JPH03191319A - Optical low pass filter - Google Patents

Abstract

PURPOSE:To reduce a difference in MTF characteristics caused by wavelength and to prevent color moire and a pseudo color from being generated by making the dispersion by a member with a large refractive index smaller than the dispersion by a member with a small refractive index. CONSTITUTION:A phase grating is formed on the surface of a 1st member with a refractive index n1 and a 2nd member 2 with a refractive index n2 is joined with the surface of the 1st member 1 so as to hold a phase grating therebetween. Then inequalities I and II are satisfied. Here, n1c and n2c, n1d and n2d, and n1g and n2g are the refractive indexes of the 1st and 2nd members 1 and 2 on lines (c), (d), and (g) respectively. Consequently, the difference in MTF characteristics caused by the wavelength becomes small and the color moire and pseudo color are prevented from being generated.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical low-pass filter, and particularly to a phase-type optical low-pass filter used in video cameras, electronic still cameras, fiberscopes, etc. .

[Conventional technology]

In a camera that uses an image sensor with a regular pixel arrangement such as a CCD, if a subject contains a frequency component higher than the Nyquist frequency, false signals called moiré fringes are generated. Also, in a fiberscope, moiré fringes occur because the image guide fibers have a regular arrangement, similar to the COD. Therefore, in order to prevent the occurrence of moiré fringes, an optical low-pass filter that limits high frequency components has been placed within the imaging optical system. As such optical low-pass filters, many phase-type optical low-pass filters that utilize the diffraction effect of light have been proposed.

However, since the diffraction effect differs depending on the wavelength, the MTF characteristic, which is the transfer function of the optical system, differs depending on the wavelength.
As a result, there have been problems such as a decrease in resolution and the occurrence of color moiré and false colors. The reason is as follows.

The MTF characteristic of a phase-type optical low-pass filter changes depending on the phase difference δ determined by the height of the phase section. The phase difference δ is given by δ=(nl nt )h/λ (i). However, λ is the target wavelength, nl is the refractive index of the material of the phase part of the low-pass filter at wavelength λ, n
2 is the refractive index at wavelength λ of the medium in contact with the phase section surface of the low-pass filter, and h is the height of the phase section.

Therefore, as shown in the above formula (i), since δ changes depending on λ, the MTF characteristic changes depending on λ. Therefore, for example, JP-A-50-131543
In the optical low-pass filters described in Japanese Patent Application Publication No. 63-311323, the difference in MTF characteristics depending on the wavelength is reduced by devising the shape of the phase part (protrusion).

However, since the height h of the phase part of a phase-type optical low-pass filter is as low as the wavelength used, that is, about 0.6 μm, the ratio of the manufacturing error Δh to h becomes large, which is clear from the above equation (i). As shown in FIG. There is a problem in that the precision must be extremely high and manufacturing is troublesome.
In the second embodiment described in Japanese Patent Publication No. 63, parts with different refractive indexes are bonded together, and in the optical low-pass filter described in Japanese Patent Application Laid-Open No. 149923/1983, parts with different refractive indexes are separated by ion exchange method etc. By forming by this method, the value of (nl-n2) in the above formula is reduced and the height l of the phase part is increased, thereby making manufacturing easier by not requiring very high precision. .

That is, the refractive index n1 of the phase part is generally about 1.5,
If the medium in contact with the phase plane is air, its refractive index nt
is rz''=1, and the refractive index difference nn2 is quite large, but in the second embodiment described in JP-A-53-119063 and the optical low-pass filter described in JP-A-61-149923, By using a member in which , n, is larger than l, the refractive index difference n + n t can be made small, and the difference can be made large when a certain δ is given.

As a result, even if the manufacturing error amount Δh of h is the same, the variation in δ becomes smaller than when n2''+1, so it is possible to reduce the variation in MTF characteristics.

Note that the case of combining two members in which n1+ng is larger than 1 in this manner will be referred to as "bonding" hereinafter. The relationship between the two members is just that they are optically in contact with each other at the phase plane,
It may be in any state of bonding, adhesion, or close contact.

[Problem to be solved by the invention]

However, the refractive index n++nz changes depending on the wavelength λ. There is what is called dispersion. Therefore, when "bonding" members with different refractive indexes as in the conventional example above, unless the dispersion of each member is taken into account, an optical low-pass filter will be constructed with a combination that will result in a large difference in MTF characteristics depending on the wavelength. You may end up doing this. On the other hand, by actively utilizing this dispersion, it is possible to control the change in the value of δ depending on the wavelength and to reduce the difference in MTF characteristics depending on the wavelength.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide an optical low-pass filter that can reduce the difference in MTF characteristics depending on wavelength and prevent the occurrence of color moiré and false colors.

[Means and effects for solving the problems] The optical low-pass filter according to the present invention is a phase-type optical low-pass filter made by "bonding" two members with different refractive indexes, and the optical low-pass filter is a phase-type optical low-pass filter made by "bonding" two members with different refractive indexes. This satisfies the condition that the dispersion of the material is smaller than the dispersion of a member having a small refractive index.

This will be explained in more detail below.

One of the optical low-pass filters according to the present invention has a phase grating formed on the surface of a first member having a refractive index nl, and a second member having a refractive index n2 bonded to the surface of the first member so as to sandwich the phase grating. The following conditional expression (IL (21
This is an optical low-pass filter that satisfies the following.

m n z> 1211 (21nl nlc <ntt n2c but 5n
lc+ n2c+ n1ar n2a+ n+
g+n2g is the refractive index of the C line, d line, and 1g line of the first member and the second member, respectively.

First, let us consider the case where the dispersion of both members is not taken into consideration, that is, the refractive index does not change depending on the wavelength, and the cross-sectional shape of the phase grating is a triangular wave as shown in FIG. In addition, in the figure, l is the first member and 2 is the second member. In this case, δ=(nl n2)h/λ+nl 11. = constant. And the wavelengths of the g-line, d-line, and c-line are λ.

(=435nm), λa (=587nm
) + λc (656 nm), and when the magnitude of the phase difference δ at λ is 1, it becomes 35 56 and the MTF characteristics are as shown in FIG.

Therefore, the MTF characteristics vary greatly depending on the wavelength.

On the other hand, in order to satisfy the above conditional expressions (11, (2)) of the present invention, a member having a refractive index as shown in the following table is used, for example.

However, n++nz is the refractive index of the first member and the second member, respectively. Therefore, if the magnitude of the phase difference δ when λi is 1, then δ=1.21 when λ1 and δ=0.91 when λ, and the MTF characteristics are as shown in FIG.

Therefore, the difference in MTF characteristics depending on the wavelength is reduced, and the occurrence of color moiré and false colors can be prevented.

In addition, if the above conditional expression m, (2) is not satisfied, that is, if the dispersion of a material with a high refractive index is larger than that of a material with a low refractive index, the opposite result will be obtained, and the difference in MTF characteristics will increase depending on the wavelength. This results in a decrease in resolution, color moiré, and false colors.

Another optical low-pass filter according to the present invention forms a phase grating on the surface of the first member having a refractive index of n+,
An optical low-pass filter comprising a second member having a refractive index n2 bonded to the surface of the first member so as to sandwich the phase grating, the optical low-pass filter having a regular pixel arrangement that receives light that has passed through the optical low-pass filter. The three wavelengths within the effective sensitivity wavelength range of the photodetector having
．． λ, an optical low-pass filter (3) that satisfies the following conditional expression (3), (41) n
l z t >fi2A I(4) nl
λ + n+1s<nt +
n 2 x + However, nl λl! n21
In n11 □.

n! Enter! + n1xsr ntzs are the wavelengths λl, λ2 . λ, is the refractive index of light.

For example, if the imaging optical system includes an optical low-pass filter and an infrared cut filter, and the light-receiving element placed on the image-forming surface is a solid-state image sensor, the light-receiving element of the solid-state image sensor itself Even if the sensitivity is from 200nm to 900nm, the imaging optical system is made of glass and the sensitivity is 400nm.
If the infrared cut filter only allows light beams with wavelengths above 700 nm to pass through, and the infrared cut filter cuts wavelengths above 700 nm, then the effective sensitivity wavelength range is 200 nm, which the light receiving element itself has.
The wavelength range of the actually incident light beam is not 400 nm to 700 nm, but 400 nm to 700 nm.

In addition, if the light receiving element placed on the image forming plane is an optical fiber bundle, the wavelength range of the light beam transmitted through the optical fiber bundle is 35.
If it is 0 nm to 750 nm, even if the wavelength range of the light beam incident on the light receiving element is 300 nm to 800 nm,
The effective sensitivity wavelength range is 350 nm to 750 nm.

It is desirable that the optical low-pass filter according to the present invention further satisfies the following conditional expression (5).

(5) -0,5<ΔD/Δfig-o, osHowever,
ΔD=(nl n+c) (nt, -ngc)Δ
n+d nia or 80” (DBl-nBi-(Lll-n+1+)
Δn=(n+□tnxxl).

This conditional expression (5) defines the amount of dispersion of the members to be bonded together, and if the lower limit of conditional expression (5) is exceeded, the effect of the present invention will be weakened, and the difference in MTF characteristics depending on the wavelength will be insufficiently corrected. Moreover, if the upper limit of conditional expression (5) is exceeded, the correction will be excessive, and the difference in MTF characteristics depending on the wavelength will become larger.

For example, in the case of the combination shown in the table above, Δn It is.

〔Example〕

Hereinafter, the present invention will be described in detail based on the illustrated embodiments.

As shown in FIG. 4, this is the first member 1 with a refractive index of n.
A phase grating having a triangular wave cross section is formed on the surface of the first member l, and a second member 2 having a refractive index n2 is bonded to the surface of the first member l so as to sandwich the phase grating. Note that LaK11 is used for the first member 1, and polycarbonate is used for the second member 2, and the height h of the phase portion is 4.2 μm.

λ1. λ4. The refractive index at λ, nl+n! and refractive index difference n
1n* and phase difference δ are as shown in the following table.

This satisfies conditional expressions (11, (2), and λ,.

λ4. The MTF characteristics at λ6 are as shown in Figure 5,
It can be seen that the difference in MTF characteristics depending on wavelength is small. Therefore, the occurrence of color moiré and false colors can be prevented.

Also, as is clear from the values in the table above, ΔD/Δn=-
0, 1, 8, and conditional expression (5) is also satisfied.

As shown in FIG.
A phase grating having a sine wave cross section is formed on the surface of the first member 1, and a refractive index n,
This is an optical low-pass filter formed by joining the second member 2 of. Note that PSKI is used for the first member 1 and urethane acrylate ultraviolet curing resin is used for the second member 2, and the height h of the phase portion is 11.4 μm.

λ1. λ4. Refractive index nl+n! at λ0! and refractive index difference n1
-nt and phase difference δ are as shown in the following table.

This satisfies conditional expressions (1) and (2), and λ.

λ2. The MTF characteristics at λ are as shown in Figure 7,
It can be seen that the difference in MTF characteristics depending on wavelength is small. Therefore, the occurrence of color moiré and false colors can be prevented.

Also, as is clear from the values in the table above, ΔD/Δn=-
0.18, and conditional expression (5) is also satisfied.

! ""0 Tadaken This is the first member 1 with a refractive index n, as shown in FIG.
A phase grating having a triangular cross section is formed on the joint surface of the
, and a phase grating whose cross section is a triangular wave is formed on the joint surface of both 2.3, and at least the above conditional expressions (1) and (2) are satisfied with respect to adjacent members.
) or (3), which satisfies f41.

Note that the present invention is not limited to the above embodiments, and that adjacent members satisfy at least the above conditional expressions (1), (2), or (
Various materials can be used as long as they satisfy 3) and (4). Also, when choosing materials.

It is preferable to use a material whose refractive index changes equally depending on temperature and humidity, since variations in MTF characteristics due to changes in temperature and humidity will be reduced. In addition, the shape of the phase section is not limited to triangular waves, sine waves, but trapezoidal waves.

It may be an arcuate wave, a two-dimensional waveform, a combination thereof, or a random pattern.

〔Effect of the invention〕

As described above, the optical low-pass filter according to the present invention has the practically important advantage that the difference in MTF characteristics depending on wavelength is reduced, and the occurrence of color moiré and false color can be prevented.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram of the main parts of the optical low-pass filter according to the present invention, and FIGS. 2 and 3 are MTFs of the optical low-pass filter of FIG. 1 when the conditions of the present invention are not satisfied and when the conditions of the present invention are satisfied, respectively. Figures 4 and 5 are sectional views of the main parts of the first embodiment and diagrams showing the MTF characteristics, and Figures 6 and 7 are sectional views of the main parts and MTF of the second embodiment, respectively. FIG. 8, a diagram showing the characteristics, is a perspective view of the third embodiment. 1...First member, 2...Second member, 3...
Third member. Fig. 1 Fig. 2 O 6 Fig. IIF 1 Rifouho 4 Fig.

Claims (4)

[Claims] (1) A phase grating is formed on the surface of a first member having a refractive index n_1, and a second member having a refractive index n_2 is bonded to the surface of the first member so as to sandwich the phase grating, and the following conditional expression ( An optical low-pass filter that satisfies 1) and (2). (1) n_1_d>n_2_d (2) n_1_g-n_1_C<n_2_g-n_2_
C However, n_1_C, n_2_C, n_1_d, n_2
_d, n_1_g, and n_2_g are the refractive indices of the C-line, d-line, and g-line of the first member and the second member, respectively. (2) An optical low-pass filter comprising a phase grating formed on the surface of a first member having a refractive index n_1, and a second member having a refractive index n_2 bonded to the surface of the first member so as to sandwich the phase grating. Then, three wavelengths within the effective sensitivity wavelength range of the light receiving element having a regular pixel array that receives the light that has passed through the optical low-pass filter are sequentially set to λ from the shortest wavelength.
When _1, λ_2, λ_3, the following conditional expression (3),
An optical low-pass filter that satisfies (4). (3) n_1_λ_2>n_2_λ_2 (4) n_1_λ_1−n_1_λ_3<n_2_λ_
1-n_2_λ_3 However, n_1_λ_1, n_2_λ
_1, n_1_λ_2, n_2_λ_2, n_1_λ_
3, n_2_λ_3 are the refractive indexes of light of wavelengths λ_1, λ_2, and λ_3 in the first member and the second member, respectively.