The degradation of optical properties of transparent plastics is the single greatest cause for in-service removal and replacement. Some optical qualities are inherent in the geometry, manufacturing process, and materials, and remain relatively unchanged after manufacture, while others are subject to gradual change during exposure to the service environment. Factors having an influence on the crazing of transparent plastics include stress, ultraviolet (UV), moisture, and temperature. Sufficient data has been generated to make it evident that real-world conditioning must be experienced by developmental test specimens, as opposed to testing new unexposed material to determine durability, prior to in-service usage. However, the laboratory simulation of natural weathering, and especially accelerated simulation, is imprecise and correlation of results obtained for different plastics or from using different exposure apparatus should not be attempted until a valid database has been generated for such cross-correlation.