Determination of the infrared irradiance caused by stars and other objects outside the earth's atmosphere by means of sensors located within the atmosphere requires correction for atmospheric effects. Subtraction of the path radiance is often inherent in the sensor design. However, determination of the atmospheric transmittance is more difficult, frequently requiring auxiliary measurements of profiles of species concentration, temperature, and pressure along the optical path. A procedure has been developed for determining the atmospheric transmittance, without reference to atmospheric profiles, from the measured path radiance augmented by surface meteorological observations. The dependence of the effective atmospheric radiating temperature on elevation angle for a given sensor band can be described by a function of elevation angle in which the parameters are determined from surface observations. When the effective radiating temperature of the atmosphere is known, the transmittance is obtained from the emissivity defined by the ratio of the observed path radiance to the Planck function for the effective atmospheric temperature. This algorithm, which is useful only for clear conditions in the infrared spectral windows, has reproduced, in test situations, the transmittance computed with models such as LOWTRAN to within 10% for all portions of the sky with elevation angles greater than 10 deg.