The present invention is directed to a method and apparatus for reducing the effects of window clouding on a viewport window in a reactive environment. One or more clouded viewport windows are obtained for testing, in which the clouding results from exposure to the reactive environment. The clouding typically appears as a coating film on the test windows. The clouded viewport windows are analyzed for one or more spectral regions having good transmission. The threshold level of light transmission is determined by the particular application in which the window is used. The spectral regions of good transmission are evaluated for their usefulness with a particular algorithm that will use the spectral data in a production environment. Spectral regions that cannot be evaluated using the subject algorithm are eliminated from consideration. Spectral regions that can be evaluated using the subject algorithm and exhibit low absorption are selected for monitoring in the production environment.