Polymeric backsheets are an important component affecting the performance and durability of photovoltaic modules. The optical properties of the backsheet should be considered in the design and performance of a photovoltaic module and the stability and durability of optical properties have an impact on power, safety and appearance. Changes in optical properties in fielded modules and accelerated durability testing are compared. IR analysis was conducted on various backsheet materials in accelerated durability testing and compared to outdoor performance to better understand the relevant chemical changes and associated degradation mechanisms. The connection between optical properties and chemical changes is discussed.