A helmet system including a head-up display system wherein the combiner comprises a tuned optical coating (17) carried on the inner surface of a transparent vizor (15) provided on the helmet (13), and light from the display source (27) is directed onto the combiner, for reflection to the helmet wearer's eyes (29), after passing through a body of light refractive material (25). Light from the display source passes into the body via a cylindrical surface (33), is reflected within the body at a planar surface (35) and exits the body via a convex surface (37). The system provides a high quality collimated display with minimal change of the physical size and aerodynamic performance of the helmet and vizor so as to provide improved fast jet ejection capability.