Head-up display systems

Claim 1

A head-up display system comprising: an object surface (21); a holographic combiner (17) which has been constructed using two coherent sources respectively located adjacent to entrance and exit pupils of the combiner (17) so as to deviate an axial ray coming from the object surface through an off-axis angle into an exit pupil of the system and which has a focal surface with axial coma and axial astigmatism; an optical sub-system (19) along the path of said axial ray between the object surface (21) and the combiner (17) which at least partially compensates said axial coma and said axial astigmatism; and reflector means (25) whereby rays from the object surface (21) are redirected at the exit of the optical sub-system (19) to the combiner (17) for reflection thereby to the exit pupil; characterized in that said reflector means (25) comprises: a first reflective optical surface (27); and a second optical surface (29) possessing both light reflective and light transmissive properties; the two optical said surfaces (27, 29) defining a space (S) tapering in the direction of ray propagation from the object surface (21); and the arrangement being such that rays from the object surface (21) are first reflected at said first surface (27) to said second surface (29), are then reflected from said second surface (29) back to said first surface (27), and are then reflected again at said first surface (27) to said second surface (29), the rays consequently transmitted by said second surface (29) being incident at the combiner (17) at an angle to which the hologram of the combiner (17) is tuned for reflection.