Effect of surface degradation on optical performance of potassium dihydrogen phosphate optics

We have measured scatter produced by roughening of bare potassium dihydrogen phosphate (KDP) surfaces and by surface degradation (etch pits) that develop beneath a porous solgel coating on KDP after exposure to ambient relative humidity. The etch pits that form on coated KDP scatter incident light into strongly anisotropic angular distributions characteristic of the defect size and shape. The total integrated scatter (TIS) can be as high as 9% for a crystal with etch pits as compared with 0.05% for the as-manufactured crystal. The amount of TIS correlates with the area obscured by defects as measured by optical microscopy.