Deformation Of The Imaging Devices Induced By The Self-heating And Its Influence On The Photomechanics Method

Photomechanics is the general term of the methods in experimental mechanics which measure the mechanical quantities,i.e.stress,strain,displacement,etc.,based on the light interference principle or the image analysis methods.When using a digital camera to develop experiments,the temperature variation of the imaging system will lead to the "virtual deformation" of the recorded images,resulting in non-ignorable errors from displacement and strain measurement.At present,there is no comprehensively systematic or thorough understanding of the physical mechanism and the regularity varying with the temperature generated by "virtual deformation".Therefore,the quantitative evaluation for the displacement and strain measurement error in the photomechanics caused by temperature varying of the imaging system has run into a stone wall.In this study,starting with the constitution of the image-forming system which takes the digital camera as the core,the transmission route of the heat transmission path and the resulting ways of thermal deformation of the key imaging devices in the imaging system hot deformation mode of key image-forming device in the image-forming system are analyzed.Spontaneously,the thermal deformations of the camera body(interface)and target surface caused by the camera self-heating are mainly studied,it is found that thermal expansion and spinning will happen in all of the three directions of the digital camera body,and there will be a heat-resulted translation in parallel with the fixed direction caused by the thermal deformation of the fixing device.Meanwhile,the parameters of heat-resulted deformation are generally smaller in the tripod-fixed mode than which in the magnetic table-bar-fixed mode,thereby it means that the former fix pattern is better than the latter one.At the same time,a translation along the optical axis direction and a very tiny rotation relative to the camera body of the target surface of the digital camera will occurred.As for the cameras of different brands,different forms of fixed patterns and installation errors of the target surface will cause different errors in translation and rotation as well.Therefore,it can be concluded that the image "virtual distortion" is induced by the heat-resulted motion or deformation of camera body and target surface in the imaging system.Furthermore,the camera imaging model which takes the thermal deformation of the key devices of the imaging system into consideration is established on the basis of pinhole imaging model,and the influence rules of different parameters of the model for images are analyzed and summarized.It is found that thermal induced motions of the camera body and target surface will lead to the movement of pixels in the image.According to that,a quantitative model has been built,which characterizes the relationship between the displacement and strain measurement error induced by the self-heating of camera and the thermal deformation parameters.Through standard experiments,it is found that no matter fringe analysis method or image analysis method,both of which will be affected by the thermal deformation of the devices,and the experimental result and the model calculation are in good agreement,which proves rationality of the relationship model given in this research.In this study,the mechanism that the "virtual deformation" of images induced by the camera self-heating is revealed,and the law which the thermal deformation of imaging devices influences the displacement and strain measurement errors in photomechanics is summarized,which can provide guidance for improving the measuring accuracy of the photomechanics.