Fig. 1. Research progress of AR-3D display system based on off-axis optical elements technology and free-form surface optical elements technology. (a) Schematic of AR-3D display based on off-axis optical elements ^[36]; (b) photos by AR-3D display based on off-axis optical elements ^[36]; (c) schematic of AR-3D head-up display based on free-form surface optical elements ^[41]; (d) photos by AR-3D head-up display based on free-form surface optical elements^[41]

Fig. 2. Research progress of AR-3D display system based on sparse reflective membrane array technology and double 4f lens array technology. (a) Structure diagram of AR-3D display based on sparse reflective membrane array^[42]; (b) photos by AR-3D display based on sparse reflective membrane array^[42]; (c) structure diagram of elementary imaging system based on double 4f lens array system^[43]; (d) schematic diagram of experimental system based on double 4f lens array system^[8]; (e) superposition of 3D image on real object by experimental system^[8]

Fig. 3. Research progress of AR-3D display system based on microlens array technology. (a) Schematic of electrically-focus-tunable-lens-based AR-3D display experiment^[50]; (b) (c) photos by electrically-focus-tunable-lens-based AR-3D display^[50]; (d) schematic of vari-focal-liquid-lens-array-based AR-3D display^[51]; (e) photos by AR-3D display when camera focusing on 30 cm^[51]; (f) photos by AR-3D display when camera focusing on 50 cm^[51]

Fig. 4. Research progress of AR-3D display system based on pinhole array technology and reflective polarizer technology. (a) Schematic diagram of reflection-type glasses-free AR-3D display based on pinhole array technology^[52]; (b) photos by reflection-type glasses-free AR display based on pinhole array technology^[52]; (c) schematic diagram of reflection-type glasses-free AR-3D display based on a reflective polarizer^[53]; (d) photos by reflection-type glasses-free AR-3D display based on a reflective polarizer^[53]

Fig. 5. Research progress of AR-3D display system based on holographic optical element technology. (a) Schematic diagram of glasses-free AR-3D display using a lenticular lens array holographic optical element^[62]; (b) experimental results of glasses-free AR-3D display using a lenticular lens array holographic optical element^[62]; (c) schematic diagram of glasses-free AR-3D display based on a pixelated volume holographic optical element^[65]; (d) experimental results of glasses-free AR-3D display based on a pixelated volume holographic optical element^[65]

Fig. 6. Research progress of AR-3D display system based on holographic technology. (a) Schematic diagram of dual-view holographic AR-3D display based on CGH projection^[75]; (b) (c) captured experimental images at viewing zone 1 when digital camera focused at 300 mm^{and 400 mm, respectively[75]; (d) schematic diagram of transparent holographic AR-3D display based on CGH[72]; (e) photos by transparent holographic AR-3D display based on CGH[72]}

Fig. 7. Research progress of AR-3D display system based on pixelated diffractive optical element technology. (a) Schematic diagram of glasses-free AR-3D display based on spatially multiplexed pixelated nanogratings^[80]; (b) transmittance curves of holographic based on spatially multiplexed pixelated nanogratings^[80] ; (c) photos by glasses-free AR-3D display based on spatially multiplexed pixelated nanogratings^[80]; (d) schematic diagram of glasses-free AR-3D display based on pixelated multilevel blazed gratings^[81]; (e) schematic diagram of viewpoints formed by pixelated multilevel blazed gratings in different focal planes^[81]; (f) photos by glasses-free AR-3D display based on pixelated multilevel blazed gratings ^[81]