| CPC G01N 21/3504 (2013.01) [G01N 21/314 (2013.01); G01N 21/61 (2013.01); G01N 21/93 (2013.01); G01N 33/0006 (2013.01); G01N 35/00693 (2013.01); G02B 5/281 (2013.01); G02B 5/285 (2013.01); G01N 2021/933 (2013.01); G01N 2035/00702 (2013.01); G01N 2201/0686 (2013.01); G01N 2223/638 (2013.01)] | 17 Claims |

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1. An optical device comprising:
an optical filter having a substrate and a multilayer film having a plurality of layers with different refractive indexes formed on at least one side of the substrate; and
an infrared light emitting and receiving device; wherein
the optical filter includes a wavelength range having an average transmittance of 70% or more with a width of 50 nm or more in a wavelength range of 2400 nm to 6000 nm, and has a maximum transmittance of 5% or more in a wavelength range of 6000 nm or more of a mid-infrared range.
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