JPH0261004B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to an antireflection film provided on the surface of an optical element such as an optical lens to improve its transmittance. 2. Description of the Related Art In recent years, various automatic focus detection devices for photographic lenses have been proposed in the fields of still cameras and video cameras. In particular, one of the promising methods for automatic focus detection devices applied to video cameras is to project near-infrared light as a distance measurement signal onto the subject to be photographed (hereinafter referred to as the subject), and then reflect it from the subject. There is a method of receiving the distance measurement signal and driving the photographing lens to an appropriate focusing position based on the received distance measurement signal. The above-mentioned near-infrared light is often projected or received through a photographic lens, but since the amount of near-infrared light greatly affects the distance and accuracy of distance measurement, the photographic lens has a It is necessary to form an antireflection film. On the other hand, conventional photographic lenses use antireflection coatings with a number of layers ranging from a single layer to about 4 layers, and are used in the vicinity of a single wavelength or in the visible wavelength range (and 400 to 600 nm,
Approximately 400nm to 700nm)
Many of them have an anti-reflection effect.
In addition, although the details of the structure are not clear, it is expected that more multilayer films (more than 10 layers) will be used to provide antireflection effects from the visible wavelength range to the near-infrared wavelength range (approximately 400 to 1000 nm). There are also examples where this has been done. (For example, "Optical Thin Films" (1985) edited by Shiro Fujiwara)
Pages 98-110, "Optronics" (1984) No.
6, pp. 44-45) Problems to be Solved by the Invention However, with the above-mentioned configuration, that is, with an anti-reflection coating of about one to four layers, it is difficult to solve the problem for the photographing lens used in the automatic focus detection method. The effective wavelength range is insufficient for anti-reflection coatings, and in particular, the reflectance of multilayer anti-reflection coatings increases rapidly on both sides of the effective wavelength range, making it difficult to use near-infrared light for distance measurement. It is not suitable as an antireflection film for In addition, in order to provide anti-reflection effects from the visible wavelength range to the near-infrared wavelength range, more multilayer films (10
However, it is expected that the manufacturing cost will be high and the manufacturing yield will be poor, and this is also not suitable for productivity. In view of the above-mentioned problems, the present invention provides an antireflection film that is effective in preventing reflection of visible light and near-infrared light for distance measurement and has excellent productivity. Means for Solving the Problems In order to solve the above problems, the antireflection film of the present invention has a 6-layer structure for a glass substrate with a refractive index (nd) of 1.58 to 1.62 for the d-line. The first layer, third layer, and fifth layer have refractive indexes in order toward the substrate side.
Consisting of a low refractive index material in the range of 1.35 to 1.40,
In addition, the second layer, fourth layer and sixth layer have a refractive index of 1.90~
It is made of a material with a high refractive index in the range of 2.40, and the optical thickness of each layer satisfies the following conditions. n ₁ d ₁ = (0.2758±0.0138) λ ₀ n ₂ d ₂ = (0.1069 ± 0.0107) λ ₀ n ₃ d ₃ = (0.0468 ± 0.0047) λ ₀ n ₄ d ₄ = (0.4969 ± 0.0248) λ ₀ n ₅ d ₅ = (0.0461 ± 0.0046) λ ₀ n ₆ d ₆ = (0.0895 ± 0.0090) λ ₀ However, λ ₀ ; design wavelength effect in the visible wavelength range. ~600nm) and near-infrared wavelength range (840~900nm)
Provides effective anti-reflection for light of wavelengths
The present invention provides an antireflection film suitable for a focus detection method that uses near-infrared light around 880 nm. Example The following is an example of an antireflection film of the present invention.
This will be explained with reference to the drawings. FIG. 1 shows the structure of the antireflection film of the present invention. In FIG. 1, 1 is a glass substrate,
The anti-reflection film is composed of six layers, in order from the air side to the substrate 1 side: a first layer 11, a second layer 12, a third layer 13, a fourth layer 14, a fifth layer 15, and a sixth layer 16. The constituent materials and optical film thicknesses are shown in Table 1.

[Table] Figure 2 is a diagram showing the spectral reflectance characteristics, and curve a is the optical film thickness λ ₀ /4 (λ ₀ = 520 nm).
The MgF ₂ single layer film is formed on the surface of the glass substrate 1, and the curve b is for this example. From Figure 2, the anti-reflection film of this example has approximately 400
It can be seen that it has a good antireflection effect in the visible wavelength range of ~600 nm and the near-infrared wavelength range of approximately 820 to 920 nm. In particular, the average reflectance in the visible wavelength range is smaller than that of a single MgF ₂ film, making it a good antireflection film. In addition, in the above embodiment, the first layer 11, the third layer
The high refractive index material forming the layer 13 and the fifth layer 15 is
Although ZrTiO ₄ was used, the high refractive index material was ZrO ₂ ,
It may be TiO ₂ or a mixture of at least two of the above substances. Effects of the Invention As described above, the present invention provides a six-layer antireflection film on a glass substrate with a refractive index (nd) for d-line of 1.58 to 1.62, and the first layer, the third layer, and the like from the air side to the substrate side. and the fifth layer is made of a low refractive index material with a refractive index in the range of 1.35 to 1.40;
The layers are made of a high refractive index material with a refractive index in the range of 1.90 to 2.40, and the optical thickness of each layer satisfies the following conditions to ensure good antireflection against visible light and near-infrared light. can be done. n ₁ d ₁ = (0.2758±0.0138) λ ₀ n ₂ d ₂ = (0.1069 ± 0.0107) λ ₀ n ₃ d ₃ = (0.0468 ± 0.0047) λ ₀ n ₄ d ₄ = (0.4969 ± 0.0248) λ ₀ n ₅ d ₅ = (0.0461±0.0046) λ ₀ n ₆ d ₆ = (0.0895 ± 0.0090) λ ₀ λ ₀ ; Design wavelength in the visible wavelength range In addition, the antireflection film of the present invention has excellent productivity. Its practical value is extremely great.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of the antireflection film of the present invention,
FIG. 2 is a diagram showing spectral reflectance characteristics. 1... Glass substrate, 11... First layer, 12...
2nd layer, 13...3rd layer, 14...4th layer, 15
...Fifth layer, 16...Sixth layer, a...Curve showing the spectral reflectance characteristics of the MgF ₂ single layer film, b...Curve showing the spectral reflectance characteristics of the example of the present invention.

Claims (1)

[Scope of Claims] 1. An anti-reflection film formed on the surface of a glass substrate having a refractive index (nd) for the d-line in the range of 1.58 to 1.62, the anti-reflection film having a six-layer structure, From the substrate side, the first, third and fifth layers are made of a low refractive index material with a refractive index in the range of 1.35 to 1.40, and the second, fourth and sixth layers have a refractive index of 1.90 to 1.90.
2.40, and the optical thickness of each layer is as follows: n ₁ d ₁ = (0.2758±0.0138) λ ₀ n ₂ d ₂ = (0.1069 ± 0.0107) λ ₀ n ₃ d ₃ = (0.0468±0.0047)λ ₀ n ₄ d ₄ = (0.4969±0.0248) λ ₀ n ₅ d ₅ = (0.0461±0.0046) λ ₀ n ₆ d ₆ = (0.0895±0.0090) λ _0However , here, ni: refractive index of constituent material of i-th layer (i=1,2,...,6) nidi: optical thickness of constituent material of i-th layer (i=1,2,...,6) _λ0 : visible An anti-reflection film that satisfies the design wavelength in the wavelength range. 2. The antireflection film according to claim 1, wherein the low refractive index substance constituting the first layer, third layer, and fifth layer is _MgF2 . 3. Claim 1, characterized in that the high refractive index material constituting the second layer, fourth layer and sixth layer is ZrTiO ₄ , ZrO ₂ , TiO ₂ or a mixture of at least two of the above substances. Anti-reflective coating as described in section.