A viewing device for viewing a first image overlain with a second image comprising a view of an outside scene in reduced lighting conditions, wherein the second image comprises radiation in a second, infra-red wavelength range, the first wavelength range and the second wavelength range being separate wavelength ranges; comprising: an image projector for generating the first image comprising radiation in a first wavelength range characteristic of green light; an image combiner comprising a waveguide having an input diffraction grating for receiving radiation in the first range of wavelengths from the image projector and for directing the received radiation along the waveguide towards an output diffraction grating of a lower diffraction efficiency than that of the input diffraction grating for the radiation from the image projector, the output diffraction grating being arranged to couple the radiation in the first range of wavelengths out of the waveguide along a viewing direction, the waveguide and the output diffraction grating being substantially transparent to radiation in the second, infra-red range of wavelengths passing therethrough when received along the viewing direction from an external scene; and a housing incorporating: a lens arrangement having an inlet aperture for receiving, along the viewing direction, a combined radiation of the first image in the first range of wavelengths combined with radiation in the second, infra-red range of wavelengths received from the outside scene; an image intensifier arranged to receive the combined radiation via the inlet aperture and to render discernible to a viewer the radiation in the second, infra-red range of wavelengths; and an outlet for outputting radiation intensified by the image intensifier such that a viewer may view the first image overlain upon a discernible view of the external scene in reduced lighting conditions. an image projector for generating the first image comprising radiation in a first wavelength range characteristic of green light; an image combiner comprising a waveguide having an input diffraction grating for receiving radiation in the first range of wavelengths from the image projector and for directing the received radiation along the waveguide towards an output diffraction grating of a lower diffraction efficiency than that of the input diffraction grating for the radiation from the image projector, the output diffraction grating being arranged to couple the radiation in the first range of wavelengths out of the waveguide along a viewing direction, the waveguide and the output diffraction grating being substantially transparent to radiation in the second, infra-red range of wavelengths passing therethrough when received along the viewing direction from an external scene; and a housing incorporating: a lens arrangement having an inlet aperture for receiving, along the viewing direction, a combined radiation of the first image in the first range of wavelengths combined with radiation in the second, infra-red range of wavelengths received from the outside scene; an image intensifier arranged to receive the combined radiation via the inlet aperture and to render discernible to a viewer the radiation in the second, infra-red range of wavelengths; and an outlet for outputting radiation intensified by the image intensifier such that a viewer may view the first image overlain upon a discernible view of the external scene in reduced lighting conditions. a lens arrangement having an inlet aperture for receiving, along the viewing direction, a combined radiation of the first image in the first range of wavelengths combined with radiation in the second, infra-red range of wavelengths received from the outside scene; an image intensifier arranged to receive the combined radiation via the inlet aperture and to render discernible to a viewer the radiation in the second, infra-red range of wavelengths; and an outlet for outputting radiation intensified by the image intensifier such that a viewer may view the first image overlain upon a discernible view of the external scene in reduced lighting conditions.