The reliability of a 266 nm cw (continuous-wave) solid-state laser under the influence of purge gas was considered. Scatterers on a mirror during long-term operation were ammonium sulfate (NH4SO4). The synthesis of ammonium sulfate was related to the amount of water in the purge gas. UV power decreased by scatterers when the purge was not conducted.

We succeeded to develop a reference light source in the range of very low luminance using a double integrating sphere system, and calibrated a commercial spectrophotometer below 110-5 cd/m2 levels, which is 1/100 lower than the specified limit for measurement. And we improved measurements in the ultra low luminance range of displays using the calibrated commercial spectrophotometer and a dark sphere to suppress the influence of the surround.