Infrared thermal cameras have applications in a range of fields including security, agriculture, inspection, and health. In this paper, we are interested in their sensing properties, where we model and estimate the camera parameters required to transform digital counts to temperature values. Specifically, we propose a method to characterize the spatial deviation for the irradiance observed at the image plane for an uncooled focal-plane array camera deprived of an internal measurement of temperature. In our approach, we establish the relationship between the radiance emitted by a temperature varying blackbody chamber and the uncooled camera digital output. The model approximates this relationship as a quadratic polynomial whose variations are themselves approximated with another polynomial expression. Our results suggest a significant improvement over the commonly used Plancklike expression.