The simulated grating for absolute measuring the transfer function of large-aperture Fizeau interferometer is proposed, combining with the surface and homogeneity absolute measurement data, the simulated grating absolute surface data without aberration can be obtained more accurately to achieve the full aperture transfer function absolute measurement at all spatial frequency. A 300mm aperture Fizeau interferometer experimental results show that the absolute surface errors of the three flats are less than λ/20(PV) and λ/100(RMS), and the homogeneity measurement is better than 1*10-6. The optimized transfer function is better than 0.86 at 1 mm-1 spatial frequency, which is a 10% improvement.
A dual-beam Fizeau interferometry with both small and large aperture two measurement modes is proposed. The two modes of the interferometer were measured and analyzed using three-flat four-step absolute measurement and three-flat simulated sinusoidal phase grating. An integrated 4″-18″ aperture dual-beam Fizeau interferometer was used to perform experiments on large and small aperture by the above two measurement methods. The experimental results show that the absolute surface errors of the three flats are less than λ/20(PV) and λ/100(RMS), and the transfer function is better than 0.78 at the 1 mm-1 spatial frequency, which satisfies the specification.
Chromatic focal shift is used to characterize the variation of focal wavelength of optical system with wavelength. It
is an important tool and index to analyze the color difference of optical system. In this paper, the function relationship
between focal length and wavelength of optical system is analyzed by using chromatic focal shift. According to the
dispersion characteristics of the optical system material, the Conrady formula which fits the refractive index curve well
with less data is selected. Simulations show that the chromatic focal shift of most of monochromatic wavelength systems
in the 400-1000 nm wavelength range is monotonically increasing and can be expressed by the Conrady dispersion formula.
The chromatic focal shift of achromatic systems which consists of a variety of glass materials usually has an inflection
point, and the shapes of most of the achromatic chromatic focal shift are the same, so the formula for the curve should be
consistent.The calculation results show that the Conrady formula can also solve the curve of the achromatic system
effectively in a relatively short band, such as 400nm~700nm. In fact, the chromatic aberration correction of most
achromatic systems is limited, and the effective working band is short. Therefore, the Conrady formula is a very good
expression for both monochrome and achromatic systems. By studying the chromatic focal shift analytical equation of the
optical system, it provides a reference for the theoretical calculation and detection of focal length at a specific wavelength.
Interferometry repeatability is an important indicator for measuring instrumentation and test results. The precondition of improving the detection accuracy is to ensure high-precision repeatability, so high-precision repeatability is one of the most important indicators in precision detection. In the Fizeau interferometer, common light paths are used for the distances from the laser light source to the reference surface, from the reference surface to the detector CCD, and the reference beam and the test beam. However, the strict sense of the co-beam does not exist, and it is not absolute ideal that reference plane has high-precision surface. The test beam reflects from the measured surface will have a certain angular deviation from the reference beam. If so, it may make the reference beam and the test beam reach the CCD detector along different optical paths and generate return error accordingly. And finally it makes effects in sampling and wave surface reproduction. Therefore, using the Zygo GPI system, the 4D system and the H and L system to make research on the same optical platform for relative interferometry repeatability comparison, ensuring that the cavity length is the same, and the standard reference mirror and the tested mirror is the same. Measurements were repeated 50 times for zero-stripes, five-stripes, and ten-stripes. The PV values and RMS values of the 50 measurements make mean processing to reflect the measurement repeatability. Further repeatability error analysis is performed on the phase shift algorithm and PZT phase shifter.
Digital interferometer is widely used for evaluating optical surfaces due to its outstanding sub-nanometer accuracy and precision. In this paper, we will summarize its advantages and then describe its applications in industry, especially in both absolute flat and cylindrical surface and measurements. Inner surfaces measurement of cylindrical ring can be achieved without map stitching, by a Fizeau interferometer with a 90° conical mirror. The alignment of this arrangement, however, is very crucial to the accomplishment. Any small misplacement of 90° cone or hollow cylinder from their ideal settings may result in large measurement errors. These errors are not intuitive and hard to be removed if their origins are not well understood. In other words, it is very important to know how these measurement errors are generated from the optical misalignment in order to eliminate them. Transmission flat has normally 1/20 wavelength PV. However, when a flat surface under test is better or much better than the transmission flat, we need the absolute flat measurement. We developed a new method to be easily able to achieve the accuracy of 1/100 wavelength PV. We have dedicated our efforts to do so. The theoretical analysis, computer simulations, and experimental validation are presented in the paper.
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