In the measurement of aspheric surfaces, the vertex sphere and the best fit sphere are often used as reference sphere to
calculate the non-null compensation deviation. In traditional interferometry, the detected wavefront is equal to twice of
the deviation; but it is true only in the null condition or with a certain tolerance in the near null condition. In the non-null
condition, when reference spherical wavefront (the best fit sphere in this paper) incidences to the aspheric surface, the
rays will not return in the same path but deviate certain angles which cause normal longitudinal aberration. If the normal
longitudinal aberration is small enough, for example, much smaller than one wavelength, the wavefront aberration can be
equalized to twice of the deviation between the aspheric surface and the reference sphere. However, if the normal
longitudinal aberration can not be negligible, the wavefront aberration should not be equalized to twice of the deviation.
In this paper, the distribution of the deviation between the aspheric surface and the reference sphere is modeled, and the
relationship between the wavefront aberration and the normal longitudinal aberration is discussed. Two paraboloids, one
with small asphericity and the other large, are analyzed respectively to compare the different result when whether
considering the influence of the normal longitudinal aberration. Computer simulation is also carried out in optical tracing
software.
In the measurement of aspheric surfaces, spatial phase modulation technology is always combined. With a tilt, the linear
spatial carrier is introduced to enhance the signal noise ratio (SNR). Then using DFT on the interferogram, after
spectrum shifting, the first order spectrum filter and IDFT, the wavefront phase of aspheric surface is obtained. In this
paper, the traditional way to introduce spatial carrier, namely that the center of the first order spectrum is positioned on
the x axis, is analyzed. This method exist a limitation which may influence the testing result. Thus a new method is put
forward here. Make sure the slope of spatial carrier is unvaried, and transfer the first order spectrum to the bisector of x
axis and y axis. This change can not only reserve the low frequency components of the first order spectrum, but also
keep the high frequency components at a large extent. So in some circumstance, the wavefront phase of aspheric surface
with a large PV value can't be recovered accurately by the traditional spatial carrier way, but it can be recovered
accurately by the new way. This greatly expands the testing range of wavefront of aspheric surface.
This paper describes a novel synchronous control system of high speed imaging, which combines a common path
interferometer system modulated by the space phase. The system can continuously grab multiple frame interferograms,
which contain transient flow field distortion. The study of this system will provide a fire-new means for the research of
aerodynamics. The light source of the system is Nd: YLF semiconductor pump solid pulsed laser of which wavelength is
1053 nanometers. The laser pulse width is less than 30 nanoseconds, far less than the exposure time of the camera
shutter. Thus the laser pulse can freeze the flow field within several dozen nanoseconds and catch the biggish change of
turbulent flow. The pulsed laser beam containing the information of turbulent flow enters a cyclical radial shearing
interferometer. The emergent lights, being respectively contracted and expanded, re-combine and form fringe pattern in
high space frequency, modulated with a definite carrier frequency. The fringe pattern is formed on the high speed CMOS
camera at last. An accurate short time delay circuit is provided for synchronization matching of the pulsed laser and
camera exposure. The speed of image acquisition in full pixels with 1280×1024 can reach 450 frames per second. This
interferogram acquisition system with compact configuration and strong anti-disturbance capability, has successfully
grabbed clear transient interferograms that provided reliable image information for follow-up image processing and flow
field density calculating.
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