the optical selective film of the present invention is an optical selective film that includes at least an ag - containing layer and an ag anti - diffusion layer situated adjacent to the ag - containing layer , wherein the ag anti - diffusion layer includes fesi x ( x = 1 to 2 ). the optical selective film of the present invention can contribute to efficient conversion of light to heat . specifically , the ag anti - diffusion layer composing the optical selective film of the present invention prevents the ag ( silver ) in the ag - containing layer from diffusing into the other layers composing the optical selective film when the optical selective film of the present invention has been heated , thus contributing to maintenance of stable optical characteristics . the temperature at which the optical selective film of the present invention is heated by collected sunlight and the heating medium may be 400 ° c . or higher , and is preferably 600 ° c . or higher and more preferably 700 ° c . or higher . the optical selective film of the present invention can maintain stable optical characteristics even when exposed to high temperature during use . the ag - containing layer composing the optical selective film of the present invention contains ag ( silver ), and the ag ( silver ) may be pure silver composed of simple elemental silver , or it may be a silver alloy composed of elemental silver and another metal element , or it may be metallic silver composed of elemental silver and a non - metal element . examples of other metal elements include gold ( au ), copper ( cu ) and pd ( palladium ). si ( silicon ) is an example of a non - metal element . the ag - containing layer composing the optical selective film of the present invention may contain ag ( silver ) in any desired proportion , however preferably it contains it at at least 80 vol % to 100 vol %. the ag anti - diffusion layer composing the optical selective film of the present invention is situated adjacent to the ag - containing layer . being situated adjacent means that at least a portion of the ag anti - diffusion layer and at least a portion of the ag - containing layer are situated in contact with each other . in the optical selective film of the present invention , the ag anti - diffusion layer is preferably layered on the ag - containing layer . the ag anti - diffusion layer composing the optical selective film of the present invention may include fesi x ( x = 1 to 2 ) in any desired proportion , however it preferably includes it at at least 80 vol % to 100 vol %. the fesi x in the ag anti - diffusion layer composing the optical selective film of the present invention is preferably fesi 2 ( x = 2 ). by including fesi 2 ( x = 2 ) in the ag anti - diffusion layer , the optical selective film of the present invention can further contribute to efficient conversion of light to heat . the fesi 2 ( x = 2 ) in the ag anti - diffusion layer composing the optical selective film of the present invention is preferably β - fesi 2 . by including β - fesi 2 in the ag anti - diffusion layer , the optical selective film of the present invention can still further contribute to efficient conversion of light to heat . the fesi 2 ( x = 2 ) in the ag anti - diffusion layer composing the optical selective film of the present invention is also preferably α - fesi 2 . by including α - fesi 2 in the ag anti - diffusion layer , the optical selective film of the present invention can yet further contribute to efficient conversion of light to heat . the ag - containing layer composing the optical selective film of the present invention is preferably an infrared - reflecting film . the thickness of the infrared - reflecting film may be any desired thickness so long as the effect of the present invention is exhibited , and it is preferably a thickness of at least 100 nm . the ag - containing layer composing the optical selective film of the present invention may also be a photoabsorbing layer . if the ag - containing layer composing the optical selective film of the present invention is to be a photoabsorbing layer , it may be a cermet layer ( ceramic + metal = cermet ). a cermet layer composing the optical selective film of the present invention may be composed of a composite material of silver particles or silver nanoparticles and aluminum oxide ( al 2 o 3 ), it may be composed of a composite material of silver particles or silver nanoparticles and zirconium oxide ( zro or zro 2 ), or it may be composed of a composite material of silver particles or silver nanoparticles and silicon dioxide ( sio 2 ). the thickness of the cermet layer is preferably 60 nm to 180 nm and more preferably 80 nm to 150 nm . the optical selective film of the present invention may include a transparent dielectric layer as an anti - reflection layer . the transparent dielectric layer composing the optical selective film of the present invention is not particularly restricted , and examples include a sio 2 layer , al 2 o 3 layer , aln layer or the like , with a sio 2 layer being preferred . the thickness of the transparent dielectric layer composing the optical selective film of the present invention may be any desired thickness so long as the effect of the present invention is exhibited , and it is preferably a thickness of 10 nm to 500 nm . the optical selective film of the present invention can be obtained by any desired publicly known production method . the optical selective film of the present invention can be produced , for example , by physical vapor phase deposition ( pvd ), sputtering or the like , with the substrate temperature of the substrate of the optical selective film of the present invention at room temperature . the optical selective film of the present invention will now be explained in greater detail with reference to fig1 and fig2 . incidentally , the optical selective film of the present invention is not limited to the embodiment of the present invention shown in fig1 and fig2 , so long as it is within the scope of the object and gist of the present invention . fig1 is a cross - sectional schematic drawing of an optical selective film 1 , as a preferred embodiment of an optical selective film of the present invention . the optical selective film 1 according to an embodiment of the present invention has an ag anti - diffusion layer ( 12 ) laminated on an ag - containing layer ( 11 ). the optical selective film 1 is an optical selective film in which the ag anti - diffusion layer ( 12 ) is laminated on one side of the ag - containing layer ( 11 ). fig2 is a cross - sectional schematic drawing of an optical selective film 2 , as another preferred embodiment of an optical selective film of the present invention . the optical selective film 2 of this embodiment of the present invention has an ag - containing layer ( 22 ) laminated on an ag anti - diffusion layer ( 21 ), with an ag anti - diffusion layer ( 23 ) additionally laminated on the ag - containing layer ( 22 ). the optical selective film 2 is an optical selective film in which the ag anti - diffusion layer ( 21 ) and the ag anti - diffusion layer ( 23 ) are laminated on either side of the ag - containing layer ( 22 ). examples will now be provided for a more concrete explanation of the present invention . incidentally , the present invention is not limited to these examples , so long as it is within the scope of the object and gist of the present invention . the optical selective film 3 illustrated in fig3 was fabricated . an ag layer ( 31 ) was laminated on a quartz substrate ( not shown ) at room temperature by sputtering ag ( silver ) as the target , and then fesi 2 was used as the target for sputtering to form a fesi 2 layer ( 32 ) over the ag layer ( 31 ), forming an optical selective film 3 . the optical selective film 3 was then heated at 700 ° c . for 1 hour . fig4 shows a cross - sectional schematic drawing of the optical selective film 3 before and after heating . as shown in fig4 , the ag ( silver ) particles ( 33 ) in the ag layer ( 31 ) are not diffused in the fesi 2 layer ( 32 ) after heating . next , in order to verify that the ag ( silver ) particles ( 33 ) in the ag layer ( 31 ) had not diffused in the fesi 2 layer ( 32 ) after heating , the measured value for the optical characteristics of the optical selective film 3 after heating was compared with the calculated value for the optical characteristics of the optical selective film 3 and evaluated . first , single - layer films were formed of each of the ag layer ( 31 ) and fesi 2 layer ( 32 ), as the constituent layers of the optical selective film 3 after heating , and measured with a spectroscopic ellipsometer and a spectrophotometer , and the optical constants ( refractive index n 1 and extinction coefficient k 1 ) of the ag layer ( 31 ) and the optical constants ( refractive index n 2 and extinction coefficient k 2 ) of the fesi 2 layer ( 32 ) of the optical selective film 3 were each calculated from the measurement data from the spectroscopic ellipsometer and the reflectance properties and transmittance properties measured by the spectrophotometer . based on the optical constants ( refractive index n 1 and extinction coefficient k 1 ) of the ag layer ( 31 ) and the optical constants ( refractive index n 2 and extinction coefficient k 2 ) of the fesi 2 layer ( 32 ) which were calculated , an analogous multilayer film was used to calculate the optical reflectance (%) of the optical selective film 3 ( corresponding film thicknesses of 25 nm for the fesi 2 layer and 100 nm for the ag layer ). fig5 shows the results of comparing the measured value for the optical reflectance (%) of the optical selective film 3 measured with a spectrophotometer , and the value calculated for the optical reflectance . as shown in fig5 , the measured value and the calculated value for the optical reflectance (%) of the optical selective film 3 essentially coincide over the entire wavelength range , and it was confirmed that the infrared ray reflection function of the ag layer ( 31 ) was not reduced after heating . thus , it was verified that the ag ( silver ) particles ( 33 ) in the ag layer ( 31 ) were not diffused in the fesi 2 layer ( 32 ) after heating , as illustrated in fig4 . the film 4 shown in fig6 was fabricated . an ag layer ( 34 ) was laminated on a quartz substrate ( not shown ) at room temperature by sputtering ag ( silver ) as the target , and then sio 2 was used as the target for sputtering to form a sio 2 layer ( 35 ) over the ag layer ( 34 ), forming a film 4 . the film 4 was then heated at 700 ° c . for 1 hour . fig7 shows a cross - sectional schematic drawing of the film 4 before and after heating . as shown in fig7 , the ag ( silver ) particles ( 36 ) in the ag layer ( 34 ) are diffused throughout the entire film 4 after heating . next , in order to verify that the ag ( silver ) particles ( 36 ) in the ag layer ( 34 ) had diffused throughout the entire film 4 , the measured value for the optical characteristics of the film 4 after heating was compared with the calculated value for the optical characteristics of the film 4 and evaluated . first , single - layer films were formed of each of the ag layer ( 34 ) and sio 2 layer ( 35 ), as the constituent layers of the film 4 after heating , and measured with a spectroscopic ellipsometer and a spectrophotometer , and the optical constants ( refractive index n 1 and extinction coefficient k 1 ) of the ag layer ( 34 ) and the optical constants ( refractive index n 3 and extinction coefficient k 3 ) of the sio 2 layer ( 35 ) of the film 4 were each calculated from the measurement data from the spectroscopic ellipsometer and the reflectance properties and transmittance properties measured by the spectrophotometer . based on the optical constants ( refractive index n 1 and extinction coefficient k 1 ) of the ag layer ( 34 ) and the optical constants ( refractive index n 3 and extinction coefficient k 3 ) of the sio 2 layer ( 35 ) which were calculated , an analogous multilayer film was used to calculate the optical reflectance (%) of the film 4 ( corresponding film thicknesses of 100 nm for the sio 2 layer and 100 nm for the ag layer ). fig8 shows the results of comparing the measured value for the optical reflectance (%) of the film 4 measured with a spectrophotometer , and the value calculated for the optical reflectance . as shown in fig8 , the measured value and the calculated value for the optical reflectance (%) of the film 4 differed considerably , the measured value of the optical reflectance (%) being reduced to about 40 % from that at a wavelength of about 1000 nm , and it was confirmed that the infrared ray reflection function of the ag layer ( 34 ) was reduced after heating . thus , it was verified that the ag ( silver ) particles ( 36 ) in the ag layer ( 34 ) were diffused throughout the entire film 4 after heating , as illustrated in fig7 . in addition , as shown by the optical microscope photograph in fig9 , it was confirmed that the ag ( silver ) particles ( 36 ) in the ag layer ( 34 ) had diffused through the entire film 4 after heating , with the ag ( silver ) particles ( 37 ) and sio 2 particles ( 38 ) being mixed by the diffusion of the ag ( silver ) particles ( 37 ).