A varifocal augmented reality head-up display using Alvarez freeform lenses

A crucial requirement of augmented reality head-up displays (AR-HUDs) is continuously adjustable virtual image distance (VID), which allows adaptation to various depths in road environments and thereby avoids visual fatigue. However, usual varifocal components for near-eye displays are unavailable because AR-HUDs require the varifocal component's aperture to be larger than 10 cm. This study considers the Alvarez lenses, which change the optical power by in-plane sliding two freeform lenses. Under the paraxial assumption, classic Alvarez lenses can create a quadratic wavefront profile, but the large aperture and extensive diopter variation range required by AR-HUDs lead to significant aberrations. Thus, the classic paraxial Alvarez lens design is extended by co-optimizing Alvarez lenses with high-order surface profiles and a primary freeform mirror. Therefore, a novel varifocal AR-HUD containing Alvarez lenses with apertures larger than 15 cm is proposed. The AR-HUD generates a varifocal plane whose VID can be continuously adjusted between 2.5 and 7.5 m, and another focal plane with a fixed VID at 7.5 m. In addition, merely one display panel is used for compactness. Finally, an AR-HUD prototype with a reduced volume of 9.8 L was built. The expected varifocal performance and qualified imaging quality were experimentally verified through the field of view, VID, and image sharpness.