A temperature invariant narrow bandpass filter 5 to transmit mid-infrared radiation that may be used in a mid-infrared sensor for monitoring a species, which may be a component of a fluid or a solid material. The filter has a substrate 14, a first stack deposited on the substrate, a cavity 16 deposited on the first stack and a second stack deposited on the cavity. The first and second stacks are made from alternating high 10 and low 12 refractive index materials, and the cavity is a low refractive index material. By providing a high ratio of low refractive index material in the filter with respective to high refractive index material, the filter is configured so that wavelength transmission remains constant with varying temperature. The high refractive index material has a refractive index that decreases with temperature and the cavity is at least three full wavelengths or three half wavelengths of the transmission wavelength or the narrow band of transmission wavelengths
