Changes in Physical Radiation Properties of Reflection Standards after UV-Irradiation.

Tables pressed of BaSO//4 are superior to those pressed of MgO powder with respect to their temporal stability. In the past they were proved in the visible spectral range as useful (white) reflectance standards. But by exposing them to UV-radiation one can observe a decrease of the radiance factor below lambda equals 250 nm (up to 7% at 312 nm). Although the radian e factor increases after a storage of the tablets in a desiccator, is does not reach the initial value before UV-irradiation. Responsible for the change of the reflection properites is the radiation below lambda equals 270 nm. some processes that may be responsible for the decrease of the radiance factor and for the ″recuperation″ are briefly discussed. Up to now they could not be proved.