BAND-PASS FILTER

1 . A band-pass filter comprising a plurality of films, having a reflective band centered on a predetermined wavelength λ and having a transmission band in a part within the reflective band,

wherein the band-pass filter further comprises an additional layer formed of two continuous layers of a high refractive index film with an optical film thickness of λ/4.5 or less and a low refractive index film with an optical film thickness of λ/4.5 or less, inserted in at least one boundary out of boundaries of the plurality of films.

2 . The band-pass filter according to claim 1 , comprising a plurality of the additional layers,

wherein each of the additional layers inserted in an arbitrary plurality of boundaries out of the boundaries, respectively.

3 . The band-pass filter according to claim 1 , comprising two or more sets of the additional layers continuously inserted in an arbitrary boundary.

4 . The band-pass filter according to claim 1 , wherein at least one boundary out of boundaries reflecting at the wavelength λ is a boundary interposed between two metal films, and

wherein the additional layer is inserted in the boundary interposed between the two metal films.

2 . The band-pass filter according to claim 2 , wherein at least one boundary out of boundaries reflecting at the wavelength λ is a boundary interposed between two metal films, and

wherein the additional layer is inserted in the boundary interposed between the two metal films.

6 . The band-pass filter according to claim 3 , wherein at least one boundary out of boundaries reflecting at the wavelength λ is a boundary interposed between two metal films, and

wherein the additional layer is inserted in the boundary interposed between the two metal films.

7 . The band-pass filter according to claim 4 , comprising a plurality of the boundaries each interposed between two metal films,

wherein at least one set of the additional layer is inserted in each of the plurality of the boundaries each interposed between two metal films.

8 . The band-pass filter according to claim 5 , comprising a plurality of the boundaries each interposed between two metal films,

wherein at least one set of the additional layer is inserted in each of the plurality of the boundaries each interposed between two metal films.

9 . The band-pass filter according to claim 6 , comprising a plurality of the boundaries each interposed between two metal films,

wherein at least one set of the additional layer is inserted in each of the plurality of the boundaries each interposed between two metal films.

10 . The band-pass filter according to claim 1 , which is imparted with a characteristic capable of transmitting a specific band in one or both sub-transmission bands on a shorter wavelength side and a longer wavelength side than the wavelength λ.

11 . The band-pass filter according to claim 10 , wherein the sub-transmission band is a visible light band, and

a first wavelength band on a longer wavelength side than the visible light band has, from the shorter wavelength side, a first transmission prevention band exhibiting transmission prevention characteristics, the transmission band, and a second transmission prevention band exhibiting transmission prevention characteristics.

12 . The band-pass filter according to claim 11 , wherein the first wavelength band is on 650 nm or more.

13 . The band-pass filter according to claim 11 , wherein the second transmission prevention band is on 1100 nm or more.

14 . The band-pass filter according to claim 11 , wherein the transmission band has a central wavelength within a range of from 780 nm to 950 nm.

15 . The band-pass filter according to claim 11 , wherein the transmission band has a band width of 200 nm or less.

16 . The band-pass filter according to claim 11 , having a transmittance in the visible light region and a transmittance in the transmission band both being 60% or more.

11 . band-pass filter according to claim 11 , having a transmittance in the first transmission prevention band and a transmittance in the second transmission prevention band both being 30% or less.

18 . The band-pass filter according to claim 1 , comprising a substrate and a multilayer film on one main surface of the substrate,

wherein the multilayer film comprises the additional layer.