Power stabilized DL's represent today convenient sources for radiometric applications, as transfer laboratory standards. Stability measurements were carried out and reported by other authors for different time intervals, but only for low optical power levels (max. 16 mW). For calibrating usual optical radiometers, such low emitted power DL's are useless. This paper reports stability measurements carried out on several collimated DL's with λ around 980 nm and emitted power up to 265 mW in near-field/150 mW in far-field. Stabilities of the order 1e-4 for short-time intervals (tens of seconds) and 1e-3 for medium-time intervals (1 hour) were found for a non-thermostated structure, having the control photodiode (PD) in the same enclosure with the DL (at the rear of the structure). The corresponding stabilities for an external control PD resulted of the order 1e-5 for both short and medium time intervals. The schematic of the optical power stabilizer is presented