By taking advantage of the fact that all of the depth cues of a three-dimensional (3D) object are contained in the complex wavefront of that object, researchers at the Beijing Institute of Technology (Beijing, China) have developed a see-through 3D head-mounted display using compact optics that essentially eliminates the viewer fatigue that so often plagues these head-worn technologies.
Unlike two-hologram interference, cascaded holograms, and modified iteration algorithms that require at least two spatial light modulators (SLMs) and bulky crystals or additional filtering, the grating filter method applied by the researchers needs only a grating and a 4f lens system to reconstruct the complex wavefront. Essentially, a laser illuminates a SLM (1920 × 1080 pixels with an 8 μm pixel pitch) through a beamsplitter and the beam then passes to the 4f system consisting of two lenses with a grating at the back focus of the first lens acting as a frequency filter. A mirror reflects the beam to another beamsplitter that combines the modulated image with the real-world scene for a true see-through display with a compact size of around 134 × 40 × 35 mm. Experimental observation of small 3D objects using the display produces very clear and fatigue-absent images at different focus depths ranging from about 130 to 220 mm. And because the modulation depth range depends on the coherence length of the laser source, typical semiconductor lasers with nanometer linewidths and several-meter coherence lengths are more than sufficient for a realistic 3D experience. Reference: Q. Gao et al., Opt. Express, 25, 7, 8412-8424 (2017).