Abstract:
In a polarization beam splitter that allows effective polarization separation in a wide wavelength range with low dependency on angle of incidence, between two prisms is sandwiched a dielectric multilayer film composed of a first and a second multilayer portion that are designed with respect to a first and a second wavelength λ 1 , and λ 2 , respectively. Moreover, the formula λ 1&lt;λ2≦155·λ1  is fulfilled, and which is greater of the difference between the refractive indices of the high-refractive-index and low-refractive-index layers in the first multilayer portion and the same difference in the second multilayer portion coincides with which is greater differences of the angles θ 1  and θ 2  from the prism vertex angle θ.

Description:
This application is based on Japanese Patent Application No. 2002-280188 filed on Sep. 26, 2002, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a polarization beam splitter that separates P- and S-polarized light. 
     2. Description of the Prior Art 
     Polarization beam splitters that separate polarized light components that are polarized on mutually perpendicular polarization planes are used in optical systems of image display apparatuses and optical disk apparatuses. A polarization beam splitter is provided with a dielectric multilayer film having two types of dielectric with different refractive indices alternately laid on top of one another so that each layer has an optical film thickness equal to ¼ of the wavelength of light to be separated. Of the light obliquely incident on this dielectric multilayer film, P-polarized light is transmitted therethrough and S-polarized light is reflected therefrom. This makes possible the separation of the two differently polarized light components. 
     For efficient separation of P- and S-polarized light, it is advisable to make the angle of incidence at which light is incident on the dielectric multilayer film as close to the Brewster angle as possible. Moreover, it is preferable that the refractive index of the dielectric multilayer film and the angle at which the light travels therethrough fulfill formula (0) below. 
     
       
         sin 2   φ=NH   2   ·NL   2   /[NE   2 ·( NH   2   +NL   2 )]  (0) 
       
     
     where NE represents the refractive index of the medium that is located contiguous with the dielectric multilayer film and from which light enters the dielectric multilayer film, NH represents the refractive index of the high-refractive-index layers of the dielectric multilayer film, NL represents the refractive index of the low-refractive-index layers of the dielectric multilayer film, and φ represents the angle of the light relative to a normal to the dielectric multilayer film. 
     Moreover, it is necessary to protect the dielectric multilayer film and to package it into an easy-to-handle optical device. 
     In consideration of these requirements, a polarization beam splitter is generally composed of a dielectric multilayer film sandwiched between two transparent media, and the dielectric multilayer film is designed to be used at an angle of incidence of 45° and to fulfill formula (0). Moreover, to minimize the deflection of light at the interface between the transparent media and air, and to make easy the handling of separated light by making P- and S-polarized light components travel in mutually perpendicular directions after separation, the two transparent media are each formed into the shape of a prism of which the section has the shape of a right-angled isosceles triangle, with the dielectric multilayer film sandwiched between the hypotenuse surfaces of those prisms. 
     A conventional polarization beam splitter built as described above exhibits, when light is incident on the dielectric multilayer film at an angle of incidence of 45°, high transmissivity for P-polarized light and high reflectivity for S-polarized light over a wide wavelength range about a reference wavelength, and thus it separates the two differently polarized light components very effectively. However, a conventional polarization beam splitter exhibits high dependency on angle of incidence; that is, when the angle of incidence at which light is incident on the dielectric multilayer film deviates even slightly from the design value of 45°, the polarization beam splitter separates P- and S-polarized light less effectively, exhibiting particularly lower transmissivity for P-polarized light. That is, unless light is incident at 45°, the wavelength range in which differently polarized light components are separated effectively is extremely narrow. 
     Some optical systems employing a polarization beam splitter handle only light of a single wavelength. However, many such optical systems handle light spreading over a certain range of wavelengths. For example, optical systems for use in image display apparatuses handle light spreading all over the range of wavelengths of visible light. To miniaturize such optical systems, it is preferable to use as few polarization beam splitters as possible, and to make a convergent or divergent beam of light incident on a polarization beam splitter. 
     However, because of the above-mentioned dependency on angle of incidence, it is impossible to effectively separate light spreading over a wide wavelength range when the light is in the form of a convergent or divergent beam. Even when the principal ray of a convergent or divergent beam of light is made incident on a dielectric multilayer film at an angle of incidence of 45° as designed, the rays other than the principal ray are incident at angles deviated from 45°. Thus, only the portion of the beam quite near its principal ray is separated effectively into P- and S-polarized light, and the other portion, particularly the peripheral portion, of the beam is separated markedly less effectively. 
     FIGS. 25 to  27  show the relationship between the wavelength of light and transmissivity in a conventional polarization beam splitter. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light. FIG. 25 deals with the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism (i.e., in the layer of air). This ray is incident on the dielectric multilayer film at an angle of incidence of 45°. FIG. 26 deals with the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism. FIG. 27 deals with the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film before entering the prism. These three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45° with the dielectric multilayer film. The refractive index Nd of the prism is 1.84. 
     As FIG. 25 clearly shows, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45°, it is possible to separate P- and S-polarized light effectively over a wide wavelength range of from about 440 nm to about 640 nm. However, as FIG. 26 shows, with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible only in wavelength ranges of from about 490 nm to about 540 nm and from about 600 nm to about 670 nm. Moreover, as FIG. 27 shows, with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible only in wavelength ranges of from about 380 nm to about 430 nm and from about 490 nm to about 600 nm. 
     Thus, the wavelength range in which the whole beam of light having an f-number of 2.8 is effectively separated into P- and S-polarized light is extremely narrow, namely from 490 nm to 540 nm. The greater the f-number of the beam of light, the wider the wavelength range in which effective polarization separation is possible, but increasing the f-number poses a demanding requirement on the beam of light to be subjected to polarization separation. This makes it difficult to miniaturize optical systems incorporating polarization beam splitters. 
     As one way to overcome this inconvenience, there have conventionally been proposed (for example, in Japanese Patent Applications Laid-Open Nos. H7-281024, H11-211916, and 2001-350024) polarization beam splitters in which the dielectric multilayer film is composed of two multilayer portions, of which the first is designed to fulfill formula (0) at the angle (φ 1 ) of a first ray with respect to a first reference wavelength and of which the second is designed to fulfill formula (0) at the angle (φ 2 ) of a second ray with respect to a second reference wavelength. In a polarization beam splitter built in this way, the wavelength range in which each multilayer portion allows effective polarization separation with low dependency on angle of incidence is no wider than that obtained conventionally, but it is possible to make the two multilayer portions cover different wavelength ranges so that the dielectric multilayer film as a whole allows effective polarization separation with low dependency on angle of incidence in a wider wavelength range. 
     However, simply designing a dielectric multilayer film to cope with different reference wavelengths and different angles of light does not always result in the two multilayer portions allowing effective polarization separation in continuous wavelength ranges. If the wavelength ranges in which the two multilayer portions allow effective polarization separation are not continuous, the dielectric multilayer film as a whole may allow effective polarization separation in a wider wavelength range, but this wavelength range includes, within itself, a partial wavelength range in which polarization separation is less effective. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a polarization beam splitter that allows effective polarization separation in a wide wavelength range and that exhibits low dependency on angle of incidence all over that wavelength range. 
     To achieve the above object, according to the present invention, in a polarization beam splitter provided with a first prism including a first surface on which light is incident and a second surface that makes an acute angle with the first surface, a second prism disposed so as to face the second surface of the first prism, and a dielectric multilayer film sandwiched between the surfaces of the first and second prisms that face each other and composed of a first multilayer portion having high-refractive-index layers and low-refractive-index layers laid alternately on one another so that each layer has an optical film thickness equal to ¼ of a first wavelength and a second multilayer portion having high-refractive-index layers and low-refractive-index layers laid alternately on one another so that each layer has an optical film thickness equal to ¼ of a second wavelength, the condition expressed by formula (1) is fulfilled, and in addition, for angles θ 1  and θ 2  that respectively fulfill the conditions expressed by formulae (2) and (3), the conditions expressed by formulae (4) and (5), or (6) and (7), are fulfilled. 
     
       
         λ 1 &lt;λ 2 ≦1.55·λ 1   (1) 
       
     
     
       
         sin 2 θ 1 = NH   1   2   ·NL   1   2   /[Nd   2 ·( NH   1   2   +NL   1   2 )]  (2) 
       
     
      sin 2 θ 2 = NH   2   2   ·NL   2   2   /[Nd   2 ·( NH   2   2   +NL   2   2 )]  (3) 
     
       
         | NH   1 − NL   1 |&lt;| NH   2 − NL   2 |  (4) 
       
     
     
       
         |θ 1 −θ|&lt;|θ 2 −θ|  (5) 
       
     
     
       
         | NH   1 − NL   1 |&gt;| NH   2 − NL   2 |  (6) 
       
     
     
       
         |θ 1 −θ|&gt;|θ 2 −θ|  (7) 
       
     
     where λ 1  represents the first wavelength, λ 2  represents the second wavelength, θ represents the angle that the first and second surfaces of the first prism make with each other, Nd represents the refractive index of the first prism, NH 1  represents the refractive index of the high-refractive-index layers of the first multilayer portion, NL 1  represents the refractive index of the low-refractive-index layers of the first multilayer portion, NH 2  represents the refractive index of the high-refractive-index layers of the second multilayer portion, and NL 2  represents the refractive index of the low-refractive-index layers of the second multilayer portion. The angles θ 1  and θ 2  are those formed by the light traveling through the dielectric multilayer film relative to a normal thereto. 
     In this polarization beam splitter, light is introduced into it through the first surface of the first prism, and is then separated into P- and S-polarized light by the dielectric multilayer film sandwiched between the first and second prisms. The dielectric multilayer film is composed of the first and second multilayer portions, of which the first is designed to fulfill formula (0) noted earlier at a first angle θ 1  with respect to a reference wavelength set at a first wavelength λ 1  (formula (2)) and of which the second is designed to fulfill formula (0) at a second angle θ 2  with respect to a reference wavelength set at a second wavelength λ 2  (formula (3)). 
     Here, the second wavelength λ 2  is longer than the first wavelength λ 1 , and the second wavelength λ 2  is so set as to be equal to or shorter than 1.55 times the first wavelength λ 1 . The angle θ that the first and second surfaces of the first prism makes with each other (i.e., the prism vertex angle) is equal to the angle of incidence at which the ray that is incident perpendicularly on the first surface is incident on the dielectric multilayer film. Thus, between the first angle θ 1 , which fulfills formula (2) in the first multilayer portion, and the angle of incidence θ at which the ray that is incident perpendicularly on the first surface is incident on the dielectric multilayer film, there is a difference of which the absolute value is equal to |θ 1 −1|. Likewise, between the second angle θ 2 , which fulfills formula (3) in the second multilayer portion, and the angle of incidence θ at which the ray that is incident perpendicularly on the first surface is incident on the dielectric multilayer film, there is a difference of which the absolute value is equal to |θ 2 −1. 
     These differences in angle are determined according to which is greater of the difference between the refractive indices of the high-refractive-index and low-refractive-index layers in the first multilayer portion and that in the second multilayer portion. When the difference between the refractive indices of the high-refractive-index and low-refractive-index layers is greater in the second multilayer portion than in the first multilayer portion (formula (4)), the difference between the second angle and the prism vertex angle is made greater than the difference between the first angle and the prism vertex angle (formula (5)). On the other hand, when the difference between the refractive indices of the high-refractive-index and low-refractive-index layers is greater in the first multilayer portion than in the second multilayer portion (formula (6)), the difference between the first angle and the prism vertex angle is made greater than the difference between the second angle and the prism vertex angle (formula (7)). 
     When formula (1) is fulfilled, and in addition, for the angles θ 1  and θ 2  that respectively fulfill formulae (2) and (3), formulae (4) and (5), or (6) and (7), are fulfilled, then the first and second multilayer portions allow effective polarization separation with low dependency on angle of incidence in different wavelength ranges, with an overlap secured between those two wavelength ranges. That is, it is possible to realize a dielectric multilayer film of which the dependency on angle of incidence remains low over two continuous wavelength ranges. 
     In one example, the high-refractive-index layers of the first multilayer portion is made of TiO 2  or Ta 2 O 5 , the low-refractive-index layers of the first multilayer portion is made of MgF 2  or SiO 2 , the high-refractive-index layers of the second multilayer portion is made of TiO 2  or Ta 2 O 5 , and the low-refractive-index layers of the second multilayer portion is made of a mixture of Al 2 O 3  and La 2 O 3 , a mixture of Al 2 O 3  and La 2 O, or Al 2 O 3 . 
     In another example, the high-refractive-index layers of the first multilayer portion is made of a mixture of TiO 2  and La 2 O 3 , a mixture of TiO 2 and La   2 O, or Ta 2 O 5 , the low-refractive-index layers of the first multilayer portion is made of MgF 2  or SiO 2 , the high-refractive-index layers of the second multilayer portion is made of TiO 2  or Ta 2 O 5 , and the low-refractive-index layers of the second multilayer portion is made of a mixture of Al 2 O 3  and La 2 O 3 , a mixture of Al 2 O 3  and La 2 O, or Al 2 O 3 . 
     In another example, the high-refractive-index layers of the first multilayer portion is made of a mixture of Al 2 O 3  and La 2 O 3  or a mixture of Al 2 O 3  and La 2 O, the low-refractive-index layers of the first multilayer portion is made of MgF 2 , the high-refractive-index layers of the second multilayer portion is made of TiO 2  or Ta 2 O 5 , and the low-refractive-index layers of the second multilayer portion is made of Al 2 O 3 . 
     In another example, the high-refractive-index layers of the first multilayer portion is made of Al 2 O 3 , the low-refractive-index layers of the first multilayer portion is made of MgF 2 , the high-refractive-index layers of the second multilayer portion is made of TiO 2  or Ta 2 O 5 , and the low-refractive-index layers of the second multilayer portion is made of SiO 2 . 
     The first and second prisms may each have the shape of a right-angled isosceles triangle. 
     The first and second prisms may have equal refractive indices for the same wavelength. 
     The ratio λ 2 /λ 1  of the wavelength λ 2  to the wavelength λ 1  may be equal to or higher than 1.1. 
     The light introduced through the first surface may be visible light spreading over a predetermined wavelength range between 480 nm and 750 nm. 
     The light introduced through the first surface may be in the form of a convergent or divergent beam of which the f-number as observed in the layer of air is 2.8 and of which; the principal ray makes an angle of 45° with the dielectric multilayer film. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings in which: 
     FIG. 1 is a diagram schematically showing the overall structure of a polarization beam splitter embodying the invention; 
     FIG. 2 is a diagram schematically showing how the wavelength ranges of light to be subjected to polarization separation are set in a polarization beam splitter embodying the invention; 
     FIG. 3 is a diagram schematically showing the structure of the dielectric multilayer film used in a polarization beam splitter embodying the invention; 
     FIG. 4 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 1; 
     FIG. 5 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 1; 
     FIG. 6 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 1; 
     FIG. 7 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 2; 
     FIG. 8 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 2; 
     FIG. 9 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 2; 
     FIG. 10 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 3; 
     FIG. 11 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 3; 
     FIG. 12 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 3; 
     FIG. 13 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 4; 
     FIG. 14 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 4; 
     FIG. 15 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 4; 
     FIG. 16 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 5; 
     FIG. 17 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 5; 
     FIG. 18 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 5; 
     FIG. 19 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 6; 
     FIG. 20 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 6; 
     FIG. 21 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle, of 55.3° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 6; 
     FIG. 22 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 7; 
     FIG. 23 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 7; 
     FIG. 24 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film before entering the prism in the polarization beam splitter of Practical Example 7; 
     FIG. 25 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism in a conventional polarization beam splitter; 
     FIG. 26 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism in the conventional polarization beam splitter; 
     FIG. 27 is a diagram showing the relationship between wavelength and transmissivity observed for the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film before entering the prism in the conventional polarization beam splitter; 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 schematically shows the overall structure of a polarization beam splitter  1  embodying the invention. The polarization beam splitter  1  is composed of a first prism  10 , a second prism  20 , and a dielectric multilayer film  30  sandwiched between the first and second prisms  10  and  20 . Light is shone obliquely on the dielectric multilayer film  30  so as to be separated into P-polarized light, which is transmitted through the dielectric multilayer film  30 , and S-polarized light, which is reflected from the dielectric multilayer film  30 . 
     The first prism  10  has a first surface  11 , a second surface  12 , and a third surface  13 , and the first and third surfaces  11  and  13  are perpendicular to each other, making the first prism  10  a right-angled triangular prism. Likewise, the second prism  20  has a first surface  21 , a second surface  22 , and a third surface  23 , and the first and third surfaces  21  and  23  are perpendicular to each other, making the second prism  20  a right-angled triangular prism. The first and second prisms  10  and  20  are made of the same optical material, and thus have equal refractive indices. 
     The first and second prisms  10  and  20  are arranged so as to face each other, with their respective second surfaces  12  and  22  located close and parallel to each other. The dielectric multilayer film  30  is sandwiched between the second surface  12  of the first prism  10  and the second surface  22  of the second prism  20 . In the polarization beam splitter  1 , light is introduced into it through the first surface  11  of the first prism  10 . The P-polarized light, having transmitted through the dielectric multilayer film  30 , exits from the polarization beam splitter  1  through the third surface  23  of the second prism  20 , and the S-polarized light, having reflected from the dielectric multilayer film  30 , exits the polarization beam splitter  1  through the third surface  13  of the first prism  10 . 
     The light that is shone into the polarization beam splitter  1  so as to be subjected to polarization separation may be in the form of not only a parallel beam but also a convergent or divergent beam. The light to be subjected to polarization separation is shone on the first surface  11  of the first prism  10  in such a way that the principal ray thereof is perpendicularly incident on the first surface  11 . The first and second prisms  10  and  20  are, as described above, right-angled triangular prisms. These prisms may be right-angled isosceles triangular prisms. Specifically, the first and second prisms  10  and  20  may be so shaped that, on the plane perpendicular to the first and third surfaces  11  and  13  of the first prism  10 , these surfaces describe equally long lines and that, on the plane perpendicular to the first and third surfaces  21  and  23  of the second prism  20 , these surfaces describe equally long lines. 
     When the first and second prisms  10  and  20  are formed as right-angled isosceles triangular prisms, it is possible to make the principal rays of the P- and S-polarized light after separation travel in mutually perpendicular directions. This makes easy the handling of the two differently polarized light components after separation. Moreover, it is also possible to make the principal rays of the P- and S-polarized light after separation exit perpendicularly through the third surfaces  13  and  23 . This permits the S-polarized light exiting through the third surface  13  of the first prism  10  and the P-polarized light exiting through the third surface  23  of the second prism  20  to maintain the same degree of convergence or divergence as that of the light before striking the first surface  11  (that is, it is possible to keep the f-number constant). This makes easy the designing of the optical system as a whole including the polarization beam splitter  1 . 
     However, it is not absolutely necessary to form the first and second prisms  10  and  20  as right-angled isosceles triangular prisms. The first and second prisms  10  and  20  may be given any shape so long as the angle θ that the first and second surfaces  11  and  12  of the first prism  10  make with each other (i.e., the prism vertex angle) is not greatly deviated from the Brewster angle. Moreover, the first and second prisms  10  and  20  may or may not have equal refractive indices. 
     FIG. 2 schematically shows how the wavelength ranges of light to be subjected to polarization separation are set in the polarization beam splitter  1 . In the polarization beam splitter  1 , the dielectric multilayer film  30  is composed of two multilayer portions, of which one performs polarization separation on light spreading over a wavelength range A and of which the other performs polarization separation on light spreading over a wavelength range B different from the wavelength range A, with an overlapping range D secured between those two wavelength ranges. Moreover, one multilayer portion is so formed as to exhibit low dependency on angle of incidence all over the wavelength range A, and the other multilayer portion is so formed as to exhibit low dependency on angle of incidence all over the wavelength range B, so that the dielectric multilayer film  30  as a whole exhibits low dependency on angle of incidence all over a wide wavelength range. 
     FIG. 3 schematically shows the structure of the dielectric multilayer film  30 . The dielectric multilayer film  30  is composed of a first multilayer portion  31  and a second multilayer portion  32 . The first multilayer portion  31  is composed of high-refractive-index layers  31 H, having a comparatively high refractive index, and low-refractive-index layers  31 L, having a comparatively low refractive index, that are laid alternately on one another. Likewise, the second multilayer portion  32  is composed of high-refractive-index layers  32 H, having a comparatively high refractive index, and low-refractive-index layers  32 L, having a comparatively low refractive index, that are laid alternately on one another. 
     Except the layer contiguous with the first prism  10  and the layer contiguous with the second multilayer portion  32 , the high-refractive-index layers  31 H and the low-refractive-index layers  31 L of the first multilayer portion  31  are each so formed as to have an optical film thickness equal to ¼ of a first wavelength λ 1 . The layers contiguous with the first prism  10  and the second multilayer portion  32  are each so formed as to have an optical film thickness equal to ⅛ of the first wavelength λ 1 . Except the layer contiguous with the second prism  20  and the layer contiguous with the first multilayer portion  31 , the high-refractive-index layers  32 H and the low-refractive-index layers  32 L of the second multilayer portion  32  are each so formed as to have an optical film thickness equal to ¼ of a second wavelength λ 2 . The layers contiguous with the second prism  20  and the first multilayer portion  31  are each so formed as to have an optical film thickness equal to ⅛ of the second wavelength λ 2 . 
     In the first multilayer portion  31 , the high-refractive-index and low-refractive-index layers  31 H and  31 L may be laid in reverse order, and, in the second multilayer portion  32 , the high-refractive-index and low-refractive-index layers  32 H and  32 L may be laid in reverse order. In the first multilayer portion  31 , the total number of high-refractive-index and low-refractive-index layers  31 H and  31 L may be even or odd. Likewise, in the second multilayer portion  32 , the total number of high-refractive-index and low-refractive-index layers  32 H and  32 L may be even or odd. Either of the first and second multilayer portions  31  and  32  may be arranged on the first prism  10  side. The layers having an optical film thickness equal to ⅛ of the wavelength λ 1  or λ 2  may have the same refractive index as the dielectric layer contiguous therewith, or may have any other optical film thickness, or may be omitted. 
     Let the refractive index of the first prism  10  be refractive index of the high-refractive-index layers  31 H of the first multilayer portion  31  be NH 1 , the refractive index of the low-refractive-index layers  31 L of the first multilayer portion  31  be NL 1 , the refractive index of the high-refractive-index layers  32 H of the second multilayer portion  32  be NH 2 , the refractive index of the low-refractive-index layers  32 L of the second multilayer portion  32  be NL 2 , a first angle relative to a normal to the dielectric multilayer film  30  be θ 1 , and a second angle relative to a normal to the dielectric multilayer film  30  be θ 2 . Then, the polarization beam splitter  1  is designed to fulfill the conditions described below. 
     The first multilayer portion  31  is so formed as to fulfill formula (2) noted earlier, i.e., fulfill formula (0) at the first angle θ 1 . The second multilayer portion  32  is so formed as to fulfill formula (3) noted earlier, i.e., fulfill formula (0) at the second angle θ 2 . 
     The first wavelength λ 1 , which is the reference wavelength of the first multilayer portion  31 , and the second wavelength λ 2 , which is the reference wavelength of the second multilayer portion  32 , are so set that the second wavelength λ 2  is longer than the first wavelength λ 1  and equal to or shorter than 1.55 times the first wavelength λ 1 . That is, these wavelengths are so set as to fulfill formula (1). When the wavelength ratio λ 2 /λ 1  is too high, there is no overlap between the wavelength ranges in which the first and second multilayer portions  31  and  32  allow effective polarization separation, and thus the wavelength range in which the dielectric multilayer film  30  allows polarization separation includes, within itself, a partial wavelength range in which polarization separation is less effective. This inconvenience can be avoided by restricting the wavelength ratio λ 2 /λ 1  to 1.55 or lower. 
     The lower the wavelength ratio λ 2 /λ 1  the wider the overlap between the wavelength ranges in which the first and second multilayer portions  31  and  32  allow effective polarization separation, and thus the narrower the wavelength range in which the dielectric multilayer film  30  allows effective polarization separation. To avoid making too narrow the wavelength range in which the dielectric multilayer film  30  allows effective polarization separation, it is advisable to set the wavelength ratio λ 2 /λ 1  at about 1.1 or higher. 
     In addition, when the difference |NH 1 −NL 1  | between the refractive indices NH 1  and NL 1  of the high-refractive-index and low-refractive-index layers  31 H and  31 L of the first multilayer portion  31  is smaller than the difference |NH 2 −NL 2  | between the refractive indices NH 2  and NL 2  of the high-refractive-index and low-refractive-index layers  32 H and  32 L of the second multilayer portion  32 , the first and second angles θ 1  and θ 2  are so set that the absolute value of the difference |θ 1 −θ| between the first angle θ 1  and the prism vertex angle θ is smaller than the absolute value of the difference |θ 2 −θ| between the second angle θ 2  and the prism vertex angle θ. On the other hand, when the refractive index difference |NH 1 −NL 1 | in the first multilayer portion  31  is greater than the refractive index difference |NH 2 −NL 2 | in the second multilayer portion  32 , the first and second angles θ 1  and θ 2  are so set that the absolute value of the difference |θ 1 −θ| between the first angle θ 1  and the prism vertex angle θ is greater than the absolute value of the difference |θ 2 −θ| between the second angle θ 2  and the prism vertex angle θ. That is, formulae (4) and (5) are fulfilled, or alternatively formulae (6) and (7) are fulfilled. 
     When designed as described above, the polarization beam splitter  1  allows effective separation of P- and S-polarized light in a wide wavelength range and exhibits low dependency on angle of incidence all over that wavelength range, as exemplified by practical examples presented below. The individual dielectric layers  31 ,  31 L,  32 H, and  32 L of the dielectric multilayer film  30  are formed by a conventionally well-established film formation method such as sputtering, plasma ion plating, ion beam assist, or the like. 
     Hereinafter, practical examples of how the polarization beam splitter  1  is designed in reality will be presented. In all of these practical examples, the first and second prisms  10  and  20  are formed as right-angled isosceles triangular prisms, and thus the vertex angle θ that the first and second surfaces  11  and  12  of the first prism  10  make with each other is 45°. The refractive index of the second prism  20  is equal to the refractive index Nd of the first prism  10 . 
     The first multilayer portion  31  of the dielectric multilayer film  30  includes, as the layer contiguous with the first prism  10  and the layer contiguous with the second multilayer portion  32 , layers each having an optical film thickness equal to ⅛ of the first wavelength λ 1 . Likewise, the second multilayer portion  32  includes, as the layer contiguous with the second prism  20  and the layer contiguous with the first multilayer portion  31 , layers each having an optical film thickness equal to ⅛ of the second wavelength λ 2 . One dielectric layer having an optical film thickness equal to ¼ of the wavelength λ 1  (or λ 2 ), together with those portions of the two dielectric layers sandwiching it from both sides which each correspond to an optical film thickness equal to ⅛ of the wavelength λ 1  (or λ 2 ), constitutes one unit, and the structure of each of the first and second multilayer portions  31  and  32  is designated by the number of units constituting it. 
     PRACTICAL EXAMPLE 1 
     Table 1 shows an overview of the design of Practical Example 1. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Prism Vertex Angle θ: 
                 45° 
               
               
                 Prism Refractive Index Nd: 
                 1.84 
               
               
                 First Multilayer Portion 31 
               
               
                 Number of Units: 
                 10 
               
               
                 First Wavelength λ1: 
                 740 nm 
               
               
                 Material of High-Refractive-Index Layers 31H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 31L: 
                 MgF 2   
               
               
                 Refractive Index NH1 of High-Refractive-Index Layers 31H: 
                 2.200 
               
               
                 Refractive Index NL1 of Low-Refractive-Index Layers 31L: 
                 1.385 
               
               
                 Refractive Index Difference | NH1 − NL1 |: 
                 0.815 
               
               
                 Angle Difference | θ1 − θ |: 
                 5.467 
               
               
                 Second Multilayer Portion 32 
               
               
                 Number of Units: 
                 10 
               
               
                 Second Wavelength λ2: 
                 1065.6 nm 
               
               
                 Material of High-Refractive-Index Layers 32H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 32L: 
                 A Mixture 
               
               
                   
                 of Al 2 O 3   
               
               
                   
                 and La 2 O 3   
               
               
                   
                 or a Mix- 
               
               
                   
                 ture of 
               
               
                   
                 Al 2 O 3   
               
               
                   
                 and La 2 O 
               
               
                 Refractive Index NH2 of High-Refractive-Index Layers 32H: 
                 2.200 
               
               
                 Refractive Index NL2 of Low-Refractive-Index Layers 32L: 
                 1.710 
               
               
                 Refractive Index Difference | NH2 − NL2 |: 
                 0.490 
               
               
                 Angle Difference | θ2 − θ |: 
                 2.156 
               
               
                 Wavelength Ratio λ2 / λ1: 
                 1.440 
               
               
                   
               
             
          
         
       
     
     When the high-refractive-index layers  31 H and  32 H are made of Ta 2 O 5 , an impurity is added thereto by ion plating or sputtering. This permits Ta 2 O 5  to have the refractive index noted above, which is higher than its ordinary value, namely 2.050. Which of TiO 2  and Ta 2 O 5  to use as the material of the high-refractive-index layers  31 H and  32 H is determined in consideration of the affinity with the material of the prisms  10  and  20 , the affinity with the materials of the low-refractive-index layers  31 L and  32 L, easiness of film formation, and other factors. 
     FIGS. 4 to  6  show the relationship between the wavelength of light and transmissivity as observed in Practical Example 1. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light. FIG. 4 deals with the ray that makes an angle of 45° with a normal to the dielectric multilayer film  30  before entering the first prism  10  (i.e., in the layer of air). This ray is incident on the dielectric multilayer film  30  at an angle of incidence of 45°. FIG. 5 deals with the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film  30  before entering the first prism  10 , and FIG. 6 deals with the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . These three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45° with the dielectric multilayer film  30 . 
     As shown in FIG. 4, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45°, it is possible to separate P- and S-polarized light effectively over a wide wavelength range of from about 480 nm to about 750 nm. Moreover, as shown in FIG. 5, also with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible over a wide wavelength range of from about 410 nm to about 720 nm. Furthermore, as shown in FIG. 6, also with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible over a wavelength range of from about 470 nm to about 670 nm. 
     Thus, in this practical example, the whole beam of light having an f-number as small as 2.8 can be subjected to effective polarization separation over a wavelength range of from about 480 nm to about 670 nm. Although not shown, with a beam of light having an f-number of 3.5, the whole beam of light can be subjected to effective polarization separation all over the wavelength range of visible light. 
     PRACTICAL EXAMPLE 2 
     Table 2 shows an overview of the design of Practical Example 2. 
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Prism Vertex Angle θ: 
                 45° 
               
               
                 Prism Refractive Index Nd: 
                 1.80 
               
               
                 First Multilayer Portion 31 
               
               
                 Number of Units: 
                 10 
               
               
                 First Wavelength λ1: 
                 720 nm 
               
               
                 Material of High-Refractive-Index Layers 31H: 
                 A Mixture 
               
               
                   
                 of TiO 2   
               
               
                   
                 and 
               
               
                   
                 La 2 O 3 , 
               
               
                   
                 a Mixture 
               
               
                   
                 of TiO 2   
               
               
                   
                 and La 2 O, 
               
               
                   
                 or Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 31L: 
                 MgF 2   
               
               
                 Refractive Index NH1 of High-Refractive-Index Layers 31H: 
                 2.050 
               
               
                 Refractive Index NL1 of Low-Refractive-Index Layers 31L: 
                 1.385 
               
               
                 Refractive Index Difference | NH1 − NL1 |: 
                 0.665 
               
               
                 Angle Difference | θ1 − θ |: 
                 6.520 
               
               
                 Second Multilayer Portion 32 
               
               
                 Number of Units: 
                 11 
               
               
                 Second Wavelength λ2: 
                 864 nm 
               
               
                 Material of High-Refractive-Index Layers 32H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 32L: 
                 A Mixture 
               
               
                   
                 of Al 2 O 3   
               
               
                   
                 and La 2 O 3   
               
               
                   
                 or a Mix- 
               
               
                   
                 ture of 
               
               
                   
                 Al 2 O 3   
               
               
                   
                 and La 2 O 
               
               
                 Refractive Index NH2 of High-Refractive-Index Layers 32H: 
                 2.200 
               
               
                 Refractive Index NL2 of Low-Refractive-Index Layers 32L: 
                 1.710 
               
               
                 Refractive Index Difference | NH2 − NL2 |: 
                 0.490 
               
               
                 Angle Difference | θ2 − θ |: 
                 3.142 
               
               
                 Wavelength Ratio λ2 / λ1: 
                 1.200 
               
               
                   
               
             
          
         
       
     
     FIGS. 7 to  9  show the relationship between the wavelength of light and transmissivity as observed in Practical Example 2. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light. FIG. 7 deals with the ray that makes an angle of 45° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . FIG. 8 deals with the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film  30  before entering the first prism  10 , and FIG. 9 deals with the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . As described earlier, these three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45° with the dielectric multilayer film  30 . 
     As shown in FIG. 7, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45°, it is possible to separate P- and S-polarized light effectively over a wide wavelength range of from about 480 nm to about 750 nm. Moreover, as shown in FIG. 8, with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible over a wavelength range of from about 410 nm to about 650 nm. Furthermore, as shown in FIG. 9, with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible over a wide wavelength range of from about 450 nm to about 710 nm. Thus, in this practical example, the whole beam of light having an f-number as small as 2.8 can be subjected to effective polarization separation over a wavelength range of from about 480 nm to about 650 nm. 
     PRACTICAL EXAMPLE 3 
     Table 3 shows an overview of the design of Practical Example 3. 
     
       
         
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
             
             
               
                 Prism Vertex Angle θ: 
                 45° 
               
               
                 Prism Refractive Index Nd: 
                 1.70 
               
               
                 First Multilayer Portion 31 
               
               
                 Number of Units: 
                 11 
               
               
                 First Wavelength λ1: 
                 750 nm 
               
               
                 Material of High-Refractive-Index Layers 31H: 
                 A Mixture 
               
               
                   
                 of Al 2 O 3   
               
               
                   
                 and La 2 O 3   
               
               
                   
                 or a Mix- 
               
               
                   
                 ture of 
               
               
                   
                 Al 2 O 3   
               
               
                   
                 and La 2 O 
               
               
                 Material of Low-Refractive-Index Layers 31L: 
                 MgF 2   
               
               
                 Refractive Index NH1 of High-Refractive-Index Layers 31H: 
                 1.710 
               
               
                 Refractive Index NL1 of Low-Refractive-Index Layers 31L: 
                 1.385 
               
               
                 Refractive Index Difference | NH1 − NL1 |: 
                 0.325 
               
               
                 Angle Difference | θ1 − θ |: 
                 5.721 
               
               
                 Second Multilayer Portion 32 
               
               
                 Number of Units: 
                 11 
               
               
                 Second Wavelength λ2: 
                 830 nm 
               
               
                 Material of High-Refractive-Index Layers 32H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 32L: 
                 Al 2 O 3   
               
               
                 Refractive Index NH2 of High-Refractive-Index Layers 32H: 
                 2.200 
               
               
                 Refractive Index NL2 of Low-Refractive-Index Layers 32L: 
                 1.620 
               
               
                 Refractive Index Difference | NH2 − NL2 |: 
                 0.580 
               
               
                 Angle Difference | θ2 − θ |: 
                 6.177 
               
               
                 Wavelength Ratio λ2 / λ1: 
                 1.107 
               
               
                   
               
             
          
         
       
     
     FIGS. 10 to  12  show the relationship between the wavelength of light and transmissivity as observed in Practical Example 3. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light. FIG. 10 deals with the ray that makes an angle of 45° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . FIG. 11 deals with the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film  30  before entering the first prism  10 , and FIG. 12 deals with the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . As described earlier, these three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45° with the dielectric multilayer film  30 . 
     As shown in FIG. 10, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45°, it is possible to separate P- and S-polarized light effectively over a wavelength range of from about 480 nm to about 600 nm. Moreover, as shown in FIG. 11, with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible over a wavelength range of from about 470 nm to about 630 nm. Furthermore, as shown in FIG. 12, with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible over a wide wavelength range of from about 440 nm to about 720 nm. Thus, in this practical example, the whole beam of light having an f-number as small as 2.8 can be subjected to effective polarization separation over a wavelength range of from about 480 nm to about 600 nm. 
     PRACTICAL EXAMPLE 4 
     Table 4 shows an overview of the design of Practical Example 4. 
     
       
         
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
             
             
               
                 Prism Vertex Angle θ: 
                 45° 
               
               
                 Prism Refractive Index Nd: 
                 1.60 
               
               
                 First Multilayer Portion 31 
               
               
                 Number of Units: 
                 11 
               
               
                 First Wavelength λ1: 
                 720 nm 
               
               
                 Material of High-Refractive-Index Layers 31H: 
                 Al 2 O 3   
               
               
                 Material of Low-Refractive-Index Layers 31L: 
                 MgF 2   
               
               
                 Refractive Index NH1 of High-Refractive-Index Layers 31H: 
                 1.620 
               
               
                 Refractive Index NL1 of Low-Refractive-Index Layers 31L: 
                 1.385 
               
               
                 Refractive Index Difference | NH1 − NL1 |: 
                 0.235 
               
               
                 Angle Difference | θ1 − θ |: 
                 4.074 
               
               
                 Second Multilayer Portion 32 
               
               
                 Number of Units: 
                 13 
               
               
                 Second Wavelength λ2: 
                 850 nm 
               
               
                 Material of High-Refractive-Index Layers 32H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 32L: 
                 SiO 2   
               
               
                 Refractive Index NH2 of High-Refractive-Index Layers 32H: 
                 2.200 
               
               
                 Refractive Index NL2 of Low-Refractive-Index Layers 32L: 
                 1.460 
               
               
                 Refractive Index Difference | NH2 − NL2 |: 
                 0.740 
               
               
                 Angle Difference | θ2 − θ |: 
                 4.200 
               
               
                 Wavelength Ratio λ2 / λ1: 
                 1.181 
               
               
                   
               
             
          
         
       
     
     FIGS. 13 to  15  show the relationship between the wavelength of light and transmissivity as observed in Practical Example 4. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light. FIG. 13 deals with the ray that makes an angle of 45° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . FIG. 14 deals with the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film  30  before entering the first prism  10 , and FIG. 15 deals with the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . As described earlier, these three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45° with the dielectric multilayer film  30 . 
     As shown in FIG. 13, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45°, it is possible to separate P- and S-polarized light effectively over a wavelength range of from about 480 nm to about 590 nm. Moreover, as shown in FIG. 14, with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible over a wavelength range of from about 480 nm to about 630 nm. Furthermore, as shown in FIG. 15, with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible over, a wide wavelength range of from about 440 nm to about 750 nm. Thus, in this practical example, the whole beam of light having an f-number as small as 2.8 can be subjected to effective polarization separation over a wavelength range of from about 480 nm to about 590 nm. 
     PRACTICAL EXAMPLE 5 
     Table 5 shows an overview of the design of Practical Example 5. 
     
       
         
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
             
             
               
                 Prism Vertex Angle θ: 
                 45° 
               
               
                 Prism Refractive Index Nd: 
                 1.84 
               
               
                 First Multilayer Portion 31 
               
               
                 Number of Units: 
                 10 
               
               
                 First Wavelength λ1: 
                 740 nm 
               
               
                 Material of High-Refractive-Index Layers 31H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 31L: 
                 MgF 2   
               
               
                 Refractive Index NH1 of High-Refractive-Index Layers 31H: 
                 2.200 
               
               
                 Refractive Index NL1 of Low-Refractive-Index Layers 31L: 
                 1.385 
               
               
                 Refractive Index Difference | NH1 − NL1 |: 
                 0.815 
               
               
                 Angle Difference | θ1 − θ |: 
                 5.467 
               
               
                 Second Multilayer Portion 32 
               
               
                 Number of Units: 
                 10 
               
               
                 Second Wavelength λ2: 
                 1147 nm 
               
               
                 Material of High-Refractive-Index Layers 32H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 32L: 
                 A Mixture 
               
               
                   
                 of Al 2 O 3   
               
               
                   
                 and La 2 O 3   
               
               
                   
                 or a Mix- 
               
               
                   
                 ture of 
               
               
                   
                 Al 2 O 3   
               
               
                   
                 and La 2 O 
               
               
                 Refractive Index NH2 of High-Refractive-Index Layers 32H: 
                 2.200 
               
               
                 Refractive Index NL2 of Low-Refractive-Index Layers 32L: 
                 1.710 
               
               
                 Refractive Index Difference | NH2 − NL2 |: 
                 0.490 
               
               
                 Angle Difference | θ2 − θ |: 
                 2.156 
               
               
                 Wavelength Ratio λ2 / λ1: 
                 1.550 
               
               
                   
               
             
          
         
       
     
     The design of this practical example is the same as that of Practical Example 1 except for the second wavelength λ 2 , with the result that here the wavelength ratio λ 2 /λ 1  is 1.550. FIGS. 16 to  18  show the relationship between the wavelength of light and transmissivity as observed in Practical Example 5. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light. FIG. 16 deals with the ray that makes an angle of 45° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . FIG. 17 deals with the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film  30  before entering the first prism  10 , and FIG. 18 deals with the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . As described earlier, these three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45° with the dielectric multilayer film  30 . 
     As shown in FIG. 16, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45°, it is possible to separate P- and S-polarized light effectively over a wavelength range of from about 490 nm to about 670 nm. Moreover, as shown in FIG. 17, with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible over a wide wavelength range of from about 410 nm to about 720 nm. Furthermore, as shown in FIG. 18, with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible over a wavelength range of from about 470 nm to about 720 nm, excluding the wavelength range of from about 620 nm to about 630 nm. 
     Thus, in this practical example, the whole beam of light having an f-number as small as 2.8 can be subjected to largely effective polarization separation over a wavelength range of from about 480 nm to about 720 nm. In the wavelength range of from about 620 nm to about 630 nm, however, when the angle of incidence with respect to the dielectric multilayer film is large, some S-polarized light is transmitted. This transmission of S-polarized light can be reduced to almost zero (not shown) by increasing the f-number to 3.5, but it can safely be said that it is preferable to restrict the wavelength ratio λ 2 /λ 1  to 1.55 or lower when a beam of light having an f-number of about 3 or less is to be subjected to polarization separation. 
     PRACTICAL EXAMPLE 6 
     Table 6 shows an overview of the design of Practical Example 6. 
     
       
         
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
             
             
               
                 Prism Vertex Angle θ: 
                 45° 
               
               
                 Prism Refractive Index Nd: 
                 1.84 
               
               
                 First Multilayer Portion 31 
               
               
                 Number of Units: 
                 10 
               
               
                 First Wavelength λ1: 
                 740 nm 
               
               
                 Material of High-Refractive-Index Layers 31H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 31L: 
                 SiO 2   
               
               
                 Refractive Index NH1 of High-Refractive-Index Layers 31H: 
                 2.200 
               
               
                 Refractive Index NL1 of Low-Refractive-Index Layers 31L: 
                 1.460 
               
               
                 Refractive Index Difference | NH1 − NL1 |: 
                 0.740 
               
               
                 Angle Difference | θ1 − θ |: 
                 3.652 
               
               
                 Second Multilayer Portion 32 
               
               
                 Number of Units: 
                 10 
               
               
                 Second Wavelength λ2: 
                 1132.2 nm 
               
               
                 Material of High-Refractive-Index Layers 32H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 32L: 
                 Al 2 O 3   
               
               
                 Refractive Index NH2 of High-Refractive-Index Layers 32H: 
                 2.200 
               
               
                 Refractive Index NL2 of Low-Refractive-Index Layers 32L: 
                 1.620 
               
               
                 Refractive Index Difference | NH2 − NL2 |: 
                 0.580 
               
               
                 Angle Difference | θ2 − θ |: 
                 0.107 
               
               
                 Wavelength Ratio λ2 / λ1: 
                 1.530 
               
               
                   
               
             
          
         
       
     
     FIGS. 19 to  21  show the relationship between the wavelength of light and transmissivity as observed in Practical Example 6. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light. FIG. 19 deals with the ray that makes an angle of 45° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . FIG. 20 deals with the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film  30  before entering the first prism  10 , and FIG. 21 deals with the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . As described earlier, these three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45° with the dielectric multilayer film  30 . 
     As shown in FIG. 19, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45°, it is possible to separate P- and S-polarized light effectively over a wavelength range of from about 510 nm to about 750 nm. Moreover, as shown in FIG. 20, with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible over a wide wavelength range of from about 430 nm to about 720 nm. Furthermore, as shown in FIG. 21, with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible over a wavelength range of from about 480 nm to about 640 nm. Thus, in this practical example, the whole beam of light having an f-number as small as 2.8 can be subjected to effective polarization separation over a wavelength range of from about 510 nm to about 640 nm. 
     PRACTICAL EXAMPLE 7 
     Table 7 shows an overview of the design of Practical Example 7. 
     
       
         
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
             
             
               
                 Prism Vertex Angle θ: 
                 45° 
               
               
                 Prism Refractive Index Nd: 
                 1.80 
               
               
                 First Multilayer Portion 31 
               
               
                 Number of Units: 
                 10 
               
               
                 First Wavelength λ1: 
                 740 nm 
               
               
                 Material of High-Refractive-Index Layers 31H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 31L: 
                 SiO 2   
               
               
                 Refractive Index NH1 of High-Refractive-Index Layers 31H: 
                 2.200 
               
               
                 Refractive Index NL1 of Low-Refractive-Index Layers 31L: 
                 1.460 
               
               
                 Refractive Index Difference | NH1 − NL1 |: 
                 0.740 
               
               
                 Angle Difference | θ1 − θ |: 
                 2.630 
               
               
                 Second Multilayer Portion 32 
               
               
                 Number of Units: 
                 11 
               
               
                 Second Wavelength λ2: 
                 1117.4 nm 
               
               
                 Material of High-Refractive-Index Layers 32H: 
                 TiO 2  or 
               
               
                   
                 Ta 2 O 5   
               
               
                 Material of Low-Refractive-Index Layers 32L: 
                 Al 2 O 3   
               
               
                 Refractive Index NH2 of High-Refractive-Index Layers 32H: 
                 2.200 
               
               
                 Refractive Index NL2 of Low-Refractive-Index Layers 32L: 
                 1.620 
               
               
                 Refractive Index Difference | NH2 − NL2 |: 
                 0.580 
               
               
                 Angle Difference | θ2 − θ |: 
                 1.275 
               
               
                 Wavelength Ratio λ2 / λ1: 
                 1.510 
               
               
                   
               
             
          
         
       
     
     FIGS. 22 to  24  show the relationship between the wavelength of light and transmissivity as observed in Practical Example 7. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light. FIG. 22 deals with the ray that makes an angle of 45° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . FIG. 23 deals with the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film  30  before entering the first prism  10 , and FIG. 24 deals with the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film  30  before entering the first prism  10 . As described earlier, these three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45° with the dielectric multilayer film  30 . 
     As shown in FIG. 22, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45°, it is possible to separate P- and S-polarized light effectively over a wavelength range of from about 520 nm to about 750 nm. Moreover, as shown in FIG. 23, with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible over wavelength ranges of from about 430 nm to about 520 nm and from about 580 nm to about 720 nm. Furthermore, as shown in FIG. 24, with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible over a wavelength range of from about 500 nm to about 670 nm. Thus, in this practical example, the whole beam of light having an f-number as small as 2.8 can be subjected to effective polarization separation over a wavelength range of from about 580 nm to about 670 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.