We demonstrate that full-field deflectometry is a viable alternative to interferometry for the characterization of free-form mirrors. Deflectometry does not require the use of a CGH. Instead of measuring the surface height map, the deflectometer measures the surface slopes in two orthogonal directions using the phase-shifting Schlieren method [1]. The surface height map is then reconstructed by integration of the slope maps. We present two instruments. The first one can be mounted in the lathe for in situ measurement. The second is adapted for the characterization of large concave mirrors.
Full-field deflectometry, which combines high-precision and robustness to external perturbations, is well adapted for the characterization of high-precision freeform mirrors. Instead of measuring the surface height map like interferometry does, the instrument will estimate the surface slopes in two perpendicular directions. The principle of the method is to measure the angular distribution by applying spatial filtering in the Fourier plane of the mirror under test. This method has been called phase-shifting Schlieren deflectometry
Inspection of mirrors in terms of slopes instead surface height offers multiple advantages. In particular, deflectometry is well adapted for the detection of waviness, which is a mid-spatial frequency topography error. Waviness detection during the diamond turning process is critical since it is hard to remove afterwards by polishing. Keeping the mirror mounted in the lathe during the measurement of its shape will simplify the process since it will avoid misalignment when re-mounting the mirror in the lathe.
The presentation will discuss the principles of phase-shifting Schlieren deflectometry, the performance specification based on the tolerance study of the four-mirror spectrometer, the design of the new instrument under development and finally preliminary measurements of freeform mirrors performed at AMOS with the mirror mounted in the lathe that demonstrate the capability of the instrument for the detection of mid-spatial frequency errors.
Full-field deflectometry, which combines high-precision and robustness to external perturbations, is well adapted for the characterization of high-precision freeform mirrors. Instead of measuring the surface height map like interferometry does, the instrument will estimate the surface slopes in two perpendicular directions. The principle of the method is to measure the angular distribution by applying spatial filtering. This method has been called phase-shifting Schlieren deflectometry Inspection of mirrors in terms of slopes instead surface height offers multiple advantages. In particular, deflectometry is well adapted for the detection of waviness, which is a mid-spatial frequency topography error. Waviness detection during the diamond turning process is critical since it is hard to remove afterwards by polishing. Keeping the mirror mounted in the lathe during the measurement of its shape will simplify the process since it will avoid misalignment when remounting the mirror in the lathe.
Several applications require the identification of chemical elements during re-entry of material in the atmosphere. The materials can be from human origin or meteorites. The Automated Transfer Vehicle (ATV) re-entry has been filmed with conventional camera from airborne manual operation. In order to permit the identification of the separate elements from their glow, spectral analysis needs to be added to the video data. In a LET-SME contract with ESA, Lambda-X has built a Fourier Transform Imaging Spectrometer to permit, in a future work, to bring the technology to the readiness level required for the application. In this paper, the principles of the Fourier Transform Imaging spectroscopy are recalled, the different interferometers suitable for supporting the technique are reviewed and the selection process is explained. The final selection of the interferometer corresponds to a birefringent prism based common path shear interferometer. The design of the breadboard and its performances are presented in terms of spatial resolution, aperture, and spectral resolution. A discussion is open regarding perspective of the technique for other remote sensing applications compared to more usual push broom configurations.
We study the resolution of an inverse problem arising in Optical Deflectometry: the reconstruction of refractive
index map of transparent materials from light deflection measurements under multiple orientations. This problem
is solved from a standard convex optimization procedure aiming at minimizing the Total Variation of the map
(as a prior information) while matching the observed data (fidelity constraint). In this process, the forward
measurement operator, mapping any index map to its deflectometric observation, is simplified in the Fourier
domain according to a modified Projection-Slice theorem.
Optical components are routinely tested with inteferometric based techniques. It is show in this paper that Fourier based
deflectometry method can be used for optical component inspection through very sensitive and precise wavefront
reconstruction. The wavefront is expressed from the raw measurements of the wavefront derivatives as a Zernike
polynomial expansion. The form of the polynomials permits absolute instrumental error characterization by repeated
measurement of the element under test oriented at several azimuthal angles. It is shown that nanometric precision of
Zernike based reconstructions can be performed and that the air turbulences are the experimental limiting factor to the
instrumental precision.
We present a new optical tomography technique based on phase-shifting schlieren deflectometry. The principle is that of
computerized tomography. The three-dimensional profile is reconstructed from the deflection angles of rays passing
through the tested object. We have investigated optical phantoms chosen in view of the characterization of dendritic
growth in a solidification process. Promising results have been obtained with a homogeneous sphere and a bundle of
200μm fibers. The deviation angles exceed two degrees with a variation of the refractive index ▵n=0.025.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.