Measurement And Evaluation Of Free-form Surfaces For Optical Windows

Hypersonic technology research is the current research hotspot but it lacks accurate experimental measurement verification.Wind tunnel experiment is the most effective means.The experiment requires optical flow display of the entire flow field through the optical window.In order to reduce the interference to the internal flow,the window adopts a new design with inner and outer surfaces as free surfaces.The surface shape is complex and the shape and position accuracy is required to reach sub-micron level.Limited by the dynamic range of the interferometer,wavefront interference methods cannot directly detect such surfaces.In this paper,the coordinate measurement error is identified and compensated based on sub-micron precision optical standard gage,and high-precision probe radius compensation and point cloud matching method are proposed to improve the coordinate measurement accuracy of free-form surface.Then the wavefront interference detection method is introduced to convert the free-form surface positioning error used in pairs into a transmitted wavefront for detection.The specific content includes the following aspects:(1)Identify and compensate the geometric error of the coordinate measuring machine based on the high-precision optical standard inspection tool.According to the homogeneous coordinate transformation and the principle of small error approximation,the geometric error model of the coordinate measuring machine is established.The six-degree-of-freedom error of each coordinate axis is described by cubic polynomial,and finally,an error conversion matrix containing a total of 57 parameters including polynomial coefficients and triaxial orthogonality errors is obtained.The optical plane,spherical surface and cylindrical surface with negligible surface error are used as standard gages to identify the above error parameters,and then the error of the measured surface shape is compensated according to the identified error model,which can improve the measurement accuracy effectively.This method was verified initially in the measurement experiment of ?110mm caliber convex spherical surface.(2)The probe radius compensation and registration method for high-steepness surfaces.The point cloud data obtained by the coordinate measurement does not coincide with the coordinate system of the point cloud data defined by the nominal surface,resulting in significant alignment error in the free-form surface evaluation result,so the point cloud data must be registered.The approximate distance calculation method for triangular patches on points to surfaces is proposed,and the three-dimensional point matching is transformed into the Delaunay triangulation of the planar point set,which improves the calculation efficiency greatly.In order to compensate the probe radius,a method of matching the point cloud data to the equidistant surface of the nominal plane is proposed,so that the registration algorithm is robust to tens of millimeters/degree of alignment error.Finally,the window free surface in the grinding process was tested and verified,and the surface shape error after registration was used to guide the compensation processing,which reduced the surface shape error.(3)Wavefront interference detection method for free-form surface lens of the optical window.Although a single surface cannaot be detected by an interferometer,the window contains two free-form surfaces that are used in pairs and it's shape and position error is only micro-level modulation of the transmitted wavefront of the window,so the wavefront interference detection method can be used.The ray tracing analysis of the free-form surface window using TracePro software verifies that the collimated beam is still approximately collimated after passing through the monolithic window.And the wavefront error is within the dynamic range of the interferometer,so that the window can then be placed between the interferometer planar lens and the high precision planar mirror for transmission wavefront detection.The aberrations caused by different misalignments in the detection optical path of the single chip and the pair of windows are further simulated,which provides guidance for the computer aided adjustment to reduce the influence of the offset aberration.