How light emitted at high temperature affects common digital image correlation algorithms

This manuscript serves as an addendum to our previous paper, ‘Change of Exposure Time Mid-Test in High Temperature DIC Measurement,’ published in 2020 (Thai et al 2020 Meas. Sci. Technol. 31 075402). Since publishing that paper, we have since discovered that our two sources of lighting affect the brightness and contrast of high-temperature images differently. While it is broadly understood that changes in camera sensitivity (for example, by reducing exposure time) produce linear scaling in lighting, here we show that changes in temperature (which produce blackbody radiation) instead produce uniform offsets in lighting. This has important implications for the correlation algorithm, as the normalized sum of squared differences (NSSD) function compensates only for linear scaling, not uniform offsets. Here, we repeat the analysis for three different temperatures (1300 °C, 1450 °C, and 1600 °C) and correlate using three different algorithms: the sum of squared differences, NSSD, and zero-normalized sum of squared differences (ZNSSD). Our findings reinforce the consensus that, when performing digital image correlation (DIC) at extreme temperatures, it is important to always select the ZNSSD algorithm due to its ability to correct for both offsets and linear scaling in lighting.