The out-of-band rejection and the optical density (OD) are two critical specifications of tunable filters. Unfortunately these properties are often misinterpreted and their definitions tend to differ from one manufacturer to another. End users need to be careful when looking over the specifications of a filter. Also, the measurement of these properties for customers can be laborious. One needs to have sensitive instruments with a high dynamic range, a wide spectral range and a high power source. In this white paper, clear and rigorous definitions of the out-of-band rejection and the OD of widely tunable filters are presented, and the steps and instrumentation needed to accurately measure those specifications are exposed.
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