Modified Köhler illumination for LED-based projection display

•An improved design of conventional Köhler illumination is proposed for projection display.•The modified Köhler illumination can significantly reduce the system volume.•An actual pocket-sized CF-LCoS projector is designed, simulated and analyzed.•High light efficiency and illumination uniformity are both available.•Tolerance analysis shows that the system is well acceptable for current machining.

Common projection optics use Köhler illumination to achieve a required lighting. These systems always prevent the realization of a compact optical configuration along with a high lumen output. Based on conventional Köhler illumination, a modified Köhler illumination system for LED-based projection display is presented in this paper, which can significantly reduce the system volume while allowing for adequate and homogeneous illumination. Equipped with the proposed system, a pocket-sized CF-LCoS projector with a physical dimension of 27.4 mm × 19.4 mm × 9.6 mm is designed, simulated and analyzed. Compared to conventional approaches, this design could offer an average 43% volume reduction with acceptable tolerance. To the best of our knowledge, the screen uniformity of 90.2% and the light efficiency of 56.5% are competitive as compared with those of the currently commercialized pocket-sized CF-LCoS projectors.