Gas sensor and optical device

A gas sensor, the gas sensor comprising:

• a light emitter that emits light as infrared light;
• a detector that detects a signal based on light emitted from the light emitter;
• a light guide that at least includes a mirror and reflects the light to form an optical path in which the light emitted from the light emitter passes through an introduced gas;
• and
• an optical filter that is disposed in the optical path to limit a transmission wavelength band of the light;
• wherein
• a base material of the optical filter has a refractive index of 1.5 or more in the transmission wavelength band,
• a ratio of an area of the optical filter in a planar view to an area of the light emitter is in a range of 1 to 1.2, and
• the optical filter satisfies 0.25<(T/Lf)≤0.3 where T [mm] is a thickness of the optical filter and Lf [mm] is a maximum length of the optical filter in a longitudinal direction in the planar view, wherein the optical filter is positioned to satisfy 0.9≤(Lf×(√d))≤2.5 where d [mm] is the shortest distance between a surface of the optical filter on a side opposite to the light emitter and the mirror of the light guide which the light first reaches.

An optical device, the optical device comprising:

• a light emitter that emits light as infrared light;
• a light guide that at least includes a mirror and reflects the light to form an optical path in which the light emitted from the light emitter passes through an introduced gas;
• and
• an optical filter that is disposed in an optical path to limit a transmission wavelength band of the light, wherein
• a base material of the optical filter has a refractive index of 1.5 or more in the transmission wavelength band, and
• the optical filter satisfies 0.3<(T/Lf)<1.3 where T [mm] is a thickness of the optical filter and Lf [mm] is a maximum length of the optical filter in a longitudinal direction in a planar view, wherein
• the optical filter is positioned to satisfy 0.9<(Lf×(√d)≤2.5 where d [mm] is the shortest distance between a surface of the optical filter on a side opposite to the light emitter and the mirror of the light guide which the light first reaches.