A finite element analysis model of a transient technique used to measure the emittance of surfaces and coatings was developed and used to estimate the uncertainty in emittance. The dimensions used in the model matched the dimensions used in the design of a low temperature calorimetric vacuum emissometer being built to characterize the thermal properties of space power materials in the temperature range 173-673 K. Radiant energy from a quartz halogen lamp impinged on an aluminum sample that was coated with a thermal control coating and suspended in a liquid-nitrogen-cooled vacuum chamber by narrow gauge thermocouple wires. After removing the heat source, the temperature of the sample was monitored vs, time and the temperature-time curve was used to calculate the emittance. Factors contributing to the uncertainty in the emittance included uncertainties in time, temperature, area of the sample, heat capacity of the sample and heat loss from the uncoated back side of the sample. Heat losses from the thermocouple wires were found to be negligible. The total probable error in the emittance obtained from the low temperature calorimetric vacuum emissometer design was estimated to be less than 4% for emittance values greater than 0.5 at temperatures between 173 and 673 K.