Mechanism And Correction Of The Error In Photomechanics Methods Induced By Temperature

Photomechanics method is a class of experimental mechanics technology for stress,strain and displacement measurement based on optical interference and image analysis.Recently, the errors on images caused by temperature variation of the digital camera havecaught the attention of various researchers. It has been found that temperature inducedstrain error can be hundreds of micro strain and the displacement error can be a few tenthspixels in photomechanics measurement. It is obviously that the error is larger enough toeffect the precision of photomechanics measurement and should be eliminated. But thephysical mechanism of temperature induced error in photomechanics measurement is stillnot clear and many problems such as transformation property of heat, temperaturedistribution and thermal deformation behavior of imaging component have not been solved.Thus it is difficult to find out the relationship between the measurement error andtemperature, meanwhile the error correction and compensation method.The principle of temperature induced error in photomechanics method was deeplystudied in this paper. I was found that the imaging parameters variation caused by thermaldeformation of digital camera case, mount and lens is the reason of this error. Thetemperature distribution and thermal deformation of camera component is different inindoor and outdoor measurement. For indoor situation, there is only one heat source inimaging system, the heat go through form the camera case to the lens. The movement ofCCD chip and lens shows a very good linear relationship with the camera outer casetemperature. So the temperature of outer case could be the characteristic temperature, andthe model to describe the relationship between characteristic temperature and measurementerror is established including camera temperature, the thermal deformation parameter ofcase and mount. For outdoor situation, the camera self-heating and environmenttemperature variation couple together so that the heat balance temperature is different underdifferent environment temperature. The temperature of lens affected most by environmentand the temperature variation forbid the same rule of environment temperature variation.The influence of environment temperature variation on camera outer case is very small and mainly affected by camera self-heating. The temperature variation rule of camera mount isthe result of both camera self-heating and environment temperature variation. So the modelto describe the relationship between temperature and measurement error in outdoormeasurement is established including camera temperature, lens camera, the thermaldeformation parameter of case, mount and lens.In order to compensate for the temperature induced error, it must first be evaluated andthen subtracted from the measured result. Two methods can be used to evaluate the error:First, the image expansion rate, which could be converted to strain error for image analysisbased methods or relative phase error for fringe analysis based methods, of the imagingsystem is calibrated before the experiment and the camera temperature is recorded duringthe whole procedure. After the experiment, the temperature induced error is obtained fromthe product of the error strain rate (or the error phase rate) and the recorded cameratemperature variation. Secondly, an unloaded compensation specimen is placed in the samefield of view, and at the same distance from the focus plane as the measured specimen. Themeasured result on the specimen could be directly regarded as the temperature inducederror. Both of these two error compensate method have their own merits, the users couldmake choice according to real conditions.The principle and rules of temperature induced error presented in this paper could notonly provide theoretical guidance and technical support for the digital camera design, butalso improve the measuring accuracy of photomechanics.