JP3654388B2 - Optical light attenuation filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical light attenuating filter such as a so-called ND filter used in general optical equipment.
[0002]
[Prior art]
In various optical devices, a light attenuating filter used for attenuating light for adjusting the amount of light is known to be made of a plate material of ND glass or ND plastic (hereinafter referred to as ND glass) that absorbs light. Yes. Ideally, such an optical attenuation filter has a uniform transmittance over the entire wavelength range of visible light. However, the above-described ND glass has a larger transmittance on the long wavelength side than on the short wavelength side. Therefore, an image obtained by a video camera or the like using a light attenuation filter is entirely reddish. Moreover, since the materials such as ND glass are limited, the degree of freedom in changing the transmittance is greatly limited.
[0003]
On the other hand, as another type of light attenuating filter, a transparent glass surface is coated with a light attenuating film such as a metal film such as chromium or inconel or a dielectric film, and this light attenuating film absorbs part of the light and attenuates it. Things are known. Such an optical attenuating filter using an optical attenuating film has the following major problems although the degree of freedom in changing the light transmittance is greater than that of an optical attenuating filter made of ND glass or the like.
[0004]
[Problems to be solved by the invention]
FIG. 9 is a diagram illustrating a first example of an optical system using an optical attenuation filter. As shown in FIG. 9, the first example of the optical system using the light attenuating filter is the imaging optical system having the imaging lens 21, the ND filter 22 coated with the light attenuating film 22A on one side, and the CCD 23 from the previous stage. 24 is arranged. The imaging optical system 24 is provided with a CCD cover glass 25 for protecting the CCD 23. The light that has passed through the imaging lens 21 is attenuated by the ND filter 22, passes through the CCD cover glass 25, and forms an image on the imaging surface of the CCD 23.
[0005]
FIG. 10 is a diagram illustrating a second example of an optical system using an optical attenuation filter. As shown in FIG. 10, the second example of the optical system using the light attenuating filter is an ND filter 32 having an optical attenuating film 32A, 32B coated on both surfaces, an imaging lens 31, and an imaging surface such as a CCD from the previous stage. 33 is arranged. Then, the light representing the image of the subject 35 is attenuated by the ND filter 32 and is imaged by the imaging lens 31 at a point P10 on the imaging surface 33.
[0006]
FIG. 11 is a diagram illustrating a third example of an optical system using an optical attenuation filter. As shown in FIG. 11, the third example of the optical system using the light attenuating filter is that the imaging lens 41, the ND filter 42 coated with the light attenuating films 42A and 42B on both sides, and the imaging surface such as a CCD from the previous stage. 43 is arranged. The light transmitted through the imaging lens 41 is attenuated by the ND filter 42 and forms an image at a point P12 on the imaging surface 43.
[0007]
However, in the optical systems shown in FIGS. 9 to 11 described above, although the light can be attenuated by the ND filters 22, 32, 42, the light attenuation film coated on the ND filters 22, 32, 42 There is a problem that reflected light is generated. For example, in the first example, a part of the light transmitted through the ND filter 22 is reflected on the CCD cover glass 25, and the reflected light L10 is further reflected on the surface coated with the light attenuation film 22A of the ND filter 22. The light enters the CCD 23 at a position different from the original position or a position deviated from the CCD 23.
[0008]
Further, in the second example, a part of the light incident on the ND filter 32 undergoes multiple reflection on both surfaces of the ND filter 32, and the multiple reflected light L11 is at a position P10 'different from the point P10 on the image plane 33. Will form an image.
Further, in the third example, as in the second example, part of the light incident on the ND filter 42 undergoes multiple reflections on both sides of the ND filter 42, and this multiple reflected light L 12 is a point on the image plane 43. An image is formed at a point P12 'in front of P12.
[0009]
As described above, the reflected light or the multiple reflected lights L10 to L12 reflected by the ND filters 22, 32 and 42 form an image at a position different from the original image formation point. Flares and ghosts occur in an image obtained when reproduced as a visual image, and the image quality deteriorates.
[0010]
The present invention has been made in view of such circumstances, can obtain a good image without causing flare and ghost due to reflected light, and has a uniform transmittance characteristic over the entire visible light region. An object of the present invention is to provide an optical light attenuating filter having a high degree of design freedom when changing the transmittance.
[0011]
[Means for Solving the Problems]
The optical light attenuating filter according to the present invention is formed by coating a light attenuating film on both surfaces of a light attenuating substrate made of ND glass or ND plastic that absorbs light, and the light attenuating film is a metal film. The light transmission characteristics of the attenuating film are added to the light transmission characteristics of the light attenuation substrate to form a substantially uniform light transmission characteristic over the entire visible light region as a whole.
[0012]
The metal film is preferably made of chromium or inconel.
[0013]
The optical light attenuating filter according to the present invention is formed by coating a light attenuating film on both surfaces of a light attenuating substrate made of ND glass or ND plastic that absorbs light, and the light attenuating film comprises at least two layers. A dielectric film and at least one metal film are alternately laminated, and the light transmission characteristic of the light attenuation film is added to the light transmission characteristic of the light attenuation substrate, so that visible light as a whole is formed. It has a substantially uniform light transmission characteristic over the entire area.
The light attenuation substrate and the light attenuation film both attenuate light by the light absorption action.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0015]
FIG. 1 is a cross-sectional view showing a configuration of an optical light attenuation filter according to the first embodiment of the present invention. As shown in FIG. 1, an optical light attenuation filter (hereinafter referred to as an ND filter) 1 according to a first embodiment of the present invention is an ND made of a metal film on both surfaces of an absorption ND filter plate 2 made of an ND glass plate or the like. It is configured by coating the coating films 3A and 3B. In the present embodiment, HOYA ND25 having a thickness of 2 mm is used as the absorbing ND filter plate 2, and chromium is used as the material of the ND coating films 3A and 3B.
As a method for forming the ND coat films 3A and 3B, various known methods such as vapor deposition and sputtering can be used.
[0016]
Here, the characteristics of the transmittance of the ND 25 with respect to the wavelength of visible light are shown in FIG. As shown in FIG. 2, the ND 25 absorbs and attenuates transmitted visible light to approximately 40% over the entire wavelength range, but the transmittance on the long wavelength side is slightly larger than that on the short wavelength side. It has characteristics. On the other hand, the transmittance characteristics of the ND coat films 3A and 3B with respect to the visible light wavelength are shown in FIG. In addition, the transmittance | permeability in FIG. 3 is the transmittance | permeability of the visible light of ND filter comprised by coating ND coat film | membrane 3A, 3B on the transparent glass plate whose transmittance | permeability is 100% over all visible light wavelength ranges. In this embodiment, the transmittance is approximately 15%. As shown in FIG. 3, the transmittance characteristics of the ND coat films 3A and 3B are slightly larger on the short wavelength side than on the long wavelength side.
[0017]
In this embodiment, the ND coating films 3A and 3B having the transmittance characteristics shown in FIG. 3 are coated on the absorption ND filter plate 2 having the transmittance characteristics shown in FIG. 2, as shown in FIG. Thus, the ND filter 1 having a transmittance characteristic of approximately 6.3% over the entire wavelength range of visible light can be obtained. That is, the absorption ND filter plate 2 made of ND25 has a characteristic that the transmittance on the long wavelength side is slightly larger than that on the short wavelength side, and the ND coating films 3A and 3B have a short wavelength side compared with the long wavelength side. Since it has a slightly larger characteristic, both characteristics cancel each other, and the ND filter 1 having a substantially uniform transmittance over the entire wavelength range of visible light can be obtained.
[0018]
The multiple reflectance characteristics of such an ND filter 1 are shown in FIG. In FIG. 5, a broken line indicates the multiple reflectance characteristic of the ND filter 1 according to the present embodiment, and a solid line indicates an ND coating film 3A having the transmittance characteristic shown in FIG. 3B shows the multiple reflectance characteristics of an ND filter coated with 3B (hereinafter referred to as a conventional ND filter). Here, the multiple reflectance in FIG. 5 is the ratio of the multiple reflected light to the incident light, and as shown in FIG. 6, both surfaces of the ND filter 1 with respect to the light L1 obliquely incident on the ND filter 1. The ratio of the light L2 indicated by a broken line emitted by multiple reflection in FIG.
[0019]
As shown in FIG. 5, the multiple reflectance of the conventional ND filter is approximately 0.04%, whereas the multiple reflectance of the ND filter 1 according to the present embodiment is 0.0025%. Compared with the ND filter, the multiple reflected light is approximately 1/16. That is, by coating the absorbing ND filter plate 2 made of ND25 with the ND coating films 3A and 3B, the transmittance is about 1 / 2.5 (compared to the transparent glass coated with the ND coating films 3A and 3B). = 6.3 / 15), but the multiple reflectance is 1/16. That is, although the transmittance is reduced, the multiple reflectance is 16 times smaller than that of the conventional ND filter. Therefore, compared with the conventional ND filter, the multiple reflectance can be greatly reduced, thereby preventing the occurrence of flare and ghost caused by the multiple reflected light and obtaining a good image.
[0020]
In the first embodiment, the absorption ND filter plate 2 is coated with the ND coating films 3A and 3B made of chromium. Alternatively, a dielectric film may be coated.
[0021]
Next, a second embodiment of the present invention will be described. FIG. 7 is a cross-sectional view showing a configuration of an optical light attenuating filter according to the second embodiment of the present invention. As shown in FIG. 7, an optical light attenuation filter (hereinafter referred to as an ND filter) 11 according to a second embodiment of the present invention coats dielectric films 14A and 14B on both surfaces of an absorption ND filter plate 12, and It is configured by coating ND coat films 13A and 13B on top and further coating dielectric films 15A and 15B. As described above, the absorption ND filter plate 12 is coated with only the ND coat films 13A and 13B by applying the laminated coating composed of the dielectric films 14A, 14B, 15A and 15B and the ND coat films 13A and 13B. In comparison, the reflectance on both surfaces of the absorbing ND filter plate 12 can be reduced. Therefore, multiple reflected light can be reduced, and flare and ghost caused by the multiple reflected light can be further reduced to obtain a better image.
[0022]
FIG. 8 is a graph showing a comparison of the ratio of multiple reflected light to incident light between the second embodiment and the prior art. In FIG. 8, the broken line indicates the multiple reflectance characteristic of the ND filter 11 according to this embodiment, and the solid line indicates the multiple reflectance characteristic of the conventional ND filter. That is, according to the second embodiment, the multiple reflectivity is 1/20 which is significantly smaller than that obtained by coating the transparent glass with the ND coating films 3A and 3B. The light transmittance of the laminated coating film itself is substantially the same as that of the ND coat films 3A and 3B of the first embodiment.
[0023]
Furthermore, in the above embodiment, chromium is used as the metal material for forming the ND coat film, but other metal materials such as Inconel can also be used.
In the above embodiment, both sides of the absorption ND filter plate 2 are coated with an ND coat film, a dielectric film, or a laminate of an ND coat film and a dielectric film. Only these may be coated.
[0024]
【The invention's effect】
As described above in detail, according to the optical light attenuating filter of the present invention, multiple light reflections can be greatly reduced by coating the light attenuating film on one or both surfaces of the light attenuating substrate. Thereby, in the image obtained by the optical system using the optical light attenuation filter of the present invention, the occurrence of flare and ghost can be greatly reduced, and a good image can be obtained. In addition, by adding the light transmission characteristics of the light attenuation substrate and the light transmission characteristics of the light attenuation film, the fluctuations of both with respect to the wavelength are canceled out to have uniform light transmission characteristics over the entire visible light region. be able to. Furthermore, it is possible to set a desired transmittance by changing the coating thickness of the light attenuating film.
Moreover, the light attenuating film is formed by alternately laminating at least two dielectric films and at least one metal film, thereby reducing the light reflectance on the surface of the light attenuating substrate. Thus, the multiple reflected light can be further reduced.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration of an ND filter according to a first embodiment of the present invention. FIG. 2 is a graph showing transmittance characteristics of ND25. FIG. 3 is a graph showing transmittance characteristics of an ND coating film. FIG. 5 is a graph showing the transmittance characteristics of the ND filter according to the first embodiment of the present invention. FIG. 5 is a graph showing a comparison of the ratio of multiple reflected light to incident light in the first embodiment and the prior art. 6 is a diagram showing incident light incident on an ND filter and multiple reflected light in the ND filter. FIG. 7 is a cross-sectional view showing a configuration of an ND filter according to a second embodiment of the invention. FIG. 9 is a graph showing a comparison of the ratio of multiple reflected light to incident light in the embodiment and the prior art. FIG. 9 is a diagram showing a first example of an optical system using an ND filter. FIG. 11 is a diagram showing a second example of the optical system used. Diagram illustrating a third example of an optical system using the filter [Description of symbols]
1, 11 ND filter 2, 12 Absorption ND filter plate 3A, 3B, 13A, 13B ND coat film 14A, 14B, 15A, 15B Dielectric film

Claims (3)

In the optical light attenuation filter,
A light attenuating film is coated on both sides of a light attenuating substrate made of ND glass or ND plastic plate that absorbs light,
The light attenuating film is a metal film;
The light transmission characteristic of the light attenuating film is added to the light transmission characteristic of the light attenuation substrate to form a substantially uniform light transmission characteristic over the entire visible light region as a whole. filter.   2. The optical light attenuation filter according to claim 1, wherein the metal film is made of chromium or inconel. In the optical light attenuation filter,
A light attenuating film is coated on both sides of a light attenuating substrate made of ND glass or ND plastic plate that absorbs light,
The light attenuating film is formed by alternately laminating at least two dielectric films and at least one metal film,
The light transmission characteristic of the light attenuating film is added to the light transmission characteristic of the light attenuation substrate to form a substantially uniform light transmission characteristic over the entire visible light region as a whole. filter.

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