Thermal radiation sources are often used in thermal imaging sensors as reference sources for offset equalization and gain normalization of the infrared detecting elements. For best results, the `t-number' by which the detector elements view these sources must be exactly equal to that with which they view the external scene. If the t-numbers for different detecting elements are not the same, the result will be inaccurate offset equalization and imperfect gain normalization. The relationship between the scene temperature, reference source temperature, and t-number differences are derived. Quantitative values for the errors in gain and level normalization are calculated.