JPS6145202B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a dichroic film that has an appropriate half-width, has a simple film structure, and is easy to manufacture. The blue reflective dichroic film of a three-color separation optical system usually has a film thickness ratio of high refractive index layer H and low refractive index layer L of 3:
1 (hereinafter abbreviated as H:L=3:1) is often used. In Figure 1, the basic film thickness configuration is H:L=3:1.
This is a diagram showing the transmittance characteristics of an example of a blue-reflecting dichroic film.As shown in the film structure in Table 1, the first layer, second layer, and final layer are arranged from the substrate side to suppress ripples. The film thickness has changed from the basic thickness. The basic configuration is H:L=3:1.

【table】

【table】

[Table] Dichroic films have the disadvantage that they cannot have a sufficient half-width. For the purpose of widening the half-width of the dichroic film, H:L=3:1 configuration and H:L=
A method of widening the half-width by mixing 1:1 configurations is shown in Japanese Patent Application No. 71896/1984 (Japanese Patent Application Laid-Open No. 7359/1989). FIG. 2 shows a transmission characteristic curve of an example of a blue reflective dichroic film whose half-width is expanded by this method, and its structure is shown in Table 2.
With this method, it is possible to expand the half-width, but in order to remove ripples, it is necessary to provide a layer with a film thickness for suppressing ripples in the intermediate layer, which requires advanced manufacturing technology. . The present invention aims to eliminate the above-mentioned drawbacks and provides a dichroic film which has an enlarged half-width, has a simple film structure, and is easy to manufacture. The dichroic film according to the present invention is composed of alternating layers of high refractive index layers and low refractive index layers, and when the reference wavelength is λ ₀ , the basic film thickness of the high refractive index layer is λ ₀ /4. The above object is achieved by setting the basic thickness of the low refractive index layer to 3/ _4λ0 . The present invention will be explained in detail below. Figure 3 shows H:
This is a diagram showing the transmittance characteristics of the L=3:1 configuration and the H:L=1:3 configuration, where the dotted line indicates the H:L=3:1 configuration (the third
Table), and the solid line is for the H:L=1:3 configuration (Table 4). In the case of normal incidence with an incident angle of zero, these characteristic curves would match if there was no dispersion, but if dispersion is taken into account, as is clear from Fig. 3, the basics of H:L = 1:3 according to the present invention This configuration has a wider half-width.

【table】

【table】

[Table] This is because high refractive index materials generally have dispersion, so their refractive index changes on the short wavelength side, and the optical film thickness changes accordingly. Therefore, the effect of enlarging the half-width is particularly remarkable in a blue reflective dichroic film having a reflection band on the short wavelength side. As described above, in the present invention, when the reference wavelength is λ ₀ , the first layer from the substrate side starts with a high refractive index layer, the film thickness of the high refractive index layer is λ ₀ /4, and the film thickness of the low refractive index layer is 3λ ₀ /
4, the half width is expanded compared to the H:L=3:1 configuration, and H:L=3:1,
This constitutes a dichroic film that is easier to manufacture than a 1:1 mixed film structure. All commonly used high refractive index vapor deposition materials have dispersion and can be used in the present invention.
For example, high refractive index materials include ZrO ₂ , TiO ₂ ,
Ta ₂ O ₅ , CeO ₂ , ZnS, ThO ₂ and mixtures thereof; low refractive index materials include MgF ₂ , SiO ₂ ,
ThF ₄ , Na ₃ AlF ₆ and mixtures thereof can be used. Figure 4 shows an example of the present invention, in which CeO ₂ was used as the high refractive index material and MgF ₂ was used as the low refractive index material.
This is a diagram showing the transmittance characteristics of a 10-layer dichroic film, and the film configuration at that time is shown in Table 5. In the H:L=1:3 configuration shown in FIG. 4, ripples can be easily suppressed by shifting a total of four layers, two layers on the substrate side and two layers on the outer medium side, from the basic film thickness. FIG. 4 shows ripple suppression performed using this method. Comparing Figure 4 with the characteristic diagram for the H:L=3:1 configuration in Figure 1, the half-width is clearly expanded, and the H:L=3:1, 1:1 configuration in Figure 2 is clearly expanded. Compared to mixed membranes, the half-width is expanded to almost the same extent, yet the membrane configuration is simplified and regular, and fabrication is almost as easy as the conventional one.

【table】

[Table] The ripple suppressing film is not limited to the values shown in Table 5, but there is a range where it is practically acceptable even if the values are slightly changed. For example, considering the first and second layers from the substrate side, if the first layer becomes thinner, make the second layer thicker, and conversely, if the first layer becomes thicker, make the second layer thinner. By doing so, the influence of changes in film thickness can be reduced. That is, the sum of the film thicknesses of the first and second layers from the substrate side is between 3.15/4λ ₀ and 3.85/4λ ₀ , and the optical thickness of the first layer is 0.25. /4λ ₀ to 0.5/4λ ₀
If it exists between the two, ripple suppression can be achieved without causing any practical problems. Similarly, for the ripple suppression film on the outer medium side, if the total number of layers is n, the sum of the film thicknesses of the n-1st layer and the nth layer is between 4.4/4λ ₀ and 5.5/4λ ₀ . exists, and the first (
n-1) The thickness of the layer may be between 0.7/4λ ₀ and λ ₀ . FIG. 5 shows transmittance characteristic curves of Examples corresponding to the upper and lower limits of the film thickness, and Table 6 shows the film configuration at that time. All of the characteristic examples listed in this explanation use CeO ₂ as a high refractive index material and MgF ₂ as a low refractive index material, and the refractive index is CeO ₂ according to the dispersion formula n (λ) = 2.2 +
11.1/λ-367, and n=1.38 for _MgF2 . As detailed above, the dichroic film according to the present invention is composed of an alternating multilayer film of high refractive index layers and low refractive index layers, and the first layer from the substrate starts with the high refractive index layer,
The optical thickness of the layer that should be the basis of the high refractive index layer is λ
₀ /4, the optical thickness of the layer that should be the basis of the low refractive index layer is 3λ ₀ /4, and ripples can be easily suppressed by shifting the two layers on the substrate side and the two layers on the external medium side from the basic thickness. It has the excellent effect of increasing the half-width, having a simple structure, and being easy to manufacture as before.

[Brief explanation of the drawing]

Fig. 1 is a transmittance characteristic diagram of an example of a conventional H:L=3:1 configuration, and Fig. 2 is a transmittance characteristic diagram of an example of a conventional H:L=3:1 configuration.
FIG. 3 is a diagram for explaining the effect of the H:L=1:3 configuration according to the present invention, and FIGS. 4 and 5 are diagrams for explaining the effect of the H:L=1:3 configuration according to the present invention. FIG. 3 is a diagram showing the transmittance characteristics of an example of such a dichroic film. T...Transmittance, λ...Wavelength.

Claims (1)

[Claims] 1. A dichroic film consisting of n alternating layers of high refractive index layers and low refractive index layers has a reflection band mainly for blue, and when the reference wavelength is λ ₀ , The basic film thickness of the high refractive index layer is λ ₀ /4, the basic film thickness of the low refractive index layer is 3/4λ ₀ , and the first layer, the second layer, and the (n-1)th layer counting from the substrate side. layer,
The n-th layer is changed from the above basic thickness, and the sum of the optical thicknesses of the first layer and the second layer is between 3.15/4λ ₀ and 3.85/4λ ₀ , and the The sum of the optical film thicknesses of the n-1) layer and the n-th layer is 4.4/4λ ₀ to 5.
A dichroic film characterized by existing between 5/ _4λ0 . 2 The optical thickness of the first layer is from 0.25/4λ ₀
0.5/4λ ₀ , and the optical thickness of the (n-1)th layer is between 0.7/4λ ₀ and λ ₀ , as set forth in claim 1. dichroic membrane.