A lens assembly 10 for an infrared window through which infrared inspection of an apparatus within a housing is performed when the lens assembly is mounted in an aperture provided in the housing. The lens assembly includes an infrared transmitting pane 11 having two generally opposed surfaces 14; and a first protective grille 12 lying against one of the opposed surfaces 14 and having an array of holes 20 formed therethrough that permit infrared inspection of the apparatus through the pane. A second protective grille 12 may be provided on the opposite face and there can be alignment means 21, 32 to ensure correct alignment of each with respect to the other.