Uncertainty is inherent in every measurement. To truly understand the value of a measurement, the uncertainty of such should be analyzed. A critical step of this analysis is to know the uncertainty contributed by the instruments used to make the measurement. The intent of this document is to provide a common approach to the uncertainty analysis for calibration of lumen measuring equipment (integrating spheres) with standard incandescent lamps, including tungsten-halogen, that have been assigned values of total luminous flux and/or total spectral radiant flux. This document provides a detailed and step-by- step method for determining uncertainty of certain calibration procedures common to instruments used in lumen measurement laboratories. It uses the structure proposed in the Guide to the Expression of Uncertainty in Measurement (GUM) for its recommendations. For each calibration procedure presented in this document, a mathematical model is developed, estimates of each input quantity are discussed, and guidance is provided for the evaluation of the standard uncertainty of each input estimate. With this information, the combined standard uncertainty is calculated, effective degrees of freedom are calculated, and finally, the expanded uncertainty is reported.