An electronic 3D-shape measurement method has been developed by combining light-in-flight recording by holography (lif) with digital holography. Lif is a technique for direct visualization of the propagation of a short light pulse when it is e.g. intersecting a 3D-shaped object resulting in contour lines of the object. This is achieved by letting the reference beam be delayed in comparison to the object beam along a CCD-sensor. A Fourier-Fresnel algorithm numerically reconstructs the hologram. A Littrow mounted reflection grating in a Twyman-Green type interferometer set-up creates the optical delay between the object and the reference beam. The 3D-shape and the position of the object can be determined by combining the contour lines of the object together to a depth map.
The recorded hologram from one point will be confined to a rectangular area on the CCD-sensor, where the delay between the reference and the object beam is within the pulse length of the laser. Due to diffraction limitation will the resolution be poor in the direction of the delay since the recorded rectangle area will have its shortest side there. The resolution can however be improved by making two recordings in two perpendicular directions. The two holographic images can then be combined keeping the best resolution of the two holograms. Increase of the resolution makes the lif method useful for more applications for shape and deformation measurements.
Point clusters of measured shapes achieved with the digital light-in-flight recording by holography method, is combined with CAD - Computer Aided Design software to obtain convertible formats for design and simulation software. The point cloud is constructed to a b-spline surface by least square fitting. The b-spline surface is converted into IGES- format in order to be readable for CAD and other software. A simple deformation experiment is made which demonstrates the advantages of combining holographic measurements with FEA- analyzes in order to adjust the constraints, the boundary conditions and loads in the simulation model to better fit experimental results.
KEYWORDS: Holography, 3D acquisition, Holograms, Digital holography, 3D modeling, Visualization, 3D image processing, Charge-coupled devices, Holographic interferometry, Finite element methods
An optical measurement system composed of an optical microscope (Olympus BX60) and an optical multichannel analyzer (EG&G OMA 1460) has been assembled and tested. The optical microscope allows the user to make measurements on a small and well defined area of the sample. The light source, a 100 W halogen lamp, and the diode array detector, result in high sensitivity in the wavelength region of 450-750 nm. The spectral resolution of the instrument is listed as 0.59 nm/channel. The full width at half maximum (FWHM) of the strongest peaks in calibration measurements on a mercury lamp is 5 channels corresponding to 3 nm. Quasifractal clusters of gold particles have been produced with electron beam lithography. The clusters consists of different numbers of particles, giving a cluster size variation from 1.6 (mu) m to 50 (mu) m. The individual gold particles are 50 nm in diameter each. The measurement system has been used to measure both absolute transmittance and the relative transmittance using the uncoated substrate as a reference.
An electronic method is proposed that can simultaneously measure an objects shape and its displacements in the micrometer range as well as in the millimeter range. This opens up the possibility to easily integrate holographic deformation measurements with FEA-modeling. It also makes it possible to record a 3D-movie. The method is a combination of digital holography and light-in-flight recording by holography. A Twyman-Green type interferometer is used with the reference mirror replaced by a Littrow-mounted reflection grating. The grating creates an optical delay across the beam profile. The deformations are projected to the 3D-shape whose gradients are determined by Sobel- operators. Large rigid body displacements are calculated by crosscorrelation of shapes measured before and after displacement. A short theoretical description is followed by experimental results of this method.
KEYWORDS: Holography, 3D modeling, Digital holography, Visualization, 3D image processing, 3D metrology, Stereoscopic cameras, Finite element methods, Computer aided design, Image resolution
In the development of new products there are two parallel lines for the development process to follow; the traditional or 'real,' and the new computer aided or 'virtual.' The traditional line is to develop prototypes that can be used for testing strength, functionality, and visual appearance of the product. In the virtual line digital (CAD) models are developed which can be tested entirely in a computer by simulations e.g., using Finite Element Analysis (FEA) and other tools. For transformation from the real to the virtual world some kind of 3D camera is needed. The shape of the model should be measured together with further data concerning the visual appearance, material properties, etc. We have developed electronic recording techniques for doing this based on the Light-in-Flight technique. This technology has all advantages of holography, i.e., apart from the shape it is also possible to measure how much light is reflected from different parts of the object along with interferometric information, which can give mechanical data for the object. This can be used for example for visualization and to give tactile and haptic information to a virtual reality system about how the object would be perceived by a person touching it.
As a part of a project for using short laser pulses and holographic techniques to measure three-dimensional shapes, measurements have been carried out of the pulse shapes of laser pulses from a mode-locked argon-ion pumped dye laser with autocorrelator, light-in-flight recording by holography and nonscanning white-light interferometer. The aim of the study is to find a reliable, inexpensive and fast method for measuring the pulse length of laser pulses in the picosecond region, when exposing and reconstructing the light-in-flight hologram, in respect to pulse width, symmetry and signal-to- noise ratio. Experimental results, along with the theoretical background, from these tests are accounted for in this presentation.
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