Fig. 1. Layout of the final OST-HMD system. S1 and S2 denote optical surfaces of the 1st auxiliary freeform element E1, S3−S5 denote optical surfaces of the main wedge-shaped prism E2, S6−S10 denote the optical surfaces of two rotationally symmetric lenses E4 and E5, S4′ and S10 denote optical surfaces of the 2nd auxiliary freeform element E5.

Fig. 2. Optical paths of the rays of different object fields. R1 is the upper marginal ray of the maximum Y-direction field, and R2′ is the lower marginal ray of the minimum Y-direction field. Pa0−Pa3 are the intersection points of ray R2′ and surfaces. Pb0−Pb2 are the intersection points of ray R2′ and surfaces. Y1 and Y2 are straight lines coinciding with marginal rays.

Fig. 3. Schematic diagram of JMRCC basic principle, wherein L is a line defined by point A of a reference ray intersected on specified surface and its direction of propagation after the surface, and P is another point formed by another specified reference ray and surface.

Fig. 4. Diopter adjustment diagram of freeform prism OST-HMD. (a) Traditional diopter adjustment scheme, where additional lens was added between human eye and prism. (b) Correct hyperopia through auxiliary prism with convex S1. (c) Correct myopia through auxiliary prism with concave S1.

Fig. 5. Schematic diagram of distortion control. (a) For rotational symmetry system, radial distance of hreal and hideal affects the distortion ratio. (b) For the off-axis system, radial and tangential distances of Preal and Pideal affect the distortion ratio.

Fig. 6. Distortion grids of the previous system and present system. (a) Distortion grid of the previous virtual image light path, maximum ratio is 12% [26]. (b) Distortion grid of the previous see-through path, maximum ratio is 1.4%. [26]. (c) Distortion grid of the present virtual image light path, maximum ratio is 0.6% without anamorphosis. (d) Distortion grid of the present see-through path, the maximum ratio is 0.4% without anamorphosis.

Fig. 7. See-through MTF plot of the previous and present systems. (a) MTF plot of previous design in Ref. [26], the value is higher than 0.4 for most fields at 50 lp/mm. (b) MTF plot of present design, the value is higher than 0.9 for all fields at 50 lp/mm.

Fig. 8. (a) Error function variation curve. (b) Final optical layout of the OST-HMD.

Fig. 9. MTF plot of the optical system. (a) The MTF plot before automatic balancing, MTF value is higher than 0.18 at 50 lp/mm for all fields. (b) The MTF plot after automatic balancing, MTF value is higher than 0.4 at 50 lp/mm for all fields.

Fig. 10. MTF plots for different eye positions. (a) MTF plot when eye locates in center of eyebox. (b) MTF plot when human eye moves 2.5 mm to the right. (c) MTF plot when human eye moves up 2.5 mm.

Fig. 11. Probable change of MTF value with four different cumulative probabilities for overall tolerance analysis using tolerances values listed in Table 4. F1–F25 denote the sampled fields the same as in Fig. 9.

Fig. 12. Components and prototype of the optical system. (a) Exploded view showing all elements of the system. (b) Overall appearance of the prototype.

Fig. 13. Testing results of the optical prototype. (a) The input image displayed in microdisplay when testing the performance of the system. (b) Output image captured by camera at exit pupil of the system. (c) The result of fusion of virtual cup and real cup.

Table 1. Comparison of Four OST-HMD Optical Solutions

Table 2. Specifications of Different Freeform Prisms

Table 3. Overall Parameter Requirements of the System

Table 4. Tolerance Items