This disclosure provides a highly accurate NDIR gas sensor and a highly accurate optical device even using a simplified optical filter. The NDIR gas sensor and the optical device include: an optical filter having a substrate and a multilayer film on the substrate; and an infrared light emitting and receiving device; where the multilayer film has a structure in which a first layer and a second layer are alternately stacked; the active layer contains Al.sub.xIn.sub.1-xSb or InAs.sub.ySb.sub.1-y; and the optical filter includes a wavelength range having an average transmittance of 70% or more with a width of 50 nm or more in 2400-6000 nm, and has a maximum transmittance of 5% or more in 6000-8000 nm and an average transmittance of 2% or more and 60% or less in 6000-8000 nm.

An optical device comprises an optical filter having a substrate and a multilayer film having layers with different refractive indexes formed on at least one side of the substrate; and an infrared light emitting and receiving device having a first conductive-type semiconductor layer, an active layer, and a second conductive-type semiconductor layer. The multilayer film has alternatively stacked first second layers each having refractive indexes of 1.2 or more and 2.5 or less, and 3.2 or more and 4.2 or less, respectively, in a wavelength range of 2400 nm to 6000 nm. 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 to 8000 nm.

An optical device includes 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. 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 the mid-infrared range.

The invention provides a material characterisation system and method for characterising a stream of material emanating from a material identification, exploration, extraction or processing activity, the system comprising: a source of incident radiation (64) configured to irradiate the stream of material in an irradiation region (18); one or more detectors (300,302,312,1701,1704,1600,1607,1608,1604) adapted to detect radiation emanating from within or passing through the stream of material as a result of the irradiation by the incident radiation (1700) and thereby produce a detection signal (313); and one or more digital processors (304-311,2000-2009) configured to process the detection signal to characterise the stream of material; wherein the source of incident radiation (64) and the one or more detectors (300,302,312,1701,1704,1600,1607,1608,1604) are adapted to be disposed relative to the stream of material so as to irradiate the stream of material and detect the radiation emanating from within or passing through the stream (1700) as the stream passes through the irradiation region (18).

The invention provides a system and method for characterising at least part of a material comprising: a source of incident X-rays (4, 28) configured to irradiate at least part of the material; one or more detectors (300,302,312,1701,1704,1600,1607,1608,1604) adapted to detect radiation emanating from within or passing through the material as a result of the irradiation by the incident radiation (1700) and thereby produce a detection signal (313); and one or more digital processors (304-311,2000-2009) configured to process the detection signal (313) to characterise at least part of the material; wherein the one or more detectors (300,302,312,1701, 1704,1600,1607,1608,1604) and one or more digital processors (304-311,2000-2009) are configured to characterise at least part of the material by performing energy resolved photon counting X-ray transmission spectroscopy analysis.

An optical device includes 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. 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 the mid-infrared range.