## 4 Conclusions  
When using a camera system sensitive to temperature variations for capturing images and performing measurements in photo mechanics methods, temperature is a significant factor that introduces considerable image errors, which further leads to measurement errors in photo mechanics. These errors must be eliminated or compensated. To address this major problem, in the present work, we first investigated the underlying mechanism of thermal-induced measurement errors in photo mechanics methods and further established a physical model to elucidate the quantitative relationship between temperature and the thermal-induced measurement error. The underlying mechanism indicates that (a) mechanical camera components, which include the camera case, mount, lens, and CCD focal plane, have thermal behaviours for temperature variation. These behaviours consist of thermal deformation, rotation, and translation. (b) In our opinion, these thermal behaviours of the camera components generate slight changes in the image geometry leading to inevitable thermal-induced errors. (c) Using the strategy of improving the camera's fixed mode, some thermal behaviours can be effectively decreased and even eliminated, which may contribute to enhancement of the measurement accuracy of photo mechanics methods. Then, based on the investigation of the underlying mechanism, we introduced three types of temperature compensation methods, namely the model compensation method, preheating method, and compensation specimen method. The model compensation method uses the quantitative model established in this study to characterise the relationship between the environmental temperature and the thermal-induced measurement error. By recording the environmental temperature during the experiment, the measurement results of photo mechanics methods can be corrected. The preheating method is suitable for indoor photo mechanics measurements, wherein the environmental temperature remains constant, which leads to a thermal balance of the camera system after the self-heating period and, hence, the elimination of the thermal-induced error. The compensation specimen method requires placing an unloaded specimen, which records the thermal-induced measurement error, next to the tested specimen. Corrected measurement results using photo mechanics methods can be obtained by subtracting the measurement result of the unloaded specimen from the tested one. This method reduces the effective resolution of the images and requires that the image error forms in the region of the tested specimen are the same as that in the region of the compensation specimen. According to the characteristics of a given measurement situation, the appropriate compensation strategy can be selected from the proposed temperature compensation methods. For the measurement situation where environmental temperature changes and high image resolution is required, the recommended strategy is the model compensation method, which requires recording environmental temperature and calibrating the thermal parameters and thermal deformation parameters of the camera system. In some special situations, such as an indoor photo mechanics measurement wherein the environmental temperature is constant, the preheating method is the primary choice if there is enough time to wait for thermal balance for the camera. In addition, for situations where the image resolution is enough and the compensation specimen is easy to prepare, the compensation specimen method is the recommended strategy, which does not consider the environmental temperature change. Figure 17: Experimental verification of the compensation specimen method for compensating the translation measurement error: (**a**) experimental arrangement and (**b**) experimental results. Lastly, it should be noted that the mechanical components of different camera systems respond differently to temperature variation, which may result in a slightly different error model whereas this can be obtained by the analysis approach fully illustrated in this paper.