激光投影系统中影响散斑抑制的参数分析

Parameters Analysis for Speckle Suppression in Laser Projection Systems

基于激光投影显示系统中散斑形成机理,从理论上分析了影响投影系统中散斑抑制的两个关键参数,即统计独立散斑图像数量N和探测器成像镜头在观测屏上一个分辨基元内投影镜头分辨基元的数量M。参数M或N值的增大,都将有效降低散斑图像的对比度,但即使投射到屏幕上的独立散斑图像数目N实现巨大,即N→∞,也只能将散斑图像对比度降至1/M。在简化投影系统中,利用旋转小散射片产生统计独立散斑图像的方法,通过系统实验,比较了同一散斑抑制技术用于激光非投影和投影系统中散斑抑制的程度,以及投影镜头F数和探测器成像镜头F数的变化对参数M和散斑抑制的影响。结果表明:对于相同N值和检测条件,投影镜头的存在使散斑图像对比度从0.146提高到了0.427,呈现的散斑更严重,且投影镜头F数的增大还会进一步恶化散斑图像,这使得激光投影系统的小型化受到挑战。

Base on speckle formation mechanism in laser projection display system, two key parameters that influence speckle suppression are theoretically analyzed, namely the number N of statistically independent speckle images and the number M of projection-lens resolution elements within a single imaging-lens resolution element on the screen. The increasing value of the parameter M or N can effectively reduce the contrast of the speckle image. However, although the number N of independent speckle images projected on the screen tends to infinity, the contrast of the speckle image can only be reduced to 1/√M. By using a rotating small diffuser to generate independent speckle image, the degrees of speckle suppression are experimentally compared without projection- lens and with projection-lens in simplified projection system. And the effects of the change of the F-number of the projection-lens or the F-number of the imaging-lens on the parameter M and the speckle suppression are experimentally analyzed. The results demonstrate that for the same value of N and under the same test conditions, the contrast of the speckle image increases from 0.146 to 0.427 and the speckle is more serious due to the presence of the projection-lens, and the increase of the F-numbers of the projection-lens can further deteriorate the speckle image, which will be a challenge for the miniaturization of laser projection system.