JP2008058561A - Optical filter and color separation prism - Google Patents

Abstract

An optical filter that is easy to manufacture and has a high effect of suppressing luminance shading and color shading is provided.
An optical filter 21 according to the present invention has a first optical film H having a first refractive index and a second refractive index on a transparent base material (substrate) 11W, 12W. The second optical film M1 is alternately laminated, and the first refractive index is higher than the refractive index of the base material, and the second refractive index is higher than the refractive index of the base material. And it is lower than the said 1st refractive index, It is characterized by the above-mentioned. By using an intermediate refractive index film having a refractive index higher than that of the base material and lower than that of the first optical film H as the second optical film M1, the refractive index difference between the first and second optical films is reduced, and light Suppresses the shift in the spectral characteristics of the optical film thickness with respect to changes in the incident angle [selection figure]

Description

  The present invention relates to an optical filter including an optical multilayer film, and more particularly to a color separation prism that separates incident light into blue, green, and red color components.

  In recent years, a three-plate video camera using three solid-state image sensors for blue, green and red has been developed. This type of video camera uses a color separation prism that separates an imaged light beam into three primary colors of blue, green, and red.

  A color separation prism for a three-plate video camera is formed by sequentially arranging three first, second, and third prism members from the light incident side, and a color is formed at two prism interfaces between the three prism members. First and second optical filters for separation are formed in order. The first and second optical filters are usually composed of an optical multilayer film in which two or more optical films having different refractive indexes are alternately stacked. As the types of color separation prisms, the first optical filter is a blue reflection film, the second optical filter is a red reflection film, the first optical filter is a green reflection film, and the second optical filter is a second optical filter. There is known a color separation prism composed of a blue reflective film (see Patent Document 1 below).

  Here, in the color separation prism in which the first optical filter is composed of a blue reflective film, a phenomenon called “color shading” in which the color tone differs from top to bottom due to the incident angle dependency of the blue reflective film and the red reflective film. It is known to occur. This is caused by increasing green and decreasing red and blue at the top of the screen, and decreasing green and increasing red and blue at the bottom of the screen.

That is, since the imaging light beam enters the color separation prism through the lens, the imaging light beam is incident at a certain angle with respect to the incident surface of the color separation prism. In general, the optical multilayer film has an incident angle dependency, and as the incident angle increases, the spectral characteristic shifts to the short wavelength side, and as the incident angle decreases, the spectral characteristic tends to shift to the long wavelength side. Therefore, the amount of light that is reflected or transmitted is different between light that is incident on the optical multilayer film at a high incident angle and light that is incident at a low incident angle. As a result, the angle-dependent characteristic of the green light quantity that is transmitted through and separated by the two optical filters is stronger than the blue light quantity and the red light quantity, and the color shading is caused. In particular, green light has a strong influence on the luminance. The luminance is high where the amount of transmitted green light is large, and conversely, the luminance is low where the amount of green light is small. This also causes the occurrence of a phenomenon called “brightness shading” in which the brightness differs between the top and bottom of the screen.

  On the other hand, in the color separation prism in which the first optical filter is formed of a green reflective film, the amount of green light that is a luminance signal is stabilized, so that luminance shading can be significantly improved. FIG. 6 shows the spectral characteristics of the green light reflecting film configured as the first optical filter. The bottom wavelength position of the transmittance changes due to the incident angle dependency. In FIG. 6, an alternate long and short dash line A1 indicates the spectral characteristics at the center (design) incident angle, a solid line B1 indicates the spectral characteristics at the high incident angle, and a two-dot chain line C1 indicates the spectral characteristics at the low incident angle. As described above, the wavelength position changes depending on the incident angle, but the entire reflected band moves, so the change in the amount of green reflected light is small.

  On the other hand, in Patent Document 2 below, in order to reduce the incident angle dependency of spectral characteristics, a high refractive index first optical film having a refractive index of 2.5 or more on a substrate, and the first optical film, Also disclosed is an optical multilayer film in which second optical films having a refractive index of 0.3 or more are alternately stacked, and a method of forming the same. With this configuration, the shift in spectral characteristics is greatly reduced, and when used as a color separation prism for a video camera or the like, the shading characteristics can be greatly improved.

JP-A-51-81520 JP-A-7-27907

  However, in the color separation prism in which the green light reflection film is formed as the first optical filter, even if luminance shading is suppressed, the angle dependence of blue light and red light becomes stronger due to the shift in spectral characteristics. There is a problem that color shading has not been improved.

  In addition, according to the configuration of Patent Document 2, it is possible to suppress a shift in spectral characteristics due to a change in incident angle. However, when a high refractive index film having a refractive index of 2.5 or more is formed, temperature conditions and vacuum conditions can be reduced. There is a problem that the process is complicated and manufacturing is not easy.

  An object of the present invention is to provide an optical filter and a color separation prism which are made in view of the above-described problems, are easy to manufacture, and have a high effect of suppressing luminance shading and color shading.

  In solving the above-described problems, the optical filter of the present invention includes a first optical film having a first refractive index and a second optical film having a second refractive index on a translucent substrate. Wherein the first refractive index is higher than the refractive index of the base material, the second refractive index is higher than the refractive index of the base material, and It is lower than the first refractive index.

  An optical filter according to the present invention includes an optical multilayer film in which a plurality of the first optical films and the second optical films are alternately stacked. Then, by using an intermediate refractive index film having a refractive index higher than that of the substrate and lower than that of the first optical film as the second optical film, the difference in refractive index between the first and second optical films is reduced, and the light The shift of the spectral characteristic of the optical film thickness with respect to the change in the incident angle is suppressed.

The intermediate refractive index film is an optical thin film having a refractive index higher than that of the substrate and lower than that of the high refractive index film, and the refractive index thereof is, for example, 1.6 or more and 1.9 or less. The intermediate refractive index film includes an intermediate refractive index material film such as Al ₂ O ₃ or La-based Al ₂ O ₃ , a high refractive index material (TiO ₂ , Nb ₂ O ₅ , HfO _2, etc.) and a low refractive index material (SiO ₂ ). ₂ , MgF _2, etc.). An intermediate refractive index film can be easily produced by a vacuum vapor deposition method or a radical assist sputtering (RAS) method using these materials as a vapor deposition material or a target material.

  The color separation prism of the present invention is arranged between the first, second, and third prism members arranged in order from the light incident side and the first and second prism members, and reflects green light. And a first optical filter that transmits red light and blue light, and a second optical filter that is disposed between the second and third prism members and reflects one of red light and blue light and transmits the other. The first optical filter includes a multilayer film in which a first optical film having a first refractive index and a second optical film having a second refractive index are alternately stacked, and the first refractive index Is higher than the refractive index of the prism member, and the second refractive index is higher than the refractive index of the prism member and lower than the first refractive index.

  In the color separation prism of the present invention, the incident angle dependence characteristic of the green light reflection film is suppressed by using the optical filter according to the present invention having the above-described configuration as the first optical filter. Thereby, luminance shading and color shading can be improved.

  According to the present invention, by forming an optical filter with an optical multilayer film of a high refractive index film and an intermediate refractive index film, manufacturing is facilitated, and luminance shading and color shading can be effectively suppressed.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of a color separation prism 10 according to an embodiment of the present invention. The color separation prism 10 is used in, for example, a three-plate video camera, and includes a first prism member 11, a second prism member 12, and a third prism member 13 in order from the light incident side. Each prism member 11-13 is formed by processing glass materials, such as BK7 (refractive index 1.518).

  A first optical filter 21 is disposed between the first prism member 11 and the second prism member 12, and a second optical filter 22 is disposed between the second prism member 12 and the third prism member 13. Is arranged. The first optical filter 21 is inclined so that the imaging light beam L is incident at a predetermined incident angle (for example, 26.5 °), reflects the green light LG, and transmits the red light LR and the blue light LB. It is composed of an optical multilayer film. The second optical filter 22 is provided at an angle with respect to the first optical filter 21 and is formed of an optical multilayer film that reflects the blue light LB and transmits the red light LR.

  A green light transmission filter 11G that transmits only the green light LG is provided on the light emission surface of the first prism member 11, and a solid-state image pickup device (not shown) for green light is disposed through the green light transmission filter 11G. Has been. Further, a blue light transmission filter 12B that transmits only the blue light LB is provided on the light exit surface of the second prism member 12, and a blue light solid-state imaging device (not shown) is provided via the blue light transmission filter 12B. Is arranged. Further, a red light transmission filter 13R that transmits only the red light LR is provided on the light exit surface of the third prism member 13, and a solid-state image sensor for red light (not shown) is passed through the red light transmission filter 13R. Is arranged.

The green light transmission filter 11G, the blue light transmission filter 12B, and the red light transmission filter 13R may be omitted as necessary. The solid-state image sensor is composed of a CCD imager, a CMOS imager, or the like.

  In the above configuration, the imaging light flux L incident on the color separation prism 10 is incident on the first prism member 11, and then the green light LG is reflected by the first optical filter 21 and the blue light LB and the red light LR are transmitted. The green light LG reflected by the first optical filter 21 is totally reflected by the incident surface of the first prism member 11, and then emitted toward the solid-state image sensor for green light through the green light transmission filter 11G.

  On the other hand, the blue light LB that has passed through the first optical filter 21 is reflected by the second optical filter 22 and is emitted toward the solid-state image sensor for blue light through the blue light transmission filter 12B. The red light LR that has passed through the first optical filter 21 passes through the second optical filter 22 and is emitted toward the solid-state imaging device for red light through the red light transmission filter 13R.

  Here, since the imaging light beam L is condensed and incident on the incident surface of the color separation prism 10 from the subject via the imaging lens optical system, the imaging light beam L is present with respect to the first optical filter 21 of the color separation prism 10. Incident light is incident within a certain angle range (for example, 26.5 ° ± 9.5 °). At this time, if the angle dependency characteristic of the first optical filter 21 is strong, the spectral characteristic of the light is greatly shifted between the case where the first optical filter 21 is incident at a high incident angle and the case where the incident light is incident at a low incident angle. The occurrence of color shading becomes significant. For this reason, it is preferable that the angle-dependent characteristic of the optical filter 21 is as small as possible.

  Therefore, in the present embodiment, the first optical filter 21 is configured as follows. Hereinafter, the details of the first optical filter 21 according to the present invention configured as a green light reflecting film will be described.

  FIG. 2 is a schematic cross-sectional view showing the configuration of the first optical filter 21. The first optical filter 21 includes a multilayer film 16 in which first optical films H having a first refractive index and second optical films M1 having a second refractive index are alternately stacked.

The multilayer film 16 is disposed between the two substrates 11W and 12W. After the multilayer film 16 is formed on one of the substrates, the other substrate is bonded via an adhesive or the like. The substrates 11W and 12W are both configured as translucent base materials. In the present embodiment, one substrate 11 </ b> W corresponds to the first prism member 11, and the other substrate 12 </ b> W corresponds to the second prism member 12. These plate-like substrates 11W and 12W are shaped into a predetermined prism shape by machining.

The first optical film H is made of a high refractive index film having a refractive index (first refractive index) higher than the refractive index (1.518) of the substrates 11W and 12W. .4 or less TiO ₂ film, Nb ₂ O ₅ film, Ta ₂ O ₅ film, HfO ₂ film, ZrO ₂ or the like.

  The second optical film M1 is an intermediate refractive index film having a refractive index (second refractive index) that is higher than the refractive indexes of the substrates 11W and 12W and lower than the refractive index of the first optical film H. By using an intermediate refractive index film whose refractive index is higher than the substrates 11W and 12W and lower than the first optical film H as the second optical film M1, the refractive index difference between the first and second optical films is reduced, The shift of the spectral characteristic of the optical film thickness with respect to the change in the incident angle of light is suppressed.

The constituent material of the second optical film M1 is, for example, an intermediate refractive index material such as La-based Al ₂ O ₃ or Al ₂ O ₃ having a refractive index of 1.6 or more and 1.9 or less, as well as a high refractive index material and a low refractive index material. It is composed of a mixed material of refractive index materials (SiO ₂ , MgF _2, etc.). Examples of the La-based Al ₂ O ₃ material include “M2” and “M3” manufactured by Merck.

The second optical film M1 can be easily manufactured by a vacuum evaporation method or a radical assist sputtering (RAS) method using the above-described intermediate refractive index material as an evaporation material or a target material.

  The film thickness, the number of layers, etc. of each of the first optical film H and the second optical film M1 are not particularly limited, and are appropriately set according to the target reflection wavelength band, reflectance characteristics, and the like. In the present embodiment, the first optical filter 21 is composed of a green light reflecting film, and is configured with a film design that can obtain a predetermined reflectance with respect to green light.

As specific film design examples, 1.0 (H) = 1.0 (M1) = λ / 4 = n × d (λ: center wavelength [nm], n: refractive index of film, d: film thickness) (Physical film thickness [nm]), the basic film structure is formed in the following film thickness ratio in order from the substrate 11W side. 0.4 (H), 1.4 (M1), 0.5 (H), 1.3 (M1), 0.6 (H), 1.2 (M1), 0.7 (H), 1 .1 (M1), 0.8 (H), 1.0 (M1), (0.9 (H), 0.9 (M1)) ^Z , 1.0 (H), 0.8 (M1) 1.1 (H), 0.7 (M1), 1.2 (H), 0.6 (M1), 1.3 (H), 0.5 (M1), 1.4 (H), 0.4 (M1). Z is an integer and means the number of repetitions.

  As described above, the film thickness composition ratio of the first optical film H constituting the multilayer film 16 is gradually increased from 0.4 to 1.4, and the film thickness composition ratio of the second optical film M1 is set to 1. By gradually reducing the thickness from 4 to 0.4, a stable reflectance characteristic with little wavelength dispersion can be obtained. In the configuration example of FIG. 2, the film configuration when Z = 1 is shown.

  In the present embodiment, the first optical filter 21 is configured by the multilayer film 16 of the first optical film H and the second optical film M1 and the two ripple adjustment layers 17A and 17B sandwiching the multilayer film 16. . One ripple adjustment layer 17A is disposed between the substrate 11W and the multilayer film 16, and the other ripple adjustment layer 17B is disposed between the substrate 12W and the multilayer film 16.

The ripple adjustment layers 17A and 17B are refractive materials having a refractive index higher than the refractive index of the intermediate refractive index film constituting the second optical film M1 and the substrates 11W and 12W and lower than the refractive index of the second optical film M1, respectively. It is composed of an optical layer in which intermediate refractive index films (third optical films) M2 having a refractive index (third refractive index) are alternately stacked. By configuring the first optical filter 21 to include the ripple adjustment layers 17A and 17B, the multiple interference (reflection ripple) in the blue light wavelength region and the red light wavelength region is reduced, and excellent transmittance of each color light is achieved. Characteristics can be obtained.

FIG. 3 shows an example of spectral characteristics of a green light reflecting film having a ripple adjusting layer and a green light reflecting film having no ripple adjusting layer. In FIG. 3, the solid line indicates the spectral characteristic of the green light reflecting film having the ripple adjusting layer, and the broken line indicates the spectral characteristic of the green light reflecting film without the ripple adjusting layer. As is clear from FIG. 3, compared to the case without the ripple adjustment layer, the presence of the ripple adjustment layer reduces the multiple interference in the blue light wavelength region and the red light wavelength region, and the flat transmittance. Characteristics are obtained.

As a specific design example of the ripple adjustment layers 17A and 17B, when 1.0 (M1) = 1.0 (M2) = λ / 4 = n × d, the ripple adjustment layers 17A are (M1, M2). ) ^X and the ripple adjustment layer 17B are designed as (M1, M2) ^Y. X and Y are integers, meaning the number of repetitions. In the configuration example of FIG. 2, the film configuration when X = Y = 2 is shown.

  FIG. 4 shows the spectral characteristics of the first optical filter 21 with respect to the imaging light flux having an F value of 2.0. The imaging light flux having an F value of 2.0 is incident on the first optical filter 21 with a center incident angle of ± 9.5 °. When the central incident angle is 26.5 °, the positive side incident angle is 36.0 degrees and the negative side incident angle is 17.0 °. Therefore, in FIG. 4, the one-dot chain line A is the spectral characteristic at the center (design) incident angle of 26.5 °, the solid line B is the spectral characteristic at the incident angle of 36.0 °, and the two-dot chain line C is the spectral characteristic at the incident angle of 17 °. Each characteristic is shown.

In the first optical filter 21, the substrates 11W and 12W are made of a glass material (refractive index 1.518), the first optical film (high refractive index film) H is Nb ₂ O ₃ (refractive index 2.30), and the second optical filter is used. The film (intermediate refractive index film) M1 is a mixed film of Nb ₂ O ₅ (refractive index 1.845) and SiO ₂ (refractive index 1.458), and the third optical film (intermediate refractive index film) M2 is Nb ₂ O _5. A mixed film of (refractive index 1.735) and SiO ₂ (refractive index 1.458) was used. In this example, the number Z of repetitions of the basic film configuration of the multilayer film 16 and the number of repetitions X and Y of the ripple adjustment layers 17A and 17B are set to Z = 2, X = 1, and Y = 2.

  In FIG. 4, the spectral characteristic A at the central incident angle has a maximum transmittance at a wavelength of 565.8 nm for a transmittance of 50%, a wavelength width of 75.7 nm for a transmittance of 50%, and wavelengths of 520 to 540 nm. It was 4.3%, the minimum transmittance at a wavelength of 400 to 470 nm was 96.5%, and the minimum transmittance at a wavelength of 600 to 700 nm was 97.4%.

Further, the spectral characteristic B at the high incident angle shifts to the short wavelength side while the spectral characteristic C at the low incident angle shifts to the long wavelength side with respect to the spectral characteristic A at the central incident angle. Arrows D and E in the figure represent the shift amount with respect to the spectral characteristic A with reference to the wavelength position of the transmittance 50% of the spectral characteristics B and C. In this example, D = 24.7 nm and E = 18. It was 8 nm.

  As described above, in the first optical filter 21 of the present embodiment, the minimum transmittance is 80% or more at a wavelength of 400 to 470 nm, the maximum transmittance is 10% or less at a wavelength of 520 to 540 nm, and the minimum transmittance is 80 at a wavelength of 600 to 700 nm. Satisfy more than%. Further, even if the incident angle changes by ± 9.5 °, the shift amount of the spectral characteristics can be suppressed to 25 nm or less, so that an optical multilayer film having extremely small angle dependency can be configured.

In addition, by configuring the color separation prism 10 using the optical multilayer film as the first optical filter 21, the angle dependency of the first optical filter 21 can be suppressed to be small, so that the luminance shading can be improved. In addition, since the spectral shift of the light beam incident on the second optical filter 22 is reduced by that amount, color shading can be effectively suppressed.

Further, since the intermediate refractive index films M1 and M2 can be formed by using a normal film forming method such as a vacuum evaporation method or a sputtering method, the optical filter 21 is configured easily and at a low cost. Can do.

  Next, FIG. 5 shows the spectral characteristics at the incident angle of 26.0 ° ± 1.3 ° of the imaging light beam in the first optical filter 21 configured as described above. Since the angle width with respect to the central incident angle is limited to ± 1.3 °, compared to the example shown in FIG. 4, the central incident angle at the high incident angle (dashed line) and at the low incident angle (dashed line) It can be seen that the shift amount of spectral characteristics with respect to time (solid line) is greatly reduced. In this example, the total shift amount of the spectral characteristics at the half-value position of the reflectance peak was about 6 nm.

  The embodiment of the present invention has been described above, but the present invention is not limited to this, and various modifications can be made based on the technical idea of the present invention.

  For example, in the above embodiment, the configuration of the optical multilayer film according to the present invention including the high refractive index film and the intermediate refractive index film is applied to the first optical filter 21 as the green light reflecting film. A similar multilayer film structure can also be adopted for the second optical filter 22 which is a blue reflective film. As a result, a blue light reflecting film having a small angle dependency can be formed also for the second optical filter.

  Moreover, although the glass base material with a refractive index of 1.518 was used as the prism members 11 to 13 or the substrates 11W and 12W, it is of course not limited to this, and a translucent substrate having another refractive index. A material may be used. In this case, the refractive indexes of the first to third optical films H, M1, and M2 are appropriately selected according to the refractive index of the substrate.

  In the above embodiment, the example in which the optical filter according to the present invention is applied to the green light reflecting film in the color separation prism has been described. However, in addition to this, a dichroic mirror that selectively reflects light in a specific wavelength range. The present invention can also be applied to an optical multilayer film in an optical component such as a polarizing beam splitter.

It is a schematic block diagram of the color separation prism by embodiment of this invention. It is sectional drawing which shows roughly the film | membrane structure of the 1st optical filter used as a green light reflection film of the color separation prism shown in FIG. It is a figure which compares and shows the spectral characteristic when there is no ripple adjustment layer in the film | membrane structure of the optical filter shown in FIG. It is a figure which shows an example of the spectral characteristic for demonstrating the incident angle dependence of the optical filter shown in FIG. It is a figure which shows an example of the spectral characteristic for demonstrating the incident angle dependence of the optical filter shown in FIG. It is a figure which shows an example of the spectral characteristic for demonstrating the incident angle dependence of the conventional green light reflection film.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Color separation prism, 11 ... 1st prism member, 11W ... Board | substrate, 12 ... 2nd prism member, 12W ... Board | substrate, 13 ... 3rd prism member, 16 ... Multilayer film, 17A, 17B ... Ripple adjustment layer (optical layer) ), 21... First optical filter, 22... Second optical filter, H... First optical film (high refractive index film), L .. imaging light flux, M1. 3 optical film (intermediate refractive index film)

Claims (7)

An optical filter formed by alternately laminating a first optical film having a first refractive index and a second optical film having a second refractive index on a translucent base material,
The first refractive index is higher than the refractive index of the substrate;
The optical filter, wherein the second refractive index is higher than the refractive index of the substrate and lower than the first refractive index. The optical filter according to claim 1, wherein the second refractive index is 1.6 or more and 1.9 or less. The optical filter according to claim 1, wherein the first and second optical films are formed with different film thicknesses for each layer. An optical layer in which a third optical film having a third refractive index higher than the refractive index of the base material and lower than the second refractive index and the second optical film are alternately stacked;
The optical filter according to claim 1, wherein the optical layer is disposed between the base material and the multilayer film of the first and second optical films. The optical filter according to claim 1, wherein a pair of the base materials are provided, and a multilayer film of the first and second optical films is disposed between the pair of base materials. First, second, and third three prism members arranged in order from the light incident side;
A first optical filter disposed between the first and second prism members for reflecting green light and transmitting red light and blue light;
A second color filter disposed between the second and third prism members and reflecting one of red light and blue light and transmitting the other, and a color separation prism comprising:
The first optical filter includes a multilayer film in which a first optical film having a first refractive index and a second optical film having a second refractive index are alternately stacked,
The first refractive index is higher than the refractive index of the prism member,
The color separation prism, wherein the second refractive index is higher than a refractive index of the prism member and lower than the first refractive index. Between the first prism member and the first filter, and between the first filter and the second prism member, the first refractive index is higher than the refractive index of these prism members and lower than the second refractive index. The color separation prism according to claim 6, wherein an optical layer in which a third optical film having a refractive index of 3 and the second optical film are alternately laminated is disposed.

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