Abstract: The following invention relates to an improved lens apparatus for use in a head up display (HUD), particularly for providing HUD with a depth of field and more particularly with providing a secondary virtual image in a different colour so as to provide a warning message. The partially reflecting combiner has a first surface S1 located closest to a display, and a second surface S2, located furthest from the display, wherein the effective radius of curvature of surface S2>S1, so as to provide two non-coincident virtual images. The selection of antireflective coatings on the surfaces S2 and S1 allows the non-coincident virtual images to have different coloured virtual images.