Absolute planarity measurement with interferometric data and iterative surface recovering approaches is briefly reviewed. Extension to the case of multiple measurements is outlined, and demonstration with synthetic data is provided. A generalized approach is finally presented, making use of operators representing the manipulations occurred with the surfaces taking part in the generation of the interferograms. The potential advantages of the new interferogram processing technique are pointed out.
Our acquaintance with rays and refraction is based on previous work carried out by scientists, practitioners, artisans, and optics users. Here a selection of illustrations from ancient books and documents is presented, testifying to the substantive development that took place at the turn of 1600. Achievements at the roots of our continuing advances in optical sciences are acknowledged.
The time stability of fused silica is investigated, reporting new data on two flats that were purchased in 1981. It is found
that their present shape is no more plane, exhibiting instead a concavity that is compatible with a very slow laminar flow
under the action of gravity. The data are examined altogether with those of prior observations, confirming the
occurrence of a relaxation process whose time constant is estimated in the order of 10 years; during such a time period,
viscosity approximately increases by a factor of three. Starting from the experimental data so far collected, the
computations accounting for the estimates above are given in detail.
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.
Mono and polycrystalline Chemical Vapor Deposited (CVD) diamond is a promising material for several
advanced topics: microchips substrate, biological applications, UV and particle detection. Commercial CVD
diamonds are available in small square size, commonly 3-5 millimeters side and 0.5-1.5 millimeters thickness.
To improve diamond reliability for described applications, it is important to have a quality control on
diamond samples, not only for electrical constants but also for optical characteristics and surface roughness.
In this paper we present an optical characterization method based on interferometric instruments, to measure
surface structure and internal homogeneity of mono ad polycrystalline commercial CVD diamonds, with
Research experiments and advanced applications often require the knowledge of the refractive index of vitreous materials under specific environmental conditions. Measurements are carried out by placing the glass sample in a cell where the required conditions are established; the probe beam enters and exits the cell through a pair of windows. A typical case is the measurement in vacuum, as it occurs when thermal cycles down to cryogenic temperatures have to be
performed. In such a case, data processing has to take into account the specific geometry of the cell and the windows. Here we present closed formulas for a symmetrical configuration, and a ray tracing approach for general cases where symmetry is not guaranteed. In particular, the experimental configuration is conveniently modeled, and ray tracing is used to find out the conditions of minimum deviation, starting from a tentative value of the refractive index. The latter is successively modified until the data actually obtained from experiments are matched. Specific software is developed,
accomplishing the computation task automatically by recursive iterations. The ray-tracing model is also used to
numerically estimate the sensitivity of the results to the influence variables, and to work out the uncertainty balance.
A laser strainmeter for in-situ monitoring of an important actively seismic area of Europe, namely Vrancea region in
Romania is proposed. Six groups from four different countries (Romania, Czech Republic, Italy and Greece) with
various areas of expertise (e.g. geophysics, lasers, optics, interferometry, and mechanics) are involved in order to sustain
the complexity of the project. This paper presents some preliminary laboratory experiments related to measuring relative
displacements with a stable interferometer. Displacements of the order of tens to hundreds of nanometers (80 to 285 nm)
were measured with uncertainty of ±1 nm. A computer algorithm was used to process the interferograms.
A method to measure basic lens parameters of intraocular lenses is described in detail. Most of the work
is performed using interferometry methods, for contactless and high-accuracy measurements of radius of
curvature, thickness and particularly wave aberrations; we perform also measurement of the focal length
through magnification method, and we compute the refractive index by formulas. The method is reported,
together with experimental results for two different intraocular lens types.
Photonic crystals are attractive optical materials for controlling and manipulating light. They are of great interest for both fundamental and applied research, and are expected to find commercial applications soon. In this work digital holography, white light interferometry and atomic force microscopy have been applied to the inspection and characterization of 1D and 2D nanofabricated LiN photonic crystals. Periodic pattern with periods ranging form several microns to a fraction of micron have been accurately analysed. Optical methods allow exploring relatively large areas while atomic force microscopy is well suited for high-resolution inspection of the small features.
A simplified version of the liquid mirror setup to serve as planarity reference standard is described in detail. Particulars of the mounting, maintenance and use of the mirror are provided. A series of measurements over a diameter of 90 mm is reported; the resulting expanded uncertainty for the peak-to-valley achieved in experiments is 7 nm (2σ).
We discuss classical experiments in the field of laser speckle interferometry. The typical optical setup consists of a laser diode and off-the-shelf optical elements, like a lens to expand the beam and plane mirrors to deflect it. A low-cost commercial CCD photocamera is used to acquire the speckle images. Items investigated are diffusing objects, illuminated with the laser diode and imaged on the CCD under appropriate experimental conditions. Pictures are taken both with the object at rest and under stress. The pictures are subtracted from one another with a standard software, pixel by pixel; the subtraction reveals fringes of the deformation occurred between the two exposures. With interferograms obtained in this way, we can study mechanical systems with an accuracy of the order of the wavelength of the light source used. Changing the optical setup, the measurement becomes sensitive to stresses along the camera optical axis direction (out-of-plane) or in the perpendicular plane (in-plane). Making long-time exposures it is also possible to study vibrational modes of suitable items (time-average). Three different setups are investigated and examples of measurement are reported.
The effects of probe beam residual power in interferometry are investigated. It is seen that residual power produces an asymmetry between the forward and backward beams that is scaled with f, i.e., the effect can be easily seen in very slow optical systems. Common methods to detect residual power are discussed, showing the effectiveness of lateral shear interferometry to carefully set the collimation of the probe beam. A device approaching the order of magnitude established by Rayleigh's quarter wavelength rule is demonstrated.
An interferometric method to simultaneously test plane parallel plates both for planarity and parallelism of end faces is reported. Measurements are taken by insertion of the plates in a reference cavity. The optical configuration uses a standard programmable interferometer, with an external right-angle prism folding back the probe beam. The prism error is determined by cavity calibration, and is subtracted at data reduction. The measuring procedure is discussed in detail, and results of a laboratory demonstration are presented.
The Solar Disk Sextant (SDS) is an instrument conceived to monitor the diameter of the Sun and its oscillations. A key component of the SDS is the Beam Splitting Wedge (BSW), whose function is to provide calibration to the geometry of the focal plane. The thermal behavior of the BSW is critical, as it affects the overall performance of the instrument. Modeling the elements of the BSW and the basic thermal processes is shown to account for experimental evidences of defocusing observed in early measurements with a balloon borne prototype. Basic requirements for accurate thermal stabilization on board of the final instrument are derived.
An interferometric method to test plane parallel plates both for planarity and parallelism of the end faces is reported. The method requires that the plates are reflective or temporarily metal coated. Measurements are taken by insertion of the plates in a reference cavity. The optical configuration uses a standard programmable interferometer with an external right angle prism folding back the probe beam. The prism error is determined by cavity calibration, and is subtracted at data reduction. The measuring procedure is discussed in detail, and results of a laboratory demonstration are presented.
Relating two different methods of data analysis for assessing the absolute planarity of reference flats is reported. Considered methods are based on Zernike representation and pixel handling, respectively. Operations to be implemented on interferometric systems to use the same data set for comparative processing are described.
The Fritz's method using Zernike polynomials to assess the absolute planarity of test plates is revisited. Such method is based on four interferometric measurements, which are assumed perfectly correlated. In experiments, due to several instability sources, the data set is missing perfect correlation. Modifications of the Fritz's method are here presented, taking into account the residual uncorrelation of the data; such modified approach is demonstrated on a data set from experiments, achieving nanometer uncertainty level.
Observations of wave-optics effects in sunlight are reported. In particular, conditions are described that allow for visual detection of diffraction phenomena from line edges. Typical fringe patterns are demonstrated, also showing color features that account for the wavelength dependence of the diffraction process. Hints to optimize the observation are given, outlining the aspects of simplicity and naturalness of the occurrence.
The present work demonstrates the use of a general purpose interferometer to measure the deformations of a scattering surface by processing speckle patterns instead of smooth wavefronts. The operation of the interferometer are re- programmed to include algorithms adapted to handling speckle patterns. Details of the measuring operation are described, limits of applicability are discussed, and experimental results are presented.
The design, manufacture and testing of a high aperture and wide field of view optical system is reported. Under specified constraints, the design is discussed in terms of aperture, studying its effects on the performance, the barrel length, the weight and the costs of the system. Some design criteria are detailed, and mounting techniques are outlined in order to obtain optimum position and alignment of the lenses. A f/1.8 objective is described in particular, and system tolerances are given. Interferometric tests on prototype systems are required, comparing the results with the design data.
A homodyne interferometer is described, using a He-Ne laser source and working in Twyman- Green configuration. By means of proper retardation plates and polarizing beam splitters, three signals at 90 degree(s) phase lag are made available. Such signals are digitized and fed into a desktop computer. Fitting procedures and computing algorithms are then implemented working out a phase angle; monitoring such a phase provides information on the optical path difference between the two arms of the interferometer. The general features of the approach are reviewed; examples of application are given, monitoring the fine displacement of mechanical parts and measuring the optical thickness variation of a soap film in air.
This work relates to the glass polishing process in optical workshops, classically based on pitch lapping of ground surfaces. The process progressively removes the asperities and lowers the surface toward the bottom of the remaining pits. According to Preston's hypothesis, the polishing rate is proportional to the velocity of the lap and to its pressure on the area of contact. A computerized imaging technique is here reported to monitor the progress of the polishing action. A Nomarski microscope with a 16X objective has been equipped with a TV camera connected to a personal computer. A frame grabber provides image data that are elaborated to work out the surface features. Statistics are obtained on the fractional area covered by the residual pits. Referring to the Preston's hypothesis, the measured parameter is related to the actual finish grade of the surface inspected.
After being processed by an optical system, either in reflection or in transmission, a probe wavefront contains information on the induced aberrations that are conveniently reported by interferometry. Yet, the probe beam also includes diffuse light, mostly produced by scattering at the optical surfaces. The actual disturbance that is studied via interferometry is a partially developed speckle field, made of a strong bias phaser plus the weak random contribution due to scattering. The standard deviation of the random contribution, normalized to the modulus of the bias phaser, is assumed as a characteristic parameter of the speckle field. Such a parameter has been measured with digital phase-shift interferometric techniques on a series of selected samples, corresponding to different optical finish. In excess of 120 samples have been studied, referred to a traceable polishing process. The results are interpreted on the basis of available models of polishing mechanisms. Data fitting to the equation of the theory is reported; the results are in fair agreement with the theory.
Phaseshifting interferometry applied to speckle fields is described. Experimental cases of both weak and strong scatterers are investigated. Statistical parameters are sought on timing of the polishing process of optical sample surfaces. 1 .