Computer-generated holograms (CGHs) are important for many tasks in modern optics. High efficiencies rely on an increasing number of phase steps which usually complicates the fabrication process. In this paper, we demonstrate an effective medium approach which uses binary subwavelength structures instead of a conventional surface relief profile to generate a specific phase pattern. Consequently, the fabrication process is simplified to one exposure and one etching step independent from the number of phase levels. This offers new perspectives in designing large-scale highly efficient diffractive elements with large deviation angles. For demonstration, a highly off-axis 3-level CGH for a projection screen application is designed, fabricated and experimentally evaluated.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.