A thermal infrared sensor for gas measurement including a sensing element. The sensing element includes a thermal detection layer that outputs an electric signal based on a temperature change, a light-receiving surface electrode disposed on a light-receiving surface of the thermal detection layer, and a back electrode disposed on the thermal detection layer opposite the light-receiving surface electrode. The light-receiving surface electrode has a periodic structure configured to selectively absorb infrared light having an absorption wavelength of a sample gas.

An optical system includes a light source, a multipass cell, and a detector. The multipass cell includes a first mirror and a second mirror spaced apart from the first mirror to form an optical cavity. A central axis extends between the first mirror and the second mirror. The light source outputs a light beam and is arranged to inject the light beam into the optical cavity of the multipass cell at an injection angle such that propagation of the light beam into the optical cavity is non-paraxial. The non-paraxial injection path and large-angle first reflection of the light beam causes the light beam to reflect back-and-forth between the first mirror and the second mirror to form a final (or cumulative) spot pattern that covers a large proportion of the surface areas of the first mirror and the second mirror before it exits the optical cavity.