An optoelectronic system based on digital holography is used to measure the three dimensional vector components and object shape of a vibrating object. Pulses from a ruby laser, with a separation in the range from 1 to 1000 microseconds, are used to record holograms on CCD sensors, which are later digitally reconstructed. Three different illumination directions are used to get the deformation along three different sensitivity vectors, that are afterwards combined into a 3D resultant deformation. To measure the shape of the object the two- wavelength method is used. The wavelength change is produced by changing the distance between the plates of the laser output etalon, thus obtaining the shape by subtracting the phases of the wavefronts recorded at those wavelengths. The data sets for the shape and 3D-deformation are combined and graphically shown. Finally, by using a non linear crystal (BBO) it was possible to double the frequency of the radiation emitted by the ruby laser allowing to get two wavelengths (694 nm and 347 nm) simultaneously and thus to record digital holograms with different sensitivities.
The subject matter of this report is the development and test of a technology for the industrial manufacturing of large format holographic optical elements (HOEs) for technical applications that facilitates the choice of an operating wavelength between UV and IR and the selection of a desired bandwidth that may vary between tens and hundreds of nanometers. Such requirements are essential for the fabrication of spectrally selective holographic lenses and mirrors for use in photovoltaics and solar chemistry. Large format HOEs are also finding increased application as facade elements in the control of radiant energy in buildings. Data is presented illustrating the methods used to control the shift of the operating wavelength and to adjust the bandwidth of the HOE. These methods are based on the understanding of the structure and properties of the DCG holographic material. For example, the recording of holograms for the blue part of the spectrum requires the use of a filler material to swell the hologram and a specific development process to shrink it subsequently to the desired thickness of the layer. Thus, the required filler material should have the same optical properties as the DCG and should be water soluble. The material should be optically neutral and play no more than a passive role as padding in the hologram fabrication process. A similar technique, based on the permanent swelling of the hologram, is used to shift the operating wavelength of a reflective hologram into the IR and to modify its bandwidth.
The objective of this research program was the development of the technology for the industrial manufacturing of HOEs for technical applications such as: holographic solar concentrators for utilization in photovoltaic energy conversion and solar photochemistry, and integrated holographic stacks for daylighting, glazing and shading in buildings. Some of the fabricated HOEs exhibit apertures in the order of 8 square meters. The accomplished technology facilitates the continuous fabrication of the holographic films on glass or plastic substrata. The standard holographic material we use for the fabrication of HOEs is dichromated gelatin (DCG) on glass or plastic film (PET) substrata. The dichromated gelatin layer could be prepared with different compositions to accommodate the desired exposures and chemical processing procedures. At present we manufacture holographic plates on glass substrata in sizes of up to 1 meter square. The holographic film on plastic substratum is 20 cm wide and could be made in lengths of hundredths of meters. The inexpensive fabrication of such large formats is attained by automation of the entire process: film manufacturing, hologram copying, development and test. We present in this paper the design considerations and the developed manufacturing procedures. These comprise the fabrication of large format reflective holograms for concentrating mirrors and the copying of transmissive holograms, such as gratings and lenses, using in-plane contact copying in checkerboard arrangement or rotating drum continuous copying onto an endless plastic film.
Pulsed digital holography is very useful for vibration analysis. In order to increase the temporal resolution a ruby laser producing up to four pulses has been used. The separate digital holograms (one hologram for each pulse) of an object under test are recorded in few microseconds on three CCD sensors. The images of the recorded holographic interference patterns are captured digitally with a framegrabber inside a Personal Computer. The reconstruction of the holograms is performed digitally by means of computer. For the later quantitative evaluation of the reconstructed holograms, the Fourier method is used. The optical phase of each hologram is obtained from the complex amplitude and the deformation between two laserpulses is calculated from phase subtraction and phase unwrapping of these two holograms. Experimental results are presented.
The optical properties of dichromated gelatin (DCG) as a material for volume holography are close to ideal. The material shows large refractive index modulation, high spatial resolution, negligible absorption, and low scattering. The inexpensive fabrication of large format HOEs is attained by automation of the entire process - film manufacturing, hologram copying, that the master hologram is extremely thin and consists of the holographic layer only. DCG layers, however, can not be easily lifted from the glass or plastic substratum. It is possible to achieve this objective by using other materials. As an alternative to gelatin we investigated the holographic properties of materials that contain hydroxyl, carboxyl or carbonyl groups. The investigated materials are: poly(vinyl alcohol) PVA, poly(acrylic acid) PAA and mixtures of these, such as PVA/PAA and chemically modified cPVA. The subject matter of this paper is the comprehensive presentation of the result of the experimental investigation of the holographic properties of the above introduced materials and their comparison to the properties of DCG holographic films. This comparison includes, but is not limited to the diffraction efficiency, grating strength and the transmission characteristics of the films.
A rectangular metal plate is randomly excited by a shaker or by the impact of a small rod. To investigate the temporal development of the induced deformation, an optical measuring system for the evaluation of transient deformations is used. A ruby laser, which is able to produce four laser pulses within a few laser pulses within a few microseconds and a setup of three CCD-cameras for recording the four pulses/interferograms were used. The images of the recorded holographic interference patterns are captured digitally with a framegrabber inside a PC. The reconstruction of the holograms is performed also digitally in the computer. For the later is obtained from the complex amplitude and the deformation between two laser pulses is calculated from phase subtraction and phase unwrapping of these two holograms. Each deformation image can be decomposed into the eigenmodes of the plate, which have to be known/measured before. By measurement of four holograms with a known pulses separation one is able to determine the deformation states between each adjacent pulse and at least the temporal development of the deformation and contributing magnitude coefficients of the eigenmodes.
Digital holographic interferometry has established as a readily available technique for measurement of surface deformations. In the method images containing the deformation of a surface as continuous function of high precision and fine resolution are obtained by computer subtraction of different holograms. On the other hand, the high precision finite elements method can be used to compute the deformation fields of elastic continua, where the resolution is similar to the resolution obtained with digital holographic interferometry. Combining both techniques, an attractive concept for testing is obtained: instead of performing FEM analysis in an isolated effort, it can be handled in parallel to the tests and preferably by the same personnel. This gives the opportunity for true interactive work, what may considerably ease the problem of harmonizing test and analysis. In order to demonstrate these features, a series of the first 45 modeforms of a rectangular plate are measured and computed. The plate is harmonically excited by a loudspeaker. Two separate digital holograms of the whole plate are recorded on a CCD sensor at each resonant frequency. Measured and computed modeforms are presented under identical format.
The performance of a holographic product for commercial applications depends uniquely on the properties of the material and the skills of the manufacturer to best use and optimize these properties in the design stage and during the various manufacturing steps. The design of the HOE should take into account divers changes in the material that are caused by the process, such as swelling or shrinking of the holographic film, since these determine the spectral characteristics of the HOE. Often the changes are intentionally induced to achieve certain hologram properties, such as wavelength shift of the range of operation, the dependence of the diffraction efficiency on the reconstruction angle, or the desire to influence specific diffraction orders. We report here the results from the evaluation of the holographic properties of HOE in dichromated gelatin. These effects lead to the establishment of force fields and material diffusion throughout the depth of the holographic layer and create a gradient of the mean index of refraction that produces a shift in the angular spectrum. Experimental result for holograms fabricated in DCG and carboxylic polyvinyl alcohol are present and discussed. We report on the development and completion of a facility for continuous layer deposition on plastic substrate, hologram exposure and development.
Dichromated gelatin layers (DCG) facilitate the design and fabrication of large format holographic optical elements (HOE) of high optical quality and diffraction efficiency. The HOEs are used for the fabrication of spectrally selective solar concentrators and as glazing materials for daylighting and passive sun control in buildings. The suitability of HOEs in these applications depends upon the achievable bandwidth, operating central wavelength, dispersion characteristics and low absorption losses. The HOEs are fabricated on glass or plastic film substrata in a DCG-layer of 5 to 30 micrometer thickness. The layer thickness and the gradient ar precisely controlled during the layer deposition and drying (plus or minus 1 micrometer and 0.1 micrometer/cm for standard layer of 10 micrometer thickness). The production process is based on the fabrication of high quality master holograms that are copied by dry copying procedure. The current manufacturing facilities allow the fabrication of 1 m2 HOEs on glass substratum and a continuous production of HOEs on plastic substratum with a width of 20 cm and length of 50 m. This technology is also used to fabricate holograms for instrumentation optics in metrology and for optical interconnects in multichip modules. The fabricated HOEs exhibit the desired operational characteristics: high diffraction efficiency, small Braggshift, large bandwidth and a central wavelength that may be freely selected over a wide spectral range. In this paper, we present the results from the experimental investigation and theoretical analysis of large number of holograms of the transmissive and reflective types. We discuss the attained angular and wavelength spectra, bandwidths, wavelength shifts and the diffraction efficiencies as functions of the holographic parameters. The HOEs are made for technical applications and are designed to operate in the 300 nm - 1500 m spectral range.
In this paper we present the results in the development of large format (20 cm by 50 cm) cylindrical holographic mirrors (CHMs) recorded in dichromated gelatin, for use in solar chemistry applications. The realized spectral reflectivity of 400 - 440 nm is adapted to the sensitizer zinctetra-phenylporphine. Effective efficiency of the mirror (i.e. referred to the incident radiation at the desired wavelength of 420 nm) is 75% with excellent homogeneity across the aperture.
Large scale holograms of the reflective type are applied on concentrating devices for photovoltaics and solar chemistry as well as in building applications. Especially in window applications holograms with best homogeneity in large scale are demanded by the house owners and architects. To meet this demand, we have used a scanning laser beam and internal total reflections in a moving prism for recording. This technique is usually applied to record holograms for the near infrared (NIR), but due to controlled mixture of gelatine and water-soluble polymer in addition with a matched thermo-chemical treatment the central wavelength can be adjusted in a wide range to special requirements. For application in solar chemistry we manufactured parabolic concentrators with holographic foils of size 50 cm by 10 cm in dichromated gelatine (DCG). The central wavelengths can be chosen between 400 nm and 600 nm with bandwidths of 50 nm as well as up to 200 nm. The comparison of the transmitted and reflected spectrum shows good agreement and consequently minimal scattering losses of the layer. A photovoltaic concentrator concept uses a silicon solar cell and a spectrally matched broad band spectral characteristic is required. A bandwidth of 600 nm can be achieved with a stack of two foil or even a single foil. Large scale e investigations (up to 100 cm by 10 cm)2 show best results of diffraction efficiency and large scale homogeneity.
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