Non-polarizing beam splitter

1. A non-polarizing beam splitter comprising:

a first prism;

a ground layer made of a dielectric material which is formed on a slant face of the first prism;

an Au layer having thickness of 15.5 to 35 nm which is formed on the ground layer;

a protective layer made of a dielectric material which is formed on the Au layer; and

a second prism which is jointed to an outermost layer of the protective layer via adhesive, wherein the non-polarizing beam splitter divides light at wavelength of 640 nm to 820 nm into transmission light and reflection light by a predetermined ratio, wherein the following conditions are satisfied: 25.2X 2 +12.4X+6.84<D<31.6X 2 .+12.6X+10.4, R+T>88 (%) where,

X=R /( R+T )

R designates an average of reflectance of P polarized light and reflectance of S polarized light at the wavelengths of 640 nm to 680 nm and 760 nm to 810 nm,

T designates an average of transmittance of the P polarized light and transmittance of the S polarized light at wavelengths of 640 nm to 680 nm and 760 nm to 810 nm,

D designates a thickness of the Au layer (nm).

2. The non-polarizing beam splitter according to claim 1 , wherein in wavelength ranges of 640 nm to 680 nm and 760 nm to 810 nm, a maximum value of a difference between the reflectance of P polarized light and the reflectance of S polarized light is 10% or less, and a maximum value of a difference between the transmittance of the P polarized light and the transmittance of the S polarized light is 10% or less.

3. The non-polarizing beam splitter according to claim 1 , wherein a maximum value of a difference between average reflectance of P polarized light and S polarized light at wavelength of 640 nm to 680 nm and average reflectance of the P polarized light and the S polarized light at wavelength of 760 nm to 810 nm is 10% or less, a maximum value of a difference between average transmittance of the P polarized light and the S polarized light at wavelength of 640 nm to 680 nm and average transmittance of the P polarized light and the S polarized light at wavelength of 760 nm to 810 nm is 10% or less.

4. A non-polarizing beam splitter comprising:

a first prism;

a ground layer made of a dielectric material which is formed on a slant face of the first prism;

an Au layer having thickness of 15.5 to 35 nm which is formed on the ground layer;

a protective layer made of a dielectric material which is formed on the Au layer; and

a second prism which is jointed to an outermost layer of the protective layer via adhesive, wherein the non-polarizing beam splitter divides light at wavelength of 640 nm to 820 nm into transmission light and reflection light by a predetermined ratio, wherein the following conditions are satisfied: 28.4X 2 +10.6X+7.55<D<25.2X 2 +17.1X+8.99, R+T>88(%) where,

X=R /( R+T )

R designates an average of reflectance of P polarized light and reflectance of S polarized light at the wavelengths of 640 nm to 680 nm and 760 nm to 810 nm,

T designates an average of transmittance of the P polarized light and transmittance of the S polarized light at wavelengths of 640 nm to 680 nm and 760 nm to 810 nm, D designates a thickness of the Au layer (nm).

5. The non-polarizing beam splitter according to claim 1 , wherein the protective layer includes:

a high refractive index layer having refractive index of 1.8 or more;

a intermediate refractive index layer having refractive index of 1.5 to 1.8 (not including 1.5 and 1.8); and

a low refractive index layer having refractive index of 1.5 or less.

6. The non-polarizing beam splitter according to claim 1 , wherein the layer adjacent to the Au layer in the ground layer and the protective layer is the intermediate refractive index layer.

7. The non-polarizing beam splitter according to claim 1 , wherein total thickness of the protective layer is 400 nm or more.

8. The non-polarizing beam splitter according to claim 1 , further comprising six or more additional protective layers.

9. The non-polarizing beam splitter according to claim 1 , further comprising one or more additional ground layers.

10. The non-polarizing beam splitter according to claim 1 , wherein a maximum value of a difference between average reflectance of the P polarized light and the S polarized light at wavelength of 640 nm to 680 nm and average reflectance of the P polarized light and the S polarized light at wavelength of 760 nm to 810 nm is 10% or less, a maximum value of a difference between average transmittance of the P polarized light and the S polarized light at wavelength of 640 nm to 680 nm and average transmittance of the P polarized light and the S polarized light at wavelength of 760 nm to 810 nm is 10% or less.

11. The non-polarizing beam splitter according to claim 4 , wherein a maximum value of a difference between average reflectance of P polarized light and S polarized light at wavelength of 640 nm to 680 nm and average reflectance of P polarized light and S polarized light at wavelength of 760 nm to 810 nm is 10% or less, a maximum value of a difference between average transmittance of the P polarized light and the S polarized light at wavelength of 640 nm to 680 nm and average transmittance of the P polarized light and the S polarized light at wavelength of 760 nm to 810 nm is 10% or less.

12. The non-polarizing beam splitter according to claim 10 , further comprising six or more additional protective layers, and

further comprising one or more additional ground layers.

13. The non-polarizing beam splitter according to claim 4 , wherein in wavelength ranges of 640 nm to 680 nm and 760 nm to 810 nm, a maximum value of a difference between the reflectance of the P polarized light and the reflectance of the S polarized light is 10% or less, and a maximum value of a difference between the transmittance of the P polarized light and the transmittance of the S polarized light is 10% or less.