction surfaces. 3. An arbitrary i among the plurality of diffraction action surfaces.
The wavelength at which the diffraction efficiency is highest on the
When i is used, λL: the shortest wavelength in the wavelength band of light used by the optical system λU: the longest wavelength in the wavelength band of light used by the optical system, wherein the following condition is satisfied. An optical system having a diffractive surface. 4. An optical system having a diffraction surface according to claim 1, wherein said diffraction surface is formed on a lens surface. 5. The optical system having a diffraction surface according to claim 1, wherein the diffraction surface is an annular diffraction grating. 6. The optical system having a diffraction surface according to claim 5, wherein the periodic structure of the diffraction surface is stepped. 7. The optical system having a diffractive surface according to claim 5, wherein the periodic structure of the diffractive surface is a kinoform shape. 8. The optical system according to claim 5, wherein the periodic structure of the diffraction surface has a triangular wave shape. 9. The optical system having a diffractive surface according to claim 1, wherein the wavelength band used is in a visible light region. 10. The optical system having a diffractive surface according to claim 1, wherein a wavelength band to be used is a visible light region and an infrared light region. 11. An optical system having a diffractive surface according to claim 1, wherein said optical system is an image forming optical system. 12. The optical system having a diffraction surface according to claim 1, wherein the optical system is an observation optical system. 13. An optical apparatus using the optical system having the diffraction action surface according to claim 1. 14. An attachment lens attached to an imaging optical system having a first diffraction operation surface, wherein the attachment lens has a second diffraction operation surface, and
An attachment lens, wherein the wavelength of light having the highest diffraction efficiency on the diffraction action surface is different from the wavelength of light having the highest diffraction efficiency on the first diffraction action surface. 15. The attachment lens according to claim 14, wherein said first and / or second diffraction action surface is formed on a lens surface. 16. The device according to claim 1, wherein the first and / or second diffraction surface is a ring-shaped diffraction grating.
Attachment lens according to 4, 15. 17. The periodic structure of the first and / or second diffractive surface is step-shaped.
Attachment lens as described. 18. The attachment lens according to claim 17, wherein the periodic structure of the first and / or second diffraction action surfaces has a kinoform shape. 19. The attachment lens according to claim 17, wherein the periodic structure of the first and / or second diffraction action surfaces has a triangular wave shape.