基于杂散光抑制要求的光学表面粗糙度控制方法研究

Method of controlling optical surface roughness based on stray light requirements

光学表面加工误差引起的散射是影响光学系统成像性能的重要因素.描述表面总散射能量的均方根粗糙度是评定光学表面粗糙度的通用指标,但因其未能体现散射能量的空间分布,在表征光学表面散射对具体光学系统杂散光性能影响时存在准确度不足的局限.本文基于全积分散射及双向散射分布函数理论,针对杂散光抑制要求提出一种光学表面粗糙度控制的新方法.首先通过分析确定光学表面纹理中影响系统杂散光的空间频率范围,然后度量该频率带限范围内的表面均方根粗糙度,作为控制光学表面粗糙度的指标.以太阳磁场望远镜(MFT)为例进行方法验证,确定主镜表面纹理有效频率范围为0—18 mm^(-1),分析了主镜表面带限均方根粗糙度对MFT杂散光性能的影响.结果表明,带限均方根粗糙度与MFT杂散光性能之间的关系稳定性能大幅提高,由此验证了采用带限均方根粗糙度描述光学表面粗糙度,能更为准确地控制其对具体光学系统杂散光性能的影响.

Scattering introduced by optical surface fabrication errors could degrade optical performance severely. Therefore, the optical designers are required to provide a roughness index for describing the specific surface or even all surfaces to ensure the final imaging performance. The surface root-mean-square（RMS） roughness is a common index to quantify surface topography. And there are also some available methods to acquire the surface RMS roughness based on bidirectional scattering distribution function theory or the angle spread function theory. However, the influence of the optical surface scattering on the optical system cannot be accurately revealed by the surface RMS roughness determined by these methods. On the one hand, the RMS roughness corresponds to an excessively wide spatial frequency range from 0 to 1/λ,where λ is the wavelength of the light. Consequently, it is difficult to measure the RMS roughness during manufacture.On the other hand, what really worsens the stray light performance of the system is only the surface profile located within a certain subinterval of the aforementioned frequency range, to put it in another way, the surface RMS roughness identified by the methods above is incompetent to quantify the amount of the energy that is surfacescattered to the detector. To address the issues above, in this paper we propose a novel approach to identifying the surface roughness.This method seeks to deduce the relation between optical surface RMS roughness and the stray light requirement of the system by dint of partial integrated scattering（PIS）. In contrast to total integrated scattering, PIS counts the scattering light energy that could reach the detector. Hence, the RMS roughness identified in this way corresponds to the effective spatial frequency range that contributes to the stray light in the system. Firstly, the effective frequency range concerned with the system stray light level is identified through the analysis of the propagation path of the scattered light. Then,the surface RMS roughness would be measured within the established range according to the stray light requirement of the system and used to control the surface roughness as the roughness index during the optical manufacture process.The method not only considers the scattering as the surface characteristic, but also takes into account the influence of scattering on the system. Taking the solar magnetic field telescope（MFT） for example, the validity of the method is verified by comparing with the traditional methods. As manifested in the outcome, the effective frequency range of primary mirror is from 0 to 18 mm^（-1）, and the surface RMS roughness identified in such a new way can stage the stray light performance of MFT in a more precise manner, which is more reliable to serve as a surface roughness index.