Choroidal melanomas occasionally respond very differently to irradiation by ruthenium beta-ray applicators. In order to clarify the role of the radiation parameters on the one hand and the tumor properties on the other, more precise data on the actual surface does rate than those furnished by the supplier are indispensable. With the aid of a measuring method based on the principle of thermoluminescence, the deviation was halved a +/- 15%. The measuring instrument is described, and its impact pact on therapeutic safety and indication limits is discussed.