JP2008180844A - Absorption type multilayer film one side nd filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an absorption type multilayer film one side ND (neutral density) filter which is less influenced with reflection light and has excellent mass productivity. <P>SOLUTION: The absorption type multilayer film one side ND filter is constituted by providing an absorption type multilayer film for attenuating transmitted light on incident side one surface of a substrate consisting of a resin film. The absorption type multilayer film is constituted of a multilayer film of four layers or more obtained by alternately laminating a dielectric film layer and a metal film layer. An evaluation function is used as a film thickness optimizing technique for increasing/decreasing film thickness of each layer in the absorption type multilayer film to approach an aiming spectral optical characteristic, film thickness of each layer in the absorption type multilayer film is also selected so that the evaluation function may become the minimum value, and further a value of 'incident side reflectivity' and a value of 'transmittance × emission side reflectivity' are almost equal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an absorptive multilayer film single-sided ND filter in which an absorptive multilayer film for attenuating transmitted light is provided on one side of an incident side of a substrate made of a resin film. In particular, the present invention is less affected by reflected light and excellent in mass productivity. Further, the present invention relates to an improvement of an absorption type multilayer single-sided ND filter.

  As this type of ND (Neutral Density Filter) filter, a reflection type ND filter that reflects and attenuates incident light and an absorption type ND filter that absorbs and attenuates incident light are known. When an ND filter is incorporated in a lens optical system in which reflected light is a problem, an absorption ND filter is generally used. In this absorption ND filter, an absorbing substance is mixed into the substrate itself (color glass ND). There are two types: a filter) type to be applied and a type in which a thin film formed on the surface of the substrate has no absorption and absorption. In the latter case, in order to prevent reflection on the surface of the thin film, the thin film is formed of a multilayer film (absorption type multilayer film), and has a function of attenuating transmitted light and an antireflection effect. This is because, even in an ND filter for the purpose of attenuating incident light, if there is reflection on the surface, the reflected light causes undesirably causing flare or image blur.

  By the way, the absorption multilayer ND filter used in a small and thin digital camera has a small installation space, so the substrate itself needs to be thinned. A very thin glass plate, resin plate or resin film is applied as the substrate. ing.

  Conventionally, as an absorptive multilayer ND filter in which the thin film is composed of a multilayer film, Patent Document 1 proposes a multilayer film in which a dielectric film layer and two or more types of metal oxide film layers having absorption are combined. Patent Document 2 proposes a multilayer film in which a dielectric film layer and a niobium film layer are combined, and Patent Document 3 proposes a multilayer film in which a dielectric layer film and a nickel film layer are combined. In these ND filters, there are a type in which the absorption multilayer film is formed on one side of the substrate and a type in which the absorption multilayer film is formed on both sides of the substrate.

  By the way, the type in which the absorption multilayer film is formed on both surfaces of the substrate has both the function of absorbing and attenuating incident light and the antireflection function on both surfaces of the substrate, but the absorption multilayer film is formed on one surface of the substrate. In the type to be coated, it is difficult to reduce the back surface reflection of the substrate in the same manner as the substrate incident side because there is no absorption multilayer film on the back surface side of the substrate.

  Therefore, in Patent Document 4, in consideration of the antireflection effect on the incident side and the back side (outgoing side) of the substrate, between the multilayer film composed of the dielectric film layer and the metal oxide film layer and the substrate. In addition, a complicated single-sided ND filter has been proposed in which a dielectric layer having a high refractive index made of a different material is interposed. However, even if such a complicated film configuration is adopted, it is difficult to set the reflectivity on the substrate entrance side and the substrate exit side to the same level, and if the reflectivity on the substrate exit side is set low, the substrate entrance There is a problem that the reflectance on the side becomes relatively high.

However, an absorption type multilayer film single-sided ND filter in which the absorption type multilayer film is formed on one side of the substrate can shorten the manufacturing time compared to an ND filter in which the absorption type multilayer film is formed on both sides of the substrate. Therefore, the manufacturing cost can be reduced. For this reason, the development of an absorption multilayer single-sided ND filter that can reduce the back surface reflection of the substrate is desired.
Japanese Patent No. 3359114 JP 2002-350610 A JP 2006-058854 A JP 2003-344612 A

  The present invention has been made paying attention to the above-mentioned problems, and the problem is that a resin film is used as a substrate, flat transmittance attenuation is obtained in the visible light region, and the influence of reflected light is small and mass production is possible. Another object of the present invention is to provide an absorption-type multilayer single-sided ND filter that is excellent in performance.

  Therefore, the present inventor conducted intensive experiments to solve the above-mentioned problems and conducted technical studies as described below, whereby an absorption-type multilayer double-sided ND filter in which an absorption-type multilayer film was formed on both sides of the substrate. As a result, the inventors have found an absorption multilayer single-sided ND filter having a function comparable to that of ND filters.

  First, since the absorption-type multilayer film single-sided ND filter has the absorption-type multilayer film formed only on one side of the incident side, it is necessary to adjust the reflectance on the substrate emission side to be low in order to suppress the back surface reflection of the substrate. . In this case, a film configuration that reduces the reflectivity on the substrate incident side as well as the reflectivity on the substrate exit side of the absorption multilayer film formed on one surface of the substrate can be considered, but as described in Patent Document 4 described above There is a problem that the film structure becomes very complicated and the reflectance on the substrate incident side becomes relatively high when the reflectance on the substrate exit side is set low.

  Under such a technical background, the present inventor pays attention to the light absorption function of the absorption film in the absorption multilayer single-sided ND filter, and there is a method capable of reducing the above-mentioned back reflection by a different concept from the transparent film. I tried an original technical study.

  That is, in the case of a filter in which a transparent film is formed on a substrate such as a resin film, the amount of emitted light Iout that passes through the transparent film and exits to the back side of the substrate is understood from the schematic diagram shown in FIG. Thus, (the amount of emitted light Iout) = (the amount of incident light Iin) × (the transmittance Tout). In this case, since the emitted light is hardly attenuated by the transparent film, the (emitted light amount Iout) is almost the same as the (incident light amount Iin). Next, when the emitted light is reflected (100%) by the reflector existing on the substrate emitting side and is incident on the filter again, the reflected light quantity shown in FIG. 1B is reflected at the interface between the substrate and the transparent film. Iout ref is (reflected light amount Iout ref) = (emitted light amount Iout) × (exit-side reflectance Rout). Here, since the (emitted light amount Iout) is almost the same as the (incident light amount Iin), (emitted-side reflectance Rout) is not the same as (incident-side reflectance Rin) shown in FIG. In this case, the (reflected light amount Iout ref) on the emission side is higher than the (reflected light amount Iin ref) on the incident side shown in FIG. 1B, which causes the above-described flare and image blurring. . That is, the above (exit-side reflectance Rout) is a factor that increases stray light, and thus needs to be adjusted to the same level as (incident-side reflectance Rin). However, since there is no solution to reduce both reflectances to 3% or less at the same time by optimizing the film thickness with a practical 4-6 layer film structure, both incident side reflectance and outgoing side reflectance are reduced simultaneously. It is difficult.

  On the other hand, in the case of a filter in which an absorption film is formed on a substrate such as a resin film, the amount of emitted light Iout that passes through the absorption film and exits to the back side of the substrate is understood from the schematic diagram shown in FIG. Thus, (the amount of emitted light Iout) = (the amount of incident light Iin) × (the transmittance Tout). In this case, since the emitted light is attenuated by the light absorption function of the absorbing film, the (emitted light amount Iout) is considerably smaller than the (incident light amount Iin). Next, when the outgoing light is reflected (100%) by the reflector existing on the substrate outgoing side and enters the filter again, the reflected light amount Iout ref shown in FIG. 1A reflected at the interface between the substrate and the transparent film is reflected. (Reflected light amount Iout ref) = (emitted light amount Iout) × (emitted-side reflectance Rout). Here, since the (emitted light amount Iout) is considerably smaller than the (incident light amount Iin) as described above, the (emitted side reflectance Rout) is (incident side reflectance Rin) shown in FIG. Even if they are not the same (incidence-side reflectance Rin), the (reflected light amount Iout ref) on the exit side is greatly different from the (reflected light amount Iin ref) on the incident side shown in FIG. Does not occur. For example, even if (incident side reflectivity Rin) is 2% and (exit side reflectivity Rout) is 20%, if (transmittance Tout) is 10%, the above (exit amount Iout) is described above. = (Incident light amount Iin) × (transmittance Tout) and (reflected light amount Iout ref) = (emitted light amount Iout) × (exit-side reflectance Rout), (reflected light amount Iout ref) = 0.1 × 0.2 × (incident light amount Iin) = 0.02 × (incident light amount Iin), and (reflected light amount Iin ref) = 0.02 × (incident when the above-described (incident side reflectance Rin) is 2%. Light amount Iin). That is, in the absorbing film, (exit-side reflectance Rout) is unlikely to increase stray light, and unlike a transparent film, a film structure emphasizing reduction in (incident-side reflectance Rin) is effective as a countermeasure against the stray light. In particular, the incident-side (reflected light amount Iin ref) and the outgoing-side (reflected light amount Iout ref) are made equal (if one target light amount is set low, the other light amount inevitably increases) It is confirmed that it is most ideal to make both reflected light amounts equal.

And the condition for setting (reflection amount Iin ref) on the incident side = (reflection amount Iout ref) on the emission side is:
(Incident light quantity Iin ref) = (incident light quantity Iin) × (incident side reflectance Rin) on the incident side,
From the equation of (reflected light amount Iout ref) = (emitted light amount Iout) × (emitted light reflectance Rout) and (emitted light amount Iout) = (incident light amount Iin) × (transmittance Tout)
(Reflection light amount Iout ref) = (incident light amount Iin) × (transmittance Tout) × (exit-side reflectance Rout) on the emission side,
It is found that an absorption film satisfying the condition of (incident side reflectance Rin) = (transmittance Tout) × (outgoing side reflectance Rout) may be obtained by a film thickness optimization technique.

  By the way, there are many methods such as Simplex method, Gradient method, and Monte Carlo method in the above-mentioned film thickness optimization method, but basically, the spectral transmittance and spectral reflectance are calculated while increasing or decreasing the film thickness of each layer. The theoretical transmittance and reflectance at each wavelength are evaluated by comparing with the target value (target value), and the theoretical transmittance and the theoretical reflectance at each wavelength are close to the target value (target value). The film thickness is obtained. In the present invention, an error function (evaluation function) for applying a weight (weight) to each target value is used as a method for comparing the theoretical transmittance and the theoretical reflectance at each wavelength with the target value (target value).

  In addition, since the metal film has an absorption coefficient larger than that of the absorbing metal oxide film, when forming an ND filter with a low transmittance, the thickness of each metal film layer is reduced to a fraction. There was a concern that it would be difficult to control the film thickness. However, with the recent progress of film forming technology, it is possible to control the film thickness of a metal film layer of several nm. In particular, when a film is formed on a flexible resin film, it is more suitable to apply a metal film layer having a smaller film thickness than an absorbing metal oxide film layer. Therefore, in the present invention, a method of forming a multilayer film with a dielectric film layer and a metal film layer is employed.

  The present invention has been completed through such technical studies.

That is, the invention according to claim 1
On the premise of an absorption-type multilayer film single-sided ND filter in which an absorption-type multilayer film that attenuates transmitted light is provided on one side of a substrate made of a resin film,
The absorption multilayer film is composed of four or more multilayer films in which dielectric film layers and metal film layers are alternately stacked, and the target spectroscopic optics is obtained by increasing or decreasing the thickness of each layer in the absorption multilayer film. The error function (evaluation function) shown in Equation (1) is used as a film thickness optimization method that approximates the characteristics, and the thickness of each layer in the absorption multilayer film so that the error function (evaluation function) becomes the minimum value. And the value of [incidence side reflectance] is substantially equal to the value of [transmittance × emission side reflectance],

請求項２に係る発明は、
請求項１記載の発明に係る吸収型多層膜片面ＮＤフィルターを前提とし、
上記誘電体膜層がＳｉＯ_２若しくはＡｌ_２Ｏ_３またはこれ等の混合物から成り、金属膜層の空気側最外層が次層の金属膜層より厚くないことを特徴とする。

The invention according to claim 2
On the premise of the absorption multilayer single-sided ND filter according to the invention of claim 1,
The dielectric film layer is made of SiO ₂ or Al ₂ O ₃ or a mixture thereof, and the air-side outermost layer of the metal film layer is not thicker than the next metal film layer.

Next, the invention according to claim 3 is
On the premise of the absorption multilayer single-sided ND filter according to the invention of claim 1,

波長４００〜７００ｎｍにおける透過率が２５±１２．５％、入射側反射率が５％以下、および、出射側反射率が２０％以下である条件を満たすことを特徴とし、
請求項４に係る発明は、
請求項１記載の発明に係る吸収型多層膜片面ＮＤフィルターを前提とし、

The transmittance at a wavelength of 400 to 700 nm is 25 ± 12.5%, the incident side reflectance is 5% or less, and the emission side reflectance is 20% or less.
The invention according to claim 4
On the premise of the absorption multilayer single-sided ND filter according to the invention of claim 1,

波長４００〜７００ｎｍにおける透過率が１２．５±６．３％、入射側反射率が２．５％以下、および、出射側反射率が２０％以下である条件を満たすことを特徴とし、
請求項５に係る発明は、
請求項１記載の発明に係る吸収型多層膜片面ＮＤフィルターを前提とし、

Characterized by satisfying the conditions that the transmittance at a wavelength of 400 to 700 nm is 12.5 ± 6.3%, the incident side reflectance is 2.5% or less, and the emission side reflectance is 20% or less,
The invention according to claim 5
On the premise of the absorption multilayer single-sided ND filter according to the invention of claim 1,

波長４００〜７００ｎｍにおける透過率が６．３±３．１％、入射側反射率が１．２５％以下、および、出射側反射率が２０％以下である条件を満たすことを特徴とする。

It satisfies the conditions that the transmittance at a wavelength of 400 to 700 nm is 6.3 ± 3.1%, the incident side reflectance is 1.25% or less, and the emission side reflectance is 20% or less.

Next, the invention according to claim 6 is:
On the premise of the absorption type multilayer single-sided ND filter according to any one of claims 1 to 5,
The absorption multilayer film is formed by a film forming method selected from a vacuum vapor deposition method, an ion beam sputtering method, a magnetron sputtering method, and an ion plating method.

The absorption-type multilayer film single-sided ND filter according to the present invention in which an absorption-type multilayer film that attenuates transmitted light is provided on one side of an incident side of a substrate made of a resin film,
The absorption multilayer film is composed of four or more multilayer films in which dielectric film layers and metal film layers are alternately stacked, and the target spectroscopic optics is obtained by increasing or decreasing the thickness of each layer in the absorption multilayer film. The error function (evaluation function) shown in the above formula (1) is used as a film thickness optimization method that approximates the characteristics, and the film of each layer in the absorption multilayer film so that the error function (evaluation function) becomes the minimum value. The thickness is selected, and the value of [incidence side reflectance] is substantially equal to the value of [transmittance × emission side reflectance].

  In addition, the absorption multilayer film single-sided ND filter according to the present invention having the above film formation conditions is provided with the absorption multilayer film as a substrate regardless of the configuration in which the absorption multilayer film is formed on one surface of a substrate made of a resin film. This has the effect of realizing the same performance as the absorption multilayer single-sided ND filter formed on both sides.

  The absorption multilayer single-sided ND filter according to the present invention has an absorption multilayer film formed on one side of the substrate, so that the absorption multilayer double-sided ND filter has an absorption multilayer film formed on both sides of the substrate. The film forming process can be further reduced, and the manufacturing cost can be reduced.

  Hereinafter, embodiments of the present invention will be described in detail.

  First, an absorption-type multilayer single-sided ND filter according to the present invention is an absorption-type multilayer single-sided ND filter in which an absorption-type multilayer film that attenuates transmitted light is provided on an incident-side single side of a substrate made of a resin film. The mold multilayer film is composed of four or more multilayer films in which dielectric film layers and metal film layers are alternately stacked, and the film thickness of each layer in the absorption multilayer film is increased or decreased to approach the target spectral optical characteristics. The error function (evaluation function) shown in Equation (1) is used as the film thickness optimization method, and the film thickness of each layer in the absorption multilayer film is selected so that the error function (evaluation function) is minimized. In addition, the value of [incidence side reflectance] is substantially equal to the value of [transmittance × emission side reflectance].

ここで、上記Error Function（評価関数）の第１項、すなわち、

Here, the first term of the above Error Function (evaluation function), that is,

は、透過率を目標とする値に近づけるため目標値と計算値の差を評価する項である。

Is a term for evaluating the difference between the target value and the calculated value in order to bring the transmittance close to the target value.

  In addition, the second term of the above Error Function (evaluation function), that is,

は、入射側反射率を目標とする値に近づけるため目標値と計算値の差を評価する項である。

Is a term for evaluating the difference between the target value and the calculated value in order to bring the incident side reflectance closer to the target value.

  Furthermore, the third term of the above Error Function (evaluation function), that is,

は、上記［入射側反射率］の値と［透過率×出射側反射率］の値を等しくさせるためその差を評価する項である。

Is a term for evaluating the difference in order to make the value of [incidence side reflectance] equal to the value of [transmittance × emission side reflectance].

  Then, by selecting the film thickness of each layer in the absorption multilayer film so that the value of the error function (evaluation function), which is the sum of the three terms, is minimized, the value of [incident side reflectance] and [ Absorbance type that has almost the same value of transmittance x output side reflectance, resin film as a substrate, flat transmittance attenuation in the visible light region, and little influence of reflected light. A multilayer single-sided ND filter can be obtained.

  Here, the reason for the “weighting” in the first to third terms in the formula (1) is as follows. That is, the spectral optical characteristics of the ND filter are closer to the transmittance required by the theoretical transmittance, and the better the theoretical reflectance on the incident side and the outgoing side is better. However, there is no solution for the film thickness that satisfies all requirements with a limited number of film layers by forming an absorption multilayer film on one side of the substrate with priority given to cost. Therefore, the transmittance and reflectance of each wavelength of the ND filter composed of the obtained film thickness is weighted according to the importance of the characteristics and integrated, and the film thickness of each layer is set as an error function (evaluation function). It is evaluated by. When the evaluation function is large, the film thickness of each layer is changed, evaluation is performed again using an error function (evaluation function), and these are repeated to obtain the film thickness of each layer having the smallest evaluation function. Whether importance is attached to the transmittance or the importance of the reflectance is determined by weighting of the evaluation function for each wavelength.

  In a general optical thin film, that is, an optical thin film composed of a transparent film, the amount of light does not attenuate by passing through the optical thin film, so it is assumed that the amount of light that has passed is reflected 100% and returned. Then, set the Error Function (evaluation function) so that the reflectance is the same on the incident side and the transmission side.

となる。

It becomes.

  However, in the ND filter, the amount of light attenuates by passing through the ND filter, so even if the amount of light that has passed is reflected 100% somewhere on the passing side and returns to the ND filter, the substrate in the ND filter The actual reflection level at the interface of the absorption multilayer film is only [transmittance × emission-side reflectivity] of the ND filter.

  Therefore, it is not necessary to make the output side reflectivity as low as the incident side reflectivity, and if the value of [incident side reflectivity] and [transmittance × output side reflectivity] are set to be substantially equal as described above. Good. With this concept, the degree of freedom in film design is increased, and the film thickness of each layer that makes the reflectance on the incident side lower reflective can be obtained.

Next, in the absorptive multilayer film single-sided ND filter of the present invention, the absorptive multilayer film is composed of four or more multilayer films in which dielectric film layers and metal film layers are alternately laminated. The body film layer is preferably made of SiO ₂ or Al ₂ O ₃ or a mixture thereof, and is set so that the air side outermost layer of the metal film layer is not thicker than the next metal film layer. When the dielectric film layer is not formed using SiO ₂ or Al ₂ O ₃ which is a low-refractive index material of an oxide dielectric thin film that has stable optical properties and is hard, the dielectric film layer and the metal film layer In some cases, the refractive index difference is reduced, and the degree of freedom in film design is reduced. Furthermore, when the air side outermost layer of the metal film layer is made thinner than the next metal film layer, there is an advantage that the average reflectance can be slightly lowered.

  Next, the error function (evaluation function) represented by the above formula (1) is used, the film thickness of each layer in the absorption multilayer film is selected so that this evaluation function becomes the minimum value, and [incident side] A specific example of the absorption multilayer single-sided ND filter according to the present invention in which the value of “reflectance” and the value of “transmittance × emission-side reflectance” are set to be approximately equal is as follows.

が例示される。

Is exemplified.

  Next, in the absorption type multilayer single-sided ND filter according to the present invention, the absorption type multilayer film composed of the dielectric film layer and the metal film layer is formed by vacuum deposition, ion beam sputtering, magnetron sputtering or ion plate. The film is formed by a film forming method selected from the coating method. Here, the characteristics of the metal film layer, such as the refractive index and the absorption coefficient, may vary greatly depending on the additive and impurities of the film material, the residual gas during film formation, the gas released from the substrate, and the film formation speed. Therefore, it is desirable to select the above conditions as appropriate so that an absorption-type multilayer single-sided ND filter having desired characteristics can be obtained.

  The material of the resin film constituting the substrate is not particularly limited, but is preferably transparent, and in view of mass productivity, a flexible substrate capable of dry roll coating is preferable. A flexible substrate is less expensive than a conventional glass substrate and the like, and is excellent in that it is lightweight and rich in deformability. Specific examples of the resin film include a film made of a resin selected from polyethylene terephthalate (PET), polyether sulfone (PES), polyarylate (PAR), polycarbonate (PC), polyolefin (PO) and norbornene, or A composite film in which one side or both sides of a resin film is coated with an acrylic organic film is exemplified. In particular, norbornene resin is preferable because it has features such as transparency and heat resistance in the visible wavelength range. Representative examples include ZEONOR (trade name) manufactured by Nippon Zeon Co., Ltd., which has a low absorption rate, and JSR Corporation, which has high film adhesion Arton (brand name) made from the company etc. can be mentioned.

Next, SiO ₂ is used for the dielectric film layer of the absorption-type multilayer film, Ti is used for the metal film layer, and the absorption-type multilayer film is constituted by four-layer films of SiO ₂ , Ti, SiO ₂ , Ti from the air side, A method for designing an absorptive multilayer ND filter having an emission side transmittance of 12.5% at a wavelength of 400 to 700 nm and an incident side reflectance of 1% at a wavelength of 400 to 700 nm will be specifically described.

  Here, an ND filter having an average transmittance of 12.5% is used as a design example, but the absorption multilayer ND filter according to the present invention is not limited to the above transmittance. The Simplex method is used as a film thickness optimization method.

また、生産の再現性を考慮し、誘電体膜層は２層とも同じ膜厚にする制限を入れており、膜厚の初期値として、各ＳｉＯ_２の膜厚を５０ｎｍ、各Ｔｉの膜厚を１０ｎｍに設定した。更に、ＳｉＯ_２の最小膜厚は２０ｎｍ、最大膜厚は１００ｎｍ、Ｔｉの最小膜厚は３ｎｍ、最大膜厚は５０ｎｍとし、膜厚は０．５ｎｍの最小ステップで変化させた。

In consideration of the reproducibility of production, the dielectric film layers are limited to the same film thickness for both layers. The initial value of the film thickness is 50 nm for each SiO _{2 and} the film thickness for each Ti. Was set to 10 nm. Furthermore, the minimum film thickness of SiO ₂ was 20 nm, the maximum film thickness was 100 nm, the minimum film thickness of Ti was 3 nm, the maximum film thickness was 50 nm, and the film thickness was changed in a minimum step of 0.5 nm.

この膜構成を以下の表１に、理論分光特性を図２に示す。

The film structure is shown in Table 1 below, and the theoretical spectral characteristics are shown in FIG.

図２のグラフ図から確認されるように、図２中、破線で示される分光透過率は波長４００〜７００ｎｍに対して１２．５％±１％以内、実線で示される入射側反射率は波長４００〜７００ｎｍに対して２％以下、一点鎖線で示される出射側反射率は波長４００〜７００ｎｍに対して約１８％以下になった。尚、出射側反射率は約１８％もあるが、透過率１２．５％の吸収型多層膜により吸収されて光量が減衰しているため、入射光に対して入射側反射率とほぼ同等の約２％（＝１８％×１２．５％）まで低下している。

As confirmed from the graph of FIG. 2, the spectral transmittance indicated by the broken line in FIG. 2 is within 12.5% ± 1% with respect to the wavelength of 400 to 700 nm, and the incident side reflectance indicated by the solid line is the wavelength. The output side reflectance indicated by the alternate long and short dash line was about 18% or less for wavelengths of 400 to 700 nm. Although the output side reflectance is about 18%, the amount of light is attenuated by being absorbed by the absorption multilayer film having a transmittance of 12.5%, so that it is almost equal to the incident side reflectance with respect to incident light. It has decreased to about 2% (= 18% × 12.5%).

  Therefore, by using the above error function (evaluation function) represented by the mathematical formula (1), it is possible to design an absorption multilayer single-sided ND filter in consideration of the output side reflectance in addition to the incidence side reflectance. However, the film design in which the value of [incidence side reflectance] and [transmittance × emission side reflectance] are almost equal to each other as in the above-described absorption multilayer single-sided ND filter is used when the transmittance is high. It is difficult, and the transmittance is preferably 25% or less. Furthermore, if the incident-side reflectance exceeds 4%, flare and blurring of the image appear remarkably when an ND filter is built in the digital camera. For this reason, the target value (target value) of the incident side reflectance is desirably 3% or less.

  In consideration of productivity and reproducibility, it is more convenient for the dielectric film layer and the metal film layer to have the same film thickness. However, when the film thickness is optimized in order to reduce the incident side reflectance over the entire visible region, the outermost layer (air side) of the metal film layer becomes thinner than the next metal film layer.

In the illustrated absorption multilayer single-sided ND filter, SiO ₂ is used for the dielectric film layer and Ti is used for the metal film layer, but Al ₂ O ₃ or SiO ₂ and Al ₂ O are used as the dielectric film layer. ₃ may be used, and Ni, Cr, W, Ta, Nb or the like may be used for the metal film layer.

  Next, specific examples of the present invention will be described.

  A 100 μm thick PET (polyethylene terephthalate) film slit to 300 mm was used as a substrate, and an absorptive multilayer film having the film structure shown in Table 1 was formed on one surface of the substrate.

A sputtering roll coater is used for film formation. When the film thickness of the oxide dielectric layer: SiO ₂ is 70 nm, the film conveyance speed during film formation is about 0.3 m / min, and the film thickness of the metal film layer: Ti. When the film thickness is 11.5 nm and 18.5 nm, the film conveyance speed during film formation is about 0.8 m / min and about 0.5 m / min, respectively, and the film conveyance speed is adjusted according to the film thickness of each layer. Yes. In addition, since Si is used for the target in forming the SiO ₂ layer, oxygen is introduced during the formation of the oxide dielectric layer.

  The theoretical spectral characteristics of the obtained absorption-type multilayer double-sided ND filter according to Example 1 are shown in FIG.

  Similar to Example 1, an absorption multilayer double-sided ND filter having an emission side transmittance of 12.5% at 400 to 700 nm was produced.

  That is, a PET (polyethylene terephthalate) film having a thickness of 100 μm slit to 300 mm was used as a substrate, and an absorption multilayer film having a film configuration shown in Table 2 was formed on one surface of the substrate.

A sputtering roll coater is used for film formation. When the film thickness of the oxide dielectric layer: SiO ₂ is 20 nm and 70 nm, the film conveyance speed during film formation is about 1.1 m / min and about 0.3 m / min, respectively. When the film thickness of Ti is 8 nm and 11 nm, the film transport speed during film formation is about 1.2 m / min and about 0.8 m / min, respectively, depending on the film thickness of each layer The conveyance speed is adjusted. Since the SiO ₂ layer is formed using Si as the target, oxygen is introduced during the film formation to form the oxide dielectric layer.

  FIG. 3 shows theoretical spectral characteristics of the obtained absorption multilayer double-sided ND filter according to Example 2.

［比較例］
比較例として従来の吸収型多層膜両面ＮＤフィルターの膜構成を表３に、この吸収型多層膜両面ＮＤフィルターの理論分光光学特性を図４に示す。

[Comparative example]
As a comparative example, the film configuration of a conventional absorption multilayer double-sided ND filter is shown in Table 3, and the theoretical spectroscopic optical characteristics of this absorption multilayer double-sided ND filter are shown in FIG.

「評 価」
次に、実施例１と２に係る吸収型多層膜片面ＮＤフィルター、および、比較例に係る吸収型多層膜両面ＮＤフィルターの波長４５０ｎｍにおける透過率、入射側反射率と出射側反射率を評価した。また、出射側反射率については、基板出射側から光を照射してその出射側反射パワーを測定すると共に、実際の使用状況に近づけるため、各ＮＤフィルターに対して基板入射側から光を照射しかつＮＤフィルターを一度透過し減衰した光をＡｌミラーに入射し略１００％反射させてほぼ完全にＮＤフィルターに戻すと共に基板出射側で反射された出射側反射率パワーの測定も実施した。

"Evaluation"
Next, the transmittance, incident side reflectance, and outgoing side reflectance of the absorption multilayer film single-sided ND filter according to Examples 1 and 2 and the absorption multilayer film double-sided ND filter according to the comparative example were evaluated. . As for the output side reflectivity, light is irradiated from the substrate output side and the output side reflection power is measured, and light is irradiated from the substrate input side to each ND filter in order to approximate the actual usage situation. In addition, light that was once transmitted through the ND filter and attenuated was incident on the Al mirror, reflected by about 100%, returned almost completely to the ND filter, and the output side reflectance power reflected on the substrate exit side was also measured.

  The results are shown in Table 4.

  In Table 4, “Transmission power” means a value obtained by making light incident from the substrate entrance side of each ND filter and measuring the power of the transmitted light on the substrate exit side, and “incidence side reflection power” means each ND filter. A value obtained by making light incident from the substrate incident side and measuring the power of the reflected light on the substrate incident side, “exit-side reflected power” means that light is incident from the substrate exit side of each ND filter and the power of the reflected light is emitted from the substrate. The value measured on the side, “exit-side reflection power of ND filter transmitted light” is the light incident from the substrate incident side of each ND filter, and once transmitted the light is incident on the Al mirror provided on the substrate exit side. The light reflected by the Al mirror is returned to the ND filter, and the power of the light reflected by the ND filter is measured on the substrate exit side. That.

  As can be seen from the data shown in Table 4, in the absorption-type multilayer single-sided ND filters according to Examples 1 and 2, “incident side reflected power” and “exit side reflected power of ND filter transmitted light” The absorption type multi-layer film single-sided ND filter was incorporated in a digital camera or the like, and it was not caused by flare caused by reflected light or image blur.

  Also, in the absorption multilayer double-sided ND filter according to the comparative example, the “incident side reflection power” and the “outgoing side reflection power” are the same “1.5 μW”. Even if the filter was built in a digital camera or the like, it was not caused by flare caused by reflected light or image blur.

  Therefore, the absorptive multilayer film single-sided ND filter according to Examples 1 and 2 in which the absorptive multilayer film is formed only on one side of the substrate is the absorptive type according to the comparative example in which the absorptive multilayer film is deposited on both sides of the substrate. It is confirmed that it has the same function as the multilayer double-sided ND filter.

  The absorption-type multilayer single-sided ND filter according to the present invention is more productive than the conventional absorption-type multilayer double-sided ND filter, and can reduce the manufacturing cost. It has a possibility.

FIG. 1A is a schematic diagram showing an incident / reflection state in consideration of transmittance and reflectance when light is incident on a filter having an absorption film on one side of the substrate, and FIG. 1B is a transparent film on one side of the substrate. The schematic diagram which shows the incident / reflection state which considered the transmittance | permeability and reflectance when light injects into the filter provided with. FIG. 3 is a graph showing theoretical spectral characteristics of the absorption multilayer single-sided ND filter according to Example 1; FIG. 6 is a graph showing theoretical spectral characteristics of an absorption multilayer single-sided ND filter according to Example 2. The graph which shows the theoretical spectral characteristic of the absorption type multilayer double-sided ND filter which concerns on a comparative example.

Claims (6)

In the absorption-type multilayer film single-sided ND filter in which an absorption-type multilayer film that attenuates transmitted light is provided on one side of a substrate made of a resin film,
The absorption multilayer film is composed of four or more multilayer films in which dielectric film layers and metal film layers are alternately stacked, and the target spectroscopic optics is obtained by increasing or decreasing the thickness of each layer in the absorption multilayer film. The error function (evaluation function) shown in Equation (1) is used as a film thickness optimization method that approximates the characteristics, and the thickness of each layer in the absorption multilayer film so that the error function (evaluation function) becomes the minimum value. And the value of [incidence side reflectance] is substantially equal to the value of [transmittance × emission side reflectance].

_2. The absorption according to claim 1, wherein the dielectric film layer is made of SiO ₂ or Al ₂ O ₃ or a mixture thereof, and the air-side outermost layer of the metal film layer is not thicker than the next metal film layer. Type multi-layer single-sided ND filter.

The absorption according to claim 1, characterized in that a transmittance at a wavelength of 400 to 700 nm is 25 ± 12.5%, an incident side reflectance is 5% or less, and an output side reflectance is 20% or less. Type multilayer single-sided ND filter.

The transmittance at a wavelength of 400 to 700 nm is 12.5 ± 6.3%, the incident side reflectance is 2.5% or less, and the emission side reflectance is 20% or less. The absorption-type multilayer film single-sided ND filter according to 1.

The transmittance at a wavelength of 400 to 700 nm is 6.3 ± 3.1%, the incident side reflectance is 1.25% or less, and the emission side reflectance is 20% or less. The absorption-type multilayer film single-sided ND filter according to 1.   The absorption multilayer film is formed by a film forming method selected from a vacuum vapor deposition method, an ion beam sputtering method, a magnetron sputtering method and an ion plating method. An absorption-type multilayer film single-sided ND filter described in 1.

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