FNDF-100 standard Neutral Density Filter (Simply called FNDF-100 Filter) is jointly designed and developed by the Optical Department Of China Metrological Scientific Research Institute and Foshan Analytical Instrument Co., Ltd. It is used for the calibrating and testing of accuracy of extinction rate N of opacimeter, the Model FTY-100 Opacimeter, with single channel or similar structure. One set of FNDF-100 Filter ,its visible light transmission rate respectively is :34%,50%,and 71%. That is ,the extinction rate is :66%,50%and 29%, which meet the requirements of our National Metrical Testing Specifications for “Opacimeter JJG976-2010”.The value-determining method adopts the integral result under the condition of standard human eye visual function V(λ). It conforms to the international calibrating method, and it is more reasonable and scientific than that just determine the value at 555nm.Every set of FNDF-100 Filter has the calibration certificate from the China Metrological Scientific Research Institute.