During this effort an instrument for calibrating the lunar irradiance will be designed. Such an instrument will lead to reliable exoatmospheric calibration for past, current, and future earth-viewing instruments and improve the accuracy of their data products, which in turn will improve climate change and weather models. The instrument will measure both the solar and lunar irradiances, which will enable cross calibration with the TSIS mission. The proposed instrument concept has been formulated to take advantage of the near-collimated nature of the input signals. The work plan is to develop detailed ray-trace and radiometric models of the instrument. The error budget for the instrument will be analyzed and pre- and post-launch calibration plans will be formulated.