WO2002097512A2

WO2002097512A2 - Glass sheet for vehicles provided with hud combiner and method of manufacturing the same

Info

Publication number: WO2002097512A2
Application number: PCT/JP2002/005331
Authority: WO
Inventors: Takashi Muromachi
Original assignee: Nippon Sheet Glass Company, Limited
Priority date: 2001-05-31
Filing date: 2002-05-31
Publication date: 2002-12-05
Also published as: WO2002097512A3

Abstract

The present invention provides a glass sheet for vehicles provided with an HUD combiner including a low-reflection film formed on a main surface of the glass sheet that includes a first layer and a second layer formed on the first layer. The first layer has a refractive index n1 in a range from 1.70 to 1.80 and a thickness d1 in a range from 110 nm to 130 nm, and the second layer has a refractive index n2 in a range from 1.40 to 1.47 and a thickness d2 in a range from 81 nm to 100 nm. The main surface has a first region where the second layer is not formed and the first layer serves as a reflectance-increasing film, and the first region is used as the HUD combiner.

Description

GLASS SHEET FOR VEHICLES PROVIDED WITH HUD COMBINER AND METHOD OF MANUFACTURING THE SAME

FIELD OF THE INVENTION The present invention relates to a glass sheet for vehicles provided with a Head-Up Display combiner and the method of manufacturing the same. In particular, the present invention relates to a glass sheet for vehicles provided with an HUD combiner, which includes a low-reflection film and is particularly useful as a windshield for automobiles, and a method of manufacturing the same.

BACKGROUND OF THE INVENTION

JP 6(l994)-305775 A discloses "a windowpane for vehicles comprising a glass sheet and a low-reflection thin film including a plurality of thin films formed on a surface of the glass sheet, wherein the first thin film formed directly on the glass sheet is at least exposed in a predetermined region and the first thin film has a refractive index higher than that of the outermost thin film having a relatively low refractive index."

It is also disclosed in JP 6(l994)-305775 A that "the low-reflection thin film includes two or three thin films with different refractive indices" and "the windowpane is used as a combiner glass for a head-up display".

In US 5,496,621, which is based on JP 6(l994)-340450 A in addition to the above-mentioned JP 6(l994)-305775 A, the refractive index and the thickness of the first and second thin films are disclosed. The first thin film has a refractive index in the range from 1.80 to 2.10 and a thickness in the range from 70 to 230 nm and the second thin film has a refractive index in the range from 1.40 to 1.50 and a thickness in the range from 110 to 130 nm.

JP 2500821 Y2 (Japanese Utility Model Registration) discloses a technique for "forming a transparent reflectance increasing layer of a metal oxide having a refractive index of 1.8 to 2.3 and an optical thickness of 40 to 150 nm" as an HUD combiner.

JP 6(l994)-340450 A and JP 6(l994)-305775 A do not disclose any specific thickness or refractive index of the low-reflection film. JP 6(l994)-340450 A only discloses that the low-reflection film having a two-layer structure includes a first thin film with a refractive index m in the range from 1.8 to 2.1 and a thickness di in the range from 70 to 230 nm and a second thin film with a refractive index n₂ in the range from 1.4 to 1.5 and an optical thickness d2 in the range from 110 to 130 nm.

In addition, in JP 6(l994)-340450 A and JP 6(l994)-305775 A, to form the low-reflection film on only one side of the glass sheet, it is necessary to "cover one side of the glass sheet entirely with a masking tape".

As a method of "at least exposing the first thin film in the predetermined region", JP 6(l994)-340450 A and JP 6(l994)-305775 A disclose a method of peehng off the second thin film in the following manner. First, the low— reflection film is formed by immersing. Then, an alkoxide dissolving paste is screen— printed onto the unnecessary region of the second thin film, followed by firing. The paste thus fired is removed with an aqueous solution of sodium hydroxide and an aqueous solution of sulfuric acid.

According to the method disclosed in JP 6(l994)-340450 A and JP 6(l994)-305775 A, it is difficult to remove only the second thin film cleanly. Even if the second thin film can be removed cleanly, it requires quite a few steps, resulting in high cost. US 5,496,621 only disclosed the same method as in JP

6(l994)-340450 A. In US 5,496,621, a thickness and a refractive index of the low-reflection film are limited. Low-reflection films satisfying the limitation include the first thin film with a high refractive index and a small thickness and the second thin film with a large thickness. Such low— reflection films exhibit a noticeable reflected color when the glass is mounted on a vehicle.

For example, when the combiner disclosed in JP 2500821 Y2 is formed on the low-reflection film with a two— layer structure, the low-reflection film exhibits a noticeable reflected color due to the varied optical conditions. Thus, one may consider forming a notch on the low-reflection film so that the notch is coated with the film disclosed in JP 2500821 Y2. However, such a method is not preferable because it increases the steps to be performed, thereby increasing the cost.

The applicant of the present invention discloses in JP 2000-256042 A a technique regarding a low-reflection film for reducing the reflection of light incident obliquely. The applicant discloses "a low-reflection glass article for automobiles comprising a transparent glass substrate and two thin layers laminated at least on one surface of the substrate by coating, a first thin layer formed on the substrate having a refractive index (m) in the range from 1.65 to 2.20 and a thickness in the range from 110 to 150 nm, a silica— based second thin layer formed on the first thin layer having a refractive index (n₂) in the range from 1.37 to 1.49 and a thickness in the range from 81 to 100 nm, wherein reflected Hghts caused by Hghts incident at 12° and 60° on the glass article from the side of the thin layers have excitation purities of not more that 22% and not more than 10%, respectively".

Further, as glass having the function of reducing the reflection of Hght incident obliquely, JP 8(l996)-152501 A discloses a technique described below, which aims to "provide glass harmless to the environment and human beings, which, when used as a windshield, prevents a dashboard and its surroundings from being reflected in the glass and reduces a glare on human beings and the environment by the use of a highly durable thin film assuming Httle interference color, thereby improving the driver's view so as to improve safety by preventing misperception and fatigue of the eyes".

JP 8(l996)-152501 A discloses "reflection-reducing glass for vehicles comprising a transparent glass substrate and two thin layers laminated at least on one surface of the substrate by coating, a first thin layer formed on the substrate having a refractive index in the range from 1.7 to 1.8 and a thickness in the range from 90 to 110 nm, a siHca-based second thin layer formed on the first thin layer having a refractive index in the range from 1.4 to 1.5 and a thickness in the range from 105 to 130 nm, ⁵ wherein, with respect to light incident at an angle of 50° to 70° with the vertical Hne of said surface of the substrate, the region where the thin layers are provided has a reflectance lower than that of the glass substrate by 4.5% to 6.5%, and wherein reflected Hght caused by the Hght incident at 50° to 70° has an excitation purity of not more that 18%."

10 DISCLOSURE OF THE INVENTION

A glass sheet for vehicles provided with an HUD combiner according to the present invention is characterized in that a first layer in a low-reflection film has a refractive index and a thickness within a predetermined range so that the first layer can serve as a

¹⁵ reflectance-increasing film to serve as an HUD combiner and so that it has a neutral color tone. The first layer also serves as a first layer of the low-reflection film. In addition, a second layer has a refractive index and a thickness within a predetermined range so that the low— reflection film itself has a neutral color tone.

^" A method of manufacturing a glass sheet for vehicles provided with an HUD combiner according to the present invention enables the glass sheet to be manufactured only through the usual process for forming a low-reflection film including two layers, for example, by the use of two types of flexographic plates. The present invention provides a glass sheet for vehicles provided with an HUD combiner that includes a low-reflection film formed on a main surface of the glass sheet. The low— reflection film includes a first layer and a second layer formed on the first layer. The first layer has a refractive index ni in a range from 1.70 to 1.80 and a thickness di in a range from 110 nm to 130 nm, and the second layer has a refractive index n2 in a range from 1.40 to 1.47 and a thickness d₂ in a range from 81 nm to 100 nm. The main surface has a first region where the second layer is not formed and the first layer serves as a reflectance-increasing film, and the first region is used as the HUD combiner.

The present invention provides a method of manufacturing a glass sheet for vehicles provided with an HUD combiner. This method includes forming a low-reflection film including a first layer and a second layer on a main surface of a glass sheet by a flexographic printing method such that the first layer has a refractive index m in a range from 1.70 to 1.80 and a thickness άι of 110 nm to 130 nm and the second layer has a refractive index n2 in a range from 1.40 to 1.47 and a thickness d2 in a range from 81 nm to 100 nm. The first layer is formed in a first region and a second region of the main surface and the second layer is formed on the second region with a flexographic plate having a recess so as not to form the second layer on the first region. The first region is used as the HUD combiner where the first layer serves as a reflectance-increasing film. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic cross-sectional view of a glass sheet for vehicles provided with an HUD combiner according to an embodiment of the present invention.

Fig. 2 shows a schematic cross-sectional view illustrating a flexographic printing device that can be used in the present invention. Figs. 3 A and 3B show schematic cross— sectional views illustrating flexographic plates. Fig. 3A shows a flexographic plate for a first layer and Fig. 3B shows a flexographic plate for a second layer.

Fig. 4 shows a schematic cross-sectional view illustrating a combiner portion in a glass sheet an embodiment according to the present invention.

Fig. 5 is a graph showing a relationship between a 60° reflectance and a film thickness of the first layer. DETAILED DESCRIPTION OF THE INVENTION

In the glass sheet of the present invention, with respect to Hght incident at 60°on the glass sheet from a second main surface on which the low-reflection film is not formed, the first region (a combiner region) and a second region where the low-reflection film are provided preferably exhibit a reflected color tone having Hunter chromaticity coordinates (a, b) both in a range from —5 to +5 under IUuminant C, respectively. This preferable color tone can provide a neutral appearance of the glass sheet when the glass sheet is observed from the outside of the vehicle.

With respect to Hght incident at 60°on. the glass sheet from a first main surface on which the low-reflection film is formed, the second region preferably has a visible hght reflectance of not more than 11% and the first region preferably has a visible Hght reflectance of at least 15%. Thus, the first region can serve as a good combiner on the inner surface of the glass sheet and the second region can produce a sufficient anti— eflection effect in the windshield. ° The glass sheet used as a transparent substrate may be a laminated glass sheet. The first layer may contain TΪO2 and Siθ2, and the second layer may contain Si0₂ as a main component. Here, a main component denotes a component that occupies 50wt% or more of the layer.

The first layer preferably contains Ti0₂ and Siθ2 in a molar ratio of 10 Ti : Si = 35 : 65 to 60 ^'• 40, and the second layer is preferably formed of Si0₂.

A low-reflection film preferably used for a glass sheet for vehicles, especially for a windshield, will be described in the foHowing.

In general, low-reflection films formed on transparent substrates are classified into several groups according to the number of layers included

1 ^X ^^D such as a single layer structure, a two-layer structure, a three— layer structure, and a multilayer structure (four layers or more).

A low— reflection film with a single layer structure should have a refractive index lower than that of the glass sheet. Examples of an appHcable low refractive index material include MgF and Siθ2. ^υ Since the low-reflection film with a single layer structure cannot produce a sufficient anti-reflection effect, a low-reflection film with a two-layer structure, which includes a layer with a refractive index higher than that of the glass sheet and a layer with a refractive index lower than that of the glass sheet, preferably is used. Further, in the case where the low— reflection film with a two-layer structure cannot provide a sufficient anti— eflection effect, a low— reflection film with a three-layer structure including a low refractive index layer, a middle refractive index layer, and a high refractive index layer, or a

° low-reflection film with a multilayer structure including four or more layers is used.

In either structure, the layer formed on top preferably has a refractive index lower than that of a transparent substrate. For example, as a material having a refractive index lower than that of a glass sheet, ^ϋ only the above-mentioned MgF2 and Siθ2 could be used. However, since MgF2 does not have a sufficient durabifity or weather— resistance and is susceptible to heat appfied during a step of bending the glass sheet, Siθ2 is a preferable material that can be used for such an appHcation.

In the case where a low-reflection film is used for a bent glass sheet ⁵ for vehicles, it is preferable to form the low-reflection film on a flat glass sheet and then bend the glass sheet by heating, on account of the uniformity of the film thickness and the like. A glass sheet for vehicles provided with an HUD combiner according to the present invention preferably is a bent glass sheet. ^υ Examples of a preferable method for providing Si0₂ coating over a large area at low cost include roU coating. When performing roU coating, an Si0₂ layer preferably is formed by a sol-gel process. The Si0₂ layer may be made porous to decrease its apparent refractive index. Further, the Si0₂ layer may contain inorganic particles having a low refection index to decrease its refractive index. The anti-reflection effect can be improved by decreasing the refractive index of the second layer. The second layer, which contains Siθ2 as a main component, also may contain B2O3 and/or

Now, in a low-reflection film with a two-layer structure producing a great anti-reflection effect by a relatively simple film structure, a refractive index of a high refractive index material that can be used in combination with a low refractive index material Siθ2 12 = 1.46) will be calculated. For example, a relationship between m and n2 is represented by the foUowing equation where n_g denotes a refractive index of glass ( = 1.52) and n₃ denotes a refractive index of air ( = 1.0). (Equation l) ni = [(n₂)² x n_g/n₃]^{1 2}

From this equation, it is found that ni is preferably 1.80. However, none of the materials that can be used for film formation by the sol— gel process has a refractive index ni in the vicinity of 1.80. Accordingly, the use of a layer containing Tiθ2 (n = 2.2) and Si0₂ (n = 1.46), which are both usable for film formation by the sol— gel process, can be considered. The above layer may further contain Zrθ2 (n = 1.95), Ceθ2, Bi₂θ3, etc. so that it can serve as a high refractive index layer. Examples (Preparation of Coating Solution)

Preparation of a coating solution used for forming a low— reflection film will be explained in the following. First, 500 g of ethyl siHcate 40 (COLCOAT Co., LTD) was hydrolyzed using 410 g of ethylceHosolve and 90 g of 1 mol/L hydrochloric acid, and the resultant mixture was stirred to prepare a solution A. Next, 65.5 g of titanium tetraisoprop oxide and 64.1 g of ⁵ acetylacetone were mixed with each other to prepare a solution B.

The solution A and the solution B were mixed at a ratio of 1 ^*• 1, and the resultant mixture was diluted appropriately with an ethylceHosolve solvent to prepare a coating solution C. In the coating solution C thus obtained, Ti and Si were present at a mole ratio of 50 ^"• 50. ^ιυ Further, the solution A was diluted appropriately with an ethylceHosolve solvent to prepare a coating solution D.

When the ratio of Ti •^" Si in the coating solution is adjusted in the range from 35 ^'■ 65 to 60 : 40 (mole ratio), a layer containing Tiθ2 and Siθ2 from the coating solution has a refractive index m of 1.70 to 1.80. ¹⁵ (Example)

First, on a surface of a soda-Hme silica glass sheet produced by a float glass process, which had been cut into a predetermined size and washed, the coating solution C was coated by a flexographic painting method. Fig. 2 schematically shows a printing device used for coating. ^ In this flexographic printing device 5, a coating solution is suppHed from a dispenser 6. The coating solution suppHed is spread by a doctor roU 71 and an annex roU 72 and is transferred to a flexographic plate 8 attached on a printer roU 73. The coating solution on the flexographic plate 8 is printed on a glass sheet 10 carried on a conveyer 9. The conveyer 9 carries the glass sheet 10 in synchronization with the printer roU 73.

Fig. 3A schematicaHy shows a flexographic plate 81for forming a first layer that has a printing surface 83. The glass sheet thus coated with ⁵ the coating solution C was dried at a temperature of about 300°C, forming a first layer.

Then, the coating solution D was coated on the surface of the glass sheet entirely, also by a flexographic painting method. As shown in Fig. 3B, a flexographic plate 82 used for coating had a printing surface that ^ιυ included a notch corresponding to a region 84 to be used as a combiner so that the coating solution D was not coated on this region. After the coating, the glass sheet was dried at a temperature of about 300°C to form a second layer. As a result, the coating solution D was coated on the main surface of the glass sheet excluding the region to be used as a combiner (combiner ¹⁵ portion).

The glass sheet was fired at a temperature of 620°C to 630°C, foHowed by bending to be processed into glass for automobiles. The bending was conducted in the foUowing manner : the glass sheet and a similarly-shaped glass sheet stacked thereon were placed in a die for sag ^ bending, which was heated in heating furnace so that they were bent by their self weight.

The glass sheet thus obtained included a low-reflection film with a two-layer structure (Fig. l) including the first layer with a refractive index ni of about 1.74 and a thickness di of about 120 nm and the second layer with a refractive index n2 of about 1.44 and a thickness d2 of about 90 nm. The combiner portion 3 included only the first layer 21 without being coated with the second layer 22, while the other portion is coated with the first layer 21 and the second layer 22 as shown in Fig.4. On the surface of the ⁵ low— reflection film, the boundary of the combiner portion was formed precisely at the predetermined position while maintaining a good appearance.

By combining the glass sheet with the low— reflection film and the above— mentioned similarly-shaped glass sheet with each other via a PVB ^ιυ film as an intermediate layer through the usual combining process in such a manner that the low-reflection film side of the former would be the inboard side and the latter would be the outboard side, a glass sheet for vehicles provided with an HUD combiner as a laminated bent glass sheet with a low— reflection film was obtained (see Fig. l). The optical

¹⁵ performance of the resultant laminated glass sheet is shown in Table 1. It is to be noted that the transmittance was measured with respect to Hght incident at 0° on the laminated glass; the reflectance was measured with respect to Hght incident at 60° on the laminated glass; and the reflected color tone was represented by the Hunter color coordinates (a, b) as

^ determined under IUuminant C. Table 1 visible visible reflected color tone of reflected color tone of light light outboard side surface inboard side surface transmi- reflectance a o a b ttance (%) (%) region with low-reflection 81.0 9.6 1.5 -1.0 1.0 -1.0 film combiner

75.1 18.1 -3.3 2.3 -1.9 3.8 portion

Table 1 shows that the reflectance is increased in the combiner portion, which means the first layer serves as a reflectance-increasing film. Thus, the combiner portion can serve as a combiner. The reason for this is that the first layer has a reflection peak in the vicinity of 550 nm because it ⁵ has a refractive index m in the range from 1.70 to 1.80 and a thickness di in the range from 110 to 130 nm. Further, both the low-reflection film and the combiner portion have a neutral color tone, which is preferable.

On a glass sheet without a film, the reflectance measured with respect to Hght incident at 0° is about 8% and the reflectance with respect " to Hght incident at 60° is about 14%. The low-reflection film works for reducing the reflectance since the reflectance of 9.6% (Table l) is lower than 14%.

The region with the low-reflectance film and the combiner portion provides visible Hght transmittance of 75% or more, which is within the ⁵ standards for windshield in the European countries in addition to US and Japan. The standards are foHows: JP: JIS 3211/3212 US: ANSI Z26 EP: ECE-R43

(Verification of HUD Combiner) In the present invention, the first layer among two layers forming a low— reflection film is used as a reflectance increasing film. Thus, characteristics of the first layer as a reflectance increasing film were determined where the refractive index ni of the first layer was 1.74 and the thickness di thereof was varied from 110 to 150 nm. Among the reflection characteristics shown in Table 2, the reflection characteristics of the region where the first layer of each thickness is provided is shown in the graph of Fig. 5. The graph shows the changes in the reflectance of the region with wavelength, the reflectance being measured with respect to Hght incident at 60° from the inboard side. (In the graph, the vertical axis indicates a reflectance (%) of the region with respect to Hght incident at 60° from the inboard side and the horizontal axis indicates a wavelength (nm).)

Table 2 reflection characteristics reflection characteristics thickness of region provided with first layer of glass surface reflectance color tone reflectance color tone

(nm) (%) a b (%) a b

110 18.60 -2.03 2.67 16.33 -3.55 1.66

120 18.11 -1.93 3.78 16.03 -3.34 2.27

130 17.42 -1.60 4.38 15.63 -2.96 2.57

140 16.62 -0.99 4.16 15.15 -2.39 2.41

150 15.77 -0.16 2.95 14.67 -1.70 1.69

As is apparent from these results, the peak of the reflectance is in the vicinity of 550 nm when the thickness di of the first layer is in the range from 110 to 130 nm. Further, when the thickness di of the first layer is in this range, the reflectance of the region provided with the first layer is not less than 17% with respect to Hght incident at 60° from the inboard side and the reflectance of the glass surface is not less than 15% with respect to Hght incident at 60° from the outboard side.

Moreover, since a reflected color tone has Hunter chromaticity coordinates (a, b) both in a range from —5 to +5 under flluminant C, it is understood that the low-reflection film has a neutral color tone.

According to the present invention, a low— reflection film provided with an HUD combiner can be obtained with only the process for forming the low— reflection film. The reason for this is that the low-reflection film with a two-layer structure is coated by a flexographic printing method in such a manner that a combiner portion would not be coated with a second layer when coating the second layer. (Comparative Example) In this comparative example, the solution A and the solution B used in the above-mentioned example were mixed at a ratio of 1 : 5, and the resultant mixture was diluted appropriately with an ethylceHosolve solvent to prepare a coating solution c. In the coating solution c thus obtained, Ti and Si were present at a mole ratio of 83 • 17. The first layer was formed using the coating solution c, and the second layer was then formed using the coating solution D. The first layer had a refractive index m of about 1.90 and a thickness di of about 100 nm, and the second layer had a refractive index n2 of about 1.44 and a thickness d₂ of about 120 nm. Except for the foregoing, a glass sheet for vehicles provided with an HUD combiner was manufactured according to the same processes as in the above-mentioned example. The optical performance of the laminated glass obtained is shown in Table 3. It is to be noted that the optical performance was measured under the same conditions as in the above— mentioned example. Table 3 visible visible reflected color tone of reflected color tone of light light outboard side surface inboard side surface tr admi^¬ reflectance a υ a b ttance (%) (%) region with low-reflection 80.5 9.0 0.1 -6.5 -0.6 -9.3 film combiner

72.1 22.6 -5.5 3.0 -1.7 5.5 portion

The low reflection film according to this comparative example has a reflected color tone of blue whereas the combiner portion has a reflected color tone of green. Both of these reflected color tone are unpreferable as a windowpane for automobiles.

The visible Hght transmittance in the combiner portion is out of the spec in the European countries while it is within the specs in US and Japan. (Modified Example)

To further improve the appearance around the combiner portion, gradation dots may be provided, for example.

The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this appHcation are to be considered in aU respects as illustrative and not Hmiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and aU changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims

1. A glass sheet for vehicles provided with an HUD combiner comprising a low-reflection film formed on a main surface of the glass sheet, the low-reflection film including a first layer and a second layer formed on the first layer, the first layer having a refractive index m in a range from 1.70 to 1.80 and a thickness di in a range from 110 nm to 130 nm, and the second layer having a refractive index n2 in a range from 1.40 to

1.47 and a thickness d₂ in a range from 81 nm to 100 nm, wherein the main surface has a first region where the second layer is not formed and the first layer serves as a reflectance-increasing film, and the first region is used as the HUD combiner.

2. A glass sheet according to claim 1, wherein the glass sheet has a first main surface on which the low— reflection film is formed and a second main surface, wherein, with respect to Hght incident at 60°on the glass sheet from the second main surface, the first region and a second region where the low-reflection film is provided exhibit a reflected color tone having Hunter chromaticity coordinates (a, b) both in a range from -5 to +5 under Dluminant C, respectively, and wherein, with respect to Hght incident at 60°on the glass sheet from the first main surface, the second region has a visible Hght reflectance of not more than 11% and the first region has a visible Hght reflectance of at least 15%.

3. A glass sheet according to claim 1, wherein the glass sheet is a laminated glass sheet.

4. A glass sheet according to claim 1, wherein the first layer comprises Tiθ2 and Siθ2, and the second layer comprises Si0₂ as a main component.

5. A glass sheet according to claim 4, wherein the first layer comprises TLO2 and Siθ2 in a molar ratio of Ti : Si = 35 • 65 to 60 : 40, and the second layer is formed of Siθ2.

6. A method of manufacturing a glass sheet for vehicles provided with an HUD combiner comprising forming a low— reflection film including a first layer and a second layer on a main surface of a glass sheet by a flexographic printing method such that the first layer has a refractive index ni in a range from 1.70 to 1.80 and a thickness di of 110 nm to 130 nm and the second layer has a refractive index n2 in a range from 1.40 to 1.47 and a thickness d2 in a range from 81 nm to 100 nm, wherein the first layer is formed in a first region and a second region of the main surface and the second layer is formed on the second region with a flexographic plate having a recess so as not to form the second layer on the first region, and wherein the first region is used as the HUD combiner where the first layer serves as a reflectance-increasing film.

7. A method according to claim 6, wherein the first layer comprises Tiθ2 and Siθ2, and the second layer comprises Si0 as a main component.

8. A method according to claim 7, wherein the first layer comprises Ti0₂ and Si0₂ in a molar ratio of Ti : Si = 35 : 65 to 60 : 40, and the second layer is formed of Siθ2.