The dependence on temperature and scan time of the optical line shapes of individual chromophore molecules in low-temperature glasses is discussed within the framework of the two-level system model. It is shown that the line shapes of different individual molecules depend differently on the temperature and the scan time, reflecting the unique configuration of two-level systems in the environment of each chromophore. The experimental measurement of the distribution of temperature and scan-time dependencies would provide a sensitive test of the two-level system model.