This test method is intended to spectrophotometrically measure the light blocking properties of window covering textiles against light passing through them and being seen by a standard observer on the darkened side of the textile (typically a curtain or drape.) 1.2 This method is applicable to all types of fabrics, including knits, wovens, and non-wovens, as well as to fabrics taken from end product items. However, it is not recommended for mesh structures with mesh holes greater than 3 mm across.