Importance of Exposure Time on DIC Measurement Uncertainty at Extreme Temperatures

Digital Image Correlation (DIC) is a popular optical method for deformation and strain measurement. At extreme temperatures, it is known that materials emit light in addition to reflecting the light supplied by a light source, and the emitted light can saturate a camera sensor. More recently, a novel variation of DIC, named ultraviolet (UV) DIC, extended the range of temperature further by using a UV bandpass filter to screen out some of the brightest glowing and external UV illumination to provide additional reflected lighting. In principle, for a given optical set-up the temperature range can be extended further by reducing the camera's sensitivity to light, and exposure time is an instrumental parameter when setting such camera configurations. In this paper, we examine the influence of multiple exposure times on the uncertainty of UV-DIC correlation measurements. Rigid-motion experiments were performed at four different temperatures: room temperature, 1300(o)C, 1450(o)C, and 1600(o)C. At each temperature level, UV images were recorded for DIC at exposure times ranging from 500 s to 61,000 s - a range of over two orders of magnitude. The results showed abrupt increases of error at extremely dark or bright exposure times, but at intermediate exposure times the errors of UV-DIC were minimal. A normalized metric was presented in order to give a general guideline when choosing exposure time for camera sensitivity. It is recommended that cameras should be set at a suitable range of exposure time (between 10,000 s and 40,000 s for the camera used in this paper) in order to perform meaningful DIC up to 1600(o)C.