## 5 Resolution Evaluation  
In this section, we evaluated the resolution to verify the resolution enhancement. A known method to evaluate resolution is to use a standard resolution target (as USAF 1951)  
Figure 7: Blurs of the globe pillar presented by (**a**) the proposed and (**b**) previous systems. or wedge chart [27]. The standard resolution target was employed to evaluate the system resolution by displaying horizontal and vertical lines with different spatial frequencies. Because the spatial frequency of the wedge chart changes with the position from a vertex of a pattern, continuous resolution changes can be evaluated. However, because the remaining black regions caused by the slit array along the slanted lens direction induced moire, we could not evaluate the resolution when the horizontal and vertical lines and the wedge chart were displayed in the prototype system. Therefore, these resolution targets were not suitable for the current prototype system. Considering the influence of the masked regions of the lens, the resolution of the proposed system was evaluated using line patterns with different line widths along the slanted lens angle. Figure 8 shows three line patterns with different line widths. The line pattern in Figure 8a comprised one line at 0.194 mm, corresponding to the virtual subpixel width within the virtual image of the slit, which was the finest resolution unit displayed by the proposed prototype. The pattern in Figure 8b comprised one line pair at 0.388 mm, corresponding to the width of one virtual image of the slit. The pattern in Figure 8c comprised one line pair at 0.904 mm, corresponding to the width of two virtual images of the slit. Figures 9 and 10 show the reconstructed images of the line patterns captured by a USB camera with a 5 mm pupil diameter when the line patterns were displayed between \(-100\) and 100 mm from the screen plane at 10 mm intervals as well as at \(\pm 150\) mm and \(\pm 200\) mm. Figure 8: Line patterns used for the evaluation: the (a) 0.194 mm, (b) 0.388 mm, and (c) 0.904 mm line widths correspond to the virtual subpixel pitch, virtual image of the slit, and two virtual images of the slit, respectively. Figure 9: Reconstructed images of the line patterns when the camera was focused on the screen. Figure 10: _Cont._  
Figure 10: Reconstructed images of the line patterns when the camera was focused on the 3D images. Based on the experimental results, the contrasts of the line pattern images were compared when the camera was focused on the screen and 3D images. Figure 11 shows the contrasts of the captured line patterns. The contrast was calculated at the line where the reconstructed line was not missed, as missed parts appeared in the image depending on the depth displayed. When the camera was focused on the screen, even the smallest pattern with the line width set to the virtual subpixel pitch could be resolved, and the contrast in the smallest line pattern decreased as the displayed position of the line pattern was moved farther from the screen plane. When the camera was focused on the 3D image positions, the contrasts in the smallest line width exceeded 0.6 in depth from \(-90\) to \(150\) mm. However, these contrasts were lower than those observed when the camera was focused on the screen. When the line widths were 0.388 and 0.904 mm, the contrasts exceeded 0.8 in all the positions displayed. The evaluation results confirmed that the maximum resolution of the prototype (a line width of 0.194 mm) was resolved. However, the contrast decreased as the 3D image position was away from the display surface due to the blurring of the ray divergence in the smallest line pattern. The proposed system presents a higher resolution when the 3D images are displayed near the screen plane than when away from the screen plane. Notably, the actual position on which the human eye focuses should be evaluated by considering human perception. Figure 12 shows the full width at half maximum (FWHM) of the line pattern of the line width at 0.194 mm calculated from Figures 9 and 10.