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Based on the diffusivity approximation theory, the infrared flux at the top of atmosphere (TOA) can be obtained by multiplying a factor of pi on the infrared radiance that was measured at a viewing zenith angle (VZA) of 53deg. This letter applies the diffusivity approximation on radiance measurements of the Clouds and the Earth's Radiant Energy System (CERES) to derive TOA infrared fluxes and compares these fluxes with the state-of-the-art CERES outgoing radiative fluxes. We find that the mean difference between the two kinds of instantaneous flux that were estimated at the window channel is ~ 1 Wmiddotm-2, with a root-mean-square error of ~ 1.7 Wmiddotm-2. This result shows that radiance measurement at a fixed VZA of 53 deg is a simple and effective method in the remote sensing of the infrared flux for satellite missions that monitor some specific climate processes and require longwave/window TOA fluxes, such as the Broad Band Radiometer instrument on EarthCARE; however, this approach may involve errors from an inhomogeneous scene or non-Lambertian emission of the surface. A careful design of the VZA and scan mode, such as a conical scan at 53deg, would produce much more convenient infrared flux measurements for the Earth-atmosphere system than other designs.