Cameras

Within this work [7] a method and model to determine the optical transfer function (OTF) for the detector chain without detailed knowledge of the internal detector and camera characteristics was developed. The expected value of the signal S0.2 is calculated with... 

The eombination in a compact system of an infrared sensor and a laser as excitation source is called a photothermal camera. The surface heating is aehieved by the absorption of the focused beam of a laser. This localisation of the heating permits a three-dimensional heat diffusion in the sample to be examined. The infrared (IR) emission of the surface in the neighbourhood of the heating spot is measured by an infrared detector. A full surface inspection is possible through a video scanning of the excitation and detection spots on the piece to test (figure 1). 

Figure 2 Surface-breaking crack signal of a Photothermal camera (theory)...

The principle of the photothermal camera is known for decades (see ref. 1) but did not offer sufficient performances to permit this instrument to find an industrial issue. The main reason of that lack of success Is the poor efficiency of the previous configurations when applied on industrial products (figures 3, 4, 5) ... 

Figure 5 Fig 4. part inspection with a conventional nhotothermal camera the crack indications can he seen hut not much than optical indications associated to the rough... 

To overcome these limitations, the new photothermal camera developed by FRAMATOME and ONERA (figure 6) ... 

Figure 6 Fig 4. part inspection with the new desien ofnhotothermal camera only the crack indications can be seen in spite of the rough surface condition (no more false alarms). 

Figure 7 New photothermal camera versus penetrant testing on a crack (width 5 pm) at the surface of Fig 4. part The large arrow shows that the PT indication almost extinguishes at thinner width (approx. I pm) locations along the crack the photothermal signal does not. The bottom little arrow indicates a place where the crack splits in two, which is not revealed on the PT image because of the spreading of the penetrant liquuid.

Figure 8 Sensitivity of the new photothermal camera to small depth defects. lmage of an EDM notch of 1mm long, 100 pm width and 200 pm depth on ANSI 304 stainless steel with a bad surface condition (ground surface, "Vi 2 -6 ).

A new design of an old concept of thermal surface inspection permits the photothermal camera to be considered as an interesting alternative to NDT methods like penetrant or magnetic testing, particularly when the testing has to be made in a remote or automatic way. 

For a few months, results obtained on a pre-industrial version of this new photothermal camera allow this technology to enter into the industrial step of its development. 

For the examination of the applied metallic or ceramic layer, the test object is heated up from the outside The heat applying takes place impulse-like (4ms) by xenon-flash lamps, which are mounted on a rack The surface temperature arises to approx 150 °C Due to the high temperature gradient the warmth diffuses quickly into the material An incorrect layer, e g. due to a delamiation (layer removal) obstructs the heat transfer, so that a higher temperature can be detected with an infrared camera. A complete test of a blade lasts approximatly 5 minutes. This is also done automatically by the system. In illustration 9, a typical delamination is to be recognized. 

Table 4 shows, the sources are available with physical sizes ranging from 1 mm x 1 mm up to 3 mm x 3 mm. They are produced from firmly compressed selenium pellets of cylindrical shape. The activities range up to 3 TBq or 80 Ci, which is the maximum allowed loading of the GammaMat SE portable isotope transport and working container, as well as the Source Projector M-Se crawler camera. 

The GammaMat M isotope pipeline crawlers previously have been used with exposure cameras for iridium the models M6 and Ml 8 used exposure units designed for a maximum loading of 2.2 TBq (60Ci) and 3.7 TBq (lOOCi), respectively. 

Practical applications [2] of a GammaMat M model using the new Selenium crawler camera loaded with approx. 1 TBq (30Ci) on a pipeline of diameter 12 and wall thickness of 0.25 showed 6-7 m axial distance to the exposed source as limit of the radiation controlled area (40pSv/h) and 22m perpendicular to the pipeline. Other authors [3] have reported about a comparison for Ir-192 and Selenium source on a 4.5 diameter pipe and 0,125 steel thickness they have found for 0.7 Tbq (18Ci) Selenium a value of 1 Om behind the film (in the unshielded beam) comparing under same conditions to approx. 40m for Iridium. 

X-ray specimen image inter sifier tandem optics camera digital image processing monitor... 

The zoom optics between the image intensifier and the video camera gives the freedom to magnify details of the image for more detailed inspection. 

Three Dimensional Defect Analysis Using Stereoradioscopy Based on Camera Modelling. 

In the following sections our approach to stereoradioscopy will be described in detail. At first, the camera model and the calibration procedure are introduced, in the second part, the calculation of 3D defect positions and the volume estimation procedure are explained. 

To describe the X-ray imaging system the projection of 3D object points onto the 2D image plane, and nonlinear distortions inherent in the image detector system have to, be modelled. A parametric camera model based on a simple pinhole model to describe the projection in combination with a polynomal model of the nonlinear distortions is used to describe the X-ray imaging system. The parameters of the model are estimated using a two step approach. First the distortion parameters for fixed source and detector positions are calculated without any knowledge of the projection parameters. In a second step, the projection parameters are calculated for each image taken with the same source and detector positions but with different sample positions. 

In this section the camera model based on a projection model and the polynomal model of the image detector distortions will be described (figure 2). 

The camera model has a high number of parameters with a high correlation between several parameters. Therefore, the calibration problem is a difficult nonlinear optimization problem with the well known problems of instable behaviour and local minima. In out work, an approach to separate the calibration of the distortion parameters and the calibration of the projection parameters is used to solve this problem. 

Based on the camera model the distortion parameters are calculated for fixed source and detector positions without any knowledge of the projection parameters. 

The epipolar constrains calculated using the estimated camera parameters restrict the search for corresponding image features in different images to a ID search. Taking the uncertainty of the epipolar constrains into account, in our approach, the search is restricted to a small area around the epipolar lines in the images. 

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