In support of multiband imaging system performance forecasting, an equation-based triangle orientation discrimination (TOD) model is developed. Specifically, with the characteristic of the test pattern related to spectrum, the mathematical equations for predicting the TOD threshold of the system with distributed fusion architecture in the IR spectrum band are derived based on human vision with the "k/N" fusion rule, with emphasis on the impacts of fusion on the threshold. Furthermore, a figure of merit Q related to the TOD calculation results is introduced to analyze the relation of the discrimination performance of multiband imaging system to the size and the spectral difference of test pattern. The preliminary validation with the experiment results suggests that our proposed model can provide a reasonable prediction of the performance for a multiband imaging system.