JPH10274705A - Diffractive optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a highly accurate positioning and improve the accuracy of measurement of a lens when a grating lens is mounted on a lens holder. SOLUTION: A projecting part 4a is provided at the central part on a surface of a grating lens 1a. The maximum tilt angle θ is set to an angle calculated from θ>7/(n-1) or larger when a refraction index of the lens is expressed by n, so that this projecting part 4a can be used for positioning of the center of the lens and a reference of measurement, and the projecting part is formed in a visually detectible form. Further, the bottom area is made to 10% of a 1st zone of the diffraction grating or smaller so that the existence of the projecting part 4a does not influence the optical function.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffractive optical element having a lens function, that is, a so-called diffractive lens, and more particularly to a diffractive optical element capable of highly accurate positioning.

[0002]

2. Description of the Related Art A diffractive optical element includes a diffractive optical element formed into a pattern in which a diffractive structure acts as a prism, and an diffractive optical element acting as a lens. Of these, the diffractive optical element having a lens function is called a diffractive lens,
It is used as a photographing lens or the like that requires compensation for chromatic aberration.

This diffractive lens is different from a conventional lens in which the refractive index and the surface profile of a medium such as glass or light-transmitting resin have an optical effect, and the regularity of a diffraction grating formed on the lens surface is different. Due to the diffraction effect of the pattern on the incident light, the incident light converges at one point, and a lens action that diverges from one point occurs. Therefore,
Since the regular pattern of the diffraction grating formed on the surface of the diffraction lens extends concentrically around one point, the diffraction grating closest to the center point is referred to as a first orbicular zone. Obi, 3rd annular zone ...

[0004] Diffractive lenses can be manufactured by injection molding, etching or laser processing using a resin molding die, but are generally employed because injection molding has the advantage of high mass productivity. I have. In recent years, in the production of a mold used in this injection molding method, cutting using a diamond tool, so-called diamond turning, has been widely used.

When a die is manufactured by this diamond turning, the peripheral speed at the time of cutting the concentric diffraction pattern becomes closer to zero as it approaches the center. As a result, a projection or a depression is inevitably generated at the center of the mold. Conventionally, a projection or a recess formed at the center of the diffraction lens has been processed so as not to be visible or removed by cutting so as not to affect the lens performance.

Meanwhile, in the process of mounting a conventional diffractive lens on a lens holder in the process of manufacturing a camera or the like, the optical axis of the lens is positioned using an optical magnifying device such as a microscope. This positioning method firstly matches the image itself obtained through the camera lens with a predetermined pattern, and secondly aligns the lens optical axis based on the fitting tolerance of the lens outer diameter. is there.

[0007]

However, in the first conventional method for matching an image itself obtained through a camera lens with a predetermined pattern, the image is formed by a plurality of lenses constituting the camera lens. For this reason, there is a restriction that the image formation position is far and near, which makes it difficult to mount the image at an appropriate position.

In the second conventional method of aligning the lens optical axis with reference to the fitting tolerance of the lens outer diameter, if the target position accuracy of the lens system is required to be high, the lens position is measured and the positioning is performed. Need to be
There is a problem that the work process becomes complicated. That is,
The most important thing when attaching the lens to the holder is to match the power of the lens, but in this second method, the outer diameter of the lens is matched to match the power, As a result, indirect alignment is performed, and it is inevitable that the positional accuracy is reduced.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides a projection or a concave portion at the center of a diffractive lens so that high-precision positioning can be performed when the lens is mounted on a lens holder. It is an object of the present invention to provide a diffractive optical element that can be realized and that improves the measurement accuracy of the lens itself.

[0010]

In order to achieve the above object, the present invention provides a diffractive optical element having a lens function in which a projection or a recess is provided at the center of the lens surface. The concave portion can be used as a reference for alignment and measurement of the center portion.

In the above configuration, the protrusion or the recess is formed in a shape that can be visually detected by an optical magnifying device such as a microscope, so that visual detection can be facilitated. In this case, the maximum inclination angle of the projection or the depression is calculated from θ> 7 / (n−1), where θ is the maximum inclination angle of the projection or the depression, and n is the refractive index of the lens. By setting the angle to be equal to or larger than the angle value, visual detection becomes possible.

The area of the bottom of the projection or the recess is:
It has been experimentally confirmed that the optical function is not affected if the area of the first annular zone of the diffraction grating is 10% or less. Further, in the case of a die cut by a diamond chip, since a projection or a recess is inevitably generated in a manufacturing process, the diffractive optical element of the present invention having the projection or the recess as an essential component is required. It is desirable to mold using the mold.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a central portion of a diffractive lens as a diffractive optical element according to the present embodiment. The diffraction type lens 1a shown in this figure is a molded lens made of a light-transmitting resin, and has a large number of diffraction gratings 2 ₁ , 2 ₂ ... Formed concentrically on one surface. The cross-sectional shape of these diffraction gratings 2 ₁ , 2 ₂ ... Is formed in a so-called Blaze shape in which right triangles are continuous. The central region 3 serving as the diffraction grating 2 in ₁ of the first annular zone is formed in a spherical shape, the center portion of the center region 3, that is, the protrusion 4a of the conical shape is formed in the center of the lens I have.

FIG. 2 shows a central portion of a diffractive lens 1b different from the above embodiment. The diffraction type lens 1b shown in this figure is also a resin molded lens, but in this embodiment, a large number of blazed diffraction gratings 2 ₁ , 2 are provided on both surfaces of the lens.
2 ₂ ... are formed in a concentric manner. An inverted conical recess 4b is formed at the center of the spherical central region 3 on both surfaces of the lens.

The present invention is characterized in that the projection 4a or the recess 4b is provided at the center of the lens surface, but the function of the projection 4a and the function of the recess 4b are substantially the same. Therefore, the diffractive lens 1a having the protrusion 4a shown in FIG. 1 will be described below.

FIG. 3 shows a state in which the mold for molding the diffractive lens shown in FIG. 1 is processed. In this figure, 5
Denotes a manufacturing die on the side of the diffraction gratings 2 ₁ , 2 ₂ ..., 6 denotes a diamond bit as a cutting bit mounted on a processing device (not shown), and the die 5 drives the processing device during processing. (Not shown), the diamond tool 6 is rotated in the direction of arrow t about the axis, and the operation of the diamond tool 6 is controlled in the X-axis direction and the Y-axis direction indicated by the cross arrows. 5 has a grating shape 2a ₁ corresponding to the blazed diffraction gratings 2 ₁ , 2 ₂ .
Are formed.

The diamond bite 6 has a tip portion of a bite body formed of a diamond tip, and has been conventionally optimized for processing a blazed diffraction grating shape. In the present embodiment, a sharp cutting tool having a radius of the tip end R surface of the order of several μm, specifically, a maximum of about 3 μm, and preferably about 0.5 μm is used. Also,
When the diamond tool 6 is used in this manner, the mold 5
Although iron-based materials can be considered as a constituent material of iron, iron-based materials have a strong chemical bond with carbon, which is the material of the cutting tool 6, and cannot be used. Those subjected to electroless Ni plating are preferably used.

The profile 4aa of the central projection 4a of the mold 5 can be easily processed by finely feeding in the Y direction in addition to the profile for producing the blazed lattice shapes 2a ₁ . In the diffractive lens 1a that needs to be machined with the cutting tool 6 having the small tip curvature as described above, since the profile 4aa of the projection 4a is particularly easy to form, the diffraction is performed by the metal mold 5 cut using the diamond tool 6. If the mold lens 1a is molded, the profile 4aa of the projection 4a can be extremely easily produced in the production process of the mold 5.

In this embodiment, the projection 4a of the diffractive lens 1a is formed for the purpose of determining the center of the concentric pattern of the diffraction gratings 2 ₁ , 2 ₂ . By using a level optical magnifier, it must be formed into a visually detectable shape. In this way, by making the projections 4a appear to such an extent that they can be easily visually observed with a microscope or the like, the projections 4a can be used as a reference position for alignment or measurement of the center of the lens. Can be mounted with high accuracy.

FIG. 4 shows the shape of the projection 4a of the diffractive lens 1a formed by using the mold 5 having the above-described structure. FIG. 7 exemplifies an inconvenient concave portion 4b 'which has conventionally occurred at the time of manufacturing a mold. The generation of the concave portion 4b at the center of the lens shown in FIG. 7 is caused by the following process.

That is, in the case of cutting with a diamond cutting tool, the peripheral speed is lower in the central region of the lens than in the peripheral region as described above. Projection 4a or recess 4
b 'is caused by the lack of shape accuracy due to the difference in processing speed. However, in the past, the main point was to make the projection 4a or the depressed portion 4b 'as small as possible in the center of the lens. Therefore, the radius of the tip end R of the diamond bite was about 1 mm. Since a large diameter is used, the projection or the concave portion does not have an extremely sharp shape.
And since it has a loosely undulating shape like the concave portion 4b 'shown in FIG. 7, it cannot be confirmed visually.

On the other hand, in the case of the projection 4a of the diffractive lens 1a of this embodiment shown in FIG. 4, when forming the molding die 5 for the diffractive lens 1a, the tip radius is set as described above. Since the small and sharp diamond cutting tool 6 is used, the visible projection 4a can be machined. As the shape of the protrusion 4a, in addition to the conical shape described above, a hemispherical shape, or a combined shape in which the tip portion has a partially spherical shape and the bottom portion has a conical shape can be considered.

The condition for forming the projection 4a into a shape that can be visually observed by a microscope or the like is determined by the maximum angle of the inclined surface of the projection 4a. That is, the inclination of light when entering the projection 4a is determined substantially by (n-1) θ, where θ (radian) is the maximum inclination angle of the projection 4a and n is the refractive index.

Now, assuming that light with a tilt angle is difficult to see into the eyes if the light spreads about FNo4, it is sufficient that the 7 ° light bends. If n = 1.5, the tilt angle θ is 14 °. It has been found that the above is sufficient. Therefore, if the maximum inclination angle θ of the projection 4a is set to an angle equal to or larger than the angle value calculated from the following equation, visual observation with a microscope or the like becomes possible. θ> 7 / (n−1) Note that the maximum inclination angle θ can be obtained for the recessed portion 4b shown in FIG.

Further, the bottom area of the projections 4a can be said not to affect the optical function is less than 10% with an area surrounded by the diffraction grating 2 of the _first annular zone. In other words, the efficiency of the diffraction gratings 2 ₁ , 2 _2, ... Is 100% in terms of calculation at the design wavelength, but generally decreases by about 10 to 15% depending on the manufacturing accuracy of the grating. If the influence on the optical function due to the above is less than 10%, it can be said that it is an allowable limit.

FIG. 5 shows a state in which the diffractive lens 1a of this embodiment is mounted on a lens holder using a microscope. In this figure, 7 is a microscope and 8 is a lens holder. When positioning the lens 1a in the lens holder 8 so that the optical axis L of the lens 1a having the diffraction grating surface is aligned, the projection 4a is used as a target of the center position. Thereby, it becomes easy to determine the center position of the lens 1a on the lens holder 8 by the microscope 7, and highly accurate positioning can be performed. The projection 4a is aligned with the center of the lens holder 8, and the optical axis L
After that, the periphery of the lens 1a is fixed to the lens holder 8 by a method such as bonding.

FIG. 6 shows an example in which a diffraction type lens manufactured according to the present embodiment is measured by a stylus type measuring instrument.
In the diffraction type lens shown in this figure, the height of the blaze of the diffraction gratings 2 ₁ , 2 ₂ ... Is about 1 μm. In the case of a lens having such minute diffraction gratings 2 ₁ , 2 ₂ , etc., a measurement error occurs unless the measurement needle of the stylus-type measuring instrument is measured accurately so as to pass through the center of the lens. Even in the case of performing a simple measurement, the presence of the projection 4a at the center makes it easy to determine whether or not the measurement was made through the center of the lens, depending on whether or not the contact of the measurement needle with the projection 4a was detected. , Accurate measurement becomes possible.

[0028]

As described above, according to the present invention, since the projection or the depression is provided at the center of the lens surface of the diffractive optical element, for example, the projection or the depression is utilized by utilizing the projection or the depression. Positioning when the diffractive optical element is mounted on the lens holder can be easily performed. Also, when measuring the lens shape of the diffractive optical element, whether the protrusions or recesses could be accurately measured, because it is a reference whether or not, it is possible to accurately measure the shape,
Exhibits excellent effects not seen before.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a central region of a diffractive lens having a diffraction grating formed on one surface as a diffractive optical element according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a central region of a diffractive lens having a diffraction grating formed on both sides as a diffractive optical element according to another embodiment of the present invention.

FIG. 3 is an enlarged view of a main part schematically showing a state during processing of a mold for molding the diffractive lens shown in FIG. 1;

FIG. 4 is an enlarged view of a main part showing a shape of a protrusion.

FIG. 5 is a diagram showing a state in which a diffraction lens is mounted on a lens holder using a microscope.

FIG. 6 is a diagram showing an example in which a diffraction type lens is measured with a stylus type measuring device.

FIG. 7 is an enlarged view of a main part showing an example of a conventional inconvenient concave portion.

[Explanation of symbols]

1a Diffraction type lens 1b Diffraction type lens 2 ₁ , 2 ₂ ... Diffraction grating 2a ₁ .

Claims (5)

[Claims] 1. A diffractive optical element having a lens function, wherein a protruding portion or a concave portion is provided at a central portion of a lens surface. 2. The projection or the depression is formed in a shape that can be visually detected by an optical magnifying device.
4. The diffractive optical element according to 1. 3. The diffractive optical element according to claim 2, wherein the maximum inclination angle of the projection or the recess is set to an angle equal to or larger than an angle value calculated from the following equation. θ> 7 / (n−1), where θ: the maximum inclination angle of the protrusion or the recess, and n: the refractive index. 4. The diffractive optical element according to claim 1, wherein a bottom area of the projection or the recess is set to less than 10% of an area of the first annular zone of the diffraction grating. 5. The diffractive optical element according to claim 1, wherein the diffractive optical element is formed by a mold cut with a diamond tip.