Optical design of an ultra-short-focus projection system with low throw ratio based on a freeform surface mirror

In order to design an ultra-short-focus objective lens with a low throw ratio, a system of ultra-short-focus, low-throw-ratio objective lenses is designed using a freeform surface and refractive-reflective optical structure. The objective lenses consist of a rotationally symmetrical refractive lens group and a freeform surface mirror. A 11.938 mm Digital Micromirror Device (DMD) is used as a spatial light modulator to generate the image source. The normal-weighted iterative optimization method is used to calculate the freeform surface. Finally, the performance of the system is analyzed. The simulation results show that the ultra-short-focus projection objective lens can achieve a 3 048 mm screen projection at a distance of 580 mm. The throw ratio of the system is 0.19, which is extraordinarily low, and the maximum distortion of the system is less than 0.72%. This can meet the design requirements of low throw ratio ultra-short-focus projection objective lenses. This projection system is advantageous for its low throw ratio, low distortion and good imaging quality, which can provide a useful reference for the further development of ultra-short-focus projection systems.