Design of ultra-thin optical systems in large screen projection display

A novel ultra-thin refract-reflect projection display system was designed to meet the severe requirements of imaging lens in large screen projection display for ultra-short focal length, larger field of view, ultra-thin, larger relative aperture and higher resolution. A 0.65 inch (16.51 cm) Digital Micromirror Device (DMD) produced by TI Inc. was employed as the spatial light modulator and an aspheric mirror was used to correct distortion and shorten projection distance. Furthermore, a fold mirror was also taken to turn back the optical path to achieve ultra-thin projection display. The system is consisted of five lenses and an aspheric mirror, of which the focal length is 2.8 mm, the F-number is 3.5 and the minimum and the maximum field angles are 55° and 78.5°, respectively. When the projection distance is set to be 120 mm, the screen size can be 65.2 inch (165.6 cm). With the design, it shows that the Modulation Transfer Function (MTF) of over 95% field of view of the system is higher than 0.6 at the Nyquist frequency of the image plane. The maximum distortion is 0.8TV%, and 90% geometric encircled energy in the point target is focused in one pixel. To verify the performance of the ultra-thin refract-reflect projecting lens, a principle prototype was fabricated and assembled. The experimental results show that the system has excellent display performance, and meets the design requirements of ultra-thin, low cost and small-scale mass production.