This paper is concerned with an application of infrared thermography to study the material behaviors on the microstructure scale. A single crystal specimen of pure nickel was investigated, and a thermographic methodology was developed for realizing a quantitative measurement on the subgrain scale. The nickel single crystal was plastically deformed in a tensile test, and notable out-of-plane deformation manifested on the material surface in the form of slip markings. The out-of-plane deformation introduced significant radiometric artifacts in the measured thermal fields, as a result of the modification of apparent emissivity on the material surface. The geometrical dependence of the apparent emissivity was analyzed quantitatively based on the measured emissivity and surface profile distributions. The study demonstrates that the apparent emissivity decreases with the increase of the variation of the angle of observation, which is in good agreement with the theoretical analysis.