为解决传统头戴式显示器大视场、大出瞳距与小型、轻量化之间的矛盾，设计了折/衍混合自由曲面式头戴显示器的光学系统。利用衍射元件的特殊色散特性校正系统色差；选取最佳的自由曲面面型组合校正系统的像散、彗差和畸变；由光学塑料(PMMA)注塑成型的自由曲面棱镜形成离轴结构，使光学系统结构紧凑、便于装调。设计得到的光学系统出瞳距离为17.4 mm，出瞳直径为4 mm，对角线视场为55°，光学系统的点斑RMS半径小于30 μm，各视场光学调制传递函数在30 lp/mm时大于0.1，系统畸变小于10%。实际分析结果表明，该系统具有良好的成像质量，系统像差特性满足目镜系统的成像要求，可以满足虚拟现实各领域的实际需求。

To resolve the contradictions between wide Field of View(FOV),large exit pupil and compact，lightweight,a hybrid refractive/diffractive optical system using Free Form Surfaces(FFSs) for a Head Mount Display(HMD) was designed.The dispersion characteristics of diffractive elements were used to correct the chromatic aberration,and the appropriate types of FFS were selected to correct the off-axial aberrations(astigmatism，coma and distortion). Furthermore,the FFS prism was made of plastic materials to form an optical structure without rotational symmetry to achieve compact and easy to align functions.Obtained optical system can offer the eye relief of 17.4 mm,exit pupil of 4 mm and the FOV of 55°.Moreover,the RMS diameters of spot diagram are less than 30 μm,the MTF are higher than 0.1 at the spatial frequency of 30 lp/mm across the entire visual field,and the maximum distortion is less than 10%. Experimental results indicate that the optical system has perfect performance and can meet the demands of eyepieces for aberration characteristics.