Refractive-reflective Ultra-short Focus Projection Lens Design

In recent years ultra-short focus projection, which is a new type of projection display technology, is becoming more and more popular. Its characteristic is that it can project a very big image into screen under a considerably short distance. Because of the short working distance, almost close to the projection screen, this technology can save the use of space and overcome the problem that projection light is easy to directly go into user's eyes under the ordinary projector working condition. It also perfectly solves the shadow problem when the lecturer stands between the projector and screen, increasing the free activity space and improving the interactivity of teaching, speech, and demonstration.The realization of this new projection technology mainly relies on the special optical design of projection lens, which our predecessors have done a lot of related research and practice. Basing on their efforts, this paper designs a refractive-reflective ultra-short focus projection lens, using the off-axis refractive-reflective imaging optical design method.First of all, this paper introduces the problems and limitations of traditional coaxial imaging system in designing the lens of large view field and ultra-short focus. And then the design theory of off-axis refractive-reflective imaging system is proposed, following which to analyze how this structure solves the problems of distortion correction, lateral color aberration and field curvature under the condition of the large field of view. Based on the target parameters of the lens to be designed in this paper, an appropriate initial structure is selected. Then after a lot of modification, adjustment and optimization under the guiding of before-mentioned design principles, a design result that meets all the requirements is finally got. Afterwards, a detailed tolerance analysis is done on the designed lens and the result demonstrates that this lens has a superior tolerance performance and high yield rate. Meanwhile, the analysis results are used to determine tolerance requirements of the optical-mechanical system.Finally, after completing the optical-mechanical design, a set of experimental prototype system is manufactured and assembled in this paper. The satisfied image quality of projection picture is obtained after several rounds of repeated debug. Then the image resolution evaluation, distortion evaluation, color aberration evaluation, throw ratio and magnification computation are done for the designed lens. These evaluation results show that the designed refractive-reflective ultra-short focus projection lens fulfills the design goals, among which the throw ratio is 0.3 and the field angle achieves 150° with a F number 2.6. The lens structure is simple and compact, in which merely 13 pieces of lens are contained. Only one aspheric lens and one concave aspheric mirror are employed in the final lens system.