Geometric Calibration of Thermographic Cameras

Abstract

Commission V, WG V/5KEY WORDS: Thermography, Thermal Imaging, Calibration, Accuracy, Application ABSTRACT:The paper gives an overview of thermal imaging sensors for photogrammetric close-range applications. In particular, it presents results of the geometric calibration of thermographic cameras as they are used for building inspection and material testing. Two different test fields have been designed providing point targets that are visible in the thermal spectral band of the cameras.Five different cameras have been investigated. Four of them have solid state sensors with pixel sizes between 25 and 40 µm. One camera is working in scanning mode. The lenses for thermographic cameras are made of Germanium. Conventional imaging configurations (typically 20 images) have been used for camera calibration. Standard parameters for principal distance, principal point, radial distortion, decentring distortion, affinity and shear have been introduced into the self-calibrating bundle adjustment. All measured points are introduced as weighted control points. Image coordinates have been measured either in the professional software package AICON 3D Studio (ellipse operators), or in the IAPG software system Stereomess (least-squares template matching).The calibration results differ significantly from camera to camera. All lenses show relatively large decentring distortion and deviations from orthogonality of the image coordinate axes. Using the plane test field with lamps, the average image precision is 3/10 th of a pixel while the 3D test field with circular reflecting targets results in imaging errors of 1/20 th pixel.

Multispectral Imaging in Cultural Heritage Conservation

“…The internal orientation, through which focal length, principal point and geometric distortion are obtained, are usually solved before the radiometric calibration of the sensors (Luhmann et al, 2013 Bay et al, 2006) algorithms. Accordingly, as an alternative to this process, there is the possibility of reprojecting the different images onto a common plane using a 2D projective transformation (Hartley et al, 2003).…”

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“…For pixels that cover the edge of the squares the temperature is averaged over the black and white parts thus blurring the edges. [21] have explored the calibration procedure using different types of thermal cameras. Generally an object with a unique pattern having distinct targets is used which eases labeling and increases accuracy of the calibration process.…”

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“…Therefore, special calibration field for thermal sensing was proposed for a geometric calibration. Some inspiration and examples of thermal calibration can be found in (Luhmann, 2010). Table 6.…”