This work presents how deflectometry can be coupled with a light-field camera to better characterize and quantify the depth of anomalies on specular surfaces. In our previous work,1 we proposed a new scanning scheme for the detection and 3D reconstruction of defects on reflective objects. However, the quality of the reconstruction was strongly dependent on the object-camera distance which was required as an external input parameter. In this paper, we propose a new approach that integrates an estimation of this distance into our system by replacing the standard camera with a light-field camera.
Widely used for surface slopes measurements and for three-dimensional shape reconstruction, deflectometry is a particularly powerful technique that can also be applied for defects detection on specular surfaces. In the visible domain, deflectometry is usually based on the projection of complex encoded light patterns and necessitates heavy processing that makes it not suitable for inline inspection. In this paper, A new deflectometry based approach that is more adapted for inline inspection of linearly moving parts (parts on conveyors) is proposed. Based on a more affordable and a simpler hardware setup, the new approach allows at the same time for a proper localization and a precise geometrical quantification of any defects on the scanned specular surfaces. The proposed approach uses a fast and simple processing algorithm that lends itself very well to real-time inspection. The new method is tested and validated in laboratory for the inspection of defects on specular surfaces of plastic parts.
Within the framework of a French research program, we have developed an optical system dedicated to the measurement
of works of art. The purpose is to record the actual optical characteristics of the objects' surfaces in order to be able to
display the art object on a screen with a high degree of realism. Three types of data are recorded: 3D shape, true colours
and texture.
The 3D shape is obtained using a structured light sensor that gives a dense point cloud. A specific procedure
allows automatic registration of several point clouds without any contact with the surface.
The colours maps are recorded with the structured light sensor's RGB camera and using a built-in lighting
system. Both camera and lighting are calibrated using a well-defined procedure. Merging the colour data with the 3D
data is straightforward because the same camera is used for both tasks.
The texture information is related to the so-called BRDF (Bi-directionnal reflectance distribution function): at
each point on the surface, the reflectance is a function of the direction of observation and the direction of illumination.
To record this complex texture information, several illumination sources are used, as well as an additional moving
camera. Thus, for one 3D point cloud, a complete set of colour pictures is processed to produce texture files that are
directly linked to the 3D points.
The paper details the measurement procedure as well as the associated data processing. Several results are presented.
Monuments are continuously submitted to external events like water infiltration or condensation, temperature variation,
soil instability, that lead to internal damage of the structure itself as well as of its surfaces. Wall paintings are then
submitted to stresses that may cause cracks, internal de-lamination of the plaster or de-bonding between canvas and
plaster.
In the frame of the restoration of the "galerie des glaces" in the "château de Versailles", TV-Holography and IR
Thermography have been used to investigate the wall paintings of the vault. The surfaces to control were either direct
paintings on the plaster or paintings on canvas backed on the plaster.
IR Thermography for art work and in particular for wall paintings has only recently been used. The technique
allows to record transient temperature maps, when slightly heating the surface during a short time. Then, nonhomogeneities
in the conductive heat transfer are related to de-bonding or de-lamination. The time parameter gives
information on the depth of the defect. A calibration procedure has to be carried out to ensure reliable defect detection.
Speckle interferometry is a Non Destructive Testing technique that is currently used in industry. For the wall
paintings, we have used TV-Holography associated with a continuous wave laser. The technique allows, 13 metres away
from the surface, to detect parts of the paintings that were vibrating due to an acoustic excitation.
The control processes based on these two technologies is detailed as well as the results obtained and a comparison with manual investigation is done.
Since the early 1990's, holography has been used worldwide to study the vibration of mechanical parts at the design stage. The so-called <<time average>> technique on holographic plates was able to give accurate information on the vibration modes of structures. TV-Holography has simplified the data capture, and also has the ability to easily produce amplitude and phase maps. This optical method is a powerful tool for vibration analysis but it needs to be used carefully to gain the full benefit of the data recorded. Then several types of analysis of these data may bring to the mechanical designer key information for the future life of the designed mechanical part. In this paper, we present a complete vibration analysis of a turbocharger turbine wheel, including the two main following points: the holographic recording method and the data post-processing that is done by the vibration experts. Concerning the data recording we will emphasize the experimental conditions that lead to data that are useful for the mechanical engineer: wheel preparation, wheel boundary conditions, method of excitation, geometrical conditions, tests complementary to the holographic recording. Experimental results are reported, showing the effect of the experimental conditions on the eigenfrequencies, eigemodes and damping factor. Concerning High Cycle Fatigue (HCF) on turbine blades of turbochargers, Holography is of gret help in two instances, predictive behavior at design stage and field failures analysis. For the first task, Holography confirms/refines the 3 or 4 first modes predicted by FEA models, it gives the high order modes not predictable by models (especially coupled inducer/backdisc modes) and also the damping factors that are not accurately predicted. Those data are then fed into an "Harmonic Analysis" which allows the prediction of a forced response and, subsequently, an answer about robustness with respect to HCF. For the second task, Holography provides accurate nodal lines which can be easily correlated with location of field cracks or fractures. A Campbell diagram can be used to identify the order of the aero excitation responsible for the failure. Corrective actions to the product design or recommendations for speed limit can then be taken accordingly.
Structural intensity is a powerful tool which give transfer path of the vibration energy within a structure. Additional data processing allows then to localize sources and sinks of that energy. The measurement of these quantities is possible using classical means (accelerometers, stress gauges, ...) but the data processing is complex and require a lot of accurate sensors because it is based on spatial derivatives of high order. The optical techniques (laser vibrometry, holography, ...) are more suited for that purpose because of the high density of measuring points and because of the well-known advantages of these methods: reduced measurement time and no modification of the mass parameters of the structure as it is the case when using contact sensors.
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