Thanks to its robustness and reduced sensitivity to vibrations and air turbulence, spatial phase-shift interferometry (SPSI)
is a measuring technique of particular value in industrial environments. Making use of a commercial CCD camera
connected with a PC we have set up an essential system that acquires and processes the fringe pattern, extracting the
relevant features of the phenomenon being observed. The basic algorithms for phase recovery are available from the
literature. Here we present a variant of one of such algorithms and describe in detail its implementation in our SPSI
system. Experimental results are presented, showing the effectiveness of the overall measuring chain.
Aspherics up to 500 nm diameter in optical glass or in ceramic substrates have been fabricated using area- compensated polishing tools and conventional optical shop machines. The tool forms are derived starting from the actual shape of the part under figuring. The figure error is measured using an interferometer mounted on-line with the polishing machine. Measurements are taken after each polishing step to compute the new tool form. The process speeds up the fabrication of aspheres and it improves repeatability in the manufacturing of axisymmetrical optics using moderate cost equipment's up to astronomical requirements. In the paper we present some examples of polishing results using the above mentioned approach on different aspherics for space applications.
The in-flight radiometric calibration of satellite multispectral sensor for earth and atmospheric observations can be conveniently based on solar diffusers. Theoretically, a knowledge of the spectral bi-directional scatter distribution function (BSDF) of the diffuser panel, and the solar incidence angle is all that is needed to allow the retrieval of the earth albedo in the observed direction. At the request of the ESA, the Centre Spatial de Liege, with the support of Officine Galileo as subcontractor, is currently designing a high-versatility high-accuracy BSDF measurement set-up with application to the calibration of space solar diffusers. This instrument will allow a BSDF measurements uncertainty within 1 percent for any angle in the wavelength range from 200 nm to 2400 nm. Vacuum measurements, polarization analysis capabilities and thermalization of the test sample between 200K and 300K are other unique features of this set-up.
We present a nondestructive, high resolution optical technique to obtain the surface topography of ancient paintings. The method is based on the projection of a grid of parallel lines, using white light, onto the surface to be analyzed. When viewed at an angle different from the projection angle, the grid pattern appears deformed by the surface shape. This pattern is digitized and then processed using a spatial-carrier phase-shifting algorithm that allows recovering the surface height map with high precision. The method relies on very simple equipment, gives immediate quantitative results, and is suitable for in situ measurements. It permits the recording of topographic maps of ancient paintings as well as the measurement of local detachments of the paint layer. A theoretical analysis of the method is discussed, together with a description of the data analysis algorithm. Finally, we provide some experimental results concerning the diagnostic of a twelfth-century masterpiece.
We realized a digital interferometer for measuring the shape of optical surfaces in an unstable environment. The instrument performs good quality measurements even if it is subjected to severe mechanical vibrations. Interferograms are decoded using a spatial-carrier phase-shifting technique. The system has been mounted over a polishing machine and used for on-line testing of aspheric mirror surfaces.
We present a technique for interferometrically testing aspheric surfaces without the use of compensating elements. The method consists of recording successive overlapping phase maps from a set of annular interferograms of an aspheric surface, obtained using a conventional phase-shifting interferometer and a micropositioning translator stage. These maps are then sewn together with a suitable algorithm we developed, and the whole surface error is recovered. Experimental results are shown to be in good agreement with the null lens test performed for comparison.
We present a technique for interferometrically testing aspherical surfaces without the use of compensating elements. The method consists of recording successive overlapping phase maps from a set of annular interferograms of an aspherical surface, obtained using a conventional phase-shifting interferometer and a micropositioning translator stage. These maps are then sewn together with a suitable algorithm we have developed, and the whole surface error is recovered. Experimental results are shown to be in good agreement with the null lens test performed for comparison.
Zernike circle polynomials generally employed in interferometric mastermind fitting software fail on an annular dominion. Therefore, when testing catadioptric optics erroneous results may be obtained. We performed interferometric measurements on annular pupils at increasing obscuration ratios, and we fitted the resulting aberration wave-fronts using either Zernike circle polynomials and another polynomial set orthogonal over an annulus. The results obtained with the two different sets are compared and the limits of the standard interferometric fitting software are pointed out.
In the present paper the optical transmittive properties of different thickness CVD ZnSe and CVD ZnS Cleartran samples are analyzed using BTDF at the He-Ne laser wavelength. The measurements obtained determine the combined surface and bulk scattering. Surface scatter dominates over bulk scattering for ZnSe, while for ZnS Cleartran the main contribution to the scattered field is given by the bulk. A criteria to assess the material thickness as a function of its scattering characteristics when it is used to realize multispectral imaging system windows is discussed. The results suggest that the thickness of ZnS Cleartran windows in a multispectral imaging system must not exceed few millimeters.