A simple shell model theory is developed in terms of the double-time Green function method to calculate the one-phonon infra-red absorption in a monatomic crystal with a small concentration of uncharged point defects. The theory allows for a defect change in atomic mass, change in core and shell charge, and change in internal core-shell force constant. The frequency-dependent absorption is found in terms of a one-phonon density of states; the integrated optical absorption is expressed in a closed form similar to that found by Doniach and Huggins. Fitting the theoretical parameters to data for krypton and xenon defects in solid argon results in a theoretical absorption of the order of magnitude found experimentally by Jones and Woodfine. Anharmonic effects in the experiments are thought to account for the principal discrepancies between theory and experiment.