sec_5_2_5__para
Any neutral density filter used in conjunction with opacimeter calibration, linearity measurements, or setting span shall have its value known to within 1,0 % opacity. The filter's nominal value must be checked for accuracy at least yearly using a reference traceable to a national or international standard.
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Source: EUR-Lex (Cellar) · retrieved 2026-11-01 · Text as adopted (Official Journal); later amendments are not incorporated in this text.