Apparatus, including a housing that is configured to be mounted in proximity to a human eye. The apparatus further includes optics that is retained by the housing and that is configured to focus optical radiation from a scene to an image viewable by the human eye. The apparatus also includes a tunable wavelength filter that is retained by the housing and that is configured to sequentially filter the optical radiation from the scene by a first wavelength and by a second wavelength different from the first wavelength.