光学系统杂光抑制研究

【作者】 廖胜；

【导师】 余学才； 沈忙作；

【作者基本信息】 电子科技大学 ， 光学工程， 2003， 硕士

【摘要】 空间光学系统，特别是空间红外光学系统、低温光学系统以及某些大型空间望远镜，都工作在系统视场外有强烈辐射源(如太阳等)的“恶劣”环境中，这些强烈杂光辐射经过光学系统孔径的衍射，以及结构与光学件表面的散射，到达系统像面形成杂光。随着空间光学技术的以及多种类光电探测器件技术的飞跃发展，杂光的抑制与分析被重新提高到了一个重要的地位，系统杂光抑制的成功与否，甚至成了某些空间环境使用的光学系统性能进一步提高的“瓶颈”。因此，对系统杂光的产生和传输、分析和计算、杂光抑制措施的采用与遮光系统的设计、材料表面BRDF以及系统杂光PST的定量测试等方面进行深入研究，是非常有必要的。本文首先介绍了光学系统杂光的抑制措施，从光辐射在两个表面传递的基本能量传输方程出发，指出影响辐射能量传输的三个因子：双向反射分布系数BRDF、投影立体角PSA和入射辐射通量。并据此，提出了从系统像面出发，向前搜寻杂光传输的关键和间接表面，并设法移除或减少的遮光系统设计思路和方法。同时，文章具体研究了遮光罩和叶片结构在杂光抑制中特殊作用，以及不同光阑对系统散射和衍射杂光的抑制作用。表示光学系统杂光抑制能力的点源透过率PST和材料表面散射特性的双向反射分布系数BRDF，这两个物理概念是光学系统杂光分析与计算的基础。本文介绍了两种系统杂光计算方法，各自给出了计算实例。采用公式法计算系统杂光，精度虽然稍差，却可清楚地了解系统七种类型杂光的成因和影响，便于有针对性采取抑制措施，优化遮光设计。精确的杂光分析计算还得“改进”的蒙特-卡洛法，它需要追迹足够多数量的光线，建立的三维模型的物理特性与实际符合，才能获得符合统计规律的计算结果。材料表面的BRDF，与波段、入射及出射角都有关系，几乎不可能用解析表达式表示，只能通过实验手段测试获得。本文在长波红外波段和可见波段，在实验室建立了材料表面BRDF的专门测试装置，测得了可见和长波红外波段两个光学镜面和5片“黑色”吸收涂层的BRDF的空间分布值。光学系统杂光指标PST的测量，包含了系统各种安装误差的影响，体现了系统杂光抑制的真实水平。本文在实验室，建立了光学系统杂光指标PST的专用测量装置。采用锁相放大器的调制检测法，成功地抑制了背景噪声，提取出了非常微弱的杂光信号，测得了口径D=180mm的卡塞格林系统的红外和可见杂光PST。在国内第一次成功获得光学系统PST的可靠测试结果。更多还原

【Abstract】 Space optical system, especially space infrared optical system, cryogenic optical system and some huge space-based telescopes, all work in the environment where there is strong radiation (such as Sun etc.) out of the field of the system. Because of diffraction of the entrance pupil and diffuse scatter of all inner surfaces, the strong stray radiation will reach the detector of the system.With fast development of space optical system and photoelectric sensor technology, stray light analysis and suppression are regarded more and more important. Whether stray light can be successfully suppressed or not indeed becomes a "bottle-neck" to the further performance improvement of some optical systems used in the outer space. Thus, it is quite necessary to deeply investigate some related problems, such as stray light transfer, stray light analysis and calculation, stray light suppression method, and baffle system design, bi-directional reflectance distribution function (BRDF) of material surface, and accurate quantitative analysis and measurement of stray light performance of optical systems, like point source transmittance (PST) etc.This paper introduces stray light suppression method. The equation of the radiation transfer between two surfaces indicates that there are three factors that have influence on radiation transfer: BRDF, projected solid angle (PSA), and incident radiation flux. Thereafter, a new baffle system design idea is put forward. Looking forward from the detector position of the optical system to the object space, one can find critical surfaces and indirect surfaces in the system, and then try to remove or reduce the area of these surfaces. In addition, this paper discusses in details the special effect of the baffle and the vane structure on the stray light suppression, and effects of different stops on restraining diffraction and scatter stray light.PST denotes optical system capability of suppressing stray light, and BRDF represents material surface scatter and reflectance property. These two physical values are the base of stray light analysis and calculation. Two methods of calculating stray radiation are discussed in the paper, and calculation examples are given for each method.Although analytical formulas are not accurate enough to calculate stray light, they can help designer to distinctively understand the cause of formation of seven types of stray radiation and their influences, so as to adopt proper methods to restrain stray light and to optimize baffle design. Accurate stray light analysis is made by "advanced" Monte-Carlo technique. In order to make the calculation coincident with statistical rules, it needs establishing a 3-D model with real physical property and tracing enough number of light rays.BRDF of material surface has relations with wavelength, incidence angle and scattering angle. It can barely be expressed by analytical formula, and can only be obtained through experiments. We have built a special experimental set-up to measure BRDF at the infrared wavelength and visible wavelength . The spatial BRDF distributions of 2 optical surfaces and 5 absorption coat samples<WP=6>were obtained. The PST of stray light includes the influence of system assemble error, and reflects the real capacity of stray light suppression. A specific set-up has been established for measuring optical system’s PST in the laboratory. In the experiment, we used correlation measurement method, which can successfully hold back background thermal noise, and detect very week stray light signals. PST of a Cassegrain system with the entrance pupil diameter of 180mm has been obtained at the infrared and visible wavelength. It is the first actual test result of the optical system’s PST measurement in China.更多还原