To accomplish the interference testing to an off-axis parabolic mirror, we provided a kind of hybrid compensation modal combining compensator with stitching testing. To verify the validity of the above modal, we measured a Φ1450mm off-axis parabolic mirror with the above method. It can be seen from the stitching map that the stitching map is smooth and continuous in the full aperture. At the same time, to evaluate the stitching testing accuracy, we compared the stitching testing map and the subaperture testing map. It shows that the RMS of the residual map between them is 0.003λ, verifying the validity and accuracy of the model.
In large aperture component’s dark-field scattering defects imaging system, the component’s size is large and part with a
wedge. When the component is in the completely level position, the surface defects image can be clearly acquired by a
high magnification microscope. Otherwise, fuzzy defects image would be gained because of defocusing which makes
digital identification can’t be able to be done. For the problem of leveling large aperture, wedge component, this paper
proposes a method that using image information entropy as focusing evaluation function for leveling large aperture
components. Firstly, in three different points of component surface acquiring multi-images by the same continuous steps.
Then calculating the images’ entropy and fitting a curve to it. Based on minimum image information entropy value
criterion, the focal plane can be found and each point’s defocusingamount of the fist acquisition position can be gained.
Relay on the relation model of acquisition points, adjust points and defocusingamount that has been built, each adjust
point’s adjustment can be got. The component’s level position can be achieved by adjusting the adjust points. In the
experiment that using a high magnification (of 16) microscope scans over the whole surface of the component with the
size of 430mm×430mm. The image microscope is always in the depth of focus which shows that the leveling precision
has achieved 20μm. Until now, this method has been successfully used in large aperture component’s dark-field
scattering defects imaging system.
The partial null interferometric aspheric testing technique, based on the Twyman-Green interferometer system, is very
useful and of good versatility. In this technique, the under-test aspheric needs to be located precisely. Taking advantage of ray tracing and digital image processing technique, a new method to locate aspheric is proposed. Firstly, model and simulate the Twyman-Green interferometer system in the ray tracing software ZEMAX, find an optimal test position and generate an optimal referenced interferogram. Record the interferogram and make it a target for the experimental interferogram to achieve. At the same time, an experimental interferogram can be obtained by building the same testing system experimentally. Process the one-dimensional gray scale data in X-axis of the two interferograms, two curves, indicating the black and white change of the interference fringes, are obtained. By comparing the normalized X coordinates of the peaks of the two curves, we can determine whether the under-test aspheric is positioned well. In order to locate the aspheric precisely, the aspheric has to be moved repeatedly to get a perfect interferogram whose peaks of interference fringes match well with those of the target interferogram. An experiment for testing a paraboloid with diameter 100mm and asphericity 50μm is carried out. The result shows that this kind of locating method has an Accuracy of 3-5μm, which demonstrates that the method is practicable and high-precision.
A digital calibration method for defect dimension of the optical surface is put forward to get the correspondence between the actual scale of defect on optical surface and the number of pixels of the defect image captured by CCD. Standard scratches, with their width ranging from 0.5μm to 40μm, are fabricated by electron beam exposure and reactive ion beam etching on two kinds of standard calibration board, quartz calibration board with and without chromium film. Calibration experiments are accomplished in five different microscope magnifications. Threshold segmentation, morphological operation and feature extraction are carried out in the images of calibration board to obtain the width of standard scratches in pixels. Interpret the theoretic trend of the calibration function as well as the linear range of it, and fit the calibration function based on the experimental results. According to the analysis and comparing of the calibration results in different microscope magnifications, error source and the factors limiting the resolving accuracy of the calibration system are analyzed. Ultimately, a standardization process including fabrication of the standard scratch, establishment of the standard calibration library for different microscope magnifications and the rapid calibration of actual detect is established. The calibration of the defects on the optical element in the size of 450mm× 450mm is successfully realized.
It is hard to quantitate the micron-scale defects on large aperture (102mm×102mm) optical components by the
conventional optical testing methods. This paper proposes a super-smooth surface defects measurement and evaluation
system, achieved by using microscopic dark-field scattering imaging device, two-dimensional sub-image scanning
mechanism and multi-cycle image mosaic algorithm. The defects detecting system, with a lateral resolution of 0.5μm,
applies a large field of view design (largest FOV: 15mm×15mm). In order to test the largest element (430mm×430mm),
however, over 1000 sub-pictures are captured. It takes more than 30 minutes to process these sub-pictures by multi-cycle
image mosaic algorithm. This paper also presents a distortion correction method to revise the image mosaic mismatch
caused by the optical distortion in the defects testing system on the platform of MATLAB. A binary optical grid plate
(BOE) is fabricated as standard board to evaluate distortion. The proposed method applies image division multi-steps to
build a look-up matrix of distortion parameters. According to the look-up matrix, all pixels on a sub-image are repositioned
from the distortion Cartesian coordinates to the ideal Cartesian coordinates. Finally, feasibility of the
distortion correction method is demonstrated by comparing the mosaic results of defect images before and after this
process. The full field view distortion is reduced from more than 4% to less than 0.1%. After distortion correction, subimages
can be directly mosaicked without using multi-cycle image mosaic algorithm, which improves test efficiency
significantly. The method mentioned in this paper may also apply to other optical testing systems for image mosaic.
In this article, an interference digital testing method for measuring spatial density distribution of transmissive objects is
presented. This method applies a radial shearing interferometer to test the density field from 8 projections in the same
plane. By taking advantage of the regularized phase-tracking technique (RPT), the single interferogram will be
demodulated to two-dimensional phase distribution of the corresponding projection beam. Then the phase data on one
given cross-section of every projection is selected to form 8 curves, which describe one-dimensional phase variation on
the given cross-section from each projection. Regarding these curves as computer tomography projection data, the
refractive index distribution of the given cross-section can be reconstructed utilizing the algebraic reconstruction
technique (ART). Thus, a three-dimensional distribution of refractive index can be obtained by applying the method
above to different cross-sections in order. Finally, we are capable of calculating the spatial density distribution with the
relation between density and refractive index of the substance tested. In addition, the density field testing for hypersonic
flow field is investigated as an example in this article. Considering the fact that the target model in the optical window
center of a wind tunnel will inevitably block some testing beams, which will lead to the sharp decline in accuracy of the
testing results, a modified algebraic reconstruction technique which improves accuracy by introducing biharmonic
spline interpolation is presented. In simulation, an error less than 3% in non-block situation is reached while an error less
than 8% in small-area-block situation is also obtained.