This European Standard specifies the scale numbers and transmittance requirements for filters for protection against infrared radiation. The other applicable requirements for these types of filters and the frames/mountings to which they are intended to be fitted are given in EN 166. Guidance on the selection and use of these filters is given in annex B. NOTE The protective filters specified in this standard are not suitable for the direct or indirect observation of an electric arc. For this purpose a welding filter appropriate to the source being observed should by used.