JPH11202127A - Dichroic mirror - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To reduce a shift of a half-value wavelength due to an increase in an incident angle of a dichroic mirror. SOLUTION: A TiO ₂ film 21 as a high refractive index film and an Al ₂ O ₃ film 2 as a medium refractive index film are formed on the surface of a substrate 10 of BK7.
2 are alternately stacked to provide a first selective transmission multilayer film 20 having a design center wavelength of 720 nm and an incident angle of 38 degrees and having an 18-layer structure, and a TiO ₂ film 31 as a high refractive index film and a low refractive index film An SiO ₂ film 32 as a refractive index film is alternately laminated to provide a second selective transmission multilayer film 30 having an eight-layer film configuration with a design center wavelength of 880 nm and an incident angle of 38 °. By combining the medium refractive index film, the shift of the half-value wavelength due to the increase of the incident angle is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a camera, a copying machine,
The present invention relates to a dichroic mirror used for an optical device such as a printer.

[0002]

2. Description of the Related Art Conventionally, light in a specified wavelength range is reflected,
As a dichroic mirror that transmits light in other wavelength ranges, a high-refractive-index film and a low-refractive-index film are each designed at the center wavelength λ.
There is known a permselective film having a multilayer structure in which layers are alternately stacked with an optical film thickness of 1/4 of the above.

In such a selective transmission film, the larger the refractive index ratio (H / L) between the high refractive index film and the low refractive index film, the wider the bandwidth of the reflection band and the higher the reflectance. In general, TiO ₂ is used as a dielectric material for forming a high refractive index film, and SiO ₂ or the like is used as a dielectric material for forming a low refractive index film. Further, as a broadband dichroic mirror having a wider reflection bandwidth, a double or triple film configuration having a design center wavelength changed by a laminated structure of a TiO ₂ film and a SiO ₂ film has been developed.

[0004]

However, according to the above-mentioned prior art, the 50% transmission wavelength (half-value wavelength) between the transmission band and the reflection band of the permselective film increases as the incident angle of the light beam increases. Tend to shift to In particular, as the refractive index of the low refractive index film is low and the refractive index ratio (H / L) with respect to the high refractive index film is large, the shift amount of the half-value wavelength increases, and the shift between the transmission band and the reflection band is increased. Since the gradient of the average transmittance in the divided area becomes gentle, when such a dichroic mirror is used in a finder optical system, problems such as color unevenness due to a difference in incident angle of light rays occur.

The present invention has been made in view of the above-mentioned unsolved problems of the prior art, and has a small shift of the half-value wavelength even when the incident angle of a light beam increases. It is an object of the present invention to provide a high-quality dichroic mirror having a spectral characteristic of a steep transmittance in the divided region of (1).

[0006]

In order to achieve the above-mentioned object, a dichroic mirror of the present invention comprises a transparent substrate,
A first layer in which high-refractive-index films and medium-refractive-index films each having an optical film thickness equal to １ ／ of the first design center wavelength are alternately laminated;
Of the selective transmission multi-layered film and one of the second design center wavelengths
A second selective transmission multilayer film in which high refractive index films and low refractive index films having an optical film thickness equal to / 4 are alternately laminated.

The medium refractive index film is preferably made of a material selected from dielectric materials including Al ₂ O ₃ , ZrO ₂ and a mixture thereof.

[0008]

The first selective transmission multilayer film composed of the high refractive index film and the middle refractive index film has a small increase in the refraction angle when the incident angle of light increases. By combining such a selective transmission multilayer film with a second selective transmission multilayer film having a high refractive index film and a low refractive index film having a large refractive index ratio, the amount of shift of the half-value wavelength of the dichroic mirror is reduced.

It is possible to realize a high quality dichroic mirror in which the optical characteristics do not greatly change with a change in the incident angle.

[0010]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a film configuration of a dichroic mirror according to one embodiment. This is because a high refractive index film of Ti is formed on the surface of an optical glass substrate 10 such as BK7.
And O ₂ film 21, provided with a first selective transmission multilayer film 20 formed by laminating the Al ₂ O ₃ film 22 is the refractive index film in a higher refractive index than the substrate 10 alternately thereon a high refractive index film Ti
This is a dichroic mirror provided with a second permselective multilayer film 30 in which an O ₂ film 31 and a SiO ₂ film 32 as a low refractive index film are alternately stacked.

In general, the dichroic mirror has a wider reflection band and a higher reflectance as the refractive index ratio between the high refractive index film and the low refractive index film increases. _{Use of} a dielectric material such as ₂ causes deterioration of optical characteristics in a divided region between a transmission band and a reflection band, such as a shift of a half-value wavelength with an increase in an incident angle. Therefore,
A double-layer structure in which a first permselective multilayer film composed of a high refractive index film and a middle refractive index film and a second permselective multilayer film composed of a high refractive index film and a low refractive index film are combined. Thus, the above troubles are reduced. An extremely high-quality dichroic mirror that does not cause color unevenness or the like in the viewfinder due to a shift in a divided area or the like can be realized without impairing the advantages of a high reflectance and a wide reflection band.

(First Embodiment) A substrate made of optical glass of BK7 is placed in a vacuum evaporation apparatus, and the surface temperature of the substrate is set to about 300 ° C.
Then, after evacuating to a vacuum degree of 1 × 10 ⁻³ Pa or less, a TiO ₂ film having a high refractive index and an Al ₂ O ₃ film having a medium refractive index are formed at a first design center wavelength of 720 nm and an incident angle of 38 nm. A first permselective multilayer film having an 18-layer structure is formed alternately with a film thickness of λ / 4. Furthermore, a high refractive index T
An iO ₂ film and a low-refractive-index SiO ₂ film are alternately laminated to a λ / 4 film thickness at a second design center wavelength of 880 nm and an incident angle of 38 ° to form a second selective transmission multilayer film having an eight-layer structure. After forming the film, a dichroic mirror having a total of 26 layers is manufactured and taken out from the vacuum evaporation apparatus.

FIG. 2 is a graph showing the spectral transmittance of the dichroic mirror of the present embodiment at an incident angle of 38 degrees as a curve A, and the spectral transmittance at an incident angle of 48 degrees as a curve B. As can be seen from this figure, the dichroic mirror of this embodiment has a wavelength 88 on the long wavelength side at an incident angle of 38 degrees.
The half-value wavelength that reflects light having a wavelength of 0 nm and a wavelength of 690 nm and has a transmittance of 50% of the transmittance in the visible light region on the shorter wavelength side is 660 nm. When the incident angle increases from 38 degrees to 48 degrees, the shift amount of the half-value wavelength is 19 nm toward the short wavelength side, and the wavelength of 80% transmittance and the transmittance indicating the inclination of the divided region between the transmission band and the reflection band. The 20% wavelength difference was 22 nm.

(Second Embodiment) An optical glass substrate of BK7 is placed in a vacuum evaporation apparatus, and the surface temperature of the substrate is set to about 300 ° C.
After vacuum evacuation to a pressure of 1 × 10 ⁻³ Pa or less, a high refractive index TiO ₂ film and a medium refractive index Al ₂
The mixture film containing O ₃ as a main component is formed at the first design center wavelength 7
A first selective transmission multilayer film having an 18-layer structure is formed by alternately laminating 20 nm and an incident angle of 38 degrees to a λ / 4 film thickness.
Furthermore, a high refractive index TiO ₂ film and a low refractive index S
An iO ₂ film is alternately laminated to a λ / 4 film thickness at a second design center wavelength of 880 nm and an incident angle of 38 ° to form a second 8-layer film structure.
Is formed, and a dichroic mirror having a total of 26 layers is manufactured and taken out from the vacuum evaporation apparatus.

The spectral characteristics of this dichroic mirror are such that the half-value wavelength of the transmittance at an incident angle of 38 degrees is 660 nm, and the shift amount of the half-value wavelength when the incident angle increases from 38 degrees to 48 degrees is 18 nm toward the short wavelength side. The difference between the wavelength of the transmittance of 80% indicating the inclination of the divided area and the wavelength of the transmittance of 20% is 20n.
m.

(Third Embodiment) A substrate made of optical glass of BK7 is placed in a vacuum deposition apparatus, and the surface temperature of the substrate is set to about 300 ° C.
After vacuum evacuation to a pressure of 1 × 10 ⁻³ Pa or less, a high-refractive-index TiO ₂ film and a medium-refractive-index ZrO 2
_{The two} films were subjected to a first design center wavelength of 720 nm and an incident angle of 38 nm.
The first permselective multilayer film having a 20-layer structure is alternately stacked to have a film thickness of λ / 4. Further, a TiO ₂ film having a high refractive index and a SiO ₂ film having a low refractive index are alternately laminated thereon to a λ / 4 film thickness at a second design center wavelength of 880 nm and an incident angle of 38 ° to form an eight-layer film structure. Is formed, and a dichroic mirror having a total of 28 layers is manufactured and taken out from the vacuum evaporation apparatus.

The spectral characteristics of this dichroic mirror are such that the half-value wavelength of the transmittance at an incident angle of 38 degrees is 660 nm, and the shift amount of the half-value wavelength when the incident angle increases from 38 degrees to 48 degrees is 17 nm toward the shorter wavelength side. The difference between the wavelength of the transmittance of 80% indicating the inclination of the divided area and the wavelength of the transmittance of 20% is 19n.
m.

(Comparative Example) A substrate made of optical glass of BK7 was placed in a vacuum evaporation apparatus, and the surface temperature of the substrate was set to about 300 ° C.
After vacuum heating and evacuation to a pressure of 1 × 10 ⁻³ Pa or less, a high-refractive-index TiO ₂ film and a low-refractive-index SiO ₂ film were formed at a first design center wavelength of 730 nm and an incident angle of 38 °. A first permselective multilayer film having a 16-layer structure is alternately stacked to have a film thickness of λ / 4. Furthermore, a high refractive index T
An iO ₂ film and a low-refractive-index SiO ₂ film are alternately laminated to a λ / 4 film thickness at a second design center wavelength of 880 nm and an incident angle of 38 ° to form a second selective transmission multilayer film having an eight-layer structure. After forming a film, a dichroic mirror having a total of 24 layers is manufactured and then taken out from the vacuum evaporation apparatus.

The spectral characteristics of this dichroic mirror are such that the half-value wavelength of the transmittance at an incident angle of 38 degrees is 660 nm, and the shift amount of the half-value wavelength when the incident angle increases from 38 degrees to 48 degrees is 25 nm toward the short wavelength side. The difference between the wavelength of the transmittance of 80% indicating the inclination of the divided area and the wavelength of the transmittance of 20% is 32n.
m. This is because the SiO ₂ film is used as the low-refractive-index film in each of the first and second permselective multilayer films, and therefore, compared to the case where a medium-refractive-index film having a relatively high refractive index is combined. It is presumed that the shift amount was large and the inclination in the divided area became gentle.

The shift amount of the half-value wavelength at the incident angle of 38 degrees and the incident angle of 48 degrees of the dichroic mirrors of the first to third embodiments and the comparative example, and the wavelength of the transmittance of 80% indicating the inclination of the divided area and the transmittance of 20 Table 1 summarizes the wavelength differences in%.
Shown in

[0022]

[Table 1]

[0023]

Since the present invention is configured as described above, it has the following effects.

A dichroic mirror capable of reducing the shift of the half-value wavelength due to an increase in the incident angle of light and maintaining a steep spectral characteristic in a divided region between a transmission band and a reflection band can be realized. As described above, by using a high-quality dichroic mirror in which spectral characteristics are hardly degraded due to a change in the incident angle, it is possible to greatly contribute to high performance of various optical devices.

[Brief description of the drawings]

FIG. 1 is a diagram showing a film configuration of a dichroic mirror according to an embodiment.

FIG. 2 is a diagram illustrating spectral characteristics of transmittance of a dichroic mirror according to the first embodiment.

[Explanation of symbols]

10 substrate 20 first selectively permeable multilayer film 21 and 31 TiO ₂ film 22 Al ₂ O ₃ film 30 and the second selective transmitting multilayer film 32 SiO ₂ film

Claims (3)

[Claims] 1. A first selective transmission multilayer film in which a transparent substrate and high-refractive-index films and medium-refractive-index films each having an optical film thickness equal to １ ／ of the first design center wavelength are alternately laminated. And a dichroic mirror having a second selective transmission multilayer film in which high-refractive-index films and low-refractive-index films each having an optical thickness equal to １ ／ of the second design center wavelength are alternately stacked. 2. The dichroic mirror according to claim 1, wherein the first design center wavelength is a wavelength on the transmission band side as compared with the second design center wavelength. 3. The medium refractive index film according to claim 1, wherein the medium refractive index film is made of a material selected from dielectric materials including Al ₂ O ₃ , ZrO ₂ and a mixture thereof. Dichroic mirror.