Design of confocal off-axis two-mirror system for head-up display.

We present a new optical system for a combiner-type head-up display (HUD) without any asymmetrical elements by employing a confocal, off-axis, two-hyperboloid mirror as an aberration corrector. From an off-axial aberration analysis, we initially obtain an off-axis two-mirror system corrected for linear astigmatism and spherical aberration by configuring its parameters to satisfy the confocal condition. In addition, to compensate the down angle in the HUD, the image display plane is tilted to satisfy the Scheimpflug condition. This design approach enables us to easily balance the residual aberrations without asymmetrical components, which results in an excellent starting design. The final optical system for an HUD has a virtual image of 7.3 in at 2 m away from an eye box having an area of 130×50  mm2.