1. A method for detecting a target gas in a target scene comprising: capturing a first image of the target scene using a detector element sensitive over a range of wavelengths of light including a first wavelength, the first wavelength being such that the target gas does not significantly absorb light of the first wavelength; illuminating a first portion of the target scene with light of the first wavelength; capturing a second image of the target scene, the second image being representative of the target scene illuminated by light of the first wavelength; comparing corresponding portions of the first image and the second image; generating a gas detection image in which, for a given one or more pixels in the corresponding portions, in the condition that the corresponding one or more pixels in the first image and the second image are not sufficiently different, considering such one or more pixels to be inoperable gas detection pixels; and in the condition that the corresponding one or more pixels in the first image and the second image are sufficiently different, considering such one or more pixels to be operable gas detection pixels; and displaying the gas detection image in which such pixels considered to be inoperable gas detection pixels are presented in a distinguishing way from such pixels considered to be operable gas detection pixels.